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ZWe: ~om~ous Effects of Substitional Alloying on the MechanicalBehavior of Molybdenum Disilicide

Author(s): Amit Misrq MST-8Adel A. Sharif, MST-8John J. Petrovic, MST-8Terence E. Mitchell, MST-8

Submitted to: gth Internation~ Symposium on Processing and Fabrication ofAdvanced MaterialsAmerican Society of Metals10/9-12, 2000St. Louis, MO

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Anomalous Effects of Substitutional Alloying on theMechanical Behavior of MoSi2

A. Misra, A.A. SharilY, J.J. Petrovic, and T.E. Mitchell,Materials Science and Technology Division,

Los Alamos National Laboratory,Los Alamos, NM 87545

* University of Michigan, Department of Engineering Science,303 East Kearsley Street, Flint, Michigan 48502

Abstract

Intermetallic compounds such as MoSi2 are potential materials for structuralapplications at temperatures above the melting points of typical superalloy.However, monolithic intermetallic compounds typically have very low strengthsat temperatures > 1200 “C and low ductility at near-ambient temperatures. Wehave investigated the effects of substitutional solid solution alloying with Re andAl on arc-melted, polycrystalline MoSiz with Cl lb body-center tetragonalstructure and found some anomalies in the mechanical behavior of alloyedmaterials. For example, (i) addition of only -2 at. % Re results in an order ofmagnitude increase in compressive strength at 1600 ‘C, (ii) minor additions ofAl cause solution softening at near-ambient temperatures, and (iii) quaternaryMoSiz-Re-Al alloys show strengthening at elevated temperatures and reductionin flow stress with enhanced plasticity at near-ambient temperatures incompression. Transmission electron microscopy studies of the dislocationsubstructures are used to gain insight on the deformation behavior. Themechanisms of anomalous solution hardening and softening are discussed.

Introduction

MoSiz–based materials have been applied in a variety of high temperatureindustrial applications, e.g., heating elements for air furnaces up to 1900 ‘C,blade outer air seal in hot sections of the aerospace engine gas turbines, dieselengine glow plugs, industrial gas burners for Oz-natural gas mixtures, moltenmetal lances and glass processing, etc. [1, 2]. These applications utilize one ormore of the following properties of MoSi2: high melting point (2020 ‘C), lowerdensity (6.3 g/cm3) compared to superalloy, excellent oxidation resistance,

.

high thermal conductivity, and thermodynamic compatibility with manyceramic reinforcements [1-4]. However, low fracture toughness at near-ambienttemperatures, low strength at elevated temperatures (> 1200 “C) in themonolithic form and accelerated oxidation (“pest”) at -500 “C have seriouslylimited the development of MoSiz-based structural materials. Recently it hasbeen shown that pest resistant MoSiz-based materials may be developed usingsilicon nitride reinforcement [5] or alloying with Al [6].

With regard to the improvements in mechanical properties, most of the earlywork focused on composites with ceramic reinforcements such as Zr02, SiJNJ,SiC, etc. [1-4]. These ceramic reinforcements enhance the high temperaturestrength of MoSi2 but have little effect on the room temperature fracturetoughness. Ductile metallic reinforcements such as Nb react with the MoSi~matrix to form other silicides and have limited practical use for long-term hightemperature applications, although improvements in toughness have beendemonstrated in these composites [7]. Little work hasbeen to done to examine the effects of alloying on themechanical behavior of MoSi2 with the body-

m

●centered tetragonal Cl lb structure (Fig. 1). Ito et al.

[8] and Maloy et al. [9] observed a variety of slip ~ “- --systems in Cl lb MoSi2 deformed in compression ~ o ‘ _,i

over a range of temperatures: (013 }<331>, F “----- “’----

{11O}C111>, {013} <100>, {011) <100>. Most ●

importantly, compressive plasticity was observed atMo

temperatures below ambient in single crystalorientations other than [001] [8,9]. The extent to 3.204A

which the critical resolved shear stresses and Fig. I Unitcell of MoSi2

operation of these slip systems are affected byalloying needs to be studied in more detail.

Recently, several investigations have been performed on the effects ofalloying on the structure and properties of MoSi2, as summarized in Table 1.Note that in general solid solubilities in the Cl lb MoSiz are rather limited,except for W. Elements such as Al, Nb, Ta, V and Cr change the structure ofMoSi2to hexagonal C40 when added in excess of the volubility limit. It has beenshown that the hexagonal structure of MoSi2 has fewer slip systems and is notdeformable in compression, even in the single crystalline form below 1100 ‘C[18]. Hence the behavior of alloys with Cl 1, structure needs to be studied. Thereis indication, mostly through hardness testing, that elements that change thestructure to C40 also cause solution softening [10-12, 14, 17]. With regard tostrengthening at elevated temperatures, W causes solid solution hardening.However, Re causes rapid hardening at eievated temperatures. In fact, the yield

2

strengths of (MoO.gTReO.OJSizand (MoO,SWO.JSizalloys were almost equalindicating that Re is an order of magnitude more potent than W in increasing thehigh temperature strength. In this investigation, we have attempted to combinethe beneficial effects of Al and Re. The mechanical behavior, in compression, of(Mo,Re)(Si,Al)z alloys is evaluated over a range of temperatures and comparedto that of pure MoSiz and ternary (Mo,Re)Siz alloys.

Table 1 Solution Hardening, Solution Softening and Volubility ofDifferent Alloying Elements in MoSi2

Elem Volubility RT effect High T effect Ref.ent (at.%)Al 2-3 softening insignificant 10-12, 14

Ti o.4Ao.l nr nr 13

Zr

v

Nb

Ta

Cr

w

Re

1.1 minor insignificant 11softening

1.4 * 0.4 minor insignificant 11, 13softening

0.8-1.0 softening modest 11, 13hardening

3-5 softening insignificant 11at lowcone.

1-1.4 minor insignificant 11, 13softening

complete hardening hardening 15

2.5 rapid rapid 16hardening hardening

I I I I I I

* Fe, Co, Ni, Ru, Hf are insoluble [13, 17]; nc not reported

Experimental

Pure MoSi2, ternary (Mo,Re)Si2, and quaternary (Me, Re)(Al,Si)z alloyswere prepared by arc-melting high-purity elemental Mo, Re, Si, and Al in an

3

argon atmosphere. The starting melt compositions were (Me, Re)(Si, Al)x,where x=2.01 to account for the Si loss during melting. The buttons were turnedover and remelted 4 times to ensure homogeneity. Alloying levels were in the1-2 at.% range. Compression testing was performed on 2 mm x 2 mm x 4 mmsamples, polished to 0.05 pm finish, in air using an Instron 1125 machine at aninitial strain rate of -1 x 104 /s. Transmission electron microscopy (TEM) wasperformed on a Philips CM30 microscope operating at 300 kV.

Results

The 0.2% offset compressive yield strengths as a function of temperature forthe MoSi2, (Moo.g25Reo.oT5)Siz and (MoO.nReO.OJ(SiO,gTA&.OJzalloys are shown inFig. 2. A log-linear plot is used in Fig. 2, where linear fits to the data would beconsistent with a Peierls stress-controlled dislocation glide behavior [19].

p....,..%

.

I;

H MoSi2 ‘a

O ‘M00.92SRe0.07,) si2

●

●

m (MoO ~7Re0~3)(Si0~7A1..03)2 ‘.. .-b

10 I Io 500 1000 1500 2000

Temperature rC)

Fig. 2 0.2% offset yield strength, in compression, of MoSi2, (MoO.9@e0.mJSi2>(Moo 97Reo,03)(Si0,97Alo,03)2atloys asa function of temperature. Note the rapiddrop ;n strength of MoSiz above 1400 ‘C and Iack of compressive plasticity atlow temperatures.The MoSi2 -Re-Al alloys exhibit compressive plasticity atroom temperature and strengthening at high temperatures ascompared to MoSi2

For MoSi z, low temperature compressive plasticity is not observed. In thetemperature range of 900-1400 “C, the yield strength of MoSiz decreases withincreasing temperature with a linear fit to the data in the log-linear plot.However, above 1400 “C, the strength of MoSi2 drops considerably, presumablydue to the predominant effect of diffusive processes. For the (MoO.gzsReO.OTJSiZalloy, the strengths are very high and no plasticity is observed at temperatures <1100 ‘C. Previously, we have demonstrated, through hot hardness experiments,the hardening by Re in the 25-1300 ‘C temperature range [20]. In thetemperature range of 1200-1600 ‘C, the trend in decreasing strength is linear inthe log-linear plot. It appears that Re alloying delays the onset of diffusion-controlled processes to higher temperatures. Even at 1600 ‘C, the strength of(MoO.gzSReO.07JSi*alloy was an order of magnitude higher than that of MoSi,.The (MoO.gTReO.OJ(SiO.gTAIO.OJzalloy could be deformed in compression even atroom temperature, although the plastic strain to fracture at room temperaturewas typically 0.5-1 %. In the log stress-linear temperature plot, the strengthdecreases linearly in the temperature range of 25-600 “C and 1000-1600 “C. Inthe intermediate temperature regime, the strength increases with increasingtemperature. This anomalous yielding behavior has also been observed in MoSizsingle crystals deformed in compression [8].

The strong influence of Re on the mechanical behavior of MoSi *at elevatedtemperatures is also reflected in the dislocation substructures (Fig. 3). In MoSiz,well-pronounced ceil sub-structure is observed (Fig. 3(a)), typical of “puremetal” behavior. Due to the extremely low yield stress of MoSi2 at 1400 “C,dislocation glide is very fast and so climb of dislocations past the substructuralobstacles becomes the rate-controlling factor. In contrast, addition of only 2.5at.% Re changes the dislocation substructure significantly (Fig. 3(b)). Here nosignificant tendency to form sub-grains is noted and the dislocations aresmoothly curved and randomly distributed. Such a substructure is consistentwith the solid solution hardening exhibited by Re and similar to viscous glide-controlled behavior in alloys.

The observations of the side surfaces of the deformed compression specimensof the quaternary MoSiz-Re-Al alloys revealed slip traces as shown in Flg.4. Atroom temperature (Fig. 4(a)), the slip traces are coarse and heterogeneous, withonly some grains exhibiting slip. At relatively higher temperatures (Fig. 4(b)),slip traces are more finely spaced. TEM observations of a compression specimenof MoSi2-Re-A1 alloy deformed at 400 ‘C reveal a high density of dislocations(Fig. 5). The dislocation substructure consisted of bothl/2<1 11> and <100>dislocations.

5

Fig. 3 TEM microgmphs showing dk.location substructures after -2% plastic strain in(a) MoSi2,1400‘Cand(b)(Mo0,W~Re0.0~JSi2,1600“C.Notethecellsub-smrctu~inMoSj,typical of pure metal behavior. Alloying with Re tends to suppress cell sub-structure formationto higher temperatures (or, higher stress levels at a given tempemture).

Discussion

Anomalous Solution Hardening by Re

The rapid solution hardening of MoSi ~,even in the presence of Al, by Re atelevated temperatures is anomalous and may not be explained within theframework of the classical solution hardening theories based on atomic size andmodulus misfits [21]. Since rhenium “disilicide” has a Si-deficient stoichiornetryof ReSi ,.75,a Si vacancy may form with every four Re atoms added [20]. Weinterpret this anomalous hardening in terms of point defect complexes that mayresult due to the pairing of Re substitutional with Si vacancies [16]. Thetetragonal distortions around these point defect complexes are expected to resultin a stronger interaction between the defects and dislocations, hence, a higherrate of solution hardening [22]. We expect the same hypothesis to be valid forthe elevated temperature strengthening of Mo(Si,Al)z alloys by Re.

SoIution Softening by Al

Before discussing the softening by Al of ( Mo,Re)Siz alloys, we discuss themechanisms of softening by Al in MoSiz As shown in table 1, elements such asNb, Al, Ta, etc, which change the structure to C40 at higher concentrations,soften the Cl 1bMoSiz at low temperatures. Solution softening may result from

6

Fig.4 NomarsklinterferencecontrastotXicalmicrommhsshowimzsli~linesin~he (h400~7Reo~3)(SiO~7A10,03)2alloy ’deformed in-compression ~t, (~) room

temperature and (b) 600”°C, to the onset of cracking.

(i) scavenging of interstitial impurities by solutes, or (ii) lowering of Peierlsstress andlor easier double kink nucleation in the presence of solutes. Harada etal [11] have shown, through hot hardness experiments, that softening is onlyobserved at temperatures <-700 ‘C. Above this temperature, when the yieldstress starts to anomalously increase with increasing temperature, softening isnot observed. These authors have also shown that elements such as Zr withstrong affinity for interstitial impurities were found to have insignificant effecton the room temperature hardness. This led Harada et al to conclude that thescavenging of interstitial solutes by Al (or, Nb) may not be the dominantmechanism for solution softening in MoSi2. Peralta et al [10] had suggested thatthe softening due to Al may be attributed to lowering of CRSS for slip on the{110] 1/2<111> system. It is known that the 1/2<111> dislocations dissociateinto two 1/4<1 11> partials separated by a stacking fault on {110} [9]. Thestacking sequence in the fault is ABC that is also the stacking sequence in theC40 structure. Thus, the alloying elements that stabilize the C40 structure withrespect to Cl lb, are likely to segregate to the stacking faults at 1/2<1 11>dislocations in Cl lb MoSiz [23]. This may lower the stacking fault energy. Theresulting change in the stacking fault energy surface (y-surface) by alloying islikely to affect the Peierls stress and dislocation nucleation and mobility [24].

Fig. 5 TEM microgmphshowingthe dislocationsubstnrcurre,consistingof1/2<111>and c1OO>dislocations, in the (Mo0,97Rqo3)(Si0,97A10.O&ailoydeformedin compressionat 400 “C.

Waghmare et al. [24] have shown, through first-principles calculations, thatelements such as Nb, Al, Mg, and V change the y-surface of 1/2<331>dislocations in MoSiz resulting in a lower Peierls stress. Since 1/2<331>dislocations are only observed in uniaxial deformation along [001] and softeninghas been observed in polycrystals where 1/2<1 11> and <100> dislocationsoperate (Fig. 5), such calculations are needed for other dislocations as well.Recently, Mitchell et al. [25] have used atomistic simulations, complemented byfirst-principles calculations, to study the core structures and y-surface of1/2<331> dislocations in MoSiz and similar studies are needed for alloys. It doesappear that solution softening in Cl lb MoSi2 by solutes that stabilize the C40structure may be related to the effect of solutes on the dislocation core structuresand Peierls stress. We further speculate that at elevated temperatures, changes inthe core structure (e.g., assisted by climb) and/or solute mobility may reduce theeffect of solute on the y-surface and the Peierls stress.

Softening at Low Temperatures and Hardening at High Temperatures inQuaternary MoSi,-Re-AI AIIoys

The mechanical behavior of the quaternary alloys is most complex. At lowtemperatures, the behavior can be described as softening of Cl lb (Mo,Re)Si2 by

.

Al. At elevated temperatures, where Al does not soften MoSi2, hardening by Reof Cl lb Mo(Si,Al)z is observed. We speculate that in MoSi2, Re is able to formstrong point defect complexes with Si vacancies; however, in the quaternaryalloys Al may occupy the vacant Si sites, and reduce the concentration ofavailable vacancies to pair with Re atoms. This may reduce the hardening by Reof Mo(Si,Al)2 at low temperatures. At elevated temperatures, where the strengthof Mo(Si,Al)2 is comparable to or slightly higher than that of MoSi2, Readditions cause hardening. However, the hardening rate at high temperatureswith increasing Re concentration is greater for MoSi2-Re alloys as compared toMo(Si,Al)2–Re alloys, presumably due to the lower concentration of point defectcomplexes in the quaternary alloys. To conclusively prove these hypothesesregarding anomalous alloying effects, we need more atomistic simulations andfirst principles calculations of y-surface and Peierls stress, and experimentalstudies of solute segregations/clusters through atom-probe field ion microscopy.

Summary

Microalloying with Re and Al leads to several anomalies in the mechanicalbehavior of solidification processed polycrystalline MoSi2:(i) elevated temperature strengthening by Re is rapid. We hypothesize thatsubstitutional Re atoms, possibly by pairing with Si vacancies, lead to tetragonaldistortions in the lattice and hence, strong interactions with dislocations.(ii) softening by Al leads to enhanced compressive plasticity at roomtemperature, and may result from the reduction in Peierls stress caused by alowering of the stacking fault energy by Al.(iii) solution softening is only observed at temperatures <800 “C and at highertemperatures, solution hardening is observed in (Mo,Re)(Si,Al)2 alloys.

AcknowledgementsThis research was funded by DOE, Office of Basic Energy Sciences.

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