An overview of powder processing of silicides and their composites

Materials Science and Engineering A261 (1999) 169 – 180

An overview of powder processing of silicides and their composites N.S. Stoloff Rensselaer Polytechnic Institute, Troy, New York, NY 12180 -3590, USA

Abstract The use of powders to prepare fully dense silicides and their composites is reviewed. Since most of the techniques are well known, we concentrate on recent developments in this field, including the effects of processing techniques on composition, microstructure and mechanical properties. The use of hybrid processes to produce silicides is described, as are hybrid materials resulting from these processes. Included in the latter are functionally graded materials (FGM). The paper concludes with a brief discussion of in-situ melt processing as a competitive means to produce aligned composite structures © 1999 Elsevier Science S.A. All rights reserved. Keywords: Hybrid processes; Silicides; In-situ melt processing; Powder; Mechanical alloying

1. Introduction The purpose of this paper is to review the powder techniques that have been used to produce silicide alloys and their composites. The high melting points and brittleness of the silicides has naturally led to a large number of studies of powder techniques, principally hot pressing [1 – 12], variants of reactive sintering (RS) [13–21], thermal or plasma spray [22 –28],and mechanical alloying (MA) [29 – 36]. Most of the alloys produced from powder have been based upon MoSi2 and Cr3Si, although efforts have been made to produce Ti5Si3, Nb5Si3, Mo5Si3 and, more recently, boron-containing silicides. A list of powder techniques appears in Table 1. In general, it is not difficult to fully densify the silicides by powder techniques, especially when secondary processing is involved, but contamination by impurities such as oxygen has often been a problem. High oxygen contents, exacerbated when fine powders are used, are detrimental because of the resulting formation of amorphous silica at grain boundaries. The SiO2 degrades both low and high temperature mechanical properties. However, good results can be obtained by adding carbon and/or by making use of displacement reactions in which a silicide or composite is produced by means of a chemical reaction. One of the objectives of this paper is to compare the microstruc-

tures and properties of silicides made by powder techniques with those of cast materials, including in-situ composites, where such data are available. It will be shown that cracking of samples is much more likely in conventional castings compared to powder products, but contamination is more likely to be encountered in conventional powder processes [37,38]. Other differences in the microstructure and properties of silicides prepared by different processes will be discussed. A major difficulty in clearly defining the merits of the various powder consolidation methods is the frequent combination of two or more processes, e.g. hot pressing + HIP or MA + HIP to prepare a monolithic or composite silicide. In fact, it is rather unusual to find accounts of processing involving only one major operation.

2. Silicides The silicides of interest for structural applications fall into three groups, the disilicides, typified by MoSi2, the 5–3 compounds such as Ti5Si3 and Mo5Si3 and the 3–1 compounds such as Nb3Si, Mo3Si and Cr3Si; the latter crystallize as cubic A15 compounds and therefore are more likely to exhibit some ductility or toughness than the other, non-cubic silicides. The disilicides can be

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Table 1 Powder metallurgical techniques Hot pressing Reacti6e processes Sintering Combustion HIPing Injection molding Displacement reactions Mechanical alloying Vapor infiltration Liquid infiltration (Ceracon) Powder cloth Extrusion XD™ Plasma spray Rapid solidification rate

further grouped by their heats of formation, as follows: the lowest group, CrSi2 and WSi2, have heats of formation between 80– 90 kJ mol − 1, followed by a group of five compounds, among them MoSi2, that fall within the range 130910 kJ mol − 1, and ZrSi2, which has the highest heat of formation at 160 kJ mol − 1 [39]. The self-propagating high temperature synthesis (SHS) reaction of MoSi2 is considered to be on the borderline of being self-sustaining, and there have been many variants of SHS or RS, initiated thermally, that have been attempted for this compound. Yen et al. [19], have pointed out that mechanically-induced SHS reactions can occur during MA, following an induction period during which powder particles are repeatedly fragmented and welded while being intimately mixed. The process is composition and particle size dependent, as are thermally ignited SHS reactions. Although thermally and mechanically initiated SHS reactions show a number of other similarities, the two processes will be described separately.

Fig. 1. Backscattered image of reaction zone at MoSi2 – Nb interface developed during hot pressing [44].

hand layup prior to hot pressing or HIPing. Fiber tows as well as individual filaments have been used [40]. In this respect, in-situ processing from the melt offers distinct advantages in terms of ease of preparation. Short fibers can be aligned by injection molding, although among the silicides only MoSi2-base composites have been produced. A list of matrix-reinforcement combinations, (excluding hybrid composites, see below) which have been reported appears in Table 1. Metallic and intermetallic reinforcements such as the refractory metals: Ta, Mo and Nb and Nb–Al–Ti alloys, have been shown to improve room temperature toughness of MoSi2, especially when added as fibers or lamellae [41–43]. However, interfacial reaction layers are commonly observed (see Fig. 1, [44]), usually consisting of an even more brittle intermetallic than the matrix, as shown clearly in Fig. 2 by the cracks around a hardness indent in the (Nb,Mo)5Si3 reaction zone [44]. Further, long-term, high temperature stability of such reinforcements is very unlikely. Pt alloy reinforcements have

3. Strategies for consolidating composites The extreme brittleness of the silicides has led to many attempts to improve toughness without degrading high temperature strength and oxidation resistance. Fortunately, many potential ceramic reinforcements, including SiC, TiC, Si3N4, Al2O3, TiB2, Y2O3 and ZrB2, have been shown to be reasonably stable in MoSi2. However, reinforcements must be compatible both chemically and in terms of thermal expansion mismatch with the matrix. Fibrous reinforcements must also resist break-up by the stresses induced in consolidation under pressure; this is a problem primarily with ceramic fibers. Alignment of long fibers usually is achieved by

Fig. 2. Cracking in reaction zone at MoSi2 – Nb interface [44].

N.S. Stoloff / Materials Science and Engineering A261 (1999) 169–180 Table 2 Reinforcements for silicidesa MoSi2

Cr3Si

Ti5Si3

Nb5Si3

SiC (f, p, w) TiC (p) TiB2 (p) HfB2 (p) ZrB2 (p) Si3N4 (p) ZrO2 (p) Y2O3 (p) Nb–Ti–Al (l) Nb (f, p, l) Ta (f, p) Mo(f, p)

Al2O3 (f) Pt–Rh (f) Pt–ZrO2 (f)

TiC (p)

Nb (p)

a

f, fiber; p, particle; w, whisker; l, lamella.

been incorporated into Cr3Si alloys, but interdiffusion and plastic flow have been too rapid to maintain the integrity of the fibers [14]. Another major shortcoming of the use of metallic fibers is their inferior creep resistance compared to the silicides, coupled with poor oxidation resistance (except for Pt). Refractory metal fibers need to be coated to prevent oxidation and reaction with silicon, but coatings will not solve the problem of inferior creep resistance. Consequently, metallic reinforcements have been most effective at room temperature in improving either strength or toughness, or both. Ceramic reinforcements such as TiB2, SiC and Al2O3 have proven to be much less effective in improving low temperature mechanical properties of MoSi2, especially when added as particulates. Only a few studies have been carried out on reinforcing other silicides, and these have utilized particles, see Table 2. The influence of reinforcement morphology has been studied by several groups, usually with Nb as the reinforcement. A clear improvement in toughness of MoSi2 was noted by Chen et al. [4], in the order: particles, fibers of increasing diameter and laminates. Alman and Stoloff [44], in a similar study, showed that while Nb fibers neck as fracture of the composite occurs, particles of Nb fail by cleavage. The difference in fracture mechanism with size and shape of reinforcement is related to two factors, constraint on the reinforcement [45], and interfacial bond strength. Although Nb particles are not very effective for improved toughness and actually degrade fatigue crack growth resistance [46], abrasive wear resistance is significantly enhanced by both Nb fibers and particles [47]. By contrast, SiC additions to MoSi2 do not improve wear resistance [47]. Finally, it has been reported that Nb mesh is more effective than wires in improving the fatigue crack growth resistance of MoSi2 [46]. The relatively high CTE of MoSi2 has led to several attempts to combine SiC particles or platelets to reduce

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the CTE mismatch between the matrix and fibers of SiC or Mo. Such efforts have led to only limited success, although Deve´ et al. [48], reported improved creep strength and fracture resistance of MoSi2 when 20 vol.% SiC particles and 20 vol.% SiC platelets were added to the matrix of a Mo-fiber reinforced alloy. Si3N4 particles have been shown to eliminate cracking in SCS-6 reinforced composites, as shown in Fig. 3 [2]. The resulting composite was shown to be pest resistant and much stronger than monolithic MoSi2 The temperature dependence of the tensile strength of a composite containing 50% Si3N4 is compared to that of several other materials in Fig. 4, [31]. A hybrid approach to composite design seems to be a promising path to improved properties, and many studies of the MoSi2 –Si3N4 system as a matrix material have recently been reported [11,12,31].

4. Hot pressing The least complicated powder consolidation method, hot pressing of pre-alloyed powders, has been utilized in many studies of silicides and their composites. Higher temperatures are needed to press pre-alloyed powders to full density than would be required in pressing elemental powder; typical temperatures for hot pressing are in the range 1600–1850°C for unalloyed MoSi2 as well as its alloys with W. Temperatures from 1600–1900°C have been utilized for MoSi2 composites reinforced with SiC particles. Both MoSi2 and Cr3Si are readily prepared from pre-alloyed powders with much less cracking than would be observed in arc melted or induction melted product of the same composition. See, for example, the microstructure of a hot pressed (Cr,Mo)3Si alloy in Fig. 5(a), [14]. However, contamination is much more severe, especially in terms of oxygen content, in hot pressed material as was reported by Raj [37], for Cr3Si and (Cr,Mo)3Si alloys. Hot pressing of pre-alloyed (Cr,Mo)3Si reinforced with various ceramic and metallic fibers also has been carried

Fig. 3. SEM image of as fabricated 30 vol.% SCS-6/MoSi2 –30Si3N4 composite [2].

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Fig. 4. Temperature dependence of ultimate tensile strength of SCS-6/MoSi2 – 50Si3N4-base composites [31].

out, with microstructures as in Fig. 5(b) for a Pt–Rh reinforced alloy [14]. Note the loss of fiber integrity due to induced stresses and inter-diffusion. Limited mechanical properties testing revealed little improvement in high temperature stiffness or strength with any reinforcement. Vacuum hot pressing has been utilized also to facilitate preparation and densification of MoSi2 by solidstate displacement reactions, as described in a later section. Recent investigations of hot pressing have been carried out in the system Mo5Si3 – Mo5SiB2 – Mo3Si [38,49]. A section of the ternary phase diagram at 1873K is shown in Fig. 6, [50]. Samples hot pressed from MoSi2, Mo, B and graphite particles showed three phases: Mo3Si, T1 and T2, but complete equilibrium was not established. Comparison of these specimens with others produced by ingot metallurgy or by mechanical working subsequent to casting, illustrated that powder product was free of macrocracks, and had the fewest microcracks. However, while strengths were high for the PM product, significant porosity and oxygen contamination were noted. The strength of PM product varied widely, due to a range of processing conditions. Another recent study of hot pressing was carried out by Pan et al. [9]. Hot pressing of pre-alloyed powders was compared with the product of an in-situ reaction between Mo2C and Si. The latter exhibited a very fine

SiC size and the composite displayed improved hardness, fracture toughness and flexural strength, (see Fig. 7 [9]). A variant of hot pressing has been used by Shah and Anton [40] to prepare MoSi2 composites with aligned FP alumina fibers. Their technique involved infiltrating fiber tows with fine elemental powders, followed by hot pressing at the relatively low temperature of 1425°C. Fiber tows of SiC or Al2O3 also have been used to prepare MoSi2 composites by a slurry method, followed by HIPing [51]. Hot pressing also has been used to produce nano-composites of MoSi2 and SiC [52]. Prealloyed powders of MoSi2, Mo–Si–Al and SiC were first ball milled, then pressed at 1500–1600°C. The addition of Al to the silicide markedly improved high temperature bend strength. Functionally graded Mo–MoSi2 composites have been produced by hot pressing mixtures of 99.9% Mo powder with varying quantities of 99.5% MoSi2 [53]. Samples consisting of five layers of varying %MoSi2(0, 25, 50, 75 and 0%) were pressed at a pressure of 17.4 MPa, with temperature raised from room temperature to 1650°C at 25°C min − 1, holding at this temperature for 4 h. Deevi and Lilly [12] have also produced FGM MoSi2 –Si3N4 compacts by hot pressing, and preliminary mechanical properties data have been reported at this conference.

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Fig. 5. (a) Hot pressed (Cr, Mo)3 Si alloy, pressed at 1400°C, showing (Cr, Mo)3Si and (Cr,Mo)5Si3 phases [14]. (b) Hot pressed (Cr,Mo)3 Si alloy, reinforced with Pt-6% Rh fibers [14].

5. Hot isostatic pressing Hot isostatic pressing can be employed as the primary consolidation method, or can serve as a secondary process to complete a reaction or improve density obtained from some other process, such as hot pressing or RS. Since the HIP process is well known, no specific description will be presented here, except to point out that either elemental or pre-alloyed powders

Fig. 6. Section of ternary Mo–Si–B diagram at 1873 K [50].

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can be HIPed. Temperatures for HIPing generally are in the range 1350–1700°C for both MoSi2 and its composites with SiC. Full density of MoSi2 has been achieved at temperatures as low as 1400°C [4]. A combination of vacuum hot pressing and HIPing has been used to produce MoSi2 –Si3N4 composites [2]. Later, a combination of MA (see below for description), vacuum hot pressing and HIP was used to consolidate MoSi2 –Si3N4 plates which in turn, were made into composites with SCS-6 SiC fibers by means of the powder cloth method [31]. The function of Si3N4 was to reduce the CTE of the MoSi2 sufficiently to prevent cracking when the fibers were incorporated, as is shown in Fig. 3. Fully dense material was obtained, and the fibers showed little reaction with the matrix. While many studies of hot pressing and HIPing of MoSi2 and its composites have been carried out, the data generally do not permit prediction of the optimum pre-treatment, consolidation temperature and applied pressure in order to achieve full density. However, Sastry et al. [54], have applied a powder consolidation model of Fischmeister and Arzt [55], and a HIP model of Ashby [56], to construct HIP diagrams for unreinforced MoSi2 as well as MoSi2 –20 vol.% Nb and MoSi2 –20 vol.% SiC particulate composites. Analysis of the diagrams showed that power law creep is the dominant densification mechanism for these materials, thereby suggesting that high pressures are advantageous in achieving full density. At 207 MPa and HIP times of 1–4 h, the temperature required to achieve near-theoretical density for MoSi2-based materials is 1600– 1700K. Nb particles promote densification at lower temperatures than for monolithic MoSi2. SiC reinforced MoSi2 is less dense than the monolith for all consolidation conditions studied.

Fig. 7. Flexural strength (sFS) and fracture toughness (KIC) of three MoSi2 materials [9].

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6. Reactive processing

6.1. Reacti6e sintering

Fig. 8. Schematic of combustion synthesis reaction [57].

Most studies of silicide matrix composites made from powder have utilized particulate or fibrous reinforcements. However, it has been demonstrated that MoSi2 powders can be reinforced with foils of Nb [42], or a ductile Nb–15Al– 40Ti alloy [43]. The fracture toughness of monolithic MoSi2 was increased from 3.6 to 18 MPa by producing laminates with 200 mm thick Nb lamellae [4]. Foils were HIPed with MoSi2 powder at 1400°C and a pressure of 107 MPa for 2 h, resulting in a 15 mm thick reaction layer. Laminated composites utilizing 20 vol.% of three different thicknesses of the ternary reinforcement were HIPed at 1400°C under 207 MPa pressure for 4 h. The layered interfacial reaction zones, about 25– 35 mm thick, consist of Mo5Si3 as in the case of hot pressed pre-alloyed powders shown in Fig. 1. Unfortunately, interdiffusion between the two phases resulted in embrittlement of the foils, especially with the thinnest (100 mm) foils.

The simplest form of RS or SHS is to simply mix unalloyed powders and allow an exothermic reaction to proceed without superimposed pressure, as shown schematically in Fig. 8 [57]. The reaction may be set off by an ignition coil, by a modest increase in temperature, or by imposing mechanical forces, such as shock loading [39]. Potential advantages include low energy consumption, shorter processing times, lower capital costs and higher purity of the product. However, there are also shortcomings, including sometimes the inability to control the reaction and lack of achievement of full density unless pressure is applied during or after the reaction. When one of the reactants melts, or the system contains a low temperature eutectic which forms during heating, the process is known as liquid phase sintering.

6.2. Variants of reacti6e sintering Variants on this process include reactive hot pressing, reactive HIPing, MA, reactive vapor infiltration of a powder preform, reactive thermal spraying and, most recently, shock reactive synthesis, see Fig. 9 [39]. The latter process has been utilized by Deevi and Thadhani [58], Yen et al. [39], and Meyers et al. [59], to produce single phase silicides. The advantage of shock processing is that the solid state chemical reactivity of the reactants is enhanced by increasing the defect density, intimate mixing and surface cleaning. Solid state reactions, while producing single phase silicides with refined microstructures, do not produce fully dense materials. Densification in the presence of liquid silicon requires

Fig. 9. Schematic diagram of one-stage gun used for shock reactive synthesis [39].

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the simultaneous application of pressure and temperature [58]. Another variant on SHS processes has been reported by Raman et al. [60]. By combining SHS with the patented Ceracon process in which a compacted preform is used the time of exposure can be reduced by orders of magnitude and canning for HIPing can be avoided. The process uses a ceramic particulate material as a pressure transmitting medium. Oxygen contents actually are lowered by the process, but small amounts of Mo5Si3 are formed, unlike in standard SHS processing. However, the authors report that higher reactant temperatures should reduce the chances of forming Mo5Si3 Composites also are readily consolidated by reactive hot pressing. Stoloff and Broglio [1] have produced MoSi2 reinforced with both SiC particles and whiskers by pressing a blend of Mo, Si and the reinforcements at 1400°C for 3 h, with pressure increased in steps. Functionally graded materials (FGM) also have been produced by SHS in the system MoSi2 +X SiC. As expected, the reaction quenches out at high concentrations of SiC, with X \0.5 [20].

6.3. Displacement reactions A displacement reaction is a solid-state process involving a chemical reaction that leads to the formation of fully dense silicides. As such, it may be considered a variant of both hot pressing and RS. Henager and co-workers [15–17,61,62], Pan et al. [9], and others [30], have shown that the process is capable of producing MoSi2 composites with SiC or MoB particles. A typical reaction is: Mo2C+ 5Si“SiC +2MoSi2 Fine, homogeneous microstructures of the order of 1 mm grain size are obtained, and the composites display high strength combined with improved toughness, as shown in Fig. 10 and Fig. 11, due to the fine microstructures and high purity of the product. However, at high temperatures the finer grain size provides inferior strength, also shown in Fig. 11, [30]. The displacement process requires optimization, and it has not yet been demonstrated that it is possible to form large quantities of a composite by this method (quantities of 130 g of blended powder have been reported by Henager et al. [62]). Nevertheless, the advantages of a solid state reaction, combined with high purity, fine microstructures and good mechanical properties, demonstrate that further work is warranted on this technique. A similar in-situ process involving co-synthesis of SiC particles in a MoSi2 matrix was reported by Alman and Stoloff [13]. Starting materials were elemental powders of Si, Mo and C.

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Fig. 10. Yield stress in four point bending in temperature for MoSi2 composites containing 5 and 20 vol.% SiC, the latter without SiO2 [30].

7. Mechanical alloying MA is another solid state process, typically carried out in a high energy ball mill, that may involve either pre-alloyed or elemental powders. When the latter are used the reaction occurs sporadically due to the impact and grinding of the balls on the powder, initiating and propagating without the formation of a liquid phase [29]. A schematic of the process appears in Fig. 12, [63]. Attributes of MA include extension of solubility limits, refinement of the microstructure down to nano-scale, the synthesis of novel crystalline or amorphous phases as well as the ability to induce chemical reactions at relatively low temperatures [35]. MA of silicides from elemental powders has been carried out in several investigations. Hardwick et al. [32], reported very high oxygen levels, \1% in MA MoSi2. This produced a very high level of SiO2 particles, which resulted in diminished high temperature strength; low temperature toughness also is adversely affected by SiO2. However, Schwarz et al. [34] have shown that MoSi2 with low oxide content can be produced by MA. Recently, MoSi2, as well as particulate composites containing SiC or Al2O3 have been consolidated by MA of elemental powders, followed by hot pressing in the range of 1100–1700°C. Lower pressing temperatures gave less porosity and better compaction. Also, less porosity was noted in the mechanically alloyed material than in hot pressed commercial powders [36]. The compressive yield stress of MA material containing 5 vol.% Al2O3 was twice that of monolithic material, while SiC dispersoids did not increase strength at all. Another investigation showed that SiO2 formation during processing could be suppressed by combining MA with carbon additions and an in situ reduction reaction [30]. The resulting microstructure consisted of uniformly dis-

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Fig. 11. Four point bend chevron-notched fracture toughness, Kc, versus temperature for MoSi2 – SiC materials produced by displacement reactions + vacuum hot pressing at 1700–1800°C [62].

tributed micron size particles of SiC in the MoSi2 matrix. Composites containing TiC or SiC also have been produced by MA by Kush et al. [64], to study thermal fatigue behavior. SiC particulates provided the best thermal fatigue resistance of the alloys tested. Yen et al. [19] have studied the MA behavior of Mo–Si alloys in the range 10 – 80 at.% Si. Depending upon composition and processing conditions, three compounds formed during MA, MoSi2, Mo3Si and Mo5Si3. MoSi2 formed at Si-rich compositions of 54, 67 and 80 at.% Si, while Mo5Si3 formed at 25, 38 and 54 at.% Si. MoSi3, on the other hand, formed only in dilute Si alloys (10, 25 at.%) after a subsequent anneal at 800°C for 1 h. Yen et al. [39], have combined MA with shock loading to consolidate nine silicides. All could be directly produced from their elemental powders by high energy ball milling for 40 h or less. The end product in most cases is a single phase silicide, but for TiSi2 a metastable phase was also noted. A difference in formation mechanism of the various silicides was noted, MoSi2 NbSi2 and ZrSi2 are formed by a mechanicallyinduced self propagating reaction (MSR), similar to a thermally activated SHS reaction. Patankar et al. [18], have pointed out that the occurrence of MSR during MA is preceded by an incubation period, the length of which depends upon the experimental conditions and the materials involved. Yen et al. [19] further report that the products of MA in the Mo – Si system vary with composition, with Mo5Si3 and Mo3Si competing with MoSi2 at high Mo/Si ratios. These results differ from those of an investigation of SHS of Mo –Si alloys carried out by Zhang and Munir [65]. The latter found

that Mo5Si3 and Mo3Si cannot be synthesized from their respective stoichiometric mixtures by SHS alone. Short et al. [41], have reported the successful use of MA and RS to produce Nb5Si3. Laminates of this compound reinforced by Nb foils then were produced in a vacuum hot press. The toughness of the laminate was five times that of the unreinforced silicide, confirming the conclusion above that laminated microstructures offer the highest toughness of any composites tested.

8. XD™ processing The XD™ process is another example of an in-situ reaction, involving the precipitation of ceramic particles in a metallic or intermetallic matrix. A combination of the XD™ process and powder processing was used by Aikin [66] to prepare MoSi2 reinforced with 30 vol.% of either SiC, HfB2 or TiB2. Yield and flow stresses in the range 1000–1400°C were up to 75% higher than those of the monolithic material. As much as 45% SiC or TiB2 has been added to MoSi2 by the XD™ process [67].

9. Thermal spray Plasma or thermal spray techniques involve melting feedstock powders or wires in a hot flame and propelling the droplets onto a substrate which causes very rapid solidification of deposits that can be removed without damage from the substrate. Cooling rates are very high, but enough self annealing occurs to provide

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Fig. 12. Schematic of MA process [63].

relief of residual stresses and improved interparticle bonding. When spraying is done in air, the deposits are more likely to be porous and to contain oxide inclusions compared to product sprayed in vacuum (actually at pressures of around 50 mbar.) The process lends itself to the preparation on near-net-shape components. Variants of plasma spray have been used to prepare both monolithic MoSi2, as well as composites reinforced with SiC or TiB2 particles [26], and Al2O3 plates [23,28]. Castro et al. [28], have explored the possibility of using plasma spray to produce tubular components of MoSi2 and MoSi2 composites. Dual powder feeding was utilized to produce alternating layers of MoSi2 and Al2O3 Wall thicknesses ranged from 2 to 4 mm in tubes of varying lengths. Mechanical properties were determined in diametral compression tests. All failures initiated at surfaces and were associated with residual porosity. Newman et al. [24], have compared the microstructures and properties of MoSi2 prepared by various techniques, including SHS, hot pressing, HIP of pre-alloyed powders and of elemental powders and vacuum plasma spray (VPS). Samples prepared by SHS and by plasma spray showed much finer grain sizes, apparently due to the effects of SiO2 impurities on grain boundary motion and, in the case of plasma spray, the rapid solidification rate of the molten droplets. HIPing of elemental powders resulted in the lowest oxygen levels, 0.64 wt.%, compared to the very high level of 4.4 wt.% resulting from plasma spray. This is in contrast to results with VPS reported earlier by Tiwari et al. [22]. In that work, powders with oxygen content of 0.24% were sprayed with a rise of oxygen content to only 0.30%, while hot pressing produced an oxygen level of 0.29%. Recently, Lawrynowicz et al. [27], reported that a reactive deposition process using methane as the carrier gas had been used to produce an in-situ MoSi2 – SiC composite with significantly improved toughness relative to material sprayed with argon as the carrier gas. A combination of low pressure plasma spray and HIPing has been used to consolidate a ternary Ni–Si– Cr alloy based on the Ni3Si phase [25]. Substrate temperature was shown to have a significant effect on the

development of the sprayed microstructure. A high temperature results in reduced porosity and improved bonding between the particle due to interdiffusion. The HIP process eliminated porosity, resulting in 2% ductility and a higher ultimate strength. The main advantage of the plasma spray step was to avoid the need for canning prior to HIP.

10. Vapor infiltration Another variant of reactive processing involves vapor-phase infiltration of a powder preform of Mo by SiCl4 vapor in order to produce MoSi2 [21]. By incorporating Nicalon SiCfibers into the preform, composites were also formed. An extensive study of the reaction kinetics during vapor infiltration was carried out. It was shown that the reaction front resulting from the presence of Si-rich vapor consisted of a Mo5Si3 layer that propagated through the powder preform and was eliminated when the reaction went to completion. The reaction followed parabolic kinetics in the temperature range 1100–1400°C, as shown in Fig. 13, [21]. The most significant advantages of this process were the relatively low reaction temperatures compared to hot pressing and HIP operations, and the low oxygen levels of the product, of the order of 0.3 at.% or less.

11. Effects of processing on properties Reviews of processing methods and their effects on properties of MoSi2 have recently been published by Hardwick [68], and by Jayashankar et al. [30]. It was pointed out by Hardwick [68], that the level of oxygen played a major role in strength at elevated temperatures, with low oxygen material produced by RS or reactive HIP maintaining higher strength than high oxygen material produced by MA. (Note: there are conflicting conclusions in the literature concerning the extent of oxygen contamination during MA) Samples produced from commercial powders fell between the two extremes, but HIPing led to higher strengths than did hot pressing. HIP material also displayed better

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creep resistance at 1200°C. Material containing large amounts of silica (from high oxygen contents) creeps preferentially at grain boundaries due to the presence of glassy grain boundary films. Such films also contribute to sub-critical crack growth at 1200°C in an Mo0 5W0 5Si2 alloy [69]. Fracture toughness, unlike strength properties, seems to be less dependent upon process variables. Higher toughness values for MoSi2 have been obtained on plasma sprayed specimens when the toughness was measured in the spray direction (through thickness), rather than in the direction parallel to the substrate. [70] A HIP cycle at 1200°C reduced the toughness of the sprayed deposit from 5.7 to 3.6 MPa m1/2. The addition of 20 vol.% Ta layers increased the toughness to 6.9 MPa m1/2, but this value was also reduced by a subsequent HIP cycle [70]. Comparisons of properties of Cr3Si and MoSi2 produced by powder and melt techniques showed much higher creep rates of PM material relative to arc melted material, presumably because of the finer grain size of the former [27].

12. Comments on melt processing of composites Several studies of the formation of silicide-matrix composites by unidirectional solidification of eutectic compositions have been reported by Bewlay and coworkers [71,72]. A complex Nb – Ti – Hf – Si – Al–Cr alloy based on the Nb3Si – Nb eutectic was shown to have toughness values in excess of 20 MPa m1/2, combined with stress-rupture resistance comparable to that of advanced nickel-base alloy single crystals, see Fig. 14,

[71,73]. The high thermal stability of such eutectic alloys, together with the single step preparation procedure, provides significant advantages over composites prepared from powder. However, the high melt temperatures involved, and potential crucible problems arising from the high reactivity of the silicides, suggest that there are still significant drawbacks to the melt techniques.

13. Summary and conclusions A wide variety of powder processes, often with two or more in combination, have been successfully applied to the consolidation of silicides, especially in the case of MoSi2. Principal advantages of powder processes include versatility in varying composition and processing conditions and the use of lower temperatures than in melt processing. Improvements in purity, principally the reduction of oxygen, have been achieved by the use of solid-state displacement reactions, by use of reactive processing, and at least in some cases by careful use of MA. However, the fine grain sizes resulting from MA or displacement reactions are a detriment to high temperature yield strength and creep resistance. Significant improvements in mechanical properties of silicides have been achieved by fibrous and laminated composites, while results for particulate-reinforced silicides have been mixed. Thermal expansion mismatch between matrix and reinforcing fibers or plates can be reduced by decreasing the CTE of the matrix with either SiC or Si3N4 particles. The latter have the added advantage of reducing pesting of MoSi2 and suggest that hybrid composites may provide a good balance of mechanical

Fig. 13. Parabolic reaction kinetics (x 2 vs. t) in the vapor infiltration of an Mo powder preform by SiCl4 [21].

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Fig. 14. Stress rupture behavior of directionally solidified Nb – Ti – Hf – Si – Al – Cr alloy (MASC) compared to nickel base superalloy single crystals CMSX-4 and CMSX 10 [71,73]. (a) Stress versus Larson– Miller parameter. (b) Density-normalized stress versus Larson – Miller parameter.

properties together with oxidation resistance. The production of fibrous or laminated composites from powders remains a very slow and labor intensive process; when metallic reinforcements are involved, the lack of thermodynamic equilibrium causes interdiffusion or chemical reaction at the interface between reinforcements and matrix. Ceramic fibers are favored for oxidation resistance and creep resistance. Production of composites by unidirectional solidification of eutectic alloys containing silicides is an attractive alternative, but is limited to certain compositions and proportions of phases. A major drawback of most processes, melt or powder, is that they have not been optimized. In the case of HIPing, progress in modelling behavior has been made, but there is a distinct need for more data on other powder techniques.

Acknowledgements The author is grateful for financial support provided by the Office of Naval Research under Aasert Grant No. N00014-94-0723 and by ONR/DARPA for support under Contract No. N00014-92J-1779, Dr Steven Fishman, Program Monitor.

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