Strong monolithic and composite MoSi2 materials by nanostructure design

The nanocomposite concept is briefly reviewed at first, and the approach to fabricating strong and tough MoSi2-based materials through the nanocomposite technology is demonstrated. Strong MoSi2-based nanocomposites such as MoSi2/SiC and MoSi2/Mo(Si,Al)(2)/SiC were developed by conventional powder metallurgical techniques. Effects of grain boundary and interface structure on mechanical properties were investigated for these composites. In the MoSi2/SiC nanocomposites, both MoSi2 matrix and siliceous grain boundary phases were simultaneously controlled, nano-sized SiC particles (50-200 nm) were dispersed into both MoSi2 matrix grains and glassy SiO2 located at triple junctions. High-resolution TEM observation revealed that the intragranular SIC particles directly bonded to MoSi2 without any impurity phases, resulting in toughening by the thermal expansion mismatch between MoSi2 and SiC. The intergranular SiC particles played important roles to inhibit the grain growth of MoSi2 matrix, and they concurrently toughened the glassy SiO2. For Al-alloyed systems, alpha-Al2O3 was dispersed in the compacts instead of glassy SiO2. The matrices became di-phasic, that is, mixture of tetragonal MoSi2 and hexagonal Mo(Si,Al)(2). SiC dispersion in the Mo-Si-Al alloys effectively improved their mechanical properties. By means of such material designs, all composites showed very high strength (more than 1 GPa by the three-point beading test). In particular, MoSi2/Mo(Si,Al)(2)/SiC composites possessed the improved high-temperature strength of about 1 GPa at 1100 degrees C. (C) 1999 Published by Elsevier Science S.A. All rights reserved.