Molybdenum – Silicon – Tantalum

Abstract

Mo-Si-Ta alloys are interesting as possible materials for high temperature applications. The main reason for this is the high strength and high resistance to oxidation of the molybdenum silicides, especially MoSi2, at these temperatures [1956Bre, 1966Ver, 2001Wei1, 2006Kim]. However, MoSi2 has little plasticity at lower temperatures and its creep strength is poor. Alloying molybdenum silicides with other metals is one way of improving their properties, and tantalum is considered as one of the most useful alloying elements [1966Ver, 2006Kim]. [1956Kud] used X-ray diffraction to investigate the phases formed in MoSi2-TaSi2 alloys after annealing at 1300°C. The studies indicated that only the MoSi2 (C11b) and TaSi2 (C40) solid solutions were in equilibrium with each other. Solubilities of 60 mol% MoSi2 in TaSi2 and 15 mol% TaSi2 in MoSi2 were determined, and a eutectic reaction between them with a eutectic point at about 30 mol% TaSi2 was found. The existence of extended solid solutions of TaSi2 in MoSi2 and MoSi2 in TaSi2 at 1300°C had been reported earlier by [1955Now]. A significant solubility of MoSi2 in TaSi2 and TaSi2 in MoSi2 was confirmed later by [1966Ver]. The interaction between the compounds Mo5Si3 and Ta5Si3 was studied by [1956Now] by investigation of alloy samples sintered at 1900°C. Limited solubility of the compounds in each other was revealed with ~35 mol% Ta5Si3 dissolving in Mo5Si3 and ~35 mol% MoSi2 in TaSi2. [1992Boe] investigated experimentally the MoSi2-TaSi2 section revising the results of [1956Kud]. The MoSi2-TaSi2 section was recognized to be quasibinary. [1992Boe] established the invariant reaction in the system to be peritectic through observation of the classic peritectic microstructure visible in as-cast samples containing about 30 mol% TaSi2. [1956Kud] however, had suggested a eutectic invariant reaction in this section. TaSi2 crystals in the as-cast structure were seen to decompose during annealing with the formation of the MoSi2 precipitates suggesting a decrease in the MoSi2 solubility of TaSi2. [1992Boe] presented their version of the MoSi2-TaSi2 quasibinary phase diagram with revised extensions for the MoSi2 based and TaSi2 based solid solutions and tentative liquidus and solidus lines constructed by fitting the data of [1956Kud] on the liquidus surface of the MoSi2-TaSi2-TiSi2 quasiternary system. [1994Har] studied the microstructure of the MoSi2 based alloys that had been slowly cooled from 1200°C. The alloys contained up to ~10 at.% Ta instead of Mo and up to ~10 at.% Ta instead of Si. Extensions of the MoSi2 and TaSi2 solid solutions and the phase equilibria between them and other solid phases were determined for these conditions. [1994Har] constructed a partial isothermal section of the Mo-Si-Ta phase diagram at ~35-100 at.% Si assuming the formation of limited solid solution between the Mo5Si3 and Ta5Si3 compounds and some homogeneity with respect to Si. Taking into account that the alloy samples had been cooled slowly from 1200°C, it is not possible to ascribe an exact temperature to this section. This temperature can be estimated, however, to be about 1000°C. The assumptions of [1994Har] in relation to the limited solid solutions between Mo5Si3 and Ta5Si3 agree, in general, with the results of [1956Now], but the extension of the Ta5Si3 based solid solution indicated by [1994Har] differs significantly from that obtained by [1956Now]. [1999Tor] investigated the composition of the diffusion layers formed during interdiffusion in Ta-MoSi2 couples at 1500 and 1600°C. Analysis of the composition change across the diffusion layers indicated formation of the (Ta,Mo)5Si3 solid solution on the MoSi2 surface suggesting equilibrium between them. The crystal structure of the (Ta,Mo)5Si3 solid solution was not studied however. [2001Wei1] investigated the microstructure of alloys of the MoSi2-TaSi2 section by different methods. The microstructural investigation confirmed the existence of MoSi2 and TaSi2 phases in equilibrium with each other and a significantly larger extension of the TaSi2 solid solution phase field, as compared with extension of the MoSi2 solid solution phase field. The two-phase MoSi2+TaSi2 field at 1400°C turned out to be somewhat narrower, as compared with that determined by [1992Boe].