Deformation mechanisms of polycrystalline MoSi2 alloyed with 1 at.% Nb

Molybdenum disilicide (MoSi2) is a high Peierls stress material with a strong temperature dependence of the yield stress at low homologous temperatures. We have investigated the effects of alloying with 1 at.% Nb on the mechanisms of deformation of polycrystalline MoSi2 by compression testing from room temperature to 1600 deg. C. While polycrystalline unalloyed MoSi2 fractured before plastic yield at temperatures ≤900 deg. C, the Nb-alloyed MoSi2 could be deformed in compression even at room temperature. Transmission electron microscopy (TEM) investigations indicated that the presence of 1 at.% Nb increased the stacking fault width of 1/2<111> dislocations and promoted 1/2<111> slip at low temperatures. The solution softening is interpreted in terms of easier kink nucleation on partial dislocations (especially in the vicinity of solute atoms) with increasing partial spacing. With increasing deformation temperature, thermal activation makes kink nucleation easy, and presumably, kink migration is hindered by the solute atoms leading to the expected solid solution hardening at elevated temperatures. At very high temperatures (∼1600 deg. C), the yield strengths of Nb-containing alloys may be up to an order of magnitude higher than pure MoSi2; the related hardening mechanisms are discussed