Microstructure and mechanical properties of MoSi2 single crystals and directionally solidified MoSi2-Based alloys

Progress m MnrerralsScience Vol 42. pp 193-207. 1997 ,(” 1997 Elsevler Science Ltd. All rights reserved PrInted in Great Brrtatn 0079~6425/97 $32.00

Pergamon

PII: soo79-6425(97)ooo15-7

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MoSi, SINGLE CRYSTALS AND DIRECTIONALLY SOLIDIFIED MoSi,-BASED ALLOYS Kazuhiro Ito,* Takayuki Yano, Takayuki Nakamoto, Masaya Moriwaki, Haruyuki Inui and Masaharu Yamaguchi Department

of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-01, Japan CONTENTS

INTRODUCTION PLASTIC DEFORMATION OF MoSi, SINGLE CRYSTALS MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DIRECTIONALLY FIED MoSi,-BASED ALLOYS 3.1 Microstructure 3.2. Crystal Orientatron Relattonship Between MoSi, and Reinforcement Phase 3.3. Mechanicul Properttes 3.3.1. Fracture toughness 3.3.2 High-temperature strength SUMMARY ACKNOWLEDGEMENTS REFERENCES

193 194 SOLIDI199 199 200 203 203 204 206 206 206

1. INTRODUCTION MoSi, is one of the most promising candidate materials for structural applications at temperatures above 1500°C and considerable effort has been devoted to develop MoS&-based alloys of engineering utility. (‘w This stems from its excellent oxidation resistance, high melting point (202O”Q relatively low density (6.24 g cm-‘), high thermal conductivity and thermodynamic compatibility with many ceramic reinforcements.‘2A’ However, there are still many obstacles to structural applications of MoSi,-based alloys. Monolithic MoSi, exhibits only a modest value of fracture toughness of around 3 MPa m’12 at room temperature and inadequate strength at elevated temperatures.‘2~3’ Thus, the mechanical properties of MoSi, and their improvement have been investigated intensively from both fundamental and engineering points of view. Considerable progress has recently been made in understanding the fundamental aspects of the mechanical properties of MoSi,. In particular, operative deformation modes and their critical resolved shear stress (CRSS) have been investigated intensively as a function of crystal orientation and temperature on single crystals. One of the most impressive results of these studies is that MoSi, single crystals except for those with orientations close to [0 0 11, which can be deformed only above 900°C exhibit significant plastic deformability *To whom

correspondence

should

be address&d. 193

194

Progress in Materials Science

at temperatures as low as room temperature. The uniqueness of the [0 0 1] orientation is a consequence of a very high value of CRSS for slip on (0 1 3)(3 3 11, which is the only slip system with a non-zero value of Schmid factor for orientations near [0 0 11.The CRSS for the slip system is highest for the exact [0 0 1] orientation and it decreases quickly with any deviation from the exact [0 0 11,although the reason has yet to be clarified. Such results of recent studies on single crystals will be reviewed in this paper. Engineering studies for developing MoSi,-based alloys have focused on improving their mechanical properties, in particular the poor fracture toughness at ambient temperatures. This has been improved by a factor of two to three in composites formed by adding ZrOi@ or SiC3,” reinforcements. However, the high-temperature strength of such composites still remains inadequate. This is probably because powder-metallurgy processing has been employed to produce MoSi,-based alloys and oxygen contamination, resulting in the formation of silica at their grain boundaries, is unavoidable. We have thus made some attempts to produce MoSi,-based alloys with composite-type microstructures without silica at grain boundaries by directional solidification. Results of these attempts, in particular their microstructures, orientation relationships between the matrix MoSi, and reinforcement phases and mechanical properties, will also be reviewed in this paper. 2. PLASTIC

DEFORMATION

OF MoSi, SINGLE

CRYSTALS

The plastic behavior of MoSi, single crystals has been investigated intensively by four different research groups. Umakoshi et c~l.‘~’made the first systematic study on this subject. They reported that MoSi, single crystals can be plastically deformed above 1000°C regardless of crystal orientation, but the strength depends strongly on crystal orientation: orientations close to [0 0 1] are much harder than other orientations. They have attributed this to the difference in operative slip systems, i.e. (0 1 3)(3 3 11, which is hard, is operative for orientations close to [0 0 I] while (1 1 0)(3 3 11, which is easy, is operative for other orientations. The mixed notation {h k I) and (u v w] is used to differentiate the first two indices from the third index which does not play the same role as the first two because of the tetragonality of MoSi, with the Cl 1, structure. The results reported by Kimura et al.“” are essentially similar to those reported by Umakoshi et a1.‘9’ On the other hand, Mitchell and Malay”‘) and Maloy et al.“*’ have reported results in conflict with those reported by the former two groups regarding the operative slip systems, although there is a general agreement in all studies about the lowest temperature above which plastic flow is possible and the orientation dependence of yield stress. The slip systems identified by the last group are {013)(331] (<13OO”C) and (01 1)(1 111 (>13OO”C) for the [OOl] orientation (hard slip systems), and { 1 1 0)( 1 1 1] and (0 1 I)(1 0 0] for other orientations (easy slip systems). Recently, we have reported that plastic flow is possible even at temperatures as low as room temperature for single crystals with orientations away from [0 0 11, while single crystals with orientations near [0 0 1] can be plastically deformed only at high temperatures above 900°C.“3’ Five slip systems-{ 1 1 0)(1 1 I], (0 1 1)( 10 01, (0 10)( 10 01, (0 2 3)( 10 0] and (0 1 3)(3 3 I] (Fig. I)--are identified as being operative depending on crystal orientation,““’ but dislocations with b = l/2(3 3 I] corresponding to the { 1 1 0)(3 3 1] slip system identified by Umakoshi et al.“’ and Kimura et ~1.“~’ have been rarely observed.

195

MoS& Single Crystals and MoS&-based Alloys

{Oll)
00

Fig. 1. Operative

slip systems

in MoSi,

The results of the identified operative slip systems are summarized in Fig. 2. Critical resolved shear stresses for these slip systems are shown as a function of temperature in Fig. 3. Slip on (0 1 I)( 10 0] and (0 1 3)(3 3 l] occurs at temperatures as low as room temperature. Slip on (1 1 0)(1 1 I] is operative also at temperatures above 300°C. On the other hand, slip on (0 1 0)( 10 0] is observed only in the narrow temperature range 600-900°C and slip on (0 2 3)( 10 0] at temperatures above 800°C. An anomalous increase in CRSS is observed in an intermediate temperature range for slip on these slip

AAAA

..*.xx g* x x l **e** f . I

A

(9O')[ilO] , {011)<100]

{011)<100]~&~

.

AAAAAArn

' @11)~100]

1

'

(ll')[i13] (5')[ii7] I

0°C

I

31

I

I1

500°C

(12')[012]

I

I

I

I

I6

1 OOO’C *aem

I

II

I

1500°C

@13)<331]"

@13)<331] t

(15')[023]

‘5 ‘1

1 F{oii)
.FLLc{013)<331] ulo)-=lllJ mm.* t110)~1111r{013)<3311

(5")[0151

(~l’)N

..m.:::

’

(0’)W11

(39')[021]

xx

@10)~1001 {013)<3311 to23)~lool

(76")[771] (49')[221]

I(

{110)<111] t l eeeemmmmmmmmmmm 1 1{013)<331] ’

{llO)
Fig. 2. Operative slip systems observed in MoSi, single crystals with orientations [I 1 0] zone circles in the temperature range from liquid nitrogen temperature

on [l 0 0] and to 1500°C.

Progress in Materials Science

196

0i

0

500 Temperature

1000

1500

(‘C)

Fig. 3. Temperature dependence of CRSS for slip on five slip systems operative in MoSiz single crystals.

systems except for (0 2 3)( 10 01. The characteristics of such anomaly as well as the anomalous temperature range vary with slip system. The Schmid law is generally valid for the {1 1 0)( 1 1 11, (0 1 l)( 10 0] and (0 2 3)( 10 0] slip systems. This is the case even in the anomalous temperature range for the {1 1 0)(1 1 I] slip system. In contrast, the CRSS for slip on (0 1 3)(3 3 l] depends strongly on crystal orientation. It is considerably high for orientations near [0 0 11, although it is not very much different from CRSS values for other slip systems for orientations away from [0 0 1].(‘3) As pointed out by Maloy et al., (14)the number of independent slip systems is at most four for the combination of the most commonly observed (0 1 1)(1 0 0] and {1 1 0)(1 1 l] slip systems. This is also the case for any combination of the (0 1 l)( 10 01, { 1 1 0)( 1 1 I], (0 1 0)(10 0] and (0 2 3)( 10 0] slip systems. Slip on (0 1 3)(3 3 1] has to be activated to achieve five independent slip systems and thus the (0 1 3)(3 3 l] slip plays a key role in polycrystalline ductility, in particular at low temperatures where mass transportation is not sufficiently high to achieve plastic flow. However, the CRSS for the (0 1 3)(3 3 l] slip depends strongly on crystal orientation and exhibits a highest value at exactly [0 0 l] that is about three times larger than that at [i lo] at 900°C and still increases with decreasing temperature. Thus, single crystals with the exact [0 0 l] orientation cannot be plastically deformed below 900°C and therefore polycrystalline ductility may not be achieved until 13OO”C,where plastic flow on a macroscopic scale becomes possible even for the hardest [0 0 l] orientation.“*’ In order to improve mechanical properties such as ductility (deformability) and fracture toughness of polycrystalline MoSi, at low temperatures, both the CRSS for slip on (0 1 3)(3 3 l] and its strong orientation dependence should be reduced. At temperatures in the range 900 to lOOO”C,yielding for orientations near [0 0 l] is not obvious and the work-hardening rate is enormously high. Specimens were always broken into small pieces (often pulverized) when compression strain reached about 0.5%. On specimens deformed to 0.243% strain, fine and wavy slip lines are observed. In sharp contrast to orientations near [0 0 11, orientations away from [0 0 l] such as [0 15 l] and [T lo] exhibit obvious yielding, a low work-hardening rate, deformability to about 2% strain, and fine but straight slip lines. Dislocations with b = l/2(3 3 l] tend to align along (3 3 l] (screw), (1 001 (mixed) and/or (3 3 I] (mixed) orientations depending on crystal

MoSiz Single Crystals

and MoSi:-based

Alloys

197

orientation and temperature. Dislocations aligned along such orientations are decomposed or dissociated.““’ At low temperatures, where slip on (0 1 3)(3 311 is observed only in specimens with orientations near [0 1 01, which is far away from [0 0 11, l/2(3 3 1] dislocations tend to align along the screw and (1 0 0] mixed orientations. Boldt et ~1.“~ have examined dislocation structures formed by microindentation at room temperature and reported that dislocations with b = l/2(3 3 1] dissociate into three partials as described by the following reaction: l/2(3

3 1] -+ l/6(3

3 I] + l/6(3

3 1] + l/6(3

3 11

(1)

In contrast, our weak-beam study for a [0 15 II-oriented specimen deformed at 300’-C shows that 1/2[33 l] dislocations dissociate into two collinear partials on (0 1 3).(15,“) If the dissociation represented by eq. (1) occurs in a planar manner, two different antiphase boundaries (APBs) with displacement vectors of l/6(3 3 1] and l/3(3 3 1] are involved. However, Rao et ~l.“~’ have calculated the energies of some different faults on (0 1 3) --and shown that the 1/6[3 3 1] APB has a much lower energy than the 1/3[3 3 1] APB. Thus, the planar dissociation given by eq. (1) is expected to be reduced to the following -two-fold dissociation involving 1/6[3 3 1] APB: ---1/2[3 3 1]+1/6[3 3 1] + 1/3[3 3 I] (2) -This is believed to be the two-fold dissociation that we observed for 1/2[3 3 l] dislocations lying along [33 1] (screw orientation) and [ 1 0 0] directions. ‘15.“I The observed separation distance for the screw orientation on (0 1 3) is about 4.9 nm. The energy of l/6(3 3 I] APB on (0 1 3) is calculated to be 824 mJ rn- ’ from the separation distance and the anisotropic er al.“” K factor calculated by Rao et al. (‘*) from elastic constants given by Nakamura Besides the mobile form of dissociation given by eq. (2), l/2(3 3 1] mixed dislocations aligned along (1 0 0] often exhibit the decomposition described by the following reaction and become sessile: l/2(3

3 l] -+ l/2( 1 1 1] + (1 IO]

(3)

The decomposition observed in a [0 15 II-oriented single crystal deformed at 300 C by means of high-resolution transmission electron microscopy (HREM) will be presented elsewhere.“s, “I In specimens with crystal orientations near [00 1] deformed at temperatures above 1000°C the same decomposition occurs for l/2(3 3 1] dislocations lying along not only (1 0 0] mixed but also the screw and (3 3 1] mixed orientations. At 1 lOO”C, the decomposition can occur whatever orientation dislocations are aligned along.“” At temperatures higher than 13OO”C, we observed dislocations with b = l/2( I 1 1] in abundance. They are possibly produced by the decomposition of l/2(3 3 l] slip dislocations since slip traces observed on the deformed specimen correspond to slip on (0 1 3).‘15)Mitchell and Maloy have identified slip on {O 1 I)( 1 1 1] above 13OOC for the exact [0 0 1] orientation.“‘. “I In addition to the dissociation by eq. (2) and the decomposition by eq. (3) we observed to eq. (4) in the narrow l/2(3 3 l] screw dislocations dissociated on {l 1 0) according temperature range 90s1OOO’C: l/2(3

3 1] -+ l/n,(3

3 I] + fault on i 1 10) + l/n2(3 3 l]

(4)

198

Progress in Materials Science

The immobility of the configuration formed by the dissociation is obvious since dislocations with b = l/2(3 3 I] may not slip on { 1 1 0). This dissociation on {1 1 0) was observed for l/2(3 3 l] screw dislocations in specimens with orientations both near and away from [0 0 1].(‘5’ Thus, depending on crystal orientation, dislocation line orientation and temperature, l/2(3 3 l] dislocations may possess some different configurations corresponding to different dissociation and decomposition reactions. The locking of l/2(3 3 l] dislocations by the decomposition according to eq. (3) or the dissociation described by eq. (4) can occur for three dislocation line orientations in specimens with crystal orientations near [0 0 11, while it occurs for two line orientations in those with crystal orientations away from [0 0 11. This difference may be related with the difference in the CRSS and, in particular, the work-hardening rate for slip on (0 1 3)(3 3 l] between crystal orientations near and away from [0 0 11. The dissociation in the mobile form described by eq. (2) may also contribute to the crystal orientation dependence of CRSS for slip on (0 1 3)(3 3 I] since which of the two partial dislocations to lead their motion on (0 13) depends on crystal orientation. However, the implications of these observations on the dissociation and decomposition of l/2(3 3 l] dislocations for the deformation of MoSi, have yet to be clarified. We believe that atomistic studies of the core structure of l/2(3 3 l] dislocations would provide a new and suggestive insight into the problem. Dislocations carrying out slip on { 1 1 0)(1 1 l] possess Burgers vectors of l/2( 1 1 l] whose magnitude is smaller than that of l/2(3 3 l] but much larger than that of (10 0] dislocations carrying out slip on (0 1 l), (0 IO) and (0 2 3) planes. Dislocations with b = l/2( 1 1 l] have also been observed to dissociate as described by l/2( 1 1 l] + l/4( 1 1 l] + SISF on {1 1 0) + l/4( 1 1 l]

(5)

in a wide temperature range,“‘) where SISF stands for superlattice intrinsic stacking fault. Measurements of the separation of two l/4(1 1 I] partial dislocations were made on a specimen deformed at 1100°C. The observed separation distance for screw 1/2[i 1 l] dislocations on (1 10) was 4.4 nm. The separation for the screw orientation was always widest on (1 1 0) and the scatter was very small. From this value of separation, the energy of SISF on (1 10) was estimated to be 365 mJ m- * .(13)Evans et al.(“) have also identified the two-fold dissociation of l/2( 1 1 l] dislocations on {1 10) at 1400°C. The corresponding SISF energy for the screw orientation is about 261 mJ m-* and much smaller than our result. An anomalous increase in CRSS has been observed for {1 1 0)( 1 1 11, (0 1 3)(3 3 11, (0 1 l)( 10 O] and (0 1 0)( 10 0] slip systems. However, the anomalous temperature range as well as the extent of anomaly vary with slip system. Most of the characteristics of the anomaly observed for the { 1 1 0)( 1 1 l] slip system in the temperature range 800-1100°C have been found to be interpreted in terms of the Portevin-Le Chatelier effect,(‘3) in accordance with the suggestions by Rao et al. (W The anomalous increase in CRSS for slip on (0 1 3)(3 3 I] is observed only in orientations near [0 0 I] at 1000 to 1200°C. The immobilization of l/2(3 3 l] dislocations by their decomposition is believed to be associated with the anomaly. (‘*.‘*)However, nothing in detail is been known on the mechanism. The anomaly for the (0 1 l)( 10 0] and (0 10)( 10 0] slip systems occurs in the range 600-9OO”C, which is much lower than the corresponding temperature ranges for slip on (1 1 0)( 1 1 l] and (0 1 3)(3 3 11. Mechanisms for the anomaly for these slip systems have yet to be identified.“”

MoSi? Single Crystals and MoS&-based Alloys 3. MICROSTRUCTURE DIRECTIONALLY

199

AND MECHANICAL PROPERTIES OF SOLIDIFIED MoSi,-BASED ALLOYS 3.1. Microstructure

The inadequate strength of MoSi,-based alloys at elevated temperatures has been believed to be due the existence of silica at grain boundaries.“.’ “.“’ If this is the case, their inadequate high-temperature strength may not be significantly improved when they are produced by powder-metallurgy methods since during this process they are unavoidably contaminated with a large amount of oxygen, leading to the formation of silica at grain boundaries. When they are produced by ingot-metallurgy methods under a purified argon gas flow, their oxygen content and therefore the amount of silica at grain boundaries are expected to be considerably lowered. Thus we have attempted to produce MoSi,-based alloys by directional solidification in a purified argon gas flow with the purpose of improving both fracture toughness and hightemperature strength, employing the optical floating zone furnace used to grow single crystals of MoSi,. Three materials for reinforcement were selected: SIC (B3), Mo,Si, (D8nz) and Mo(Si,Alh (C40), where B3, D8m and C40 are the structure types of the three reinforcement materials. On the basis of Mo-Si-C and Mo-Si-Al ternary phase diagrams, eutectic solidification is expected to occur in MoSiT-SiC’23’and MoSirMo,Si, pseudobinary systems and peritectic solidification in the MoSi2-Mo(Si,Al)z pseudobinary system. ‘24)Growth conditions for each of these MoSi,-based alloys are given in Table 1. Figure 4 shows microstructures of directionally solidified ingots of MoSi?-SiC and MoSi,Mo,Si, alloys and the MoSi2-Mo(Si,Al)z alloy. In the MoSi,-SiC alloy, SIC exists in the plate form in the single-crystalline MoSi, matrix. MoSi,-Mo,Si, and MoSiz-Mo(Si,Al)z alloys exhibit script-lamellar (as have been observed by Kung et al.““) and lamellar microstructures, respectively. Microstructures of the three alloy systems examined become finer with increasing growth rate, although the growth rate dependence of microstructure of the peritectic alloy system is much smaller than that of the two eutectic alloy systems. Sic is one of the most desirable reinforcement ceramics to increase the fracture toughness of MoSi,. However, the eutectic composition of the MoSi,-SiC pseudobinary system is as small as 3 ~01% SIC. Adding more than 3 ~01% SIC gives rise to the existence of coarse primary SIC dendrites which are harmful for both fracture toughness and high-temperature strength of the MoSi,-SiC system. In the case of the MoSi,-Mo(Si,Al), system, increasing the volume fraction of Mo(Si,Al), leads to a difference in microstructure between the outer and inner parts of ingot. In ingots of the MoSi,24.5 ~01% Mo(Si,Al)? alloy, a lamellar

Table Alloy system MoSL/SK

MoSL/Mo$, MoSiz/Mo(Si,Al)l

Eutectic

I. Mo&based

or peritectic

3 vol%

alloys Investigated

composition

Sic (eutectlc)

42.9 ~01% MO& (eutectlc) Unkown (peritectlc)

Remforcement 3 vol% Sic IO ~01% SIC 20 ~01% SIC 42.0 ~01% Mo$.i, 24 5 ~01% Mo(Si,Al), 43.0 ~01% Mo(Si,Al)?

Growth rate (mm h-l) 10 50 100 IO 50 100

Progress in Materials Science

200

MoSip /Sic

MoSi2 /MosSi

MoSiz

Fig. 4. Optical microstructures of directionally solidified MoS&3 ~01% Sic (a, b), MoSil 42.9 ~01% Mo$i, (c, d) and MoS&24.5 ~01% Mo(Si,Al), alloys (e, f).

structure such as that shown in Figs 4(e) and 4(f) was observed in both their surface and core regions; however in MoSi,-43.0 ~01% MO&Al), ingots such a lamellar structure was observed only in their near-surface region and their core region exhibits a cellular structure. 3.2. Crystal Orientation Relationship Between MoSi, and Reinforcement Phase In MoSi,SiC alloys produced by directionally solidification, orientation relationship between MoSi, and Sic given by

---

(0 0 1x1 i oiMaL2 II(l 1 ~1 i

oisic

we have identified the

(6)

Figure 5 shows an example of observations of MoSi,/SiC interfaces by transmission electron microscopy (TEM). The MoSi,/SiC interface in the figure is perpendicular to the foil and thus it is obvious that the interface is parallel to and (ii l)sic. (0 0 l)Moa,11 (ii l)sic and [l 1 O)MoSil /I[l 1 21sic, which is equivalent to [l i O)MoSi, I][ 1 i O]sic, are seen from the diffraction patterns of Fig. 5. Occasionally interfaces with orientation relationships deviating from the above relationship by 5-10” are observed. Figure 6 shows an example

SIC

MoSi2

[I 101 Fig. 5. Orientation

Fig. 6. MREM

relationship

image

between

of a MoSi,/SiC

MoSi, and Sic in a directionally Sic alloy.

interface in a directionally alloy.

solidified

solidified

MoSi,-3

MoSi,-3

vol%

~01% Sic

Progress in Materials Science

01 01 [I101

MoSi2 Fig. 7. Orientation

Mo(Si,Al);! eMo(Si,Al)z relationship between MoSi, and Mo(Si,Al), in a directionally MoS&-24.5 ~01% Mo(Si,Al), alloy.

solidified

of HREM images of such interfaces deviating from the orientation relationship given by eq. (6). It is seen that the interface contains many ledges, but the atomic arrangement between two neighboring ledges is consistent with the relationship of eq. (6). Reasons for the existence of such ledges have yet to be clarified. In the MoSi,Mo(Si,Al), system, the low index orientation relationship given by (1 1Oh,, II(0 0 0 1h and LO 0 llcII,IIP O~oL has been reported to exist between the Cl lb matrix and C40 reinforcement phases by Boettinger et ~1.~~)In this orientation relationship, close-packed planes and directions in the matrix phase are parallel to the corresponding planes and directions in the reinforcement phase. However, in directionally solidified ingots of MoSi,-Mo(Si,Al), alloys, we observed such an orientation relationship as that shown in Fig. 7. The [0 0 0 l] direction in Mo(Si,Alh is inclined at about 20” to [l lo] in MoSi,. This is always the case in directionally solidified MoSi,-Mo(Si,Al), alloys provided their microstructure is in the lamellar form. The flatness of MoSi,/Mo(Si,Al), interfaces is seen even on a HREM scale (Fig. 8). Table 2 summarizes the observed and reported orientation relationships between the matrix and reinforcement phases in the three alloy systems investigated. c5,23,‘>*‘)Interestingly, there is considerable discrepancy between the results for the arc-melted alloys and the directionally solidified alloys. In the directionally solidified MoSi,-based alloys investigated, the interfaces between the matrix MoSi, and reinforcement phases are always parallel to (00 1) in the matrix MoSi,. This might be related with the fact that MoSi, single crystals are grown with (0 0 1) facets. However, much more work is needed to clarify the reasons for the discrepancy.

MoSiz Single Crystals and MoSiz-based Alloys

Fig. 8. HREM

image of a MoSi,/Mo(Si,Al),

interface in a directionally Mo(Si,Al), alloy.

203

solidified MoSi,24.5

~01%

3.3. Mechanical Properties 3.3.1. Fracture toughness

Fracture toughness of the directionally solidified alloys was measured at room temperature by means of the indentation fracture method.@) In this method, a polished specimen is indented with a Vickers hardness indenter and the length of the resultant median cracks is measured. The fracture toughness is given as a function of the indentation load, the size of the median cracks, the elastic modulus and hardness of the material. Specimens were cut from as-grown ingots so that the surface to be indented is parallel to (1 1 0) of the matrix MoSi, phase. This is to make a direct comparison between the fracture toughness value of MoSi, single crystals and those of MoSi,-based alloys. All the specimens were indented with a load of 9.8 N at room temperature. The results are summarized in Table 3. The crack length is generally shorter when the crack propagates intersecting lamellae of the reinforcement phase than when it propagates parallel to them. Thus, the fracture toughness is anisotropic with respect to the lamellar orientation except for the MoSi,-Mo,Si, alloy with a script-lamellar structure. The higher fracture toughness Table

2. Orientation

Orientation

relationships

relationship

matrix and MoSi,-based

reinforcement alloys

Arc-melting

MoSi,/SiC

ikc randomly (1 i 0),,,,,11(3 5 o),,,,,,

tlMoSII IILO1 t1WYor

oriented(26’

P 0 IlM0L* II[l 1OlM,,S~,‘25 MoSi,/Mo(Si,Al),

phases Directional

(1 1 o),,,,,//(Ii

LO0 MoSiJMo&

between the MoSi, directionally solidified

(1 1 O)t&s,,ll(O00 l)MoW,I (0 0 ~kS~,ll(~0 1 a”fOWAI,,‘5’

in arc-melted

and

solidification

(0 0 l)MaL1lI(iT usNc II i OI,,~,,IIu i OI,,, (0 0 2)M0L*II(2 2 Q405SIl

111 chJs,,I/[I 1 OlMD5SII(?7’ see IMAGE

Progress in Materials Science

204 Table

3. Fracture toughness of MoSi, single crystals directionally Solidified MoSi,-based alloys Growth rate (mm h-l)

Alloy MoSi,J

~01% SIC

MoSir42.0 ~01% Mo,S& MoSi,-24.5 ~01% Mo(Si,Al)> MoSi,-43 ~01% Mo(Si,Al), Single-crystal MoSi? (I I 0)

50 100 IO 50 50 50 100

and

(Mkm’!‘) 2.2-2.0 2&l .8 3.2 3.0 4.9-3.3 2.61.5 2.5-1.6 1.9

value corresponding to the former case and the lower value corresponding the latter case are given as the upper and the lower bounds of fracture toughness in Table 3. In comparison with the fracture toughness of MoSi, single crystals, MoSi,-based alloys investigated exhibit high values of fracture toughness. Of the alloys investigated, the MoSi,-24.5 ~01% Mo(Si,Al), alloy shows the highest fracture toughness. However, the improvement in fracture toughness by adding 3 ~01% Sic is not significant, in particular when compared with the reported fracture toughness for MoSi,/SiC and other MoSi,-based composites prepared by powder-metallurgy processing.“.3,8’ This is simply because of the difference in the volume fraction of Sic. For the purpose of comparison, the fracture toughness of MoSi, was measured at room temperature on specimens of dimensions 16 mm x 3.5 mm x 3 mm cut from an as-grown single crystal and notched parallel to (1 1 0) and (0 0 1). The specimens were tested in three-point bending with a span length of 14 mm in air at a load-line displacement rate of 0.01 mm min-‘. The loading direction was perpendicular to the surface with dimensions 16 mm x 3 mm and was crystallographically parallel to [i 1 0] for specimens notched parallel to (1 1 0) and (0 0 1). K,, values of 1.7 and 3.1 MPa rn”’ were obtained for the former and the latter specimens, respectively. The specimen size satisfies specimen size requirements for the validity of the K,, test procedure. Since the fracture surfaces of specimens with these two different notch planes are almost equally flat (Fig. 9(a), (b)), the difference between fracture toughness of the two specimens is believed to reflect the anisotropy of the fracture toughness of MoSi, single crystals. The difference in fracture toughness may be roughly interpreted in terms of the difference in the number of broken atomic bondings per unit area of notch plane and the difference in spacing between {1 10) and (00 1) planes. 3.3.2. High-temperature

strength

Specimens with the compression axis parallel to [i 1 0] in the matrix MoSi, phase were cut from directionally solidified ingots of the MoSi,-3 ~01% SIC alloy, and were tested in vacuum at various temperatures. The MoSi,-3 ~01% Sic alloy can be deformed only at temperatures above 1000°C. As seen from Fig. 10, where the temperature dependence of yield stress of the [i IO]-oriented MoSi, single crystal is included for comparison, no significant difference in yield stress is found for the single-crystalline MoSi, and the MoSi,-3 ~01% Sic alloy at temperatures lower than 1100°C. However, above 1100°C the alloy is superior to the single-crystalline MoSi? in yield strength. The results of measurements of fracture toughness and high-temperature strength of the

MoSiz Single Crystals and MoSL-based Alloys

Fig. 9. Fracture

surfaces

of MoSiz single crystals notched planes.

parallel

to (a) (1 1 0) and (b) (0 0

MOS&-3 ~01% SIC alloy indicate the potential of the directional solidification tee:hnique for producing MoSi,-based alloys achieving higher values of both fracture tot lghness and high-temperature strength. More work is needed to develop ingot-met :allurgy 600 @ MoSi&3vol.%SiC 0 MoSin

t z

400 iC

z ? tj $ F

200 -

0

900

1200 Temperature

1500 (“C)

Fig. 10. Temperature dependence of yield stress for a MoSi,-3 oriented MoSi, single crystals. The single-crystalline MoSi> matrix [i 1 O]-oriented.

~01% SIC alloy and of the alloy specimens

[i 1 O]is also

206

Progress in Materials Science

techniques to produce MoSi,-based such as Sic.

alloys with a large volume fraction of dispersoids

4. SUMMARY 1. MoSi, single crystals exhibit room-temperature deformability in orientations away from [0 0 11. In contrast, [0 0 II-oriented single crystals cannot be plastically deformed below 900°C. Five slip systems, (1 1 0)( 1 1 11, (0 1 l)(l 001, (0 1 0)(1001, (02 3)(100] and (0 13)(3 3 11, are identified. An anomalous increase in CRSS for these slip systems except for (0 2 3)( 10 0] is observed. The anomalous temperature range as well as the extent of anomaly vary with slip system. The CRSS for (0 13)(3 3 l] depends strongly on crystal orientation. The CRSS is the highest for the exact [0 0 l] orientation and decreases quickly with any deviation from [0 0 11. A decomposition and two dissociation reactions for the (0 1 3)(3 3 l] slip system are identified. Two different sessile configurations of l/2(3 3 l] dislocations formed by the decomposition and dissociation reactions are identified. 2. MoSi,-Sic, MoSi,-MoSSi, and MoSi,-Mo(Si,Al), alloys were prepared by directional solidification, and their microstructures, orientation relationships between the matrix MoSi, and reinforcement phases and mechanical properties were investigated. Results of fracture toughness and high-temperature strength measurements for the MoSi,-3 ~01% Sic alloy indicate the potential of directional solidification for producing MoSi,-based alloys achieving higher values of both fracture toughness and high-temperature strength. More work is needed to develop ingot-metallurgy techniques to produce MoSi,-based alloys with a large volume fraction of dispersoids such as Sic.

ACKNOWLEDGEMENTS This work was supported by Grant-in-Aid for Scientific Research on the Priority Area “Intermetallic Compounds as New High Temperature Structural Materials” from the Ministry of Education, Science and Culture, Japan, the Japan Society for Promotion of Science grant on advantage high-temperature intermetallics (JSPS-RFTF96R12301) and in part by research funds from NEDO, the R&D Institute of Metals and Composites for Future Industries and the Japan Ultra-High-Temperature Materials Research Center. The authors would like to thank Dr Y. Yukawa and Mr T. Shiraki, Nikko Superior Metals, Co. Ltd, and Drs H. Shiraishi and S. Okamoto, Sumitomo Sitix Co. Ltd, for supplying high-purity MO and Si, respectively. One of the authors (KI) greatly appreciates the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.

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