Molybdenum – Silicon – Tungsten

Abstract

The silicides of Mo, especially MoSi2, are of great importance as materials for high temperature applications. They are characterized by a combination of high temperature strength, moderate density (6.23 g@cm–3) and excellent oxidation resistance in environments containing oxygen. However, there are some shortcomings in the use of these silicides for practical applications, for example, insufficient impact strength at ambient temperature. Alloying the molybdenum silicides is one of the ways of improving their refractory properties, and tungsten is considered as one such useful alloying element [1952Now, 1966Ver, 2003Gne, 2004Zha]. Molybdenum and tungsten form a continuous series of solid solutions [Mas2]. In the Mo-Si and Si-W binary systems, there are compounds with similar formulae and crystal structures, i.e. MoSi2 (low-temperature modification) and WSi2, and Mo5Si3 and W5Si3 [Mas2]. Therefore, continuous and much extended solid solutions between these pairs of compounds can be anticipated. Experimental investigations have confirmed this. [1952Now] observed continuous solid solution between the tetragonal (low-temperature) MoSi2 and WSi2 through X-ray diffraction study of MoSi2-WSi2 alloys that had been obtained by sintering the disilicide powders at 1400-1500°C followed by annealing at 1800°C. [1955Now] reported the existence at 1300°C of a continuous solid solution between MoSi2 and WSi2 with a tetragonal crystal structure. [1994Har] confirmed this observation experimentally by studying samples that had been cooled slowly from 1200°C. The continuous MoSi2-WSi2 solid solution was confirmed also by [2003Gne]. The existence of a continuous solid solution between Mo5Si3 and W5Si3 was reported by [1962Kie, 2003Gne]. In [2003Gne], this conclusion was the result of optical microscopy observations, SEM analysis and X-ray diffraction investigations of alloys that had been prepared by melting in a graphite resistance furnace under a pure argon atmosphere. The complete mutual solid solubility of Mo5Si3 and W5Si3 was recently confirmed by [2005Hay1, 2005Hay2]. Some experiments were conducted in an attempt to establish equilibria involving Mo3Si and the supposed W3Si with other phases [1956Bre, 1962Kie]. According to [1956Bre], Mo3Si in the presence of W was found without any apparent change in lattice parameters, suggesting insignificant solubility of W in Mo3Si. [1962Kie] revealed no solubility of the supposed W3Si compound in Mo3Si at 1900°C. On the other hand, equilibrium at 1900°C was found by [1962Kie] to exist between Mo3Si and W. According to [1962Kie], there were also no signs of the formation of a W rich (Mo,W)3Si solid solution. By taking into account the (Mo)-(W), Mo5Si3-W5Si3 and MoSi2-WSi2 solid solutions and the phase relations involving the Mo3Si compound, [1962Kie] constructed an isothermal section for the Mo-Si-W system at 1900°C. According to this section, the Mo3Si compound is in equilibrium with the (Mo)-(W) and Mo5Si3-W5Si3 solid solutions enriched by Mo, and the W3Si compound is absent. [1994Har] constructed a partial isothermal section of the Mo-Si-W phase diagram through observation of alloys cooled slowly from 1200°C. The section was limited to the triangle MoSi2-Si-WSi2. It presented the equilibria between pure (Si) and the MoSi2-WSi2 continuous solid solution as determined in this work. [2003Gne] presented hypothetical equilibria for part of the Mo-Si-W phase diagram in the concentration limits from Mo5Si3-W5Si3 to MoSi2-WSi2. The diagram shows two quasibinary sections, Mo5Si3-W5Si3 and MoSi2-WSi2, and a monovariant eutectic line is drawn between them connecting the eutectic points of the Mo5Si3-MoSi2 and W5Si3-WSi2 quasibinary systems. [2003Gne] also determined experimentally the liquidus temperatures along the vertical section between the points of the binary eutectics Mo5Si3-MoSi2 and W5Si3-WSi2, recognizing that the monovariant eutectic line in the ternary system could deviate from the vertical section.