On the ductile-to-brittle transition temperature in MoSi2

Abstract

This paper reports that the intermetallic compound MoSi[sub 2] combines excellent oxidation resistance, a high melting point (2020[degrees]C), reasonable strength and density, and good thermal conductivity, making it a promising material for structural applications above 1200[degrees]C. The ductile-to-brittle transition temperature of MoSi[sub 2] is generally believed to be about 1000[degrees]C, above which the material is reported to deform plastically. However, according to some recent four-point flexural test results, the transition temperature could be much higher, in the range 1300 to 1400[degrees]C. The tensile properties of hot pressed MoSi[sub 2] with two different grain sizes and a density of 94% of theoretical has been reported as a function of temperature. Both materials contained large amounts of a second phase, presumably silica. For the small-grained material, plastic elongations were undetectable at room temperature and 1090[degrees]C, 0.1% at 1204[degrees]C, and 0.5% at 1316[degrees]C. For the larger grained material, no plastic elongation was measured up to the maximum test temperature of 1316[degrees]C. Transmission electron microscopy of deformed single crystals and polycrystal samples (containing SiC whiskers) [11] reveals significant dislocation activity at elevated temperatures. Although aspects of the deformation substructure of MoSi[sub 2] is an area of active research, and some controversy, some of themore » results suggest increased plasticity at temperatures above 1300[degrees]C. UmaKoshi and co-workers [9,10] report that slip on is activated at low temperatures for single crystals deformed in compression. At 1300[degrees]C, they report a sharp increase in compressive ductility and and slip. Further they observe that the climb and the mobility of these and dislocations are very high.« less