Silicide Microstructures and...

PARTI

Silicide Microstructuresand Deformation

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Cambridge University Press978-1-558-99221-4 - Materials Research Society Symposium Proceedings Volume 322: High Temperature Silicides and Refractory Alloys: Symposium held November 29-December 2, 1993, Boston, Massachusetts, U.S.A.Editors: C.L. Briant, J.J. Petrovic, B.P. Bewlay, A.K. Vasudevan and H.A. LipsittExcerptMore information

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OVERVIEW OF HIGH TEMPERATURE STRUCTURAL SILICIDES

J.J. PETROVIC* AND A.K. VASUDEVAN***Materials Division, Group MTL-4, Los Alamos National Laboratory, Los Alamos, NM87545••Office of Naval RTesearch, Code 4421, 800 North Quincy St., Arlington, VA 22217-5660

ABSTRACT

High temperature structural silicides represent an important new class of structuralmaterials, with significant potential applications in the range of 1200-1600 °C under oxidizingand aggressive environments. Silicides, particularly those based on MoSi2, are considered tobe promising due to their combination of high melting point, elevated temperature oxidationresistance, brittle-to-ductile transition, and electrical conductivity. Possible structural uses forsilicides include their application as matrices in structural silicide composites, asreinforcements for structural ceramic matrix composites, as high temperature joining materialsfor structural ceramic components, and as oxidation-resistant coatings for refractory metalsand carbon-based materials. The historical development of structural silicides, their potentialapplications, and important issues related to their use are discussed.

INTRODUCTION

High temperature structural materials that can be used in oxidizing environments in therange of 1200-1600 °C constitute an enabling materials technology for a wide range ofapplications in the industrial, aerospace, and automotive arenas. Potential uses includeindustrial furnace elements and fixturing, power generation components, high temperatureheat exchangers, gas burners and igniters, high temperature filters, aircraft turbine engine hotsection components such as blades, vanes, combustors, nozzles, and seals, and automotivecomponents such as turbocharger rotors, valves, glow plugs, and advanced turbine engineparts. There is increasing interest in silicide-based compounds for such applications. In thistemperature range, for oxidation and strength reasons, the choice of materials is limited to thesilicon-based structural ceramics such as Si3N4 and SiC, and to the new class of "high

temperature structural silicides" [1].While the number of known silicide compounds is large, potential silicides for elevated

temperature applications are essentially those based on refractory and transition metals, suchas MoSi2, WSi2, TiSi2, CrSi2, CoSi2, Mo5Si3, and Ti5Si3. Of such materials, MoSi2 ispresently the most promising and the most developed, due to its combination of high meltingpoint, superb elevated temperature oxidation resistance, brittle-to-ductile transition, andelectrical conductivity [2]. Structural uses for silicides include their application as matrices instructural silicide composites, as reinforcements for structural ceramic composites, as hightemperature joining materials for structural ceramics, and as oxidation-resistant coatings forrefractory metals and carbon-based materials. The purpose of the present discourse is toprovide an overview regarding these materials.

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Mat. Res. Soc. Symp. Proc. Vol. 322. ©1994 Materials Research Society






HISTORICAL PERSPECTIVE:

A brief historical perspective on the development of structural silicides is given here.A more detailed history of the development of structural silicides, with emphasis on MoSi2materials, is given in Reference [1]. W.A. Maxwell was the first person to suggest the use ofa silicide, MoSi2, as a high temperature structural material. He performed interesting researchon structural silicides in the early 1950's at NACA, the predecessor to NASA [3].Unfortunately, Maxwell's thoughtful initial work was not continued due to the fact that, atthat time, the high temperature structural materials community was not yet ready to deal withbrittle materials. In the early 1970's, E. Fitzer in Germany began examinations of MoSi2matrix composites reinforced with additions of AI2O3, SiC, and Nb [4]. This work ledFitzer's colleague, J. Schlichting, to publish a detailed review article in 1978, suggesting theuse of MoSi2 as a matrix material for high temperature structural composites [5].

Two important structural silicide articles were published in 1985. The first was anarticle by Fitzer and Remmele describing work on Nb wire-MoSi2 matrix composites withimproved room temperature mechanical properties [6]. In the second article, Gac andPetrovic indicated the feasibility of SiC whisker-MoSi2 matrix composites, showingimprovements in room temperature strength and fracture toughness [7]. In 1988, Carterdemonstrated SiC whisker-MoSi2 matrix composites with mechanical property levels withinthe range of high temperature engineering applications [8]. In 1990, Umakoshi et. al.published investigations of the mechanical behavior of MoSi2 single crystals, which indicatedinteresting elevated temperature properties [9]. As a result of the growing interest instructural silicides, the First High Temperature Structural Silicides Workshop, sponsored bythe Office of Naval Research, was held in November 1991 at the National Institute ofStandards and Technology in Gaithersburg, Maryland. This Workshop consisted of 32presentations in the areas of silicide materials, processing, processing-properties,microstructures, oxidation, mechanical properties, and coatings. Reference [1] contains theproceedings from this Workshop.

SILICIDE MATRIX COMPOSITES

For silicides to be used as a basis for high temperature structural materials, both theirhigh and low temperature mechanical properties must be improved. This requires significantimprovements in high temperature strength and creep resistance, and in low temperaturefracture toughness. However, it is important that composite strategies adopted do notdegrade either the intermediate or the elevated temperature oxidation resistance of thecomposite to a significant extent.

A number of composite approaches for silicides have been employed to date [1].Reinforcement morphologies have included continuous fibers, discontinuous particulate orwhisker phases, and microlaminates. Reinforcement materials have been both oxide and non-oxide ceramics, as well as refractory metals. Fabrication techniques for composites haveinvolved hot pressing/hot isostatic pressing, melting, plasma spraying, mechanical alloying,microlamination, in-situ syntheis, and combustion synthesis.

Composite approaches have been shown to significantly improve the mechanicalproperties of MoSi2-based structural silicides [1,2]. For example, the use of SiC whisker






reinforcements has been demonstrated to reduce elevated temperature creep rates by threeorders of magnitude over that of unreinforced MoSi2- Creep rates can also be reduced byalloying with substitutional species such as WSi2- Continuous fiber reinforcements haveyielded MoSi2~based composites with room temperature fracture toughness values in excess

of 15 MPa m*/2 Toughness values of 7.8 MPa m\^ have been obtained with discontinuousZrC>2 paniculate reinforcements.

It is useful to compare MoSi2~based structural silicides to silicon-based structural

ceramics (Si3N4, SiC), since both of these material classes are candidates for 1200-1600 °C

structural applications [2]. The two central issues for the application of such materials are

those of reliability and cost, and a comparison of these aspects is given in Table 1.

Table I. Comparison of MoSi2-Based Structural Silicides and Silicon-Based Structural

Ceramics

RELIABILITY ADVANTAGES

MoSi2-Bascd Structural Silicides:O Brittle-to-ductile transition in a useful temperature range

o Potential higher fracture toughness at operating temperaturesO Alloying may be extensively employed to improve mechanical propertiesO Thermodynamically stable with a wide range of ceramic reinforcementsO Thermal expansion coefficients a closer match to metals, thus easier to join to metals

Silicon-Based Structural Ceramics:O No intermediate temperature oxidation HpestM

O Somewhat more creep resistant, at least at presentO Lower thermal expansion coefficients, thus lower thermal stresses

COST ADVANTAGES

MoSi2-Bascd Structural Silicides:O Can be electro-discharge machined, thus lower cost machiningO Can be melted, thus more versatility in processingO Easer to density, no densification aids required

Silicon-Based Structural Ceramics:O None

Currently, there is a slight reliability advantage of the MoSi2-based structural silicidesover the silicon-based structural ceramics. With further development of the structural silicides(which are currently at an early stage in comparison to the more mature structural ceramics), amore dramatic reliability advantage is likely to emerge, as issues of structural silicide creepresistance, fracture toughness, and intermediate temperature oxidation behavior areaddressed. However, Table 1 clearly shows that there is a distinct cost advantage of thestructural silicides over the structural ceramics. Because the silicides can be electro-dischargemachined, their machining costs will be significantly lower than the structural ceramics, whichmust be diamond machined. Furthermore, unlike the structural ceramics which thermally






decompose rather than melt, the silicides can be melted, leading to more versatility inprocessing since melting techniques such as plasma spraying can be employed. Finally, thesilicides are easier to densify, and do not require the densification aids which must beemployed for structural ceramics.

SILICIDE REINFORCEMENT OF CERAMIC MATRIX COMPOSITES

An area that is little explored at present but which is potentially of major importance isthe use of silicides to improve both the reliability and cost of structural ceramics [2]. Forexample, above its brittle-to-ductile transition temperature, MoSi2 can be employed as anoxidation-resistant, ductile phase in a ceramic matrix composite. This presents theopportunity to significantly improve the elevated temperature mechanical properties of thecomposite, such as strength, high temperature fracture toughness, creep, and slow crackgrowth resistance. Such property improvements would constitute an important reliabilitybenefit. Additionally, at suitable volume fraction and morphology, the MoSi2 phase may alsoimprove the machinability of ceramic matrix composites by allowing for electro-dischargemachining (EDM). This would constitute a major cost benefit.

There is very little published work in this area to date. The work that has been done,however, definitely indicates a substantial improvement in elevated temperature mechanicalproperties by incorporating a MoSi2 silicide phase into SiC and Si3N4 structural ceramicmatrices [10,11]. Although no silicide phase work on benefits to ceramic machinability hasyet been published, such benefits are also anticipated in view of published results withelectrically conductive carbide, nitride, and boride phases in structural ceramic matrixmaterials such as Si3N4. However, these carbide, nitride and boride additions do not possess

the high temperature oxidation resistance of silicides, and this fact has limited their usefulness.

SILICIDE HIGH TEMPERATURE JOINING MATERIALS

The high temperature joining of structural ceramic components has been alongstanding difficulty. Structural ceramics such as Si3N4 and SiC possess thermal expansioncoefficients significantly lower than most metals, leading to mismatch stress problems. Inaddition, conventional brazing metal alloys do not have sufficient elevated temperatureoxidation resistance. Silicides may have potential uses as higher temperature brazingmaterials for the structural ceramics. Only one investigation has been performed to date inthis area. This study demonstrated that MoSi2 and TiSi2 may be employed as braze joining

materials for SiC [12]. Sound joints were obtained by heating in the range of 1750-1950 °C,and no reactions with SiC were observed.

SILICIDE COATINGS

Silicide based materials may have applications as advanced high temperature coatingsfor refractory metals. Important factors for coatings of this type include coating oxidationresistance, thermal stability and strength, high temperature chemical compatibility of the






coating with the substrate, and thermal expansion coefficient mismatch between coating andsubstrate. Recent work has shown that MoSi2 based materials can provide an excellent hightemperature coating for niobium [13]. (Mo,W)(Si,Ge)2 coatings on niobium survived 200

one hour cycles at 1370 °C in air and 60 one hour cycles at 1540 °C in air, due to formationof a protective glassy film.

The use of silicides to coat carbon and carbon-carbon composite materials has notbeen explored to any great extent as of the present time, although the potential for suchcoatings may exist. One aspect here is the fact that silicides tend to react with carbon (forexample, M0S12 reacts to form CMo5Si3, the so-called Nowotny phase), which maynecessitate the use of reaction barrier layers.

SIGNIFICANT ISSUES WITH SILICIDES

There are several significant issues which must be addressed in order to promote theuse of silicides in high temperature structural applications. These issues include minimizing oreliminating the intermediate temperature oxidation pest behavior, increasing low temperaturefracture toughness, improving high temperature creep resistance, and obtaining basic materialproperties.

Many silicides exhibit intermediate temperature accelerated oxidation, or evenoxidation pest behavior (catastrophic oxidation) under certain conditions. For example, inMoSi2 intermediate temperature (500 °C) accelerated oxidation and pest behavior occurs dueto the retention of MOO3 as a solid oxidation product, whose volume expansion can producemicrocracking. Means to minimize or eliminate this behavior include minimization of porosityand microcracking, pre-oxidation formation of a continuous SiC>2 surface layer, alloying toalter oxide characteristics and oxidation mechanisms, and the use of metal coatings.

Improving low temperature fracture toughness is a significant issue. For many

applications, room temperature fracture toughness values below 10 MPa va\^ will beadequate, but for some high performance applications higher toughness levels will benecessary. Composite strategies developed or in development for high toughness structuralceramics should also be applicable to the silicides. Improvements in elevated temperaturecreep resistance may be achieved by the minimization or elimination of glassy phases whichpromote grain boundary sliding in preference to dislocation creep mechanisms. Dispersionstrengthening through the use of nanosized reinforcement phases dispersed intragranularly in arelatively large grained material should produce a highly creep resistant microstructure.

Lastly, there are currently substantial gaps in the description and understanding of thefundamental material properties of the silicides. Areas where little basic information existsinclude self-diffusion coefficients and diffusion mechanisms, single crystal properties,characterizations of ductile-to-brittle transitions and the factors which influence them,oxidation mechanisms, and sintering behavior. It will be necessary to obtain this basicinformation in order to be able to optimize the silicide materials for engineering applications.






ACKNOWLEDGEMENTS

Research on high temperature structural silicides at the Los Alamos NationalLaboratory has been made possible by funding support from the U.S. Department of Energyand the Office of Naval Research.

REFERENCES

1. High Temperature Structural Silicides. edited by A.K. Vasudevan and J.J. Petrovic(Elsevier Science Publishers, Amsterdam, 1992); Mat. Sci. Eng., A155, 1-274 (1992).

2. J.J. Petrovic, MRS Bulletin, XVIII, 35 (1993).

3. W.A. Maxwell, NACA Research Memorandum RM-E52B06, (1952).

4. E. Fitzer, O. Rubisch, J. Schlichting, and I. Sewdas, Spec. Ceram., 6, 24 (1973).

5. J. Schlichting, High Temp.-High Press., 10, 241 (1978).

6. E. Fitzer and W. Remmele, in Proc. 5th Int. Conf. on Composite Materials. ICCM-V.edited by W.C. Harrigan, Jr., J. Strife, and A.K. Dhingra, (AIME, Warrendale, PA, 1985), pp.515-530.

7. F.D. Gac and J.J. Petrovic, J. Am. Ceram. Soc, 68, C200 (1985).

8. D.H. Carter, MS Thesis, Massachusetts Institute of Technology, 1988; D.H. Carter, W.S.Gibbs, and J.J. Petrovic, Proc. 3rd Int. Symp. on Ceramic Materials and Components forEngines. (American Ceramic Society, 1989), pp. 977-986.

9. Y. Umakoshi, T. Sakagami, T. Hirano, and T. Yamane, Acta Metall. Mater., 38, 909(1990).

10. C.B. Lim, T. Yano, and T. Iseki, J. Mat. Sci., 24, 4144 (1989).

11. J.J. Petrovic and R.E. Honnell, J. Mat. Sci. Lett., 9, 1083 (1990).

12. T.J. Moore, J. Am. Ceram. Soc, 68, C151 (1985).

13. A. Mueller, G. Wang, R.A. Rapp, E.L. Courtright, and T.A. Kircher, Mat. Sci. Eng.,A155, 199(1992).






PLASTIC BEHAVIOR AND DEFORMATION STRUCTURE OF SILICIDE SINGLECRYSTALS WITH TRANSITION METALS AT HIGH TEMPERATURES

Y. UMAKOSHI, T. NAKASHIMA*, T. NAKANO and E. YANAGISAWA

Department of Materials Science and Engineering, Faculty of Engineering,Osaka University, 2-1 , Yamada-oka, Suita, Osaka 565 Japan* Sumitomo Metal Industries, Ltd., Wakayama, Japan

ABSTRACTThe mechanical and plastic behaviors of refractory silicide single crystals with Cl l b

(MoSij), C40 (CrSi2, TaSi2 and NbSy, D88 (Ti5Si3) and Cl (CoSi2 and (CotoNioJSia)structures were investigated. The C40-type silicides were deformed by (0001)<li20> slip.Their yield stress decreased sharply with increasing temperature but NbSi2 and TaSi2 whichwere deformable even at low temperatures, exhibited anomalous strengthening around 1350°C.Deformation of Ti5Si3 whose ductile-brittle transition occurred around 1300°C was controlledby twins and the brittle fracture occurred on the basal plane. In CoSi2 the {001}<100> slipwas only activated at ambient temperatures but addition of Ni activated {110}<110> slipas secondary slip system and improved the ductility. The creep behavior of MoSi2 and CrSi2

single crystals were also investigated and was found to be controlled by the viscous andglide motion of dislocations.

INTRODUCTIONNew, extremely high-temperature tolerant materials for service at more than 1500°C are

required for aircraft gas turbines and spacecraft airframes. From the viewpoint of specificgravity, elastic modulus, high-temperature strength and oxidation resistance, several transitionmetal silicides with high silicon content are among the potential candidates for such ultra-high temperature structural materials from a compilation of about 300 binary metallic andmetal-metalloid compounds that melt above 1500°C [1]. At high temperatures the silicidesexhibit excellent oxidation resistance since silicon atoms form viscous and protective SiO2

films which can infiltrate and cover micro cracks generated during the operating process.In general, silicides have an intricate crystal structure and can rarely be deformed. From

crystal symmetry considerations, melting point and high-temperature strength, we should lookfor several silicides with the Cllb structure based on the b.c.t. lattice, and C40 and D88

structures based on the h.c.p. lattice.Some MSi2-type silicides with the elements Mo, W and Re are known to crystallize into

the Cl l b structure. Since the Cllb structure is a long-period ordered structure derived bystacking up three b.c.c. lattices and then compressing them along the long period axis, theirsilicides possess the deformation characteristics of b.c.c. crystals. The {110}<331] and{103)<331] slips which correspond to the {110}<lll> slips in the b.c.c. lattices wereobserved in MoSi2 at around 1000°C, and with increasing temperature <100]-and <110]-slipswere activated [2, 3]. From the viewpoint of activated slip systems, poly crystalline MoSi2

is expected to have ductility since the number of slip systems is enough to satisfy the vonMises criterion. However, even single crystals were brittle extremely at low temperatures andthe transition from ductile to brittle behavior occurred around 900°C.

One of the approaches to improve the ductility and fracture toughness is to developquasi-binary disilicides with two-phase microstructures composed of the Cl l b and C40phases [4]. Detailed informations are needed on the mechanical properties and plasticcharacteristics of both C40-type and Cllb-type silicides to understand the plastic behavior

Mat. Res. Soc. Symp. Proc. Vol. 322. ©1994 Materials Research Society






of the component phases in quasi-binary silicide composites. However, to our knowledgeonly limited studies on the slip system and the temperature dependence of the criticalresolved shear stress (CRSS) of CrSi2 with the C40 structure have been reported using singlecrystals [5].

For application to high-temperature structural components, not only high temperaturestrength but also creep resistance which is even more important is required. The creepbehavior of polycrystalline MoSi2 and MoSi2-based composites reinforced with SiC whiskershas been investigated [6] but studies have not been made on the creep mechanism ordislocation structure using single crystals.

In this paper, current knowledge of the slip behavior and deformation mechanism of theCllb-type and C40-type disilicides including creep deformation is reviewed based on theresults of our recent studies on MoSi2 with the Cllb structure and CrSi2, TaSi2 and NbSi2

with the C40 structure.CoSi2 with the Cl structure exhibits excellent oxidation resistance and some ductility even

at low temperature because of its f.c.c.-based structure. It is of considerable interest as oneof the duplex phases to improve the fracture toughness of silicide composites, although themelting point of CoSi2 is not high (Tm=1326°C). CoSi2 is primarily deformed by {001}<100>slip systems and when the orientation of samples is controlled for the {001}<100>-slip,some fracture strains are obtained in single crystals even at room temperature [7].Polycrystalline CoSi2, however shows high ductile-brittle transition temperature and therefore,activation of additional slip systems is required to improve ductility. The effect of theaddition of Ni to CoSi2 on operative slip systems and plastic behaviors is also described.

Ti5Si3 with the D88 structure is also a potential candidate as a refractory material. Themechanical properties of polycrystalline Ti5Si3 and unidirectionally solidified eutectic a Ti-Ti5Si3 alloys [8], and the oxidation resistance of ternary (Ti, X)5Si3 (X=V, Nb) have beeninvestigated but no detailed study on plastic behavior has been reported. Deformation andfracture mechanisms of Ti5Si3 single crystals are also presented together with the deformationstructure.

EXPERIMENTAL PROCEDUREThe master ingots of binary MSi2 (M=Mo, Cr, Ta, Nb or Co) and Ti5Si3, and ternary

(Co09Ni01)Si2 silicides were prepared by melting high-purity raw materials in a plasma arcfurnace. Single crystals of the binary and ternary silicides were grown from these ingots bythe floating zone method using an NEC SC-35HD single crystal growth apparatus at growthrates of 5 and lOmm/h under a high-purity argon gas flow. Specimens for compression andcreep tests (approximately 2.5x2.5mm in cross-section and 7mm long) with selectedorientations were cut from the single crystals by spark machining and mechanically polishedusing diamond paste to observe surface slip markings. Compression tests were conducted onan Instron-type testing machine in a purified argon gas atmosphere or in a vacuum at anominal strain rate of 1.4XKTV1 at temperature ranging from 20 to 1500°C. Slip patternswere then examined with an optical microscope using Nomarski interference contrast. Creeptests of MoSi2 and CrSi2 single crystals were conducted under compression in an argon gasatmosphere. The displacement of specimens was measured with a linear variable-differentialtransducer.

Thin-foil specimens for electron microscopy were cut first from deformed specimens byspark machining and finally ion-milled by Ar bombardment to perforation. The thin foilswere examined in a Hitachi H-800 electron microscope operated at 200KV.

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