Effect of B addition on the microstructures and mechanical properties of Nb–16Si–10Mo–15W alloy

Materials Science and Engineering A384(2004)377–384Effect of B addition on the microstructures andmechanical properties of Nb–16Si–10Mo–15W alloyC.L.Ma a,b,∗,J.G.Li a,Y.Tan b,R.Tanaka b,S.Hanada ca School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai200030,PR Chinab Japan Ultra-high Temperature Materials Research Institute,Ube755-0001,Japanc Institute for Materials Research,Tohoku University,Sendai980-8577,JapanReceived29March2004;received in revised form14June2004AbstractNb-silicide based,hypoeutectic Nb–16Si–10Mo–15W alloys doped with various amounts of boron from0to10at.%were prepared by arc melting and annealing.Microstructures and mechanical properties of these alloys were investigated by X-ray diffraction(XRD)analysis, scanning electron microscopy(SEM),electron probe X-ray microanalysis(EPMA),Vickers hardness measurement and compression test. The eutectic reaction L→Nb ss(Nb solid solution)+␤silicide(W5Si3-type,D8m)exists in B contents less than1at.%,while a new eutectic reaction L→Nb ss+␣silicide(Cr5B3-type,D8l)does in B contents higher than1at.%.The␣silicide is also found in0.44at.%(500wt.ppm) and1at.%B-added alloys.␤silicide completely transforms into␣silicide after annealing treatment at1700◦C for48h.The lattice parameter of␣silicide decreases with increasing boron content.Boron addition increases the hardness of silicide phase,but yield no appreciable change of hardness in Nb ss.Moreover,boron addition greatly improves the high-temperature strength.©2004Elsevier B.V.All rights reserved.Keywords:Silicide;Phase transformations;Mechanical properties;Casting1.IntroductionNiobium silicide(Nb5Si3)possesses attractive properties, such as high melting point[1],excellent high-temperature strength and creep resistance[2,3],required for advanced gas turbine engine uses.However,the intrinsic brittleness of Nb5Si3at ambient temperature inhibits its practical ap-plications.To overcome this critical issue,ductile Nb solid solution(Nb ss)in equilibrium with Nb5Si3up to very high temperatures has been incorporated into an Nb5Si3matrix in an in situ manner to produce composites based on the concept of ductile-phase toughening of brittle-compounds[4,5].Al-though the toughness of such multiphase alloys,so-called in situ composites,has been greatly improved,as compared to the Nb-silicide single-phase alloys[6],the high-temperature strength of Nb-silicide was decreased by the Nb ss incorpo-∗Corresponding author.Tel.:+81-22-215-2840;fax:+81-22-215-2841.E-mail address:clma2001@(C.L.Ma).ration.Moreover,the oxidation resistance of the compos-ites was degraded because of poor oxidation resistance of constituent Nb ss at elevated temperatures.Not only to in-crease high-temperature strength and low-temperature tough-ness but also to improve oxidation resistance,compositional and microstructural control is indispensable[7].Many efforts have been devoted to looking for alloying elements and opti-mizing their content so as to meet the demands.Alloying ele-ments such as Hf,Ti,Cr,Al and Ta have been investigated,for example[8].Recently,we have studied the high-temperature strength of Nb ss/Nb-silicide in situ composites alloyed with Mo and W[9–12].Of the various alloy compositions investi-gated,Nb–16Si–10Mo–15W was found to exhibit extremely high strength and excellent creep resistance at temperatures up to1500◦C[11].However,the fracture toughness and ox-idation resistance of this alloy were still insufficient for the requirements of practical applications.Boron has been re-ported to be effective in enhancing the oxidation resistance in the Mo–Si–B system at temperatures as high as1500◦C0921-5093/$–see front matter©2004Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2004.06.058378 C.L.Ma et al./Materials Science and Engineering A 384(2004)377–384due to the formation of thin film of borosilicates on alloy surface,which inhibit oxygen atoms from penetrating into the metal [13].It is very interesting to know whether a sim-ilar effect appears in B-doped Nb ss /Nb-silicide in situ com-posites.Some research results have been reported recently that the Nb 5Si 3B 2exhibits a batter oxidation resistance than Nb 5Si 3at 1250◦C [14].Our preliminary study,unfortunately,showed only a small improvement of oxidation resistance in B-doped Nb ss /Nb-silicide in situ composites at 1500◦C.It was found,however,that the strength of silicide was sig-nificantly increased,while the ductility of Nb ss was not so degraded by B addition.This feature is very attractive for producing Nb ss /Nb-silicide in situ composites with a good balance of high-temperature strength and low-temperature toughness.The aim of the present paper is to demonstrate the effect of B addition on mechanical properties of Nb ss /Nb-silicide in situ composites.Solidification behavior,phase relationship and phase stability were also investigated for comprehensive understanding of the B effect.Based on the obtained results the potential of B as an alloying element to Nb ss /Nb-silicide in situ composites is discussed.2.Experimental procedureNb–16Si–10Mo–15W is used as the base composition,which is a hypoeutectic containing primary Nb ss particles and a eutectic structure ␤silicide +Nb ss at a solidification condi-tion.The ␤silicide transforms to a stable ␣silicide through annealing.The microstructural features and mechanical prop-erties of this alloy have been reported elsewhere [11].Various amounts of boron,from 0to 10at.%,were added to the base composition by substitution of Nb.The detailed alloy compositions are listed in Table 1.The high-purity el-emental powders Nb,Mo,W,Si and B were used as raw materials.Alloy ingots were prepared by arc melting on a water-cooled copper crucible under an ultra high-purity argon atmosphere.Each ingot was remelted three times to ensure compositional homogeneity.Heat treatment was conducted at 1700◦C for 48h,followed by furnace cooling to room temperature.Microstructures of cast and heat-treated sam-ples were characterized using SEM,XRD and EPMA.The hardness of constituent phases was measured using a Vickers hardness tester.The high-temperature strength was evaluated by compression tests using specimens with a di-mension of 3mm ×3mm ×pression tests wereTable 1Compositions and constituent phases identified by XRD Alloys CompositionsPhases in cast alloys Phases in annealed alloys HA Nb–16Si–10Mo–15W␤-Nb 5Si 3,Nb ss␣-Nb 5Si 3,Nb ss HA05Nb–16Si–10Mo–15W–0.44B ␤-Nb 5Si 3,␣-Nb 5Si 3,Nb ss ␣-Nb 5Si 3,Nb ss HA1Nb–16Si–10Mo–15W–1B ␤-Nb 5Si 3,␣-Nb 5Si 3,Nb ss ␣-Nb 5Si 3,Nb ss HA5Nb–16Si–10Mo–15W–5B ␣-Nb 5Si 3,Nb ss ␣-Nb 5Si 3,Nb ss HA10Nb–16Si–10Mo–15W–10B␣-Nb 5Si 3,Nb ss␣-Nb 5Si 3,NbssFig.1.X-ray diffraction patterns of as cast B-doped Nb/Nb-silicide alloys as a function of B content.carried out at an initial strain rate of 3×10−4s −1in an argon atmosphere using an Instron-type testing machine to a plastic strain of 16%.3.Results3.1.Microstructures3.1.1.Cast structuresXRD patterns of cast Nb–16Si–10Mo–15W alloys doped with various amounts of B are shown in Fig.1.The corre-sponding cast structures are illustrated in Fig.2.The diffrac-tion peaks on these XRD profiles were indexed based on the JCPDS databases and denoted with various marks to indicate the corresponding phases.The identified phases in each alloy are summarized in Table 1.It is evident from the table and these figures that the cast structure strongly depends on the B content.For the B-free alloy (HA),two phases of bcc Nb ss and W 5Si 3-type silicide (␤)are indexed on its XRD curve.The backscattered electron image (BEI),as shown in Fig.2(a),re-veals the two phases,which have bright and dark contrasts in BEI,distribute as dendritic primary particles and a two-phase mixture of a dark matrix and irregular lamellar bright parti-cles.The EPMA analysis revealed that the bright dendritic and irregular lamellar particles have an average composition of Nb–13.5Mo–31.4W–1.9Si and the dark matrix phase has an average composition of Nb–2.5Mo–1.2W–37Si,indicat-ing that the bright phase is Nb ss and the dark phase is ␤silicide.Thus the lamellar structure is (Nb ss +␤).Note that a core behavior of W distribution in some large Nb ss particlesC.L.Ma et al./Materials Science and Engineering A 384(2004)377–384379Fig.2.Cast microstructures of Nb/Nb–Si alloys with various B additions:(a)0at.%B;(b)0.44at.%B;(c)1at.%B and (d)5at.%B.is observed,i.e.,the W content is slightly higher in the center area than that in the out area of Nb ss particles.For the low B content alloys (HA05and HA1),besides the Nb ss and ␤silicide,a Cr 5B 3-type silicide (␣)was also detected on the XRD patterns.The cast structures,as can be seen in Fig.2(b)and (c),consists of large primary dendritic particles and a very fine interdendritic multiphase mixture.The primary dendritic particles are easily recognized to be Nb ss by their bright contrast.The ␤silicide is also easily rec-ognized by its dark contrast.Some small bright Nb ss particles dispersed on ␤phase and composed (␤+Nb ss )two-phase structure.Note that this (␤+Nb ss )two-phase structure is lo-cated at the interdendritic areas that finally solidified during solidification.Around the primary Nb ss particles are other two-phase mixtures consisting of small bright particles and gray paring the (␤+Nb ss )structure,the phase boundaries in these areas are not compositionally sharp,sug-gesting the atom distributions in the two phases are far from equilibrium state.The small particles are recognized to be Nb ss by their bright contrast while the gray phase is believed to be ␣silicide,as confirmed by the XRD analysis.The mix-ture around primary Nb ss particles is (␣+Nb ss ).For the high B content alloys (HA5and HA10),only ␣and Nb ss diffraction peaks are detected on the XRD patterns,while no ␤silicide peak is seen.Fig.2(d)shows the cast structure of HA5,which contains bright dendritic particles and a two-phase mixture of small bright particle and a gray matrix.The large dendritic particles and small dendritic par-ticles on a gray matrix are easily recognized as Nb ss from the bright contrast,which was confirmed by EPMA analysis.Then the gray matrix was identified as ␣silicide based on the XRD analysis result.Note that,here,the ␣silicide solid-ifies directly from the molten alloys doped with boron.But in boron-free alloys,including Ni–Si binary and Nb–Mo–Si,Nb–W–Si and Nb–Ti–Si ternary as well as Nb–Mo–W–Si quaternary alloys,the ␣silicide is a low-temperature phase and forms through a solid–solid reaction.It is worthy to note that,here,no boride phase was identi-fied by XRD and EPMA analyses,though the boron addition is as high as 10at.%.Fig.3(a)and (b)show a BEI of HA10alloy and the corresponding B mapping image,positional mapping using EPMA reveals that B is pref-erentially solved in ␣-type silicide.Additionally,as can be seen in Fig.1,the diffraction peaks from Nb ss shift to the high angle side,implying a decrease in lattice parameter.This is consistent with our previous re-sults [11,12].Some cracks and pores are also observed in the cast microstructures of high B content alloys,which may be introduced by thermal stress during solidification and cooling.380 C.L.Ma et al./Materials Science and Engineering A 384(2004)377–384Fig.3.Mapping image of B distribution in cast HA10alloy:(a)BEI of selected area and (b)the corresponding mappingpattern.Fig.4.X-ray diffraction patterns of annealed B-doped Nb/Nb-silicide alloys as a function of B content.3.1.2.Annealed microstructuresX-ray diffraction patterns of alloys annealed at 1700◦C for 48h are shown in Fig.4.The dashed lines in thefigureFig.5.Typical annealed microstructures of alloys with B:(a)1at.%and (b)5at.%.represent the diffraction peak positions of binary ␣silicide (Nb 5Si 3).It is clear from these X-ray profiles that all the al-loys contain Nb ss phase and ␣silicide after annealing.No ␤silicide peaks are found,indicating that this metastable phase completely transforms to stable ␣silicide.Since no discernible diffraction peak from another phase is observed,Nb ss phase and ␣silicide are equilibrium in these compo-sitions.The typical annealed microstructures of HA1and HA5alloys are shown in Fig.5(a)and (b).No apparent change is seen in the primary Nb ss phase,whereas the mi-crostructure of the multiphase matrix is slightly coarsened after the annealing treatment.The silicide volume fraction is estimated to be 47%for HA1alloy and 51%for HA5alloy.The EPMA analysis reveals a Nb ss in the B-free alloy having a composition of Nb–14.8Mo–25.3W–0.5Si and an ␣silicide of Nb–1.5Mo–0.8W–37.5Si.It clearly indicates that the solubility of Si in Nb ss as well as the Mo and W in ␣is very low.Similar results have also been reported for Nb–Mo–Si [15],Nb–W–Si [16].No core behavior is seen for annealedC.L.Ma et al./Materials Science and Engineering A384(2004)377–384381Table2Lattice parameters of␣-type silicide and Nb ss in annealed alloysAlloys Nb ss␣-Type silicidea a c a/c␣-Nb5Si3 6.569811.8870.5527 HA 3.2359 6.554811.8860.5514 HA5 3.2185 6.452211.7990.5495 HA10 3.2119 6.437111.7650.5471alloys,indicating a homogenous distribution of component elements in each phase.Some measurement errors easily occur when analyzing the light element B in constituent phases using the present EPMA method,which give rise to underestimates of B concentra-tion in␣silicide.The compositions of constituent phases in boron-doped alloys are not given here.But it is worthy to point out that the Mo and W solubility measured in B-doped ␣silicide is very low(<3at.%)though having some measure-ment error,which is comparable to that in B-free␣silicide. The distribution of B in two phases is qualitatively analyzed by measuring the compositional map using EPMA.The re-sult is similar to that obtained for cast alloys shown in Fig.3, revealing that B is preferentially solved in␣silicide.The lat-tice parameters of␣silicide and Nb ss are calculated by XRD analysis.The results are summarized in Table2,where data of binary␣Nb5Si3are also listed for comparison.It is ev-ident from the table that the lattice parameters of␣silicide and Nb ss decrease with increasing B content.3.2.Mechanical properties3.2.1.Vickers hardnessVickers hardness measurements were conducted on the constituent phases in annealed alloys containing various B concentrations.At least10points were measured for each case and the results are summarized in Table3.Though there is considerable scatter in the measured data due to the brit-tleness of silicide and relatively inhomogeneous microstruc-tures,it is evident from Table3that the hardness of Nb ss phase is not so sensitive to B content,while that of silicide phase is significantly increased by the boron additions.3.2.2.High-temperature strengthCompression tests were conducted on annealed alloys to examine the boron effect on the high-temperature strength. Three samples were tested for each alloy.The average yieldTable3Vickers hardness of constituent phases in annealed alloys as a function of B contentAlloys Nb ss␣-Type silicide HA6501050HA05(500ppm)6541158HA1(1at.%)6441159HA5(5at.%)6771372HA10(10at.%)675pressive strength of B-doped alloys at1500◦C as a function of Bcontent.parison of high-temperature strength of present alloys with some advanced Nb/Nb-silicide multi phase alloys.strengths are summarized in Fig.6.No significant differ-ence in compressive yield strengths at1500◦C is found at B contents less than5at.%,while the maximum yield strength of880MPa is obtained at5at.%B addition.Temperature dependence of yield strength was examined for5at.%B-doped alloy HA5.The results are shown in Fig.7,where data for some advanced binary,ternary and high order multi-component Nb-silicide in situ composites are also included for comparison.Note that the HA5alloy exhibits the highest strength at high temperatures.At1600◦C,the HA5alloy still maintains its high compressive stress of670MPa.4.Discussion4.1.Phase separation path4.1.1.B-free alloyThe cast structure of B-free(HA)alloy consists of Nb ss primary phase and a two-phase mixture of(Nb ss+␤),indi-cating that this alloy locates on an Nb ss-rich side of Nb ss+␤eutectic valley.The solidification sequence of this alloy can382 C.L.Ma et al./Materials Science and Engineering A 384(2004)377–384be described as follows:L →Nb ss +L 1thenL 1→␤silicide+Nb ssThe eutectic reaction is similar to that observer in Nb–Mo–Si and Nb–W–Si ternary systems.But it does not exist in Nb–Si binary [1]and Nb–Si–Ti ternary system,which were exten-sively studied recently [17].Using this simple eutectic reac-tion,one can control the microstructure of Nb ss /Nb-silicide in situ composites in a liquid–solid process,e.g.,a directional solidification process.4.1.2.B-added alloysBoron addition greatly changes the phase relationship and phase separation mechanism.The cast structures of HA5and HA10alloys consist of primary Nb ss phase and a two-phase mixture of (␣+Nb ss ),indicating that these alloys locate on an Nb ss -rich side of Nb ss +␣eutectic valley.The solidifi-cation sequence of these high boron-contained alloys can be described as follows:L →Nb ss +L 1thenL 1→␣silicide+Nb ssNote that the ␣silicide is a low-temperature phase in Nb–Si binary as well as Nb–Mo–Si,Nb–W–Si and Nb–Ti–Si ternary systems and forms through a solid–solid transforma-tion.In the present case,the ␣silicide solidifies directly from melt via a eutectic reaction,indicating the existence of a pri-mary ␣phase field in this Nb–Mo–W–Si–B system.In the composition region studied here,the Nb–Mo–W–Si–B system is reasonably thought as a pseudo-ternary system (Nb,Mo,W)–Si–B because the Nb forms a continuous solid solution with Mo and W.On the basis of the structural observation,a liquidus projection of (Nb,Mo,W)-rich zone in the pseudo-ternary (Nb,Mo,W)–Si–B system is proposed in Fig.8.As revealed in the cast structure of B-free (HA)alloy,there is a eutectic reaction e 1,L →␤+Nb ss ,on the (Nb,Mo,W)-SisideFig.8.Schematic representation of partial liquidus surface of (Nb,Mo,W)-rich zone in pseudo-ternary (Nb,Mo,W)–Si–B system.near the (Nb,Mo,W)point.The eutectic groove extends from the eutectic point into the (Nb,Mo,W)-rich zone interior and divides this region into two parts of ␤and Nb ss primary phase fields.The fact of the coexistence of Nb ss ,␣and ␤phases in low boron-content alloys (Fig.2(b)and (c))indicates that a ␣primary phase field must adjoin the Nb ss and ␤regions,which generates a phase boundary bU between ␤and ␣phase fields and eU between the Nb ss and ␣phase fields,as well as a transition reaction at triple point U.The eutectic structure of (␣+Nb ss )observed in HA5and HA10indicates the boundary line eU is eutectic.The nature of boundary line bU could not be determined by the present data.But we can know that the temperate falls from b to U,because the U is on the eutectic groove ee 1.As revealed in Fig.2(b)and (c),the eutectic structure of (␤+Nb ss )locates at the finally solidified interdendritic region in the low boron-content alloys.It indicates that the eutectic reaction L →␤+Nb ss occurs at a lower temperature than that of the eutectic reaction L →␣+Nb ss .In other words,the temperature falls from e to e 1.The temperature fallen directions of phase boundaries are denoted by arrows in the partial liquidus diagram.Though the nature of boundary line bU is unclear,the transition reaction at U can be determined to be peritectic by the temperature fallen direction of the phase boundaries,which intersect at triple point U.Based on the projection diagram,the solidification sequence occurred in low boron-content alloys can be described as follows:L →Nb ss +L 1followed byL 1→␣+Nb ss +L 2followed byL 2+␣→␤+Nb ss +L 3thenL 3→␤+Nb ssIt is note that the transition reaction is peritectic and would occur on the ␣surface when the liquid composition reaches the transition point.Since the arc-melted ingot is rapidly cooled on the water-cooled copper crucible,atom diffusion in a solid phase will be suppressed or retarded.It is the reason that no clear microstructural evidence of transition reaction is seen in the cast structure of low boron-content alloys.From the above discussion,it is clear that B concentration at the transition reaction point is lower than 0.44at.%because ␣silicide appears along with ␤silicide and Nb ss in cast HA05alloy containing 0.44at.%B.In other words,the solubility of B in ␤silicide will be less than 0.44at.%.4.2.Phase stability at 1700◦CThe present result reveals that the ␤silicide completely transforms to ␣silicide after annealing treatment at 1700◦C for 48h.␣Silicide and Nb ss are equilibrium at this tempera-ture.As shown in Table 2,the lattice parameters of Nb ss and ␣silicide decrease with increasing boron content.C.L.Ma et al./Materials Science and Engineering A384(2004)377–384383In the present study,boron is added to the base alloy (Nb–16Si–10Mo–15W)by substituting for Nb.Moreover, the compositional mapping revealed that B preferentially solved in␣-type silicide.B-doped alloys have low Nb con-tent in Nb ss,compared to B-free alloy.It is known that the dissolution of Mo and W in Nb results in a decrease in lattice constant because the atomic radii of Mo and W are smaller than of Nb[12].Hence,the decrease of Nb ss lattice constant is mainly contributed by the dissolution of Mo and W in Nb.On the other hand,the solubility of Mo and W in␣silicide is very low,suggesting that the decrease in lattice constant of␣silicide is due to the boron addition.Boron atoms may occupy interstitial or substitutional positions in a␣lattice. The latter will result in decrease of lattice parameter.The fact that quite a large mount of boron dissolves in␣silicide and does not increase the lattice constant indicates that the boron atoms mainly occupy the substitutional bining the result of little Mo and W solved in this silicide,the␣silicide could be denoted as␣(Nb,Mo,W)5(Si,B)3.Perepezko et al.suggested that a T2phase(Nb5SiB2)ex-ists in Nb–Si–B ternary system,forming a continuous phase from Nb5SiB2to Mo5SiB2(Cr5B3-type)in Nb–Mo–Si–B quaternary system[18].However,the phase boundary has not been well defined in this system.Therefore,the phase relationship between␣-Nb5Si3and Nb5SiB2is still unclear at present.4.3.High-temperature strengthOne of the most interestingfindings in this study is that the high-temperature strength of Nb-silicide based alloys is remarkably increased by boron additions,as shown in Fig.7. The strengthening mechanism is not clearly understood at present,but there are some possible explanations for the strengthening.Thefirst is the solid solution hardening of␣silicide by B.As shown in Fig.3,B atoms are preferentially solved in␣silicide and locate predominantly at substitutional sites in␣lattice,which decrease the lattice parameters.The crystal lattice distortion caused by B dissolution in␣sili-cide may greatly increase the resistance to deformation.It is the reason for the increase of Vickers hardness of␣sili-cide at room temperature(see Table3).Moreover,diffusion-controlled high-temperature deformation of␣silicide may be suppressed by the B addition leading to the decrease in lattice constants.According to Subramanian et al.,creep de-formation in binary Nb5Si3is controlled by bulk diffusion of Nb in Nb5Si3[19].Although their deformation conditions are different from ours,it will be reasonable to assume that high-temperature deformation in this study is controlled by bulk diffusion of Nb.Both the geometrically decreased unit cell of Nb-silicide arising from the decrease in lattice pa-rameters due to B dissolution and the electrochemically in-creased binding force among the atoms in␣silicide due to the B being more electronegative than Si[20]will retard the bulk diffusion of Nb at high temperature,thereby increasing the high-temperature strength of␣silicide.Another impor-tant factor is concerned with the silicide volume fraction.Asmentioned above,the silicide phase in5at.%B-doped alloy(HA5)is about51vol.%,which is slightly higher than thatof B-free alloy,in which the silicide phase is about47vol.%.As expected by the rule of mixture for strength of compositematerial,the increase of silicide volume fraction will raisethe high-temperature strength of these composites.High-temperature strength of Nb ss/Nb-silicide in situcomposites has been found to increase remarkably by alloy-ing with Mo and W.However,the low-temperature toughnessof these materials was degraded by the alloying due to theductility loss of Nb ss.This work reveals that B alloying in-creases high-temperature strength of Nb ss/Nb-silicide in situcomposites through solid solution strengthening of␣-typesilicide(see Table3and Figs.6and7).In addition,B hasextremely low solubility in Nb ss,implying that ductility ofNb ss is not degraded by alloying with B.Although no evi-dence is presented in this study concerning fracture toughnessof B-doped alloys,B has a high potential to improve high-temperature strength and room temperature fracture tough-ness simultaneously.5.ConclusionsMicrostructures and mechanical properties of hypoeu-tectic Nb–16Si–10Mo–15W alloys doped with0–10at.%Bwere investigated.There exist two simple eutectic reactions:L→Nb ss+␤silicide(B content≤1at.%),and L→Nb ss+␣silicide(B content>1at.%).The␣silicide is also found in 0.44and1at.%B-doped alloys.No boride phase is identifiedeven in high boron-content alloys in both cast and annealedconditions.␤silicide completely transformed into␣silicideafter annealing at1700◦C for48h.The lattice parametersof␣silicide decrease with increasing boron additions.Nodistinct hardening of Nb solid solution is observed by boronadditions,but the hardness of silicide phase is increased.Thehigh-temperature yield stress is increased by boron additionto about880MPa at1500◦C and670MPa at1600◦C forNb–16Si–10Mo–15W–5B.References[1]T.B.Massalski,H.Okamoto,P.R.Subramanian,L.Kacprzak,BinaryAlloy Phase Diagrams,ASM,Metals Park,Ohio,1992.[2]R.M.Nekkanti,D.M.Dimiduk,High-temperature ordered intermetal-lic alloys IV,MRS Symp.Proc.194(1990)175.[3]P.R.Subramanian,M.G.Mendiratta,D.M.Dimiduk,JOM48(1996)33.[4]K.S.Chan,High-temperature ordered intermetallic alloys VI,MRSSymp.Proc.364(1995)469.[5]K.S.Chan,Metall.Trans.A27A(1996)2518.[6]M.G.Mendiratta,J.J.Lewandowski,D.M.Dimiduk,Metall.Trans.A22A(1991)1573.[7]S.Hanada,Reports of the123committee on heat-resisting metalsand alloys,JSPS38(1997)299.384 C.L.Ma et al./Materials Science 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