Effect of third elements on the pesting suppression of Mo-Si-X intermetallics (X = Al, Ta, Ti, Zr and Y)

Abstract

The pesting behaviour of Mo-Si-X (X = Al, Ta, Ti, Zr and Y) intermetallics produced by arc-melting has been examined in air at temperatures between 673 and 973 K. MoSi2 disintegrates severely at 773 K. The selective oxidation of Si to SiO2 should occur at the grain boundaries of MoSi2 because the affinity to oxygen of Mo is much smaller than that of Si. The volume expansion accom- panying selective oxidation of Si is calculated to be about +85.6%. This significant volume expansion generates a large internal stress at the grain boundaries and results in disintegration of MoSi2. The volume change (4.9 vol%) in the oxidation of Mo(Si,Al)2 is small, and the pesting of Mo(Si,Al)2 is suppressed. In the case of third elements whose affinities to oxygen are larger than that of Si (i.e. Al, Ti, Zr and Y), the third elements oxidize selectively at the grain boundaries. When the volume expansion accompanying the internal oxidation is small, the pesting phenomenon is suppressed.