CommunicationEffect of Oxygen Partial Pressureon the Cyclic Oxidation Behaviorof Mo76Si14B10

BARNA ROY, KHUSHBOO, JAYANTA DAS,RAHUL MITRA, and SANAT KUMAR ROY

Hot-pressed and arc-melted Mo76Si14B10 (at. pct)exhibits a-Mo solid solution, Mo3Si, and Mo5SiB2 inmicrostructures with varying morphologies. Cyclic oxi-dation tests performed at oxygen partial pressures of0.21 and 1 atm show the mass loss of the hot-pressedalloy to be �1.5 and �4 times less, respectively, thanthat of the arc-melted alloy. The thickness of the pro-tective silicate layer increases with an increase of bothMoss grain size and oxygen partial pressure in theenvironment.

DOI: 10.1007/s11661-013-1756-1� The Minerals, Metals & Materials Society and ASMInternational 2013

Mo-Si-B alloys have gained particular attention asmaterials for high temperature applications due to theformation of a continuous and adherent SiO2-B2O3

glassy scale, which protects them during exposure athigh temperatures.[1,2] However, the formation of such aprotective layer requires (i) higher thermodynamicstability of SiO2 than that of any other metallic oxideexpected to form on the given alloy system and (ii)sufficient Si concentration (0.05 to 2.75 at. pct) in thealloy.[3] SiO2 exhibits high viscosity below 1373 K(1100 �C), but the presence of B in the alloy decreasesboth the liquidus temperature and the viscosity of theSiO2-rich scale during oxidation, thus promoting selfhealing of either cracks or pores in the scale and therebyenhancing the protective character of the scale againstfurther degradation.[4,5] It has been reported that theMo-Si-B alloys demonstrate linear/parabolic mass lossin the temperature range of 773 K to 973 K (500 �C to700 �C) due to evaporation of MoO3 (g).

[6–11] However,at temperatures >1273 K (1000 �C), maximum massloss occurs during the transient stage of oxidationfollowed by almost negligible mass loss during subse-quent exposure.[7,12] It has been established that the

formation and nature of such a protective scale dependon (i) the formation rate of borosilicate in competitionwith evaporation of MoO3 (g) and (ii) its permeability tooxygen and other oxidizing species.[7,9,13,14] However, acomprehensive study related to variation of oxygenpartial pressure in the environment on the degradationprocess of such alloys has not been carried out so far.The present investigation is aimed at studying themicrostructure of arc-melted and hot-pressed Mo-Si-Balloy and its effect on the cyclic oxidation behavior attwo different partial pressures of oxygen (( PO2

) 0.21 and1 atm.).Arc-melted ingots (AMI) and hot-pressed (HP) disks

(4.5 mm thickness and 75 mm diameter) of Mo76Si14B10

(MSB) (at. pct) alloy were produced following the sameprocedure as described earlier.[10,11] Pre- and post-oxidation structural and microstructural investigationswere performed using a Philips PANalytical PW 3373X-ray diffraction (XRD) facility with Cu Ka radiationand a scanning electron microscope (SEM, ZEISS EVO60) equipped with an Oxford INCA-PentaFET-X3energy dispersive X-ray spectrometer (EDS). The cyclicoxidation tests were conducted either in dry air or inhigh purity flowing oxygen (99.99 pct) in an environ-ment-controlled horizontal furnace, where each cycleconsisted of heating the alloy from room temperature to1423 K (1150 �C) at the rate of ~5 K (5 �C)/minute,followed by isothermal holding at 1423 K (1150 �C) for4 hours, and subsequent natural cooling to room tem-perature. After each cycle, mass changes were recordedusing an electronic balance (Sartorius RC210D) havingan accuracy of ±0.01 mg.The XRD patterns of both HP and AMI alloys

reveal the presence of distinct peaks of a-Mo (bccMoss), Mo3Si (A15), and Mo5SiB2 (T2) phases asreported earlier.[14–19] An examination of the SEM(BSE) images depicting the microstructures of differ-ently prepared MSB (Figures 1(a) and (b)) reveals thepresence of a brighter phase surrounded by darkerregions (gray and dark gray),which represent twodistinct phases. In the case of the HP alloy, polygonalmorphologies of all the three phases are observed(Figure 1(a)), whereas for the AMI, a white dendriticphase surrounded by gray and dark gray lamellae witheutectic morphology is observed (Figure 1(b)). Further-more, EDS analysis has revealed the brighter phase asMo solid solution (Moss) with gray and dark grayregions being identified as Mo3Si and Mo5SiB2, respec-tively. Additionally, in the HP alloy, some Si- andO-rich black inclusions are found and identified to beSiO2. Figures 1(c) and (d) show the area fraction andgrain size of each phase. The area fractions of Moss,Mo3Si, and Mo5SiB2 in the two differently preparedalloys are ~47, ~27, and ~24 pct, respectively. However,the grain size of Moss is estimated to be ~10± 0.39 lmfor AMI and 3.6± 0.161 lm for HP. Thus, althoughthe volume fraction of the phases in the two differentlyprocessed alloys is identical, the length scales of theconstituent phases are different.A variation of the mass change per unit area (DW)

with exposure time for the two differently prepared alloyspecimens subjected to cyclic oxidation in dry air (HPA,

BARNA ROY, Research Scholar, JAYANTA DAS, AssistantProfessor, RAHUL MITRA, Professor, and SANAT KUMAR ROY,Professor, are with the Department of Metallurgical and MaterialsEngineering, Indian Institute of Technology Kharagpur, Kharagpur721302, West Bengal, India. Contact e-mail: j.das@metal.iitkgp.ernet.inKHUSHBOO, formerly Graduate Student with the Department ofMetallurgical and Materials Engineering, Indian Institute of Technol-ogy Kharagpur, is now Graduate Metallurgist with the MetallurgicalDevelopment, Rio Tinto Iron Ore, Perth, WA, Australia.

Manuscript submitted January 14, 2013.Article published online April 26, 2013

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AMIA) and in pure oxygen (HPO, AMIO) atmosphereis shown in Figure 1(e). HPA, AMIA, and HPO show arapid mass loss during the first cycle of oxidation,followed by negligible mass loss in the subsequentcycles. However, continuous mass loss is recorded forthe AMIO up to three cycles.The XRD patterns of the oxidized alloy as shown in

Figure 2 reveal the presence of Mo and MoO2 in theoxide scale of HPA and cristoballite in the case ofAMIA. On the other hand, distinct peaks of MoO2,MoO3, and cristoballite have been identified in the XRDpattern from HPO, while MoO3 and cristoballite areobserved in that from AMIO.Investigations of the alloy-scale cross sections formed

under different experimental conditions reveal the for-mation of a two-layered scale as shown in Figures 3(a)through (d). Furthermore, EDS analysis suggests theuppermost layer to be a silica-rich layer, with the innerlayers being mainly Mo oxides, containing 31.2± 3 at.

Fig. 1—SEM (BSE) micrographs of as-prepared (a) hot-pressed (HP) and (b) arc-melted (AMI) alloys. (c) Area fraction and (d) grain size ofconstituent phases in HP and AMI alloys. (e) Cyclic oxidation behavior of HP and AMI alloys showing mass change with cycles of exposure atPO2

= 0.21 and 1 atm.

Fig. 2—XRD patterns of the oxidized HP and AMI alloys in air(HPA, AMIA) and 1 atm. oxygen environment (HPO, AMIO).

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 44A, JULY 2013—2911

pct Mo and 68.8± 4 at. pct O. The top surface scalesformed over HPA and AMIA are found to be smoothalong with features resembling the flow of a viscousglassy silicate layer as confirmed by EDS (insetFigure 3(c)). The specks as observed in the SEM(BSE) image are found to be enriched in Mo. On the

other hand, globular wells are observed in the oxidescales of HPO and AMIO, as shown in the inset ofFigure 3(d). Since the rate of oxygen ingress throughthe scale is enhanced due to high PO2

in pure oxygenatmosphere, the formation and escape of volatileMoO3 through the viscous silicate scale cause bubblesto form on the surface, leading to creation of theglobular wells.In order to relate the silica layer thickness to the

observed mass loss, the loss of Mo and B leading toformation of volatile oxides (MoO3 and B2O3) and massgain due to formation of SiO2 have been calculated.Hence, the mass change per unit area (DW) of theoxidized alloy can be expressed as

DW ¼ �WMo þWO2ðSiO2Þ �WB ½1�

On the other hand, the mass change per unit area(DW) can be obtained from the following relationship:

DW ¼ mass change=area ¼ ðvolume� densityÞ=area¼ thickness� density ðmg=cm2Þ ½2�

Considering the atomic ratio of Mo, Si, and B in thealloy, the equivalent amount of Mo and B can beexpressed in terms of tSiO2

, and the net mass loss canbe predicted from the following relation:

Fig. 3—SEM (BSE) micrographs showing alloy/scale cross-sectional microstructures of oxidized (a) HPA; (b) HPO; (c) AMIA, inset: smooth topsurface of the scale along with the flow lines of viscous borosilicate layer; (d) AMIO, inset: globular well present on the oxide scale.

Fig. 4—Plot of scale thickness versus total mass loss of HP andAMI alloys after 6 cycles of exposure in two different partial pres-sures of oxygen.

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DW ¼ �1:9656 tSiO2þ 0:121 tSiO2

� 0:0291 tSiO2½3�

) �DW ¼ 1:874 tSiO2; ½4�

where tSiO2= thickness of the silicate scale.

The experimentally observed DW has been plottedagainst the silicate layer thickness and then comparedwith the predicted values of DW calculated using Eq. [4],as shown in Figure 4. A linear fit yields a relation,�DW ¼ 1:36 tSiO2

with R2=0.9589. It should be notedthat for a given mass loss, the evolution of silicate layerthickness is higher than the predicted one. Usually, uponoxidation, the oxide particles formed in the scale tend toundergo liquid phase sintering in the presence of B2O3

(l) or B2O3-SiO2 eutectic (l). Furthermore, MoO3 (g)escapes by forming bubbles from the alloy/oxide inter-face to the scale/gas interface by rupturing the scale.Therefore, due to the presence of gas bubbles orlocalized rupturing of the silicate layer, the measuredlayer thickness may not exactly match the predicted one.Moreover, alloy/scale cross-sectional microstructuresrepresenting different experimental conditions have alsorevealed the presence of an intermediate MoOx (x = 2and 3) layer adjacent to the alloy, as confirmed by EDSanalysis.

Oxidation in pure oxygen environment causes greatermass loss per unit area for both HP and AMI alloy, asshown in Figure 1(e). It is well understood that forcontinued oxidation of the alloy constituents, the partialpressure of oxygen at the gas/alloy interface should behigher than the equilibrium partial pressure at the scale/alloy interface.[10] As the oxygen partial pressure ishigher in pure oxygen environment than in air, the ratesof the relevant reactions are expected to be higher,thereby leading to much greater net mass loss.

The results of the microstructural investigation showthat the grain size of the Moss phase is three times largerin the case of AMI as compared to that in the HP. Assuch, it is appropriate to infer that while the borosilicatelayer flows to cover the Moss grains, the time taken forsuch coverage is more in AMI than that in the HP. If thetime taken for such coverage in the transient stage ismore, the mass loss by vaporization of MoO3 is expectedto be higher as well. Moreover, recent work by theauthors[20] has shown that vaporization of volatileMoO3 during oxidation of the Moss phase is responsiblefor the net mass loss of the alloy and controls thetransient stage oxidation kinetics. Therefore, it is intu-itive that the AMI should exhibit much higher mass losscompared to the HP, as has been experimentallyobserved.

The microstructures of the AMI and HP Mo76Si14B10

consist of a uniform dispersion of bcc a-Mo phase inMo3Si+Mo5SiB2 matrix. Although the area fraction of

each phase is found to be almost the same in the twodifferently as-prepared alloys, the size of the Moss phasein AMI is 3 times larger than that in HP. The net massloss during cyclic oxidation of HP is 1.5 and 4 timeslower than that of AMI in air and pure oxygen,respectively. Thicknesses of both silicate and Mo-oxide(MoO3 and MoO2) layers increase with increasingpartial pressure of oxygen in the atmosphere.

The authors thank S. Mondal and P. Guha fortechnical assistance at the central research facility ofIIT Kharagpur. Financial support provided by theDefence Research Development Organization, NewDelhi, India, is gratefully acknowledged.

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