Metal Science and Heat Treatment Vol. 38, Nos. 9 - It), 1996

NONFERROUS METALS AND ALLOYS

UDC 669.71'725.5

AI -Be ALLOYS: MULT IPURPOSE METALL IC COMPOSITE MATERIALS

I. N. Fridlyander, l A. G. Bratukhin, l P. Z. Gorbunov, I V. V. Gal', I K. P. Yatsenko, ~ and A. N. Fokanov ~

Translated from Metaliovedenie i Termicheskaya Obrabotka Metaliov, No. 9, pp. 23 -26 , September, 1996.

The article is devoted to composite materials created on the basis of the technology of light alloys and developments of the All-Russia Institute of Aircraft Materials concerning beryllium alloys with a high modulus of elasticity. The competitiveness of such composite materials is determined by the high level of the specific strength properties, which exceed, in some cases, the characteristics of powder materials of a similar composition and approach the properties of titanium alloys and steels.

Beryllium is a unique component of light composite ma- terials (CM). The AI - Be system is a promising base for de- veloping a distinct class of CM. The base and alloying (for example, Mg, Li) elements of this system are metals of the second kind (pyrophoric metals) and surface-active elements. Therefore, the components of these alloys have common physicochemical and metallochemical properties. The bond- ing between the aluminum plastic matrix and the hardener forms at the microscopic level. Such a hardener as beryllium possesses a unique set of technologically important properties that sometimes seem to be mutually exclusive.

Eutectic alloys based on the A I -Be system possess a structure typical for a composite material prepared by crystal- lization from the melt rather than by mechanical combination of the composition elements. An advantage of compositions prepared by crystallization over compositions fabricated by conventional methods consists in the possibility of stronger cohesion between the matrix and the hardener, which gives a more homogeneous structure and hence a material with more isotropic properties.

When eutectic alloys of the AI - Be system with a struc- ture of a composite material are welded to each other or to another material the joints are more reliable than welded joints of conventional CM.

Eutectic alloys should not differ much from conventional materials in the conditions of pressure treatment because the chemical interaction along the hardener-matrix interface in them is not complete. The microstructure of such alloys can be changed by subjecting them to plastic deformations of dif- ferent kinds. If the ratio of the fiber length to its diameter in a

I All-Russia Institute of Aircraft Materials, Moscow, Russia.

390

eutectic A I -Be alloy is sufficiently large, the structure is analogous to that of fiber-reinforced CM. If this ratio de- creases in the deformation process, the structure of the eutec- tic alloy is closer to particle-reinforced CM. In accordance with data of [ I ] fiber strengthening ceases to be efficient and particle strengthening becomes preferable if the fiber length is less than its doubled diameter. Rigid strengthening parti- cles in compositions with a ductile matrix hamper the motion of dislocations and localize plastic deformation. In order to create a high-strength CM with a high modulus of elasticity the particles of the hardener should possess maximum rigid- ity, their structure should differ substantially from that of the matrix, and they should form high potential barriers to migra- tion of dislocations. The distance between the particles of the hardener should be minimum in order to impede inflection of dislocation loops and penetration of dislocations between particles. These requirements are met by CM in the form of alloys of the A I - Be system fabricated by crystallization; they have a favorable combination of beryllium, which pos- sesses the highest modulus of elasticity of all metals, and alu- minum, which possesses a very high ductility.

Beryllium is 1.5 times lighter than aluminum and 2.5 times lighter than titanium. The rigidity of beryllium is three times higher than that of steel and 1.6 times higher than that of titanium. Beryllium possesses the highest specific ri- gidity of all metals. The high-temperature strength of beryl- lium is somewhat inferior to that of titanium but much higher than that of other light metals. The melting point of alumi- num and magnesium is about half that of beryllium. The spe- cific heat of beryllium exceeds that of aluminum by a factor of 2.5 and that of steel by a factor of 8.

0026-0673/(~/0910-0390515.00 O i 997 Plenum Publishing Corporation

AI - Be Alloys: Multipurpose Metallic Composite Materials 391

Alloys of the A! - Be system are in essence fused light metallic composite materials. They have the favorable combi- nation of the high ductility of aluminum and the high strength of beryllium. They possess high rigidity, low density, defor- mability, unique structural properties, including low-tempera- ture properties, a low sensitivity to notching, a coefficient of linear expansion close to that of steel, and high sound absorp- tion (30- 80 times higher than in aluminum alloys). The ex- perience of Russian and foreign specialists shows that the methods of discrete metallurgy (powders, granules, flakes) are progressive, but the gain in utilization of the metal does not compensate the high expenses. Compositions fabricated by methods of discrete metallurgy are much more expensive than cast materials [2].

AI - Be alloys have unique properties compared to other alloys and to compositions obtained by methods of discrete metallurgy. All alloys of the A I - Be system can be consid- ered as spontaneously or controllably formed metallic com- posite materials in which the matrix is an aluminum alloy and the hardener is the beryllium component.

At present, composite materials of the A I - Be - Cu, Ni, Mg system are being developed in which the hardener is the beryllium component and the matrix is an aluminum alloy close in composition to AI-I. In accordance with data of [3], experimental specimens made of such an alloy have a density of 2.1 g/em 3 and a strength a,, of up to 750 N/mm 2. Compos- ite materials based on the AI - Be - Li system are studied too. They have similar components, namely, a ductile matrix and a hardener with a high modulus of elasticity, but are prepared by a high-temperature method. In powder CM on a metallic base the hardener and the matrix are bonded at the

macrolevel, whereas in CM based on the AI - Be system this occurs at the microlevel.

In Table 1 we present strength characteristics of fiber CM. At first sight they have a favorable eombiuation of prop- erties. However, a more detailed analysis of the mechanical properties of CM and their comparison with the charac- teristics of high-modulus alloys of the ABM type (Table 2) have shown the following. Fiber CM have excellent charac- teristics in the lengthwise direction and low parameters (espe- cially the ductility) in the height- and widthwise directions due to clearly manifested anisotropy. We have compared CM with approximately equal amounts of hardener (about 40%) and established that metal-based CM fabricated by mechani- cal stacking of fibers have poorer homogeneity of properties than CM based on the AI - Be system.

Figure 1 presents the microstructure of alloys of the A I - Be -Mg-N i -Cu-T i system produced from metals of commercial purity (Table 2) in the initial state and aller heat treatment.

It should be noted that in the beryllium used for preparing alloy 1 the amount of Ti and Si admixtures was an order of magnitude higher than in the beryllium used for alloy 2 (Ta- ble 2).

It can be seen from Fig. 1 that the heat treatment affects considerably the morphology of the mierostrueture of the in- itially deformed alloys. Their mechanical properties (Table 2) correlate with the parameters of the mierostructure.

The structure of the AI - Be - Mg alloys is characterized by a more uniform and ordered arrangement of the strength- ening beryllium phase (gray colored) in the volume of the ductile aluminum matrix both after extrusion (Fig. la, d) and

TABLE I

Material P, g/cm3 a u crcx,m p cr_ ! E x 10- 3 a-2, km E, km N/ram 2 p P

Boroplastic 2.0 1200- 1400 1500 500 212-250 60-70 106,000- 125,000

Boroaluminum 2.65 1100- 1300 1500- 1850 700 250 45 9500

Boromagnesium 2.2 1200 - 1300 - 600 220 55 - 59 I 0,000

Carbon aluminum 2.4 - 2.5 600 - 900 - - 110 - 175 24 - 38 4580 - 7000

S iC - Ti 3 .85-4 .15 I I00 - 1200 - - 305-325 27-31 6000-6750

Notes. I. Characteristics of fiber composite materials (FCM) at 293 K generalized in [4] are presented. 2. The ranges of properties of FCM depend on the volume fraction of the fibers and the properties of the components.

TABLE 2

(3" u 0"0. 2 Alloy Heat treatment regime 5, %

N/ram 2

AI - 58.17% Be - 0.66% Mg - 1.58% Cu Initial state 585 495 3.4

- 0.9*/, Ni - 0.04% Ti (1) Hardening in water from 515"C (2 h), two-stage aging at 180"C 745 505 3.8 for 7 h + 350C for I h

AI - 60.3% Be - 0.58% Mg - 1.58% Cu Initial state 530 460 3.2

- 0.9% Ni - 0.003% Ti (2) Hardening in water from 515C (2 h), two-stage aging at 180"C 750 510 3.4 for 7 h + 2000C for 24 h

Notes. I. The initial state is extrusion with a coefficient of 8.5. 2. Alloy I contained beryllium of grade TG-56 (0.2% Ti, 0.5 - I% O, 0.13 - 0.15% Si, 0.2% Fe), alloy 2 contained beryllium of grade TShGT (0.05% Ti, 1% O, 0.02% Si, 0.2% Fe).

392 !. N. Fridlyander et al.

Fig. I. Microstructure of transverse microscopic specimens of rods 12 mm in diameter made of alloys of the AI- Be- Mg-Ni - Cu- Ti system (see Table 2): a-c) alloy 1; d - f ) alloy 2; a, d) initial state; b, c, e,f) after heat treatment; a, b, d, e) 160; c,f) x 500 (Nomarski interference contrast).

after the heat treatment (Fig. lb, c, e , f ) compared to the an analogous ABM-3 alloy. The strength of the investigated al- loys exceeds that of ABM-3 by about 100 N/mm 2. After the material is extruded, oriented microfragrnents are formed in the direction of the extrusion that consist of a mixture of be- ryllium a-phase and beryllides of transition metals. Morpho- logically, the microstructure is represented by a conglomerate of microfragrnents separated by layers of aluminum and alu-

minides of transition metals. After the heat treatment, the area

occupied by the fragments expands, but the dimensions of the

ductile matrix do not increase. In photographs taken under a

500-fold magnification with the use of a Nomarski interfer-

ence contrast (Fig. lc, f ) complex beryllides are seen along

grain boundaries and have much larger cross sections than in

the initial (postdeformation) structure.

A! - Be Alloys: Multipurpose Metallic Composite Materials 393

Results of mechanical tests show that it is possi- p, ble to obtain alloys possessing considerable ductility after a heat treatment carried out for intermediate technological operations and high strength after the final heat treatment. The properties of heat treated A I - Be alloys ate close to those of titanium alloys and steels [6]. There are data showing technological superplasticity for A I -Be alloys containing as much as 60% beryllium hardener [7], which indi- cates that their application range can be quite wide.

Figure 2 and Table3 present the specific strength parameters of different light alloys. It should be noted that in addition to the AI - Be - Mg alloys studied beryllium-alloyed Mg-L i alloys (in our case 0.036 -0.046% Be) are of no less interest.

Light CM of the A I - Be system have been cre- ated for parts of aircraft and spacecraft. At present, the range of their application has been widened and they are beginning to be used for consumer goods. The high cost of intermediate products of such mate- rials limits their range of use but in some cases they are indis- pensable. They are used for racing bicycles produced for na- tional and Olympic teams. Frames of high-class racing bicy- cles with a mass under 1 kg are being developed from alloy ABM-1. Tubes with a wall thickness of 1 mm fabricated by "rotational extrusion" are used for this purpose. Tubes fabri-

cated by the conventional technology have o u = 420 N/mm 2,

o0. 2 = 270 N/mm 2, 6 = 15%; those prepared by "rota- tional extrusion" have o u = 560 N/ram 2, o0. 2 = 460 N/mm 2, 6= 12%.

/cnl2 E / p, kn l

, llll

l, l ," ," 3lll I 1

12 45 7 12345 67

ou/p , km

30

20

'i 12345 67

Oo.2 / p, km

30'

20

10

0 I 2345 6~

F ig . 2. Specific strength properties of deformed intermediate products o f light alloys: 1, 2 ) high-strength aluminum alloys (conventional and powder metallurgy, respectively); 3 ) AI - Be - Mg (ABM a l loys) ; 4 ) AI - Be - Mg - L i ; 5 ) magnesium alloys; 6 ) Mg - L i ; 7 ) Mg- L i - Be.

It is also expedient to use tubes made of A I - Be alloys for racing motorcycles, "safety frames" for cartings, racing cars, gliders, helicopters, wind power plants, mobile tracking systems of radar units and radiotelescopes, pontoons, and derrick transport platforms. The use of A I - Be alloys makes it possible to create hinge- and plate-rod structures with mini- mum mass that maximally absorb elastic energy without faii- Ufe.

Thus, the combination of structural components with substantially different properties in alloys of the AI - Be sys- tem gives structures with minimum mass and high strength

TABLE 3

Volume fraction, %

Material Intermediate p, matrix hardener product, state g/cm 3

O u 0"0. 2 0"u/p EIp 5,%

N/ram 2 km

L W L W L W L W L W

KAS-IA4o 60 (AV alloy) 40(w im0.15mm, Sheet (0 .8 -2 .0mmL 4.74 1470-1520

[51 VNS-9) TI

VKA-I 45 -55 45 - 55 (bon3n fiber, Sheet (0 .6 - 7 mm) 2.65 1125- 1225

[5] (ADI-0, ~0.14 ram) without heat AD- i ) treatment

VKA-2 45 - 55 The same Sheet (0.6 - 7 mm), 265 1175 - 1370 [5] (AD-33) TI

VKU-I 50 - 55 45 - 50 (carbon Reinforced casting 225 734 - 880 [51 (AL-9) ribbon with TiC (0.5 - 1.5 ram)

coating) without heal treatment

VKM-I 50-55 45 - 50 (boron fiber) Sheet (0 .8 - 4 ram) 2.15 980- I175

[5] (MA2- I ) without heat tl'eatl~uen I

ABM--4 55 - 60 40 - 45 (Be) Sheet ( 1.5 ram), M 22 530 - 580 [9] (AMg-6)

Ai - 60% B 40 (A I - I ) 21 610-735

- Mg - Cu - Ni 131

AI - Cu 70 (AI - Cu

- M8/SiC - M8)

[Sl

60 (Be) Red(~ 18 ram), TI

30 (SIC) 3.0 620

275-285 1125- 1225 2 -3 - 5.9-6.2 2470

78 - 98 0.6 0.8 43.4 - 472 3. I - 3.8 8600

175-195 - 06 0.6 45.3-52.8 6 .8 -78 9000

49-78 - 03 -04 32.3-40 22-36 10.200

05 - 46-55 10,200

4500

4500

540 I 0 - 20.6 4500

510-580 410-420 410-440 7 -10 9 -20 24.6-26.8 23.6-26.8 7150 7550

450-510 27-4 .0 - 295-357 - I0,100 -

394 I.N. FHdlyander et al.

characteristics and rigidity, which makes their operation very reliable.

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1. Fracture Toughness Testing and Its Applications, Chicago (1964).

2. N. M. Sklyarov, "Quality of aircraft materials, its criteria and es- timation," in: Aircraft Materials [in Russian], ONTI VIAM, Moscow (1982), pp. 60- 66.

3. P. Z. Gorbunov, V. V. Gal', V. N. Ilyushin, et al., "Light beryl- lium-containing alloys for machine building," in: Technology. Set. Equipment, Materials, Processes. Coll. of Works [in Rus- sian], Nos. 2 -3 (1991), pp. 60-66.

4. M. Kh. Shorshorov (ed.), Fiber CM with a Metallic Matrix [in Russian], Mashinostroenie, Moscow ( 1981 ).

5. R. E. Shalin (ed.), Aircraft Materials. A Reference Book [in Rus- sian], ONTI VIAM, Moscow (1983).

6. P. Z. Gorbunov and V. V. Gal', "Promising dispersion-hardening natural composite materials," Proizv.-Tekhn. Opyt, Nos. 1 -2 , 81 - 84 (1993).

7. P. Z. Gorbtmov, V. V. Gal', I~. I. lUarionov, et al., "Effect of the joint manifestation of superplasticity and superelasticity in light alloys based on magnesium and aluminum and alloyed with be- ryllium," in: Technology. Ser. Resource-Saving Processes, Equip- ment, Materials. Coll. of Works [in Russian], No. 1, (1992), pp. 52 - 56.

8. "Composite materials of the Ai - SiC system," Ekolinlc Rossiya Mir: Nauka Tekhnol., No. 1(5), 20 (1995).

9. I. N. Fridlyander, L. P. Yatsenko, T. I. Terent'eva, et al., Beryl- lium: A Material of Modern Engineering [in Russian], Metallur- giya, Moscow (1992).