T h e I n s t i t u t i o n of P r o d u c t i o n E n g i n e e r s J o u r n a l VOL. 38 N O . 5 MAY, 1959 The Production, Fabrication, Properties and Uses of Some of the Newer Metals by Dr. N. P. INGLIS Research Director, Metals Division, Imperial Chemical Industries Ltd. Presented to the Institution of Production Engineers on 11 th March, 1959, at the University of Birmingham, as THE 1958 VISCOUNT NUFFIELD PAPER I T is perhaps important to commence by emphasising that within the compass of a single lecture the comprehensiveness of the title cannot be matched by similar comprehensiveness of the matter in the lecture itself. This is, I think, an excusable fault, often found in most named lectures of this sort where the lecturer usually takes as his subject a broad survey of a particular field and perforce must sacrifice depth for broadness. Briefly, the object of the present lecture is to emphasise the growing industrial importance of a number of metals which a few years ago were little more than laboratory curiosities and certainly not of industrial significance. It is proposed to illustrate this thesis mainly by dealing fairly fully with titanium, zirconium and beryllium, .with some more brief reference to other of the newer or less usual metals, such as niobium, hafnium, tantalum, etc. For a great many years use of metals for structural purposes has mainly involved iron, aluminium, copper, nickel, zinc and lead. It is true that alloy additions of other elements to these metals have been 219
Transcript
T h e I n s t i t u t i o n o f P r o d u c t i o n E n g i n e e r s J o u r n a l
V O L . 3 8 N O . 5 M A Y , 1 9 5 9
The Production, Fabrication,
Properties and Uses of Some
of the Newer Metals
by Dr. N. P. INGLIS
Research Director,Metals Division,Imperial Chemical Industries Ltd.
Presented to the
Institution of Production Engineers
on 11 th March, 1959, at the
University of Birmingham, as
THE 1958 VISCOUNT NUFFIELD PAPER
IT is perhaps important to commence by emphasisingthat within the compass of a single lecture the
comprehensiveness of the title cannot be matchedby similar comprehensiveness of the matter in thelecture itself. This is, I think, an excusable fault,often found in most named lectures of this sortwhere the lecturer usually takes as his subject abroad survey of a particular field and perforce mustsacrifice depth for broadness.
Briefly, the object of the present lecture is toemphasise the growing industrial importance of anumber of metals which a few years ago were littlemore than laboratory curiosities and certainly not ofindustrial significance. It is proposed to illustrate thisthesis mainly by dealing fairly fully with titanium,zirconium and beryllium, .with some more briefreference to other of the newer or less usual metals,such as niobium, hafnium, tantalum, etc.
For a great many years use of metals for structuralpurposes has mainly involved iron, aluminium,copper, nickel, zinc and lead. It is true that alloyadditions of other elements to these metals have been
219
made and considerable success achieved in suchalloying, but broadly speaking the above six havebeen the main engineering structural basic metalsuntil very recent times.
The metals referred to above—titanium, zirconium,beryllium, niobium, etc. — are none of them new inthe sense that their existence has only recently becomeknown. Thus, the existence of titanium was recognisedin 1791 and it was isolated, admittedly in impure form,in 1825. The existence of zirconium was recognisedat about the same time and it was isolated in 1824.The existence of beryllium was recognised in 1798and first isolated in 1828. The existence of niobiumwas recognised by various chemists over the period1801 - 1844. It might, therefore, be asked why thesemetals, which have been lying dormant, so to speak,for so many years should now be of interest topractical engineers.
The reason, of course, is that new inventions, newdesigns, and new processes are calling for entirely newmaterials of construction. Thus, the discovery anddevelopment of nuclear energy, which can rightlybe regarded as leading to a second IndustrialRevolution, is playing a major part in stimulatingeffort in some of these new metals. Just as thedevelopment of the use of steam in railways andshipping rather more than 100 years ago necessitateda material of construction superior to cast andwrought iron and, therefore, fathered the manufac-ture of steel in gross tonnages, so the specialrequirements of nuclear engineering demand theavailability of materials having certain special require-ments. Then again, high speed flight and the initiationof what the newspapers are fond of calling " TheSpace Age" brings the need for materials havingspecial properties, in particular, metals of low densityand high strength with the added ability of beingable to withstand higher temperatures than the earlierlight metals such as aluminium and magnesium.Thirdly, the power-producing and chemical andpetroleum industries demand materials of constructionhaving higher strength at elevated temperature thanthe metals previously available and also materialsresistant to such corrosive conditions as cannot be metby even the special corrosion-resistant steels, thediscovery and development of which in the secondand third decades of this century was one of thelandmarks in metallurgical history. And so the reasonfor our very real and active interest in such metals astitanium, zirconium, beryllium, niobium and others,is simply that the advances of mechanical and physicalscience and practice demand it.
These metals are no longer solely of academicor subsidiary interest; they are now, or will shortlybe, an essential part of the overall industrial pictureand be available choices for the engineer whenselecting the material of construction to suit therequirements of his mechanism, structure, or the like.
Before we say a few words separately about eachof these metals it might be worthwhile to look at theavailability of different metals in the earth's surface,and Table I is, therefore, presented with this object.
This Table shows the well-known fact that
aluminium, iron and magnesium, in that order, arethe most abundant of our metals, but perhaps it isless appreciated that titanium is by no means rareand that metals like zirconium and vanadium areactually more abundant than the much morecommonly used metals like nickel, copper and zinc.
titanium
extractionAlthough, as mentioned above, titanium was
isolated in 1825, it was not until exactly 100 yearslater than van Arkel and de Boer, by the very elegantmethod of thermal dissociation of the tetraiodide,produced the metal in a form sufficiently pure toallow a true assessment of its properties. It was thenshown that in addition to a low specific gravity (4.5)the metal had very great corrosion resistance andcould be readily alloyed to give material of tensilestrength equal to or greater than high tensile steels.This combination of properties is of great andimmediate interest to the aircraft and aero enginedesigner, and in the years following 1925 work wasdone on the selection and development of anindustrial scale process for the manufacture oftitanium.
The natural form of titanium is rutile (TiO2) orilmenite ( (FeTi)O3), but, on account of the extremelygreat affinity of titanium for oxygen and the profoundeffects on the properties which even a small amountof residual oxygen has, no satisfactory direct methodof reducing the oxide to the metal has been found.An indirect route was therefore necessary wherebythe oxide is first converted to the tetrachloride — anormal and well established process — and thetetrachloride is then reduced, either by magnesiumor sodium, at a temperature of 800°G - 900°C in asealed vessel under an argon atmosphere.
The process was placed on an industrial basis byKroll, the greatest name in titanium technology, whoused the magnesium route. The products of hisreduction reaction are a rather coke-like mass of
TABLE IOCCURRENCE OF VARIOUS METALS IN THE EARTH'S
titanium metal sponge and magnesium chloride. Theremoval of the magnesium chloride by leaching withacid caused difficulties and it is generally preferableto purify the Kroll reaction product of the magnesiumreduction process by vacuum distillation.
The process developed by I.C.I, in this countrydiffers appreciably from the Kroll magnesium processin that sodium is used as the reducing agent. It isbelieved that this has several advantages over themagnesium route, since the reaction mass is verymuch easier to grind and the sodium chloride by-product is readily removed by leaching. The fact thatthe titanium metal produced by the sodium process isin finely divided or granular form is advantageouswhen it comes to the subsequent compacting to makeconsumable electrodes for melting, and also hasadvantages in adding other elements in order to makethe required alloys.
Both processes produce equally pure titanium metal,but it is considered that the sodium process is cheaperin capital and running costs and it is thought to becapable of greater development. Most of the plantsbuilt in the United States operate on the magnesiumroute, although the most recent plant there uses thesodium route. In Great Britain the only large plant isthat built by I.G.I, and this, with a nominal annualcapacity of 1,500 tons, operates on the sodium route.It might be mentioned here that production capacityfor titanium metal in the United States is estimatedat 25,000 - 30,000 tons per annum.
On the subject of purity it should be noted thatthe most likely impurities in raw titanium are oxygenand nitrogen, which cause considerable increase ofhardness with corresponding increase in strength and
loss of ductility and malleability. The effect of theseimpurities on hardness is such that a measurement ofhardness is accepted as a yardstick or rough indica-tion of the purity of the raw material. The hardnessof the purest titanium yet reported was found to be50 D.P.N., whereas commercially pure titanium asnormally produced in Great Britain today is 120-130 D.P.N. (this is equivalent to 110-120 on thestandard Brinell scale, which in turn is equivalent to100-110 on the sub-standard Brinell test favouredby some U.S. producers).
meltingTitanium is so reactive when molten that it attacks
all the refractory oxides normally available for furnacelinings and crucibles and, in addition, it must bemelted out of contact with air. Consequently, themethods employed for melting titanium and the pro-duction of ingots differ very considerably from thoseemployed with the older metals.
A metal crucible, very thoroughly water-cooled, isused with an internal source of heat and the cruciblemust be sealed in such a way that melting can becarried out in a vacuum or in an atmosphere of argon.These necessary requirements have been translatedinto practice by arc melting a consumable electrodein a water-cooled copper crucible maintained undervacuum. A schematic representation of this meltingsystem is shown in Fig. 1. The consumable electrodeis made by mixing the raw titanium granules with thedesired alloying elements and compressing the mixtureinto pieces strong enough to be handled, pressures of
10-20 tons per sq. in. being required. These piecesare then welded together to give the required weightto be melted. As mentioned earlier, it has been foundin practice that the granular raw titanium producedby the sodium reduction method is very much moreeasily compacted than the coarser product of theKroll process and also allows the better incorporationof the alloying elements, with less risk of segregationin the subsequent ingot. Fig. 2 is a photograph of aproduction scale consumable electrode arc furnacewhich is believed to be the largest of its kind inEurope, and is capable of producing a 4,000 lb.titanium ingot.
The danger of puncturing the crucible wall and socausing contact between water and molten titaniummust be considered and, therefore, the furnace isisolated during melting behind blast walls and isoperated entirely by remote control, as shown inFig. 3.
wrought formsWhilst the conversion of titanium ingots into plate,
Fig. 2.Production scale consumable electrode arcfurnace used for the melting of titanium and
zirconium
rod, sheet, and other wrought formsrequired has not been without difficulty,most of the major problems involvedhave now been overcome and, ingeneral, conversion to these forms hasbeen carried out with equipment usedfor similar processes on other metals.
The difficulties experienced in hotworking have been more chemical thanmechanical, in the sense that they havebeen concerned with the prevention ofundue contamination by oxygen, nitro-gen and hydrogen, all of which havedetrimental effects on the metal. At onetime it was thought that it might be
necessary to sheath the metal for hot working in orderto prevent atmospheric contamination, but fortunatelyit was found that the diffusion of oxygen andnitrogen in titanium at hot working temperatures wasrelatively slow, and by very carefully observed heatingtimes and procedures these impurities could be con-fined in the hot worked product to a thin surfacelayer which must be removed by chemical andmechanical methods.
Hydrogen, on the other hand, diffuses very readily,and although it can be removed by subsequent heatingin vacuo the more usual course is to limit the chancesof hydrogen contamination by extremely strict controlof the furnace atmosphere.
Hot extrusion is an extremely convenient way ofproducing metal in various sectional forms, but un-fortunately it has not proved as successful for titaniumas for many other metals. This is because of the well-known feature of titanium that when forced througha die it is prone to seize and so cause imperfectionsin the extruded metal surface. A considerable amountof attention has been given to this problem, the
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solution to which clearly depends upon more effectivehigh temperature lubrication. Some success has beenachieved by sheathing the workpiece in copper andalso by the use of glass as a lubricant, and althoughthere is still room for further improvement satis-factory progress is being made.
Whilst the cold rolling of commercially puretitanium is straightforward and calls for no comment,the cold rolling of the high tensile titanium alloys isonly possible by repeated small reductions and fre-quent intermediate annealing. This difficulty is notpeculiar to titanium; it is encountered in the coldrolling of any metal with a tensile strength as highas 60 - 70 tons per sq. in. arid a high ratio of proofstress to ultimate stress. So far it has been the generalpractice with such alloys to hot roll in single sheets,or, where thicknesses of, say, 22 s.w.g. are required, topack roll a number of sheets together, sometimeswithin steel cover plates. Hot rolling methods such asthese perforce give somewhat poorer surface finishand dimensional tolerances than if finishing weredone by cold rolling, and it is not surprising, therefore,that a considerable research effort is in progress toovercome these difficulties. One obvious approachlies in developing a heat treatable alloy which in onecondition of heat treatment is soft enough to bereadily cold rolled, but which can then bestrengthened by heat treatment after rolling.Important and promising results have already beenobtained in this connection.
The cold drawing of titanium andtitanium alloys is hampered by thetendency referred to previously for thetitanium to seize in the die, but variousmethods of reducing this tendency havebeen developed. Thus, quite finetitanium wire has been successfullydrawn by silver plating the stock, or byvarious chemical treatments such astreating with cyanide or phosphate orcontrolled oxidation at high tempera-ture. All these methods solve theproblem by providing a surface whichwill hold a lubricant and which reallyprevents direct contact between themetal and the die.
Because of the sensitivity of titaniumwhen hot to atmospheric gases, fusionwelding of the metal is really onlypossible by the arc processes carried out
Fig. 3.Remote control panel of a commercial
titanium melting furnace
in an inert atmosphere as, for example, the well-known tungsten argon-arc process. Even here pre-cautions have to be taken additional to those usedwith the argon-arc welding of the older metals. Thus,it is necessary to protect those hot surfaces which arenot normally protected by argon issuing from thewelding torch. Several ingenious ways of doing thishave been devised as, for example, the use of specialbacking plates with channels to supply argon to theunderfaces of the weld. In some cases fusion weldingby this process has been carried out in a chamberfilled with inert gas. Titanium can be resistance,pressure and flash-butt welded satisfactorily withoutdifficulty and particularly good welds have beenobtained, not only in titanium, but in some of itsalloys by the well-known flash-butt welding method.
Provided suitable precautions are taken, very satis-factory welds can be made by the argon-arc processin commercially pure ^titanium, the welded materialgiving a very satisfactory bend radius and also havinga tensile strength equal to that of the unweldedmaterial. This is also true to some extent of the alphaalloys, but it is not true of the alpha-beta alloys. Evenwhere atmospheric contamination has been completelyprevented, welded joints in such alloys are lacking inductility owing to reactions which occur duringcooling. The ductility of such material can beimproved to some extent by heat treatment afterwelding but, generally speaking, welded joints in the
223
- ' ? • • •
Fig. 4 (left). Some wrought forms in titanium
Fig. 5 (below). Examples of forged, welded,machined and manipulated titanium
alpha-beta alloys must be given a lowerrating or weld efficiency factor.
That the various forming andfabricating processes, to which briefreference has been made, have provedsatisfactory in practice and havereached full commercial status isdemonstrated by Figs. 4 and 5, whichillustrate groups of miscellaneouswrought, welded, machined and mani-pulated forms.
propertiesA very great deal of investigation of
the physical and mechanical propertiesof titanium and its alloys has been doneand it is not possible to do justice tothis volume of work in a brief statement such as this.The softest grade of commercially pure titanium nowbeing regularly produced will, after melting andprocessing to wrought form, have a tensile strengthof the order of 25 tons per sq. in. with excellentductility and formability. This very soft and ductilematerial is of main interest to the chemical engineerfor whom the extreme corrosion resistance oftitanium, combined with ease of welding andmanipulation into various shapes, is particularlyattractive.
Those concerned with aircraft and aero enginedesign are more interested in the higher strengthalloys and Table II gives the room temperaturemechanical properties for a number of the titaniumalloys in commercial production or in an advancedstage of development today.
Up to 882°G titanium has a hexagonal structureand in this form is known as alpha-titanium; abovethis temperature it changes to a body-centred cubicform, known as beta-titanium. The addition of other
elements raises or lowers this change point and byappropriate alloying and heat treatment it is possibleto obtain alloys that consist entirely of the alphaphase, entirely of the beta phase, or a mixture of thetwo. Each of these types has characteristic properties.Thus, the all-alpha alloys are strong and maintaintheir strength well at higher temperatures, but aredifficult to fabricate, particularly into sheet; the all-beta alloys are normally less strong, are easier to work,but are usually unstable if used at elevated tempera-tures. In addition, they have much lower resistanceto creep at elevated temperatures than the alphaalloys. The alpha-beta alloys are intermediate in typeand give the best combination of high strength andductility.
Because of the relatively high melting point oftitanium (1660°), it was at one time thought that themetal would have particularly high strength atelevated temperatures. This has not been found to beso since the fall in strength of titanium with rise oftemperature is appreciable, e.g., the strength at
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TABLE II
TYPICAL ROOM TEMPERATURE MECHANICAL PROPERTIES OFCOMMERCIAL TITANIUM ALLOYS IN THE ANNEALED CONDITION
NominalAlloy
Content
5% Aluminium2.5% Tin
1.5% Aluminium1.5% Manganese
1.5% Aluminium3.0% Manganese
2.25% Aluminium3.25% Manganese
4% Aluminium4% Manganese
6% Aluminium4% Vanadium
5% Aluminium2.75% Chromium1.25% Iron
5% Aluminium1.5% Iron1.4% Chromium1.2% Molybdenum
2.2% Iron2.1% Chromium2.0% Molybdenum
3.0% Aluminium5.0% Chromium
8.0% Manganese
Designation
Titanium 317Hylite 30A- I I0 ATTi-5AI-2.5SnRS-II0C
400°C of commercially pure titanium is rather lessthan 40% of the strength at room temperature. Con-siderable improvement in this respect can be obtainedby alloying and a comparison of the strengths at hightemperature of a number of different alloys is shownin Fig. 6. It will be seen that this Figure gives thestresses required to cause a given amount of plasticstrain in a given period, this or a similar criterionbeing required usually by designers of hightemperature equipment, who are not only concernedthat their selected material should not actually fractureunder the selected stress, but also should not give morethan a certain allowable deformation. This Figure
shows very clearly the considerable improvements inhigh strength brought about by suitable alloying, andfurther improvements in this direction can be con-fidently expected, since, although a considerableamount of work has been done in the titanium alloyfield, there are many areas still relatively unexplored.
corrosion resistanceWhilst the major interest in the metal in its early
days was in the aeronautical field, and indeed stillis, the extremely good corrosion resistance hasnaturally attracted the attention of chemical plant
225
60
50 -
40 -
STRESS
TONS/IN*30
^0 -
\
-
•A 6. TTTANIUM 3 I 4 A .A — - A TITANIUM 3I8A.
• — • TTTANIUM3I7
0—-O TITANIUM 3I4C.
\
in o TTTANiUM EX.0I3A.
* TITANIUM EX.013.
• HYLITE 50.
V-100 200 300 400
TEMPERATURE -DEGREES CENTIGRADE.
500 600
Fig. 6. Stress to give 0.1 % total plastic strain in 300 hours
for various titanium alloys
designers. Titanium, in fact, is resistant to a widerrange of corrodents than are the chromium-nickelaustenitic steels.
It would be both tedious and too encroaching onlimited time and space to attempt any list of thecorrodents to which titanium is resistant, but to givea general indication of its class in this respect thefollowing statistics may be of value. Recently, whenexamining a table of corrosion data covering theaction of 220 different corrosive media, the writernoted that in 172 cases titanium was in class A ofthe classification used by the U.S. Electro-ChemicalSociety, i.e., attack was less than 0.005 in. per year.In 25 other cases it was in their class B, i.e., attackbetween 0.005 in. and 0.05 in. per year, which,depending on various circumstances, is often regardedas tolerable. In 23 cases only out of 220 was the rateof attack such as to make titanium unsuitable foruse. These are very remarkable figures and clearlyput titanium in what might be termed withoutexaggeration the super corrosion-resistant class.
Perhaps time might be taken to say just a littleabout its really remarkable resistance to sea water,which is so ubiquitous a corrodent. A very greatmany tests have been done in this medium and theresults confirmed by many investigators. In theordinary sea water immersion tests, even when thesea water is heavily polluted, there is virtually noattack at all on titanium, even after very long periodsof immersion; impingement tests in aerated sea waterwhen carried out for many thousands of hours havefailed to cause the slightest pitting; it is almost com-pletely unattacked under "deposit" or differentialaeration conditions which heavily pit corrosionresistant steel; perhaps most remarkable of all, itsfatigue limit in sea water is found to be actuallyslightly greater than its fatigue limit in air.
Its use as a material of construction in chemicalplant is only just commencing, but there is littledoubt that in spite of its high price this applicationwill grow.
Naturally, efforts are being made to reduce thecost of plant made in titanium by developing methodsof lining, and Fig. 7 shows a large vessel, constructedin mild steel but lined throughout, branches included,with commercially pure titanium.
Whilst it will be seen that the corrosion resistanceof plain, untreated titanium is very great indeed in alarge variety of media, the story of corrosion resistancedoes not end there. A new chapter has been openedby the discovery of the remarkable way in which thecorrosion of titanium can be minimised or preventedeven under conditions which would normally causeattack on the metal. Titanium is not a noble metaland, as with the high chromium steels and aluminium,the corrosion resistance is due to the presence of astrong, tenacious film of oxide which heals rapidlywhen damaged. Growth of this protective film can bestimulated if the electrode potential of the metal israised by connecting it to a positive source of directcurrent. Aluminium and stainless steel can be similarlytreated and, indeed, the well-known process ofanodising aluminium is simply an example of thusstimulating growth of the protective oxide film. Withmetals such as stainless steel and aluminium, theconditions under which the protective film can beincreased are limited to a few electrolytes and a veryrestricted range of potentials. With titanium, however,it has been found that the growth of the protective
Fig. 7. Titanium-lined vessel for chemical plant
226
TABLE IIIEFFECT OF IMPRESSED CURRENT ON CORROSION RATE
OF TITANIUM
TABLE IVTHERMAL NEUTRON ABSORPTION CROSS-SECTIONS OF
DIFFERENT METALS
CORRODING MEDIUM
CONCENTRATION BY
WEIGHT
4 O ° / O H 1 S 0 4
4O7. H2SO4
3 5 ° / , H3P04
3 5 % HCI
5O°/o formic acidCN2 stirred)
25° / , oxalic acid
TEMPERATURE
(°c)
6O
101
60
60
IOI
I O O - 5
CORROSION RATE IN
IN./YR.WITH
APPUED ANODC
VOLTAGE OF 1 5
OOOOO28
OOO75
OOOOI56
0005
OOO3I
OOO95
FACTORED
WHICH RATE OF
CORROSION IS
REDUCED
33,000
1,980
4OO
67O
80
33O
THERMAL NEUTRON ABSORPTION CROSS SECTIONS . (in Barns}
Material
IRIDIUM
RHODIUM
HAFNIUMGOLD
SILVERTANTALUMPLATINUM
PALLADIUM
CHROMIUM
TfTANIUM
VANADIUM
NICKEL
COPPERIRON
MOLYBDENUMNIOBIUM
ALUMINIUMZIRCONIUM
LEAD
MAGNESIUMBISMUTHBERYLLIUM
CARBON
Cross Section4 3 O
I S O
IOS9 862
218 1
8 O7-9
5-6
S I
4 6
3-692-532-51 • 1O 2 3
0 1 8
00630032O-OIOO-OO3
film can be stimulated and any damage immediatelymade good in a very wide range of media and overa very wide range of potentials.
Reference to Table III shows the amazing reduc-tion in attack on titanium when subject to thiselectro-chemical mechanism. All the corrodents listedin Table III attack plain, unprotected titanium butif a very small positive direct current voltage isapplied and maintained between the titanium and acathode, the corrosive attack is reduced by a factorwhich in some cases runs into thousands.
Another method of anodically protecting titaniumis to connect it to a more noble element, such asplatinum. This can conveniently be done by depositingon the titanium a very thin film of platinum, and itshould be emphasised that this film need not becontinuous and can, in fact, be subject to grossporosity, since the film is in no sense intended to bea protective barrier because at any discontinuities thesubsequent electro-chemical action causes the forma-tion or healing of the protective film on the titanium.
This latter development of platinised titanium isexpected to have a very big future in electrolyticprocesses, where the problem is so often to find ananode which will conduct electric current into acorrosive electrolyte without causing catastrophiccorrosion of the anode material.
zirconium and berylliumBefore dealing individually with zirconium and
beryllium it will be helpful to say something of the
requirements of certain materials of construction innuclear engineering, because the principal uses ofboth zirconium and beryllium are in this field.
The nuclear power reactors now under construc-tion and several of those still in the exploratory anddevelopment stages, are of the type in which the heatof the nuclear reaction is abstracted by a coolantwhich is circulated past the reacting fuel. Now thenuclear fuels such as uranium, plutonium andthorium, are severely attacked by all the coolantslikely to be used, such as carbon dioxide, water, orliquid metals. In addition, direct contact between thefuel and the coolant would result in radioactive con-tamination of the coolant, which is later processed inrelatively normal equipment to transform theextracted heat into usable energy. Consequently it isnecessary to protect the fuel by a sheath or can andthe material of this can must have certain properties;this limits choice quite appreciably. Briefly, theproperties required are as follows :
1. The chosen material must be compatible withboth fuel and coolant, i.e., it must not alloywith the fuel and it must be resistant tocorrosion by the coolant, in both cases over thewhole temperature range involved.
2. To achieve good neutron economy it must havelow capacity for the absorption of neutrons, orexpressed in nuclear engineering terms, it musthave a low neutron absorption cross section.Table IV shows the neutron absorption crosssections for a wide selection of metals.
227
3. It must have reasonable strength at operatingtemperature so as to resist without fracturedistortion of the fuel and also should havereasonable ductility to accommodate anyswelling of the fuel.
4. It should preferably have good thermal con-ductivity and also be capable of being processedinto the shapes required which are not, ideally,simple shapes since finning and the like of thecans can greatly improve heat transfer (see, forexample, Fig. 8, which is typical of the sort ofshape of the magnesium alloy cans presentlyused in the GO2 gas cooled reactor stations).
The importance of zirconium and beryllium fromthe point of view of the second requirement abovewill be very clear from Table IV which also showsthat, unless one can afford to be extremely prodigal inneutrons, choice is restricted to relatively few metalsat the lower end of the Table and some of whichwould obviously not fulfil other requirements. Withregard to zirconium and beryllium, details of themanner in which they fill these other requirementswill emerge later in this lecture, but it can be saidthat zirconium admirably fulfills the requirements ofcompatibility with water as the coolant, but wouldnot be so satisfactory if CO2 were the coolant.Beryllium, on the other hand, would not be satis-factory with water as the coolant, but it has excellentresistance to GO2, both wet and dry, up to a tern-
Fig. 8. Longitudinally finned cans in a magnesium alloy of thetype used in current gas-cooled reactors
perature between 500°C - 600°C, and above 600°C ifthe gas is dry.
Details of the strength, ductility, etc., of these twometals will be given later but it can be said that theyare adequate to meet reasonable design requirements.The ability to shape them into the forms required isstill being studied, but they can certainly be fabricatedinto most of the commoner wrought forms, althoughthe designer may have to be content with a somewhatsimpler final shape than that which would be possiblewith many other metals.
It will be seen, therefore, that although a principaluse for both beryllium and zirconium is for cans andother items of what is colloquially termed " reactorironmongery", they are not at present competitive witheach other, since the selection of one as comparedwith the other depends on the type of reactor.
The British nuclear power programme is based onthe gas-cooled reactor using unenriched, or onlyslightly enriched, uranium as the fuel. In such areactor neutron economy is particularly important.In the first British commercial scale nuclear powerreactors, a magnesium alloy will be used as thecanning material. It will be seen from Table IV thatwhilst magnesium has a relatively low neutronabsorption cross section it is still about six timesgreater than that of beryllium. Perhaps even moreimportant is that the strength of the magnesiumalloy at moderately elevated temperatures is notparticularly good and, therefore, the surface tem-perature of a magnesium alloy can must be limitedto about 400°C - 450°C. In order to increase thethermodynamic efficiency by increasing this tem-perature, a change must be made from this magnesiumalloy and clearly, from what has been said already,beryllium would be a natural selection.
zirconium
extractionThe extraction, fabrication and general metallurgy
of zirconium are so similar to those of titanium thatmuch of what has already been said about the latteris true for zirconium and perhaps the simplest waywould be to point out the main differences in theprocessing of the two metals.
In the first place it is important to note that as allzirconium minerals contain between 0.5% and 2% ofhafnium, and as the presence of hafnium is extremelydetrimental to the value of zirconium for nuclearpurposes — as will be clear from Table IV — oneproblem in the extraction is the removal of hafnium.Moreover, this removal must be very complete, sincethe usual specification for zirconium of reactorquality is that the hafnium content must not exceed100 parts per million.
Several methods for achieving this separation havebeen devised, such as fractional distillation of thedouble fluorides, solvent extraction and ion exchangetechniques, but all one can do within the compass ofthis lecture is to emphasise the extreme importanceof this separation, since zirconium is almost entirelyrequired for nuclear energy purposes.
228
The reduction of the metal is, as with titanium, byreduction of the tetrachloride and the magnesiumreduction route has invariably been followed. Thedifferences from the titanium reduction process arisefrom the fact that zirconium tetrachloride is a solid,whereas titanium tetrachloride is a liquid and, also,zirconium chloride is soluble in magnesium chlorideso that the salts cannot be tapped from the reactioncrucible as can be done in the titanium reductionprocess. Purification of the reaction mass is, therefore,by vacuum heat treatment, in which magnesiumchloride is removed partially by melting and partiallyby distillation and the magnesium metal is distilledand collected on condensers. A sodium reductionprocess, as for titanium, has not been worked out forzirconium. World production capacity for zirconiummetal is probably of the order of 3,000 tons perannum, chiefly in the United States.
melting and fabricationWhen it comes to melting, the equipment and
techniques suitable for titanium and previouslydescribed have been and are being used with com-plete success for zirconium, always bearing in mindthat the melting point of zirconium is 1850°C ascompared with 1650°C for titanium. The hot workingof zirconium is generally similar to that of titaniumbut does introduce some additional difficulties. Thus,oxygen and nitrogen diffuse more rapidly inzirconium than in titanium and, therefore, the depthof the hardened layer for material heated at the samehot working temperature and for the same time as fortitanium will probably be 50% greater. This naturallymeans that one uses as low a temperature as possiblefor hot working and controls the time at temperatureeven more rigidly than is the case with titanium.For the same reason there is a greater incentive inthe case of zirconium to anneal or otherwise heat treatin argon or under vacuum. Descaling is difficultenough with titanium, but it is even more difficultwith zirconium, and mechanical means are usuallyemployed. Furthermore, although surface contamina-tion by diffusion of gases must be avoided as much aspossible with titanium, the tolerance to such con-tamination is even less for zirconium. Hot-worked
and heat-treated zirconium must be very thoroughlypickled in order to remove any residual surfacegaseous contamination, because the subsequentcorrosion rate in service is adversely affected by anynitrogen contamination. Such pickling involves theuse of very strong pickling agents of the hydrofluoric-nitric acid type.
Apart from these differences the whole productionprocess must be geared to an even higher standard ofpurity than the high standard employed withtitanium. In particular it is necessary to control tominimum levels nitrogen, aluminium and titaniumbecause of adverse effects on corrosion, most otherelements because of their effect on neutron capture,and certain elements which have a long half-life afterirradiation, e.g., cobalt. Naturally, these precautionsapply to welding, where very similar methods areused as for titanium, but again with special emphasison minimising all contamination due to gaseousdiffusion and particularly nitrogen. With suitableprecautions open air argon-arc welding has provedsatisfactory but enclosed welding may be necessary insome cases. Incidentally, the unusually high standardsof purity which apply have clearly thrown a veryheavy burden on the analytical chemists involved inthis class of work. Not only have they had to workout the general analytical chemistry of zirconium, butthey have had to apply their techniques and methodsto the determination of quantities far below thosenormally required in the general metallurgicalindustry.
That these standards can be, and indeed are beingmaintained, is perhaps demonstrated by Fig. 9, whichshows some fabricated articles in zirconium requiredfor nuclear power stations.
alloying : effect on propertiesSince the main virtue of zirconium is its combination
of low neutron absorption capacity and corrosionresistance to water, a great deal of attention has beengiven to the effect of alloying in order to improvecorrosion resistance without impairing neutronabsorption characteristics. Under the conditions ofhigh pressure and high temperature involved in apressurised water cooled reactor, some corrosion of
Dr. Inglis was born in 1902 and educated at the Liverpool Collegiate Schooland the Universities of Liverpool and Illinois.
He joined Synthetic Ammonia Nitrates, Ltd. (now the BillinghamDivision of Imperial Chemical Industries, Ltd.) in 1927 as a metallurgist, andin that capacity carried out research and development of metals suitable forwhat were then novel chemical processes. These included work on the influenceon steels of hydrogen at elevated pressures and temperatures, and also onproblems connected with the use of corrosion resistant steels.
Dr. Inglis was appointed a Director of the Metals Division of I.C.I,in 1947, and took up his present position as its Research Director, in 1951.
229
Fig. 9. Fabricated articles in zirconium
zirconium takes place in the form of a uniformattack on the metal with the formation of an oxidefilm. After a certain length of time there is a dangerof what is known as break-away corrosion, whichsimply means that the film of oxide becomes dis-continuous and flakes away from the surface (thisstate of affairs applies to metals other than zirconium).Thus, when commercially pure zirconium is exposedto very high temperature water (say 400°C) aglossy, black adherent film is formed, but it has beenfound that after a length of time, which varies fromspecimen to specimen, breakaway corrosion occursand the film spalls off and corrosion increases at anaccelerated rate. This has been attributed toimpurities, particularly nitrogen, but also carbon,aluminium and titanium, although the content ofthese was very low as judged by the standardsnormally applied to metals. A very considerable alloyprogramme is therefore in progress, mainly, but notentirely, in the United States, in order to produce ifpossible an alloy which will give greater repro-ducibility in corrosion conditions, will increase thetime before breakaway occurs and, if possible, reducethe corrosion rate in any post-breakaway period.
From the considerable amount of work which hasalready been done it seems clear that only arelatively few elements are likely to be helpful as,alloying additions for improving water resistanceand these are tin, iron, nickel and chromium.The first tangible result of this work has been thedevelopment of a zirconium alloy with 1.5% tin,0.1% iron, 0.05% nickel and 0.1% chromium, whichis generally known as Zircaloy 2. The tin contentactually increases the initial corrosion rate, but it hasthe very important effect of producing an extremelyadherent oxide scale and it certainly counteracts to aconsiderable degree the harmful effect of nitrogen.Whilst the nitrogen in this alloy must still be kept lowthe allowable nitrogen level is higher than that forunalloyed zirconium. It is considered that with thisalloy a nitrogen content of 100 parts per millionmaximum can be tolerated. Even so, insufficient workhas yet been done to be quite certain that if it is usedwith a nitrogen content below that mentioned, thecorrosion resistance will be completely satisfactoryand not unduly influenced by other factors, whichmay vary from ingot to ingot. Therefore, at thepresent time it is usual for production ingots to bepassed forward for further processing on the basis of acorrosion test in pressurised water. Since such testsinvolve pressures of the order of 1,500 lb. per sq. in.and temperatures of the order of 400°C, elaborateapparatus is required such as that shown in Fig. 10,where the research worker is seen inserting into anautoclave a rack carrying the specimens to be tested.
It has already been mentioned that zirconium hasrelatively poor resistance to CO2 at elevated tem-peratures, so that despite a fairly attractive neutronabsorption character the metal is not a candidate as acanning material for the gas-cooled reactor. Even soit is not without use for the latter. For supportbrackets and components other than cans whichrequire fairly good strength and reasonably goodneutron absorption property, zirconium is a veryreasonable choice. For such purposes it can be madesomewhat more massive than the cans themselves,which have a relatively thin wall and consequently in
TABLE V
PROPERTIES OF ZIRCONIUM AND ZIRCONIUM ALLOYS
Commercially pure reactor grade zirconium
Zircaloy 2
Metropolitan Vickers alloy
Composition
1.5% Sn0.1% Fe0.05% Ni0.1% Cr
0.5% Cu0.5% Mo
0.1%Proof Stresstons/sq.in.
17.6
22
25
U.T.S.tons/sq.in.
26.4
31
33.6
Elongationon 2 in.
30
27
22
4—I
230
this somewhat more massive form its rather indifferentresistance to GO2 can be tolerated. Naturally, how-ever, its use in such cases has stimulated work aimedat the development of zirconium alloys more resistantto CO2 and most of this latter work has been donein this country. An early result of it is the alloy putforward by the Metropolitan-Vickers -Company,which is a zirconium alloy containing 0.5% copperand 0.5% molybdenum, and which has been shownto have an appreciably greater resistance to CO_>than either reactor grade zirconium or Zircaloy 2.The mechanical properties of zirconium and the twozirconium alloys mentioned are given in Table V, towhich might be added the note that both the alloyshave appreciably better creep strength at 450° C thanthe commercially pure reactor grade zirconium.
berylliumextraction
There are four main stages in the extraction ofberyllium from the ore, beryl, these being :
1. The ground ore is mixed with an alkali doublefluoride, and roasted and leached to give asoluble alkali beryllium fluoride solution.
2. The fluoride solution is treatedwith caustic soda and thisprecipitates beryllium hydroxidewhich is then calcined to give theoxide.
3. The oxide then undergoes atreatment to convert it into ahalide — a fluoride if the reduc-tion to metal is to be by thethermal route and the chloride ifit is to be by the electrolyticroute.
4. There are two methods ofreducing the halide to metal andthere is at present somecontroversy as to which methodproduces metal with optimumproperties for nuclear engineering
requirements. The pros and cons of this contro-versy are complicated by the fact that so farthe electrolytic material has been produced ona much smaller scale than thermally reducedmaterial and there may, therefore, be thedanger, not unknown in other fields, of notcomparing like with like. The thermal reductionprocess involves reducing beryllium fluoridewith magnesium at 1300°C. This reduction,which is carried out in a graphite crucibleheated by high frequency induction, isexceptionally exothermic and very carefultemperature control is necessary. The reactionmass is wet crushed and the beryllium metal isrecovered in the form of pebbles. Alternatively,if the electrolytic route is followed, the chlorideis mixed with sodium chloride to form theelectrolyte and electrolysis is carried out in anickel cell at a temperature of 350°C with anickel cathode and a graphite anode.Beryllium is deposited in the form of flakewhich is water-washed to remove the chloride,washed with caustic soda to remove aluminium,etc., and finally washed in dilute nitric acid toremove as much beryllium oxide as possible.
Fig. 10. Corrosion testing of zirconium inhigh pressure water: inserting specimens in
autoclave
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Comparison of compositions of typical flake andpebble in the reduced form indicates that the flake(electrolytic) is at that stage purer than the pebble(thermally reduced), as regards metallic impurities,but this in itself is not necessarily important becauseboth grades must be further purified by melting.
meltingThe melting process is carried out, not to con-
solidate the metal for further processing since, as weshall see, processing to wrought form is by a powdermetallurgical route, but to effect purification. Themelting process can, therefore, be regarded as- anessential step in the overall extraction process.
Many techniques have been examined for furtherpurification but vacuum induction melting isaccepted as the most suitable commercial technique.Unlike the titanium melting and zirconium meltingdiscussed previously, the consumable electrode tech-nique is not used for beryllium since a suitablerefractory, beryllia, is available for the crucible andmelting is by high frequency, or medium frequency,induction under a vacuum of about 300 - 500^. Themelting point is quite moderate (1283°C). Some ofthe impurities, being heavier than the metal, sink tothe bottom and are left behind in subsequent lippouring. Volatiles such as fluorides, chlorides andmagnesium metal, are drawn off, but as could beexpected, there is no appreciable reduction in theberyllium oxide content. Graphite, steel, or cast ironmay be used for the moulds.
As cast the metal is very brittle, due, not only tolarge grain size, but also unfavourable orientation ofthe grains on crystallisation. Attempts to refine thegrain by the normal metallurgical approach of addinggrain refining agents have not been successful and
even if they had been, their possible effect on theimportant property of low neutron absorption capacitymight have introduced difficulties.
health hazardsBefore proceeding with the processing of the metal
to wrought forms, reference must be made to thequestion of the toxic hazards of beryllium manu-facture and processing, since the safeguarding againstthese hazards naturally influences the methods andtechniques used.
Very briefly, inhalation of beryllium and its com-pounds in finely divided form can cause a serious lungcomplaint known as berylliosis. There is some evidencewhich suggests that this is in the nature of an allergyand that certain people are more susceptible thanothers, but unfortunately those allergic cannot withcertainty be determined by known medical examina-tion methods. Precautions must, therefore, be takenon the assumption that all are likely to suffer ifexposed to the concentration of beryllium above acertain amount. Skin effects can also occur but theseare much less hazardous. The solid metal is con-sidered quite safe and, therefore, precautions aredirected against safeguarding any operation involvingberyllium or its compounds in finely divided form.Such operations as grinding and machining willobviously require special ventilating conditions, butit must also be assumed that in hot working opera-tions there is the possibility that flakes of oxide orfinely divided metal might be released and, therefore,precautions must also be taken for such operations.
All concerned in the processing of beryllium go tovery great pains and expense to render the operationsquite safe and in addition to special ventilatingsystems, a system of monitoring the atmosphere in anyshop or laboratory where beryllium is present is an
Fig. 12. Beryllium tubes produced byextruding the beryllium billet in a steel
sheath at 950°C - 1150°C.(Courtesy of The Brush Beryllium Company.)
essential feature of the work. The recommendationsmade by the U.S. Atomic Energy Commission arerigidly followed as regards allowable concentrations.Briefly these are that " in plant" concentration mustnever exceed a daily average of 2 microgrammes percubic metre and at no time, even for a limitedperiod, may personnel be exposed to a temporaryconcentration greater than 25 microgrammes percubic metre. In addition, the monthly average of theatmosphere in the neighbourhood of any plants whereberyllium is processed must not exceed 0.01 micro-grammes per cubic metre of air.
In addition to very complete ventilating systemsand monitoring arrangements, all personnel engagedin a plant or laboratory involving beryllium mustchange into special clothing on entering and showerthoroughly on leaving. The special clothing islaundered within the plant.
fabricationReturning from this very important and essential
digression to the matter of the further processingof the metal to wrought forms, the ingot is notprocessed direct as with most other metals.
The first stage after casting the ingot is to machineit to swarf or chip, which is then ground, in a ball millor an attrition mill of the plate type, to powder —usually less than 200 mesh. This grinding is carriedout in an atmosphere of argon or nitrogen to mini-mise contamination. The next stage is to consolidatethe powder by compacting into the shape requiredfor further processing. This compacting can be doneunder a variety of different combinations of pressure,temperature and time. Thus, compacting at roomtemperature can be done to produce a " green"compact of fairly high density (85%-90% theoretical)but the pressures required are of the order of 80 tonsper sq. in. On the other hand, pressures as low as100 lb. - 300 lb. per sq. in. are sufficient to producesimilarly dense compacts at a temperature of 1050°C.At intermediate temperatures the compacting pressurevaries accordingly, and the selection of the pressure-temperature combination can only be taken after fullconsideration of such factors as availability and costof die materials and other economic factors.
A very interesting and recent development by theU.K. Atomic Energy Authority is to consolidatewithout pressure — what has in fact become knownas pressureless sintering, and even higher temperaturesare involved here.
Pressed compacts of the type mentioned above arethen sintered under vacuum conditions at 1100°C-1200°C and may then be further processed bymachining, rolling, extrusion and forging. Theproperties of the pressed and sintered block areunderstandably improved by further working. Thus,the tensile strength of a hot pressed and sinteredblock is of the order of 20 tons per sq. in. with anelongation of l % - 3 % , but if this block is furtherhot rolled a tensile strength as high as 40 tons persq. in. and an elongation of the order of 10% can bedeveloped.
Although the room temperature ductility of thesintered compact is low, ductility rises appreciablywith temperature (see Fig. 11), so that operations likerolling and extrusion can be readily carried out at anappropriately chosen temperature. Examination ofFig. 11 would suggest that 400°G would be a suitablehot working temperature because of the ductility peakat that figure, but the advantage of greater ductilityhas to be weighed against the lower tool pressuresapplying at higher temperatures. Assuming aproperly health safeguarded plant, it would probablybe safe to extrude at a temperature up to 700°Cwithout misgivings that this would result in such arise in the beryllium concentration as could not betaken care of by the special ventilating features ofthe plant. Study of the oxidation-temperature curveshows a very sharp increase in the rate of oxidation at750°G and, therefore, if it is decided to hot work at,say, 800°G - 1000°C then it is usual to sheath theberyllium in a sealed mild steel sheath and thenextrude or roll the composite workpiece. Mild steelhas rather similar extrusion characteristics at thesetemperatures to beryllium and in addition does notalloy with nor contaminate beryllium. In addition togiving protection against increased beryllium con-centrations in the atmosphere, sheathing also assistsin preserving the surface, since beryllium tends to galland seize locally when drawn or extruded through adie. Fig. 12 shows a number of beryllium tubes whichhave been produced by hot extruding at 950°C -1150°C from steel-clad beryllium billets.
By these methods, extruded rod and tube and hotrolled sheet and plate have been produced and alarge unit manufactured in the United States in suchwrought beryllium is shown in Fig. 13. This is areflector for use in a Test Reactor.
With regard to welding, it must be admitted that atpresent fusion welding cannot be recommended,although a very great deal of work on this subjectis in progress. Some success has been achieved by
233
pressure welding in an argon atmosphere or in vacno.Machining problems with beryllium are :(a) those associated with the fact that at room
temperature it is a brittle metal; and
(b) those associated with ensuring complete safetyas regards non-dissemination of small particlesof metal.
With regard to (a) it has been said that conditionssuitable for the machining of cast iron are generally-suitable for beryllium, although tool wear is probablyhigher. With regard to (b) a near-glove box technique,in which the work and essential part of the machinetool is enclosed in a covering or a shroud from whichair and powder are continuously drawn off andevacuated to the plant filtering system, is the idealand is probably essential for dry machining. At firstsight, wet machining has attractions in that it wouldcurtail the amount of disseminated dust, but caremust be taken in regard to what happens to the con-taminated cutting fluid, and there is the addedcomplication of cleaning of the beryllium swarf tomake it suitable for re-processing.
The writer trusts that enough has been said to
Fig. 13. Beryllium reflector unit(Courtesy of The Brush Beryllium Company.)
indicate that beryllium is an importantmaterial in the nuclear engineeringfield and particularly so for the gas-cooled reactor, where its availabilityshould bring about a considerableadvance in performance and economics.Its use in fields other than the nuclearone is perhaps more problematic,although its combination of very lowdensity (1.83), moderately high strength,and extremely high modulus (44 X10° lb. per sq. in.) are clearly of interestin the aeronautical and guided missilefields.
It has, of course, had a small butimportant use for many years in theconstruction of X-ray windows. As analloying element it is also well-known in
the 2% beryllium-copper alloy and, indeed, most of thepresent-day production of beryllium is used for thislatter purpose, although it should be added that it isnot necessary to produce the metal itself in order tomake beryllium-copper, this latter being produced bysmelting with copper at the beryllium oxide stage.
some other new metalsAn appreciable amount of attention is being
devoted to NIOBIUM but whether its undoubtedlyhigh potential will ultimately become of practicalutility depends on whether certain deficiencies anddifficulties can be overcome. The melting point is veryhigh (2468°C), and the strength at high temperaturesis outstanding. In addition the pure metal is veryreadily processed, being extremely ductile and capableof withstanding large amounts of cold deformation.Its neutron absorption property, whilst much inferiorto zirconium, magnesium and beryllium, is reasonablylow, and its corrosion resistance is very great indeed.
These properties make it a potential candidate fortwo quite different industrial applications:
1. as a canning material for a really high tem-perature nuclear reactor;
234
2. as a basis for an alloy for gas turbines and hightemperature chemical reactions capable of beingused at much higher temperatures than, forexample, the Nimonic series of alloys, the dis-covery and development of which enabled thetemperature barrier to be pushed forwardappreciably in recent years.
With regard to (1) niobium has the advantage,only exceeded by tantalum and tungsten, of com-patibility with uranium at temperatures of the orderof 700° C and in addition would be resistant to theliquid metal coolants which might well be used in ahighly rated high temperature reactor (tantalum andtungsten would not qualify for such service becauseof their high thermal neutron absorption).
With regard to (2) much research is in progress toovercome the deficiency which niobium has in that itsoxidation resistance at temperatures above about400°C is indifferent. The objective of the researchin hand is to discover an alloy possessing to themaximum possible extent the high strength at tem-perature and workability of the pure metal, buthaving much greater oxidation resistance than thepure metal. Progress has been made in this searchin that it has proved possible by alloying to increasethe oxidation resistance of niobium at 1000°C severalhundred times as compared with the pure metal,although at some expense in creep strength and work-ability. Even so, none of the niobium alloys so far
reported is good enough as yet to warrant considera-tion as a turbine blade material at temperatures of,say, 100°C higher than is satisfactorily attainable withthe present nickel-base alloys.
In common with most of the other new metals,impurities such as oxygen, nitrogen, hydrogen andcarbon have a considerable influence on niobium andthe statement made earlier that the metal is readilyworkable is only true if these impurities can bereduced to very low limits. Oxygen is the mostdetrimental as regards its effect on the workability ofthe metal and this perhaps is best and most simplyshown by reference to Fig. 14, which shows the resultsby two different investigators of the effect of oxygenon the hardness of the metal.
Oxygen can be removed by heating in the presenceof carbon at temperatures certainly above 1600°Gand at very low pressures — lower than 10"5. Treat-ments such as this have only become possible inrecent years by the considerable advances in hightemperature-high vacuum engineering.
The metal is embrittled by hydrogen, which canbe removed by heating at 700°C at 1 micron pressure.
The chief metallic impurity is tantalum, which ischemically similar to niobium and is alwaysassociated with it in the ore. The necessity or not toremove tantalum depends on the ultimate use. If themetal or its alloy is required for corrosion-resistantor high temperature purposes only, there is no need
4OO_
Fig. 14.
Effect of oxygen on hardness of niobium
loo
o 2 .Wr 1.
235
Fig. 15. Creep testing machine adapted fortests in argon or vacuo
to remove the tantalum, but for nuclear applicationsthe presence of an appreciable amount of tantalumcannot be tolerated. The suitability of the tantalum-containing metal for non-nuclear purposes is impor-tant because naturally the cost of obtaining freedomfrom tantalum is very great.
In the present state of the research work onniobium alloys, it is often necessary to carry out hightemperature creep tests without the serious complica-tions which oxidation during the test would causeand, therefore, creep machines have been adaptedso that such tests can be carried out in argon or invacuo. Fig. 15 shows a creep testing machine soadapted.
The search for a new alloy having good strengthat higher temperatures than are possible at themoment is not confined solely to alloys based onniobium. MOLYBDENUM as a base is alsoreceiving attention in the same way, but the hightemperature strength of the pure metal is rather lessthan that of niobium and it suffers from exactly thesame weakness as regards oxidation resistance. In
addition, molybdenum has the seriousdrawbacks of brittleness at normaltemperatures and a relatively highductile-brittle transition temperature, sothat there is the necessity for very hightemperatures in processing to wroughtforms. Even so, considerable attentionis being devoted in the United Statesto the development of a high tem-perature alloy based on molybdenum.
CHROMIUM might also be a base for such a hightemperature alloy and as the price of the raw materialis much lower than the other candidates it might at firstsight be considered particularly attractive. However,it is seriously embrittled by such small amounts ofnitrogen that it is difficult to envisage maintainingthis impurity at the required extremely low levelin normal commercial practice. It is possible that analloy might be found more tolerant as regardsnitrogen contamination, but no success or even near-success has yet been reported in this respect.
HAFNIUM, which was referred to earlier as amost undesirable impurity in reactor grade zirconium,is not without interest in the nuclear field for preciselythe same reason as makes it an undesirable impurityin reactor grade zirconium, namely, its strongabsorbing power for thermal neutrons. For thisreason it has great possibilities for use in nuclearreactor control equipment. As already seen, it is aby-product of zirconium extraction and the metalitself is reduced from the chloride in much the sameway as for zirconium. Its melting and fabrication to
236
the wrought form is very similar to that used fortitanium and zirconium, the technology of which isnow well advanced, so that if hafnium is required forcontrol rods and the like there should not need tobe an appreciable development effort.
Both the corrosion resistant and electricalproperties of TANTALUM make it of interest incertain special applications, despite its very high cost.It has been commercially available for a longerperiod than most of the other metals referred to inthis survey and, indeed, Fig. 16 shows a heatexchanger made in tantalum which has been inservice for over 10 years in the United States. Itscorrosion resistance is quite outstanding, since it isresistant to such strong corrodents as hot sulphuricacid, hot hydrochloric acid and hot phosphoric acid,in all cases over a wide range of concentrations. Itscorrosion resistance is, in fact, generally superior tosuch metals as titanium, zirconium and niobium, allof which are themselves resistant over a very widerange of corrodents. Recently the importance oftantalum as a material for capacitors has arisen andthe demand for the metal in the form of foil for thispurpose is likely to become appreciable.
Neither space nor the writer's competence make itpossible to say more about GERMANIUM andSILICON than that they now have an establishedplace in the electronics industry for transistors andrectifiers. Here, again, with these two metals the mainstory with regard to their production is one ofimpurities, and it should be noted that for satisfactoryperformance in certain applications impurity levelsof 1 in 10° maximum are required and achieved.
conclusionIn this very brief and far from comprehensive
survey, the writer has attempted to show the growingindustrial importance of those metals which were notin the industrial scene 10 years ago, but which havenow reached or are approaching significant industrialimportance. Their emergence in this way is due tothe special requirements of recent engineeringdevelopments and their availability has been madepossible by the progress in several branches of scienceand technology such as the advances in vacuumengineering, in powder metallurgy and in analyticaltechniques. In many of these very recent engineeringdevelopments the older metals, and improvements inthem, will still play a vital role, but some of theserecent developments can only come to full fruitionby virtue of these newer metals. To paraphrase somefamous words, it can be truly said that the newmetals are coming to the rescue of the old, and thatthe combination of new and old metals greatlystrengthens the designer's armoury and greatlyextends his range of choice and selection.
{THE REPORTOF THE DISCUSSION ON THE PAPER
COMMENCES OVERLEAF)
SELECTED BIBLIOGRAPHY
McQuillan, A. D., and M. K. " Titanium." London,Butterworth's Scientific Publications, 1956.
Kroll, W. J. " The Pyrometallurgy of Halides." MetallurgicalReviews (Institute of Metals), 1956, / , 291.
Kroll, W. J. "Titanium." Metal Industry, 1955, 87, 63,83, 105, 130, 147, 173.
Cook, M. " The New Metal Titanium." Journal of theInstitute of Metals, 1953 - 1954, 82, 93.
Inglis, N. P., and McQuillan, M. K. " A Progress Reporton Titanium." Endeavour, 1958, 17, 77.
Swainson, E., and Berry, R. L. P. " Production Problemsof Titanium and its Alloys." N.A.T.O. Advisory Groupfor Aeronautical Research and Development. Report 95,April, 1957.
Rotherham, L. " Metallurgical Problems of Atomic Energy."Journal of the Institute of Metals, 1956 - 1957, 85, 393.
Miller, G. L. " Zirconium." 2nd edition. London, Butter-worth's Scientific Publications, 1957.
White,* D. W., and Burke, J. E. (editors). "The MetalBeryllium." Cleveland, Ohio, American Society forMetals, 1955.
Williams, L. R., and Eyre, P. B. "The Metallurgy ofBeryllium: its Nuclear Applications." NuclearEngineering, 1958, 3, 9.
Mclntosh, A. B. " The Development of Niobium." Journalof the Institute of Metals, 1956 - 1957, 85, 367.
Tottle, C. R. " The Physical and Mechanical Properties ofNiobium." Journal of the Institute of Metals, 1956-1957,85, 375.
Fig. 16. Heat exchanger in tantalum(Courtesy of Fansteel Metallurgical Corporation, Chicago.)
