Abstract
The literature on the processing, microstructure and properties of MoSi2 was briefly reviewed together with earlier work on the alloying behaviour of MoSi2 based alloys. Solidification microstructures and phase selection in ingots and melt-spun ribbons of binary, ternary and quaternary MoSi2 based alloys with additions X = W, Ta, Ti and Y = Al that substitute for Mo and Si respectively were studied. Low levels of interstitials (< 200wppm O2, < 20wppm H2) were achieved using clean melting based on cold hearth non-consumable tungsten arc melting. The following alloys were prepared: Mo-66.7Si, Mo-2.1W-66.7Si, Mo-2.1W-2.1Ta-66.7Si, Mo-2.1W-6.4Ta-66.7Si Mo-22.6Al-44.1Si, Mo-30Al-36.7Si and Mo-6Ti-30Al-36.7Si (at%). Evaluation of microstructure and properties was done using a number of experimental techniques which complement each other. These includedoptical microscopy, XRD, SEM, EPMA, TEM, DSC/TG, high temperature DTA, isothermal oxidation, T-o-F SIMS, room temperature hardness, microhardness and high temperature microhardness. The ingots of the alloys with Al or Al+Ti additions exhibited a smaller number and size of cracks and lower melt viscosity compared to the other alloys. Tungsten exhibited complete solid solubility in MoSi2. The C11b-MoSi2 structure was modified to the hexagonal C40 and/or orthorhombic C54 structures via Ta and Al or Al+Ti additions respectively. The C40 structure formed with Al addition exhibited two specific compositions with 20 at% and 24 at% Al depending on the extent of substitution of Si atoms in it. Rapid solidification refined the microstructures of the alloys by two orders of magnitude. Suppression of Mo5Si3 formation was also possible by rapid solidification. The C40 structure obtained by addition of 6.4 at% Ta in Mo-2.1W-6.4Ta-66.7Si was favoured by rapid solidification over the C11b structure. In the ribbons the solubility of Ta in (Mo,W)Si2 (C11b structure) was extended to 2 at%, while the solubility of Ti was extended up to 6 at% in Mo(Al,Si)2 (C40 structure). Segregation free microstructure with 24 at% and 30 at% Al were formed in the zone A (C40 structure) of the Mo-22.Al-44.1Al and Mo-30Al-36.7Si alloys. At low temperatures (T = 773 K) the PEST oxidation phenomenon was observed in alloys with W and/or Ta addition, while suppression of PEST was achieved in alloys with Al and Al+Ti additions where a mixed Al/Si oxide was formed with traces of Al/Mo oxide. In the former alloys a silica scale (2 to 7 μm thick) was formed at high temperatures (T > 1600 K) and there was no evidence of Mo5Si3 formation beneath the silica scale. In the latter alloys an alumina scale (10 to 20 μm thick) was formed at 1623 K, while at 1873 K the scale consisted of two oxide layers, the inner being alumina and the outer mullite and silica with dissolved alumina. Formation of Mo5Si3 beneath the alumina scale depended on the Al content of the alloy. Addition of W increased the microhardness of MoSi2 and the hardness of the alloys. Addition of Ta decreased the microhardness of MoSi2 while the hardness depended on Ta content and therefore on the volume fraction of the harder C40 phase. Addition of Al and Al+Ti decreased the hardness of MoSi2. Increase in the retained microhardness of MoSi2 up to 1273 K was observed with addition of Al (C40 structure).