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Liquidus Projection Of The Nb–si–b System In The Nb-rich Region

Liquidus projection of the Nb–Si–B system in the Nb-rich region




  Liquidus projection of the Nb–Si–B system in the Nb-rich region Da ´rio Moreira P. Ju´nior a , Carlos Angelo Nunes a, *,Gilberto Carvalho Coelho a , Fla ´vio Ferreira a,b a Departamento de Eng. de Materiais (DEMAR), Faculdade de Engenharia Quı´ mica de Lorena (FAENQUIL),CP 116, 12600-000, Lorena, Sa ˜ o Paulo, Brazil  b Escola de Engenharia Industrial Metalu´rgica de Volta Redonda-UFF-TMC, Av. dos Trabalhadores 420,27260-740,Volta Redonda, Rio de Janeiro, Brazil  Accepted 5 November 2002 Abstract Alloys of the Nb–Si–B system have been evaluated aiming its use as high temperature structural materials. In this work theliquidus projection of the Nb–Si–B system in the Nb-rich region has been established based on the microstructural characterizationof arc-melted alloys through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Six different primary solidificationregions were observed: Nb ss , NbB, T 2  ( a Nb 5 Si 3 ), Nb 3 Si, T 1  ( b Nb 5 Si 3 ) and D8 8 . The following ternary invariant reactions are pro-posed to occur in the region of study: II 1 : L+NbB () Nb ss +T 2 ; II 2 : L+T 1 () Nb 3 Si+ T 2 ; II 3 : L+Nb 3 Si () Nb ss +T 2 ; III 1 :L+NbB+D8 8 () T 2 ; III 2 : L+T 1 +D8 8 () T 2 . # 2003 Published by Elsevier Science Ltd. Keywords:  A. Silicides, various; B. Phase diagram; B. Phase identification 1. Introduction Structural materials for applications at high tem-peratures must present a suitable balance of severalproperties, including mechanical strength, creep, fati-gue, and oxidation resistance among others. The useof multiphase materials has been considered the bestoption to satisfy all the requirements for structuralintegrity [1,2], concept which has been applied in thedevelopment of superalloys and special steels for hightemperature applications. Considering the recentinterest in multiphase Me–Si–B (Me—refractorymetal) alloys for the development of this class of materials [3-6], this study aimed the determination of the liquidus projection in the Nb-rich region of theNb–Si–B system.In the Nb–Si system, the phase relations involving theliquid phase are well determined [7,8]. The followinginvariant reactions are proposed to occur in the Nb-richregion: (i) L () Nb ss  (ss—solid solution)+Nb 3 Si(1915  C); (ii) L+ b Nb 5 Si 3 () Nb 3 Si (1975  C); and (iii)L () b Nb 5 Si 3  (2515   C). Concerning the Nb–B system,recent experiments carried out by Borges at al. [9] haveindicated the need for modifications in the currentlyaccepted Nb–B phase diagram [10]. With respect to theNb-rich region (0–50 at.%B), the results of Borges et al.[9] have shown that: (i) Nb ss  and NbB are the only pri-mary phases; (ii) there exist a L () Nb ss +NbB eutecticreaction, with liquid eutectic composition near 16at.%B, (iii) the Nb 3 B 2  phase is formed through theperitectoid reaction Nb ss +NbB () Nb 3 B 2 . Theseresults are in agreement with those reported by Rudy[11]. In the case of the Nb–Si–B ternary system, the onlyinformation concerning phase equilibria is due to Now-otny et al. [12] who proposed an isothermal section at1600   C, which is shown in Fig. 1. In this figure, thephase identified as T 2  corresponds to the  a Nb 5 Si 3 -phase(Cr 5 B 3 -type) of the binary Nb–Si system and exists overa large composition range, which has been confirmed byexperiments carried out in our group. Based on thecurrently accepted Nb–Si phase diagram [10] theT 2 -region in Fig. 1 should extends up to the binary a Nb 5 Si 3 . The phase identified as D8 8  in Fig. 1 is iso-morphous with Mn 5 Si 3 . 0966-9795/03/$ - see front matter # 2003 Published by Elsevier Science Ltd.doi:10.1016/S0966-9795(02)00249-2Intermetallics 11 (2003) 251–* Corresponding author. Fax: +55-12-553-3006. E-mail address:  [email protected] (C.A. Nunes).  2. Experimental procedure The Nb–Si–B alloys were produced through arc-melting under pure argon atmosphere (min. 99.995%)in a water-cooled copper hearth. The starting materialswere high purity Nb (min. 99.8%), B (min. 99.5%) andSi (min. 99.998%). For each alloy, four melting stepswere carried out to ensure chemical homogeneity. Themaximum mass loss during melting was 0.8%. Micro-structural characterization of the as-cast alloys was car-ried out through scanning electron microscopy (SEM)in the back-scattered electron mode (BSE) and X-raydiffraction (XRD). For the SEM/BSE analysis the sam-ples were prepared through standard metallographicprocedures involving hot mounting, grinding (no.120= >  no. 1200 sand paper) and polishing with col-loidal silica suspension (OP-S). None of the sampleswere etched. SEM/BSE images were taken at 20 kV in aZEISS DSM-962 and LEO 1450 VP equipment. For theXRD experiments the samples were crushed to powderand sieved to below 177  m m. The XRD experimentswere carried out at room temperature, using Ni-filteredCu- K  a  radiation according to the following conditions:angular interval (2   ) from 10 to 90  ; angular step of 0.05  ; 2 s counting time. The phases were identifiedbased on the JCPDS data file [13]. 3. Results and discussion The compositions of the produced alloys are indicatedin the liquidus projection shown in Fig. 2. In this figure,e 1 , e 2 , p 1  and c 1  represent respectively, the followinginvariant reactions occurring in the Nb–B and Nb–Sisystems: L () Nb ss +NbB; L () Nb ss +Nb 3 Si;L+ a Nb 5 Si 3 () Nb 3 Si and L () b Nb 5 Si 3 . In the fol-lowing paragraphs the microstructural features of thesamples which allowed the establishment of this liquidusprojection are presented.Alloys nos. 1–6 presented primary Nb ss . Alloys nos.2–5 showed a eutectic-like Nb ss +T 2  precipitation in theinterdendritic region after primary Nb ss  solidification.This result suggested the solidification through aL(liquid)+Nb ss +T 2  three-phase field, where the liquid Fig. 1. Isothermal section of the Nb–Si–B system at 1600   C [12].252  D.M.P. Ju´nior et al./Intermetallics 11 (2003) 251–255  composition followed the II 1  –II 3  line in Fig. 2. The sig-nificant volume fraction of Nb ss +T 2  eutectic-likeregions in alloys nos. 2–4 indicated that line II 1  –II 3 should pass close to those compositions. Alloy No. 1presented Nb ss +NbB followed by Nb ss +T 2  eutectic-like precipitation after primary Nb ss  solidification. TheNb ss +NbB precipitation is associated to solidificationthrough the L+Nb ss +NbB three-phase field and it wasused to plot the e 1  –II 1  monovariant line which descendsin temperature towards the Nb–Si side. In the case of alloy no. 6 the Nb ss  primary solidification is followed bythe Nb ss +Nb 3 Si eutectic-like precipitation associated tosolidification through the L+Nb ss +Nb 3 Si three-phasefield and then by the Nb ss +T 2  eutectic-like precipita-tion. These results suggested the existence of the mono-variant e 2  –II 3  descending in temperature towards theNb–B side. As can be noted, the results presented bythese alloys suggest a minimum along the II 1  –II 3  line.Fig. 3a and b show SEM/BSE images of alloys nos. 2and 6, respectively.Alloys nos. 7 and 8 presented primary Nb 3 Si. Forboth alloys the primary solidification is followed byNb ss +Nb 3 Si (e 2  –II 3  line) and Nb ss +T 2  (II 3  –II 1  line)eutectic-like precipitation, in this order. These observa-tions are in agreement with those reported for alloy no.6. Fig. 3c shows a SEM/BSE image of alloy no. 7.Alloys nos. 9–23 presented primary T 2 . Eutectic-likemicrostructures formed by Nb ss  and T 2  were present inthe interdendritic region of all these alloys, indicatingthat Nb ss +T 2  precipitation followed primary T 2  solidi-fication. No further reaction seemed to take place, sug-gesting a minimum along the T 2  liquidus surface that Fig. 2. Liquidus projection of the Nb–Si–B system in the Nb-rich region. D.M.P. Ju´nior et al./Intermetallics 11 (2003) 251–255  253  Fig. 3. (a) SEM (back-scattered electron image) micrograph of alloy no. 2. XRD: Nb ss+ T 2 . (b) SEM (back-scattered electron image) micrograph of alloy no. 6. XRD: Nb ss+ Nb 3 Si. (c) SEM (back-scattered electron image) micrograph of alloy no. 7. XRD: Nb ss+ Nb 3 Si+T 2 . (d) SEM (back-scat-tered electron image) micrograph of alloy no. 18. XRD: Nb ss +T 2 . (e) SEM (back-scattered electron image) micrograph of alloy no. 13. XRD:Nb ss +T 2 . (f) SEM (back-scattered electron image) micrograph of alloy no. 27. XRD: Nb ss +NbB+T 2 . (g) SEM (back-scattered electron image)micrograph of alloy no. 30. XRD: Nb ss +T 2 +D8 8 . (h) SEM (back-scattered electron image) micrograph of alloy no. 35. XRD: Nb ss +T 2 .254  D.M.P. Ju´nior et al./Intermetallics 11 (2003) 251–255  supports the existence of a minimum along the II 1  –II 3 line. The largest volume of eutectic microstructure wasobserved in alloy no. 18, which is the closest to theII 1  –II 3  line. Fig. 3d and e shows SEM/BSE images of alloy nos. 18 and 13, respectively.Alloys nos. 24–28 presented primary NbB. In the caseof alloy no. 24 the primary solidification is followed bythe Nb ss +NbB (e 1  –II 1  line) eutectic-like precipitation.Alloys nos. 25–28 presented a peritectic-type precipita-tion of T 2  following primary NbB solidification, sincethe T 2 -phase involves the primary NbB grains. ANb ss +T 2  eutectic-like microstructure is observed in lastregion to solidify of all the alloys. These results togetherwith those of T 2 -primary were used to plot the mono-variant III 1  –II 1  line associated to the L+NbB+T 2 three-phase field which descends in temperature towardshigher Nb content. Fig. 3f  presents a SEM/BSE micro-graph of alloy no. 27.Alloys nos. 29–33 presented primary D8 8 . In all thesealloys the D8 8 -phase is involved by T 2 , suggesting aperitectic-like formation of the latter. In addition,Nb ss +T 2  precipitation is observed in the last regions tosolidify. These results together with those associated toT 2 -primary were used to plot the III 1  –III 2  line in Fig. 2.A minimum at the III 1  –III 2  line is consistent with thesolidification paths of these alloys as well as with theminimum along the T 2  liquidus surface. Fig. 3g presentsa SEM/BSE micrograph of alloy no. 30.The region of   b Nb 5 Si 3  (T 1 ) primary solidification inFig. 2 is tentative. Its location is based on the composi-tion range where this phase is primary in the Nb–Sibinary system (19.5–57 at.% Si) as well as in the pri-mary regions of Nb 3 Si and D8 8  as previously discussed.It is not possible to identify the T 1 -phase in the micro-structure of the alloys since this phase undergoes allo-tropic transformation to T 2  during solid state cooling.However, the primary precipitates in alloys nos. 34–36have been considered here to be T 1.  For these alloys, theobservation of Nb ss +T 2  eutectic-like microstructure inthe last regions to solidify and the absence of Nb 3 Sisuggest a peritetic-like transition from T 1  to T 2 . This is,again, in consonance with the minimum along the T 2 liquidus surface. Fig. 3h presents a SEM/BSE micro-graph of alloy no. 35. 4. Conclusions The microstructural analysis of arc-melted Nb–Si–Balloys have indicated the presence of the following pri-mary phases in the region from Nb to 60 at.%Nb: Nb ss ,NbB, T 2  ( a Nb 5 Si 3 ), Nb 3 Si, T 1  ( b Nb 5 Si 3 ) and D8 8 . Thelast solidification event in all the alloys involves aNb ss +T 2  eutectic-like precipitation. The followingternary invariant reactions are proposed to occur in theregion of study: II 1 : L+NbB () Nb ss +T 2 ; II 2 :L+T 1 () Nb 3 Si+ T 2 ; II 3 : L+Nb 3 Si () Nb ss +T 2 ;III 1 : L+NbB+D8 8 () T 2 ; III 2 : L+T 1 +D8 8 () T 2 . References [1] Nunes CA, Sakidja R, Perepezko JH. Second International Sympo-sium on Structural Intermetallics, EUA: Pennsylvania; 1997. p. 831.[2] Ward-Close CM, Minor R, Doorbar PJ. Intermetallics 1996;4:217.[3] Schneibel JH, Liu CT, Easton DS, Carmichael CA. Mater SciEng A 1999;261:78.[4] Summers E, Akinc M. J Am Ceram Soc 2000;83:1670.[5] Summers E, Thom AJ, Cook B, Akinc M. Intermetallics 2000;8:1169.[6] Nunes CA, Sakidja R, Dong Z, Perepezko JH. Intermetallics2000;4:327.[7] Kocherzhinskiy YA, Yupko LM, Shishkin EA. Russ Metall1980;1:184.[8] Schlesinger ME, Okamoto H, Gokhale AB, Abbaschian R.J Phase Equilibria 1993;14:502.[9] Borges Ju´nior LA, Coelho GC, Nunes CA. J Phase Equilibria[accepted].[10] Massalski TB, Subramanian PR, Okamoto H, Kacprzak L. Binaryalloy phase diagrams. 2nd ed. ASM International: Materials Park,OH, USA; 1990. vol. 1, p. 505.[11] Rudy E. Compendium of phase diagram data AFML-TR-65-2Part V Air Force Materials Lab., Wright Patterson AFB, Ohio,1969.[12] Nowotny H, Benesovsky F, Kieffer R. Z Metallkd 1960;50:417.[13] Powder diffraction files of inorganics phases: JCPDS; 1988. D.M.P. Ju´nior et al./Intermetallics 11 (2003) 251–255  255