J. Japan Inst. Metals,Vol. 63, No. 5 (1999),
pp. 613-616

Effect of Niobium or Vanadium Addition on the Microstructure and Hardness of MoSi2-Mo5Si3 Eutectic Alloys

Toshihiko Fukui*, Shunkichi Ueno*, Ryohei Tanaka*, Seiji Miura** and Yoshinao Mishima***

* Japan Ultra-high Temperature Materials Research Institute, Ube 755-0001
** Division of Materials Science and Engineering, Hokkaido University, Sapporo 060-8628
*** Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502

Abstract:

Effects of Nb or V additions on microstructure and hardness of MoSi2/Mo5Si3 eutectic alloys are investigated for the improvement of room temperature toughness of multi-phase intermetallic alloys based on MoSi2. Alloys prepared by the argon arc melting process have compositions of 4-23 mol%Nb or 10-19 mol%V which substitutes Mo in MoSi2 and Mo5Si3 phases. Examinations are made on microstructural observation by back scattered electron image, X-ray diffraction for phase identification, the concentration of Nb or V in the constituent phases by EPMA, and Vickers hardness of the multi-phase alloys in both macro- and microscopic scales. The phase constitution changes from the two-phase, MoSi2(C11b) and (Mo, Nb)3Si5, to the three-phase, MoSi2(C11b)/(Mo, Nb)Si2(C40)/(Mo, Nb)5Si3, with increasing Nb additions. In the series of alloys with V additions, a similar shift from the two- to the three-phase is accompanied with increasing V addition. Macroscopic hardness of the multi-phase alloys with Nb addition exhibits a complex concentration dependence, in which with increasing Nb content, the hardness first decreases, exhibits a minimum and then increases back. The reason could be attributed to the fact that the hardnesses of the C40 and C11b phase have individual dependence on Nb concentration as revealed by micro-scale hardness measurement. For the improvement in toughness, Nb additions to the eutectic alloys may be preferred because the MoSi2/Mo5Si3 eutectic microstructure is fine as compared with that obtained by the V addition.


(Received November 19, 1998; In Final Form March 30, 1999)

Keywords:

molybdenum-silicide, eutectic materials, niobium additions, vanadium additions, microstructure, micro-hardness

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