Materials Transactions, Vol. 43, No. 12 (2002) pp. 3254 to 3261c©2002 The Japan Institute of Metals

EXPRESS REGULAR ARTICLE

Effect of Cr Addition on Microstructure and Mechanical Properties inNb–Si–Mo Base Multiphase Alloys

Won-Yong Kim1, ∗, In-Dong Yeo2, Mok-Soon Kim3 and Shuji Hanada4

1Japan Ultra-High Temperature Materials Research Institute, Yamaguchi 755-0001, Japan2Korea Institute of Industrial Technology, Inchon 404-254, Korea3School of Materials Science and Engineering, Inha University, Inchon 402-751, Korea4Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

The effects of Cr addition to Nb–22Si–5Mo alloy on phase equilibria, microstructures and mechanical properties are investigated bymetallography, X-ray diffraction, scanning electron microscopy equipped with wavelength dispersive X-ray fluorescence spectroscopy andcompression test at temperatures from room temperature to 1773 K. With increasing Cr content a duplex microstructure consisting of Nb5Si3and Nbss (niobium solid solution) is changed to three-phase microstructure consisting of Nb5Si3, Nbssand NbCr2. Chromium addition does notchange the volume fraction of constituent phases in the two-phase alloys, whereas it increases the volume fraction of NbCr2, C14 Laves phase,at the expense of mostly Nbss in the three-phase alloys. Thec/a axis ratio ofα-Nb5Si3 phase increases with increasing (Cr+ Mo) content. Itis found that Cr alloying in Nbss increases the room temperature strength due to atomic size misfit and decreases high temperature strength dueto accelerated diffusion. The existence of C14 Laves phase increases high temperature strength, but degrades room temperature deformability.High temperature strength is found to be sensitive to the volume fraction and crystal structure of constituent phases as well as microstructure.

(Received June 19, 2002; Accepted October 17, 2002)

Keywords: microstructure, mechanical property, niobium silicide, alloying element, phase equilibria, high temperature strength

1. Introduction

Niobium silicide Nb5Si3 based alloys have a high poten-tial for high temperature structural applications because ofhigh melting point (2527 K) and low density (7.1 g/cm3) ofthe silicide. Therefore, the alloys can be good candidates tobe used at temperatures above 1473 K as a replacement forNi-base superalloys whose attainable maximum temperatureis around 1273 K.1–9) Since the monolithic Nb5Si3 phase hasbeen found to be very brittle at ambient temperatures, Nb5Si3-based multiphase alloy systems containing a ductile niobiumsolid solution (Nbss) phase would be of a technological inter-est in the field of high temperature structural materials. Con-sidering the inherent difficulty in ductilizing this silicide, itis essential that an incorporation of the ductile phase into thebrittle silicide is necessarily required in order to improve theroom temperature deformability and fracture toughness.10–21)

Thus, high temperature strength can be held by the silicide,and low temperature deformability and fracture toughness canbe enhanced by the incorporated ductile phase.

From the microstructural viewpoint, the presence of the eu-tectic and eutectoid reactions in the binary Nb–Si alloy sys-tem would allow us to improve mechanical properties by op-timizing the microstructures through a proper control of pro-cessing conditions.22) While an alternative way to satisfy therequired properties such as room temperature deformabilityand fracture toughness and high temperature strength is al-loying by which microstructural and crystal structural modi-fications of constituent phases can be achieved. In previousstudies,23–25) we have found that Mo alloying is effective inimproving room temperature fracture toughness through themicrostructural modification, and high temperature strength

∗Present address: Korea Institute of Industrial Technology, Inchon 404-254, Korea.

through both the solid solution hardening of Nbss phase andthe control of volume fraction and crystal structure of Nb5Si3phase. Especially, Nb–22Si–5Mo ternary alloy has beenshown to have a reasonable balance between room temper-ature fracture toughness and high temperature strength. Inthe present study, Cr is added to Nb–22Si–5Mo ternary al-loy to form Cr5Si3 phase which possesses a complex tetrag-onal structure (D8l structure type) in the binary Cr–Si sys-tem. The Cr5Si3 phase undergoes phase transformation fromα-Cr5Si3 to β-Cr5Si3 at around 1773 K. If a pseudo-binarybetween Cr5Si3 and Nb5Si3 is formed and phase transfor-mation is closely related to diffusion-driven process, Cr mayhave a positive role to enhance the phase stability ofβ-Nb5Si3in the present quaternary alloy system, since the transforma-tion temperature of Nb5Si3 from α to β is higher than thatof Cr5Si3 phase. In our previous studies,α-Nb5Si3 has beenfound to be favorable from the viewpoint of high temper-ature strength thanβ-Nb5Si3.23) These reports indicate thatphase stability of Nb5Si3 is directly correlated with mechan-ical property. Moreover, another intermetallic phase, C14Laves phase NbCr2, can be expected to form in the Nb-richcomposition range of this alloy system.

We aim at understanding the alloying behavior ofchromium on microstructural formation, crystal structure ofconstituents and mechanical properties in a wide temperaturerange from room temperature to 1773 K.

2. Experimental Procedure

The purities of starting raw materials were 99.9 mass%Nb,99.999 mass%Si, 99.9 mass%Mo and 99.999 mass%Cr, re-spectively. Small alloy buttons of 20 mm in diameter wereprepared by arc melting on a water-cooled copper hearth un-der an argon gas atmosphere with a non-consumable tungsten

Effect of Cr Addition on Microstructure and Mechanical Properties in Nb–Si–Mo Base Multiphase Alloys 3255

Fig. 1 X-ray diffraction spectra for six as-cast alloys at constant Si and Mo contents. The alloya is ternary and the others are quaternary.

Table 1 Nominal compositions (mol%) of six alloys used in this study.

Sample No. Nb Si Mo Cr

a, a1 73 22 5 —

b, b1 68 22 5 5

c, c1 63 22 5 10

d, d1 58 22 5 15

e, e1 53 22 5 20

f , f1 48 22 5 25

electrode. The button ingots were re-melted at least five timesfor chemical homogeneity. The ingots were then heat treatedunder an argon atmosphere at 1773 K for 100 h followed byfurnace cooling to room temperature at a rate of 200 K/minin the high temperature region (1773 to 773 K) and then ata rate of 10 K/min in the low temperature region (773 K toroom temperature). Samples for chemical composition anal-ysis, microstructural observation and compression test weremade by electro-discharge machining (EDM) from both theas-cast and heat treated buttons. The nominal compositionsfor all of the samples studied here are listed in Table 1. X-ray diffraction (XRD) analysis was conducted on bulk sam-ples to determine the crystal structures and lattice parame-ters of constituent phases. Lattice parameters were deter-mined by the well-known least square method.26) Samples formetallographic observations were mechanically polished withSiC paper and Al2O3 particles with water. Scanning elec-tron microscopy (SEM) equipped with wavelength-dispersiveX-ray fluorescence spectroscopy (WDS) was used for deter-mining the chemical composition of each constituent phase.Phase analysis by XRD was performed on finely polishedbulk samples. The measurement of volume fraction of con-stituent phases was carried out using image analyzer equipped

with computer system. Compression test specimens with2.5 mm× 2.5 mm cross-section and 6 mm height were pre-pared by EDM and then mechanically polished. Compressiontest was carried out using an Instron model 8500 mechanicaltesting machine at room temperature to 1773 K in Ar atmo-sphere and at an initial strain rate of 3× 10−4 s−1.

3. Results

3.1 Phase analysis and lattice parameterThe result of phase analysis by XRD of both the as-cast and

the heat treated alloys indicate that all samples are either twophases withα-Nb5Si3 (D8l ) or β- (D8m) tetragonal structureand bcc Nbss (alloysa, b, a1, b1), or three phases composedof theα- or β-Nb5Si3, NbCr2 (C14 hexagonal Laves crystalstructure) and bcc Nbss (alloys c, d, e, f , c1, d1, e1, f1). InFig. 1, Results of XRD on six samples of as-cast alloysa to fare shown. Filled circles and squares in Fig. 1 indicate peaksfrom Nbss andβ-Nb5Si3, respecitvely. The C14 Laves phaseis marked as filled triangles. No discernible peaks are identi-fied as meta-stable Nb3Si, which appears as an intermediatephase in the Nb–Si binary alloy phase diagram,22) and as cu-bic C15 Laves phase which is stable in the low temperatureregion in the Cr–Nb binary alloy phase diagram. The XRDspectra clearly show that alloysa andb are composed of du-plex structure withβ-Nb5Si3 and Nbss, whereas alloysc, d,e and f additionally contain the C14 hexagonal Laves phase.The six alloys a tof in Fig. 1 have the same Si and Mo con-tents with the Cr concentrations from 0 to 25 mol% as listed inTable 1. No significant change is exhibited in the diffractionspectra by increasing Cr content from 0 to 5 mol%. On furtherincreasing Cr content in alloysc to f , microstructure observa-tions indicated that the C14 Laves phase appears and its vol-ume fraction increases, while the volume fraction of Nbss de-

3256 W.-Y. Kim, I.-D. Yeo, M.-S. Kim and S. Hanada

Fig. 2 X-ray diffraction spectra for six heat treated alloys at constant Si and Mo contents. The alloya1 is ternary and the others arequaternary.

Table 2 Summary of constituent phases and volume fractions in each alloy used in this study.

SampleConstituent phases

Volume fractions (%)Remarks

No. Nbss Nb5Si3 NbCr2

a Nbss+ β − Nb5Si3 46 54 —

b Nbss+ β − Nb5Si3 45 55 —

c Nbss+ β − Nb5Si3 + C14 NbCr2 32 55 13as-cast

d Nbss+ β − Nb5Si3 + C14 NbCr2 23 51 16

e Nbss+ β − Nb5Si3 + C14 NbCr2 24 45 31

f Nbss+ β − Nb5Si3 + C14 NbCr2 11 44 45

a1 Nbss+ α − Nb5Si3 43 57 —

b1 Nbss+ α − Nb5Si3 45 55 — heat

c1 Nbss+ α − Nb5Si3 + C14 NbCr2 36 52 12 treatment at

d1 Nbss+ α − Nb5Si3 + C14 NbCr2 23 49 28 1773 K for

e1 Nbss+ α − Nb5Si3 + C14 NbCr2 20 46 34 100 h

f1 Nbss+ α − Nb5Si3 + C14 NbCr2 12 44 44

creases. Another noticeable change caused by the increase inCr content is the shift of the bcc diffraction peaks to higher 2θ

indicating a contraction of the lattice parameter. This contrac-tion seen in Fig. 1 is dependent on Cr content. XRD resultson six alloysa1 to f1, which were heat-treated at 1773 K for100 h, are shown in Fig. 2. Basically, the diffraction spectrafor the heat-treated alloys are observed to be similar to thosefor the as-cast alloys. The alloysa1 andb1 are composed ofduplex phases, and the others appear to be of three-phase. Anobvious difference, however, is found in the crystal structureof Nb5Si3. The silicide isβ-Nb5Si3 phase in the as-cast al-loys, while it isα-Nb5Si3 in the heat-treated alloys. In Fig. 3,the lattice parameters of Nbss phase in the heat-treated alloysa1 to f1 are plotted as a function of (Cr+ Mo) content inNbss. The lattice parameter decreases rapidly with increasing(Cr + Mo) content up to 15 mol%, and then it gradually de-

creases with a linear slope. SEM-WDS analysis indicates thatthe decrease in lattice parameter of the Nbss phase originatesfrom a concurrent increase in Cr and Mo contents in the alloysa1 to b1, and from a decrease in Cr content at almost constantcontent of Mo in the alloysc1 to f1. Since the atomic sizes ofCr, Mo and Nb are 0.128, 0.140 and 0.147 nm, respectively,the above decrease in lattice parameter will be explained bythe substitution of Cr and Mo for Nb and their content to Nbss.

Figure 4 shows thec/a axis ratio ofα-Nb5Si3 phase as afunction of (Cr+ Mo) content. Thec/a axis ratio increasessharply with increasing content of (Cr+ Mo) especially atsmall contents, and it finally saturates.

3.2 Microstructures of as-cast and heat treated alloysBack scattered electron images (BEI) of as-cast alloysa to

f are shown in Fig. 5. It is revealed from XRD and SEM-

Effect of Cr Addition on Microstructure and Mechanical Properties in Nb–Si–Mo Base Multiphase Alloys 3257

Fig. 3 Lattice parameter of the bcc Nbss phase plotted against (Cr+ Mo)content in Nbss phase calculated from XRD results. All samples were heattreated at 1773 K for 100 h.

Fig. 4 Thec/a axis ratio ofα-Nb5Si3 phase plotted against (Cr+ Mo)content inα-Nb5Si3 phase calculated from XRD results. All samples wereheat treated at 1773 K for 100 h.

EDS analysis that microstructures shown in Figs. 5(a) and (b)consist of a light Nbss phase and a darkβ-Nb5Si3 phase, asindicated by arrows in the figure. In alloya, large primaryβ-Nb5Si3 particles with irregular shapes and a fine eutecticstructure consisting ofβ-Nb5Si3 and Nbss are observed asshown in Fig. 5(a). The volume fraction of primaryβ-Nb5Si3phase is increased by the addition of 5 mol%Cr to alloya, andthe fine eutectic microstructure is changed to coarse structureas shown in Fig. 5(b). No significant change in the volumefraction of β-Nb5Si3 and Nbss is found in the two-phase al-loys a andb, indicating that the volume fractions of the con-stituent phases is not influenced by Cr content. This resultis very similar to the effect of Mo addition reported previ-

ously.24) The major difference in microstructure by Cr addi-tion is that the volume fraction of primary Nb5Si3 phase in theternary Nb–22Si–5Mo alloy, suggesting Cr addition to Nb–22Si–5Mo ternary system gives rise to a shift of eutectic linetoward Si-rich corner in the two-phase Nbss/Nb5Si3 region.In contrast, microstructures shown in Figs. 5(c) to (f) consistof a dark C14 NbCr2 Laves phase, a light Nbss phase and agrayβ-Nb5Si3 phase. In the alloy with 10 mol%Cr (alloyc),NbCr2 appears in addition to Nbss andβ-Nb5Si3, and the eu-tectic microstructure consists of NbCr2 Laves phase and Nbss,as shown in Fig. 5(c). Theβ-Nb5Si3 phase is produced as aprimary phase with large size. The eutectic region increaseswith increasing Cr content in the microstructures as seen inFigs. 5(c) to (e). However, the eutectic region decreases at25 mol%Cr, which may be related to the large volume frac-tion of primary NbCr2 phase as shown in Fig. 5(f). The phaseconstitutions and volume fractions of constituent phases aresummarized in Table 2. The volume faction of the C14 Lavesphase increases with increasing Cr content at the expense ofmostly Nbss phase and a small amount of Nb5Si3 phase in theas-cast three-phase alloys. Figure 6 shows BEIs of the alloysheat treated at 1773 K for 100 h. The three phases in Fig. 6with the different contrasts, light, dark and gray, are identi-fied to correspond to the three phases shown in the Fig. 5.There are no additional intermediate phase such as Nb3Si andno other phase transformation as mentioned in Fig. 2. A eu-tectic structure consisting ofα-Nb5Si3 and Nbss, and primaryα-Nb5Si3 phase in the microstructures are found for the alloysa1 andb1, as can be seen in Figs. 6(a) and (b). The eutec-tic structure including a primaryα-Nb5Si3 phase tends to be-come coarse with increasing Cr content from 0 to 5 mol%, in-dicating that Cr may accelerate atomic diffusion in the presentalloy system. Two distinctive regions, consisting of a finetwo-phase mixture and a coarse primary structure, are char-acterized in the heat treated three phase alloys. With increas-ing Cr content, it is found that the volume fraction of a Lavesphase increases, whereas that of Nbss decreases, as shown inthe Figs. 6(c) to (f). Fine Nbss and NbCr2 precipitates areformed in the primary Nb5Si3 to reach an equilibrium stateat the given heat treatment condition. No visible change isobserved in the volume fractions of the constituent phases be-fore and after the heat treatment.

3.3 Mechanical propertiesCompression tests were carried out to ascertain the effect of

Cr addition on the mechanical properties of as-cast and heattreated alloys in the wide temperature range from room tem-perature to 1773 K. Yield stress or fracture stress, and com-pressive ductility are summarized in Table 3 for five as-castand four heat treated samples tested at room temperature at aninitial strain rate of 3× 10−4 s−1. The addition of 5 mol%Crincreases the yield stress of as-cast two-phase alloys (see al-loys a andb), and the heat treatment increases the compres-sive ductility without decreasing yield stress (see alloysb andb1). The increase in yield stress at room temperature is at-tributable to solid solution strengthening of Nbss, because thelattice parameter of Nbss deceases significantly by 5 mol%Craddition, as shown in Fig. 3, with almost no change in the vol-ume fraction of Nbss, as seen in Table 2. The increased com-pressive ductility of 5 mol%Cr added two phase alloy by the

3258 W.-Y. Kim, I.-D. Yeo, M.-S. Kim and S. Hanada

Fig. 5 BEI micrographs of the as-cast alloys used in this study. (a) Nb–22Si–5Mo (b) Nb–22Si–5Mo–5Cr (c) Nb–22Si–5Mo–10Cr,(d) Nb–22Si–5Mo–15Cr, (e) Nb–22Si–5Mo–20Cr, (f) Nb–22Si–5Mo–25Cr.

heat treatment would be associated with the formation of eu-ectic structure and the increase in thickness of the constituentNbss phase, since the crack propagation is retarded by thickNbss phase. No compressive ductility is shown in three-phasealloys with high Cr contents irrespective of heat treatment.This may be closely related to the decrease in volume fractionof ductile Nbss and the presence of very brittle C14 NbCr2

Laves phase.27–29)

The 0.2% offset yield stress at 1773 K is presented in Fig. 7as a function of Cr content for both the as-cast and the heattreated alloys. At all compositions investigated, the yieldstress of the heat treated alloys is seen to be higher than that ofthe as-cast alloys. With increasing Cr content, yield stress ex-hibits the minimum at around 15 mol%Cr and then increasesin both the as-cast and heat treated alloys. Fracture or yield

stress of the heat treated two-phase and the three-phase alloysis plotted against test temperature in Fig. 8. The two- andthree-phase alloys exhibit very high yield stress at room tem-perature, but are remarkably weakened above 1500 K. Thehigh Cr content alloys containing Laves phase are very brit-tle at room temperature, and especially Nb–22Si–5Mo–25Cr(alloy f1) fractures without showing yielding at room temper-ature to 1473 K. This brittleness would be ascribed to theintrinsic brittleness of Laves phase and the decrease in thevolume fraction of Nbss with an increase in Cr content.

4. Discussion

The microstructure, phase equilibria, mechanical and phys-ical properties are often sensitive to a small change in com-

Effect of Cr Addition on Microstructure and Mechanical Properties in Nb–Si–Mo Base Multiphase Alloys 3259

Fig. 6 BEI micrographs of the alloys heat treated at 1773 K for 100 h. (a) Nb–22Si–5Mo, (b) Nb–22Si–5Mo–5Cr,(c) Nb–22Si–5Mo–10Cr, (d) Nb–22Si–5Mo–15Cr, (e) Nb–22Si–5Mo–20Cr, (f) Nb–22Si–5Mo–25Cr.

position of intermetallic compound. In previous studies,we have correlated the phase equilibrium and microstructurewith the mechanical properties in the temperature range fromroom temperature to 1773 K in the Nb–Si–Mo ternary alloysystem.23–25) In the present study, we have investigated thephase formation on the Nb–22Si–5Mo alloy containing Cr,which was the heat treated at 1773 K for 100 h, using XRDand SEM-WDS. Interestingly, we found that the constituentphases are different between Nb–Si–Mo and Nb–Si–Mo–Crsystems. In the quaternary system the C14 NbCr2 Lavesphase is formed along with Nbss and Nb5Si3 phases at Crcontents of 10 to 25 mol%, indicating that the maximum sol-ubility limit of Cr to Nbss is between 5 and 10 mol% in thisalloy system. From the results of Figs. 1 and 2, it is foundthatβ-Nb5Si3, which is stable at high temperature, transformsto α-Nb5Si3 at 1773 K, which is stable at low temperature.No intermediate phase such as Nb3Si or Cr3Si is found, sug-gesting that Mo has the effect to make A3B-type compound

unstable in this alloy system. Considering the facts that theatomic sizes of Mo and Cr are smaller than Nb, and if Moand Cr atoms were substituted for Nb atoms and there was noanisotropy in the site occupancy, the unit cell size of the sili-cide would become compact and thec/a ratio would not bechanged. However, the measuredc/a axis ratio is somewhatdifferent from our expectation as shown in Fig. 4. Thec/aaxis ratio ofα-Nb5Si3 phase shows a significant increase up toabout 5 mol% of (Cr+ Mo) content. Kimet al. have reportedthat thec/a axis ratio ofα-Nb5Si3 phase increases with in-creasing Mo content and phase transformation fromα-Nb5Si3to β-Nb5Si3 takes place at around 5 mol%Mo at 1973 K.23)

Therefore, the present result is similar to the previous resultby Kim et al. This may be related to the site preference arisingfrom the anisotropy in inter-atomic bonding force dependingon crystallographic orientation. Correspondingly, Chuet al.have found that the thermal expansion is strongly anisotropicalong thea andc directions in a Mo5Si3 single crystal.30)

3260 W.-Y. Kim, I.-D. Yeo, M.-S. Kim and S. Hanada

Table 3 Summary of 0.2% offset yield or fracture stress and compressiveductility of alloys used in this study.

SampleYield or fracture stress (MPa) Compressive

Y: Yield stress ductility RemarksNo.

F: Fracture stress (%)

a 1888(Y) 2.2

b 2304(Y) 1.8

c 2627(F) 1as-cast

d 2855(F) 0.5

e 2576(F) 0

f 2196(F) 0

a1 — —

b1 2209(Y) 3 heat

c1 2231(Y) 2.1 treatment at

d1 2496(F) 0 1773 K for

e1 —- — 100 h

f1 1178(F) 0

Fig. 7 0.2% offset yield stress plotted as a function of Cr content in theas-cast and the heat treated alloys.

However, the present observations of the extended solubil-ity of alloying elements Mo and Cr and no phase transforma-tion in α-Nb5Si3 heat treated at 1773 K are different from theprevious results obtained in the Nb–Si–Mo ternary system inwhich phase transformation fromα-Nb5Si3 to β-Nb5Si3 tookplace at around 5 mol%Mo at 1973 K, indicating maximumsolubility of Mo in theα-Nb5Si3. It is very interesting to notethat the constituent phases change from two to three phasesat the transition composition, as seen in Figs. 1 and 2 andTable 2. The (Cr+ Mo) content dependence of lattice param-eter of Nbss in Fig. 3 results from the distribution of alloyingelements into the constituent phases. Using SEM-WDS anal-ysis, we have found that the contents of Cr and Mo in Nbss

increase concurrently with increasing Cr content in the two-phase equilibrium, whereas the content of Mo is kept constantin the three-phase equilibrium in this study. This means thatthe sharp decrease of the lattice parameter seen in Fig. 3 isdue to the increase of (Cr+ Mo) content in Nbss, while the

Fig. 8 Temperature dependence of fracture or yield stress for various al-loys heat treated. The data taken from the samples that fractured withoutshowing plastic yielding are indicated by arrows in the figure.

gradual decrease is attributable to the increase of Cr contentat the constant Mo content. These differences therefore, couldbe explained by considering the role of Cr alloying which sta-bilize α-Nb5Si3 stable. As shown in Fig. 3, the lattice pa-rameter of Nbss decreases along the two combined slopes, as(Cr+Mo) content increases. The composition at the transitionof the slopes is found to be about 14.5 mol% for the (Cr+Mo)content in Nbss.

The room temperature compression tests revealed that thetwo-phase alloys show plastic yielding and appreciable ductil-ity, while the three-phase alloys fracture before yielding withhigh fracture stress. This difference may result simply fromthe volume fraction of ductile Nbss. As shown in Table 2 forboth as-cast and heat treated conditions, the volume fractionsof Nbss in the two-phase alloys are higher than 40%, whilethose of the three phase alloys are less than 32%. Here, it isnoted that the as-cast three-phase alloyc shows no ductility,while the heat treated alloyc shows yielding, where the vol-ume fractions of Nbss in the as-cast and heat treated alloys are32 and 36%, respectively. Therefore, the volume fraction ofa ductile phase would play an important role to provide thecompressive ductility.

In view of plastic deformation, more ductile phase wouldbe favorable to increase ductility. The compressive ductil-ity is larger in the alloya than in the alloyb. This is be-cause the Nbss of alloy b is solid solution hardened by Cr al-loying, which is confirmed by the significant decrease in thelattice parameter, as shown in Fig. 3. At present, it is noteasy to detect the difference in room temperature deforma-bility between monolithic Nb5Si3 and NbCr2 phases, becauseboth the intermetallics are completely brittle even in compres-sion. However, it has been shown that a duplex Nbss/Nb5Si3alloy with a similar volume fraction of Nbss phase to alloycdeforms plastically at room temperature.23) This result sug-gests that Nbss/Nb5Si3 alloy has deformability superior toNbss/Nb5Si3/NbCr2 alloy at room temperature. Much more

Effect of Cr Addition on Microstructure and Mechanical Properties in Nb–Si–Mo Base Multiphase Alloys 3261

works would be required to understand the room tempera-ture deformability of those complex intermetallic phases inthe relation to microstructures as well as volume fractions ofconstituent phases.

The yield stress at 1773 K changes dramatically with Crcontent as shown in Fig. 7. The rapid decrease of yield stressby 5 mol%Cr addition would be associated with weakeningof Nbss, since the volume fractions ratio of the constituentphases is identical in alloysa andb. As shown in Fig. 3, thelattice parameter of Nbss decreases with increasing (Cr+Mo)content in Nb–22Si–5Mo–xCr alloys. Even though solid so-lution strengthening due to Cr addition occurs at room tem-perature in the alloys, Cr is well known to have high diffu-sivity in Nbss at high temperature,31) which can acceleratethe deformation controlled by a thermally activated processand thereby high temperature strength would be decreased. Incontrast, the rapid increase of yield stress by 25 mol%Cr ad-dition would be ascribed to the increase in volume fraction ofNbCr2 and the decrease in volume fraction of Nbss. As seen inFig. 7, yield stress is always higher in the heat treated alloysthan in the as-cast alloys. Although a strict explanation forthe difference is difficult because of microstructure changesby heat treatment, the decrease in the volume fraction of Nbss

(Table 2) should be responsible.As can be seen in Fig. 8, yield stress is still very high even

at around 1500 K, which results from high strength of Nb5Si3and NbCr2. The abrupt decrease of yield stress above 1700 Kmay be due to intrinsic nature of the constituent phases.

5. Conclusions

The phase equilibria, crystal structures of constituentphases, microstructures and mechanical properties were in-vestigated at room temperature to 1773 K for Cr-added Nb–22Si–5Mo–xCr alloys consisting of two-phase Nbss/Nb5Si3or three-phase Nbss/Nb5Si3/NbCr2. The obtained results aresummarized as follows.

(1) The Cr addition to Nb–22Si–5Mo ternary systemgives rise to a shift of eutectic line toward Si-rich corner be-ing related to the two-phase Nbss/Nb5Si3 region, while three-phase, Nbss, Nb5Si3 and NbCr2 Laves phase, are formed by Craddition higher than 5 mol%. The volume fraction of NbCr2

increases at the expense of mostly Nbss with increasing Crcontent.

(2) Lattice parameter of Nbss phase decreases with in-creasing Cr content at constant Si and Mo contents. The de-crease in the lattice parameter of Nbss can be related to par-titioning of Mo and Cr in the constituent phases. Thec/aaxis ratio increases with increasing content of (Cr+ Mo) upto 5.9 mol% and then is saturated.

(3) Phase transformation fromβ-Nb5Si3 to α-Nb5Si3 ex-ists at temperature above 1773 K in the present alloys.

(4) With increasing Cr content yield stress increases butcompressive ductility decreases at room temperature in thetwo-phase alloys. The presence of C14 Laves phase is notbeneficial for improving deformability at room temperaturein this alloy system.

(5) The 0.2% offset yield stress at 1773 K increases withincreasing the volume fraction of Nb5Si3 or C14 Laves phasebut decreases with increasing the volume fraction of bcc solid

solution or increasing Cr content in Nbss.

Acknowledgements

This work is supported by a grant from the New Energy andIndustrial Technology Development Organization (NEDO) ofJapan.

REFERENCES

1) D. L. Anton and D. M. Shah:Intermetallic matrix composites, eds. D. L.Anton, R.McMeeking, D. Miracle and P. Martin (Pittsburgh, PA MRS,1990) pp. 45–52.

2) R. L. Fleischer and R. J. Zabala: Metall. Trans. A21A (1990) 2149–2154.

3) M. Takeyama and C. T. Liu: Mater. Sci. Eng. AA132 (1991) 61–66.4) H. M Yun and R. H. Titran:High temperature Nb alloys, eds. I. N.

Stephens and J. J. Ahmad (Warrendale, PA TMS, 1991) pp. 83–90.5) D. M. Shah: Superalloys, eds. Antolovich,et al., (Warrendale, PA

TMS,1992) pp. 409–417.6) D. L. Davidson and D. L. Anton:High temperature ordered intermetal-

lic alloys V, eds. I. Baker, R. Darolia, J. D. Whittenberger and M. H.Yoo (Pittsburgh, PA MRS, 1993) pp. 807–816.

7) J. D. Livingston: High Temperature Silicides and Refractory Alloys,eds. C. L. Briant, J. J. Petrovic, B. P. Bewlay, A. K. Vasudevan and H.A. Lipsitt (Pittsburgh, PA MRS, 1994) pp. 395–406.

8) R. Tanaka: Mater. High Temp.17 (2000) 457.9) W.-Y. Kim, H. Tanaka, A. Kasama and S. Hanada: J. Alloys Compd.

333 (2002) 170–178.10) R. M. Nekkanti and D. M. Dimiduk:Intermetallic matrix composites,

eds. R. McMeeking, D. Miracle and P. Martin (Pittsburgh, PA MRS,1990) pp. 175–182.

11) M. G. Mendiratta and J. J. Lewandowski: Metall. Trans. A22A (1991)1573–1583.

12) J. D. Rigney, P. M Singh and J. J. Lewandowski: JOM44 (1992) 36–41.13) D. L. Anton and D. M. Shaw:High temperature ordered intermetal-

lic alloys III, eds. C. T. Liu, A. I. Taub, N. S. Stoloff and C. C. Koch(Pittsburgh, PA MRS, 1989) pp. 361–372.

14) M. G. Mendiratta and D. M. Dimiduc: Metall. Trans. A24A (1993)501–504.

15) M. Y. He and A. G. Evans: Acta Metal. Mater.41 (1993) 1223–1230.16) M. G.Mendiratta, J. J. Lewandowski and D. M.Dimiduk: Metall. Trans.

A 22A (1991) 1573–1583.17) R. Gnanamoorthy and S. Hanada: Scr. Mater.34 (1997) 999–1003.18) M. F. Ashby, F. J. Blunt and M. Bannister: Acta Metall. Mater.37 (1989)

1847–1857.19) B. P. Bewlay, M. R. Jackson and H. A. Lipsitt: Metall. Mater. Trans. A

27A (1996) 3801–3808.20) N. Sekido, Y. Kimura, F. G. Wei, S. Miura and Y. Mishima: J. Japan.

Inst. Metals64 (2000) 1056–1061.21) J. Kajuch, J. Short and J. J. Lewandowski: Acta Metal. Mater.43 (1995)

1955–1967.22) H. Okamoto, A. B. Gokhale and G. J. Abbaschain:Binary Alloy Phase

Diagrams, Second Edition, eds. T. B. Massalski,et al., (ASM, 1990)pp. 2764–2769.

23) W.-Y. Kim, H. Tanaka, A. Kasama, R. Tanaka and S. Hanada: Inter-metallics9 (2001) 521–527.

24) W.-Y. Kim, H. Tanaka, A. Kasama and S. Hanada: Intermetallics9(2001) 827–834.

25) W.-Y. Kim, H. Tanaka and S. Hanada: Intermetallics, in press.26) B. D. Cullity: Elements of X-ray diffraction, (Addison-Wesley, 1977)

p. 360.27) M. Yoshida and T. Takasugi: Mater. Sci. Eng. AA224 (1997) 77–86.28) M. Yoshida and T. Takasugi: Mater. Sci. Eng. AA234–236 (1997) 873–

876.29) T. Takasugi, M. Yoshida and S. Hanada:Design Fundamentals of High

Temperature Composites, Intermetallics, and Metal-Ceramic systems,eds. R. Y. Lin, Y. A. Chang, R. G. Reddy and C. T. Liu (Warrendale, PATMS, 1995) pp. 235–249.

30) F. Chu, D. J. Thoma, K. McClellan, P. Peralta and Y. He: Intermetallics7 (1999) 611–620.

31) F. Roux and A. Vignes: Rev. Phys. Appl.5 (1970) 393.