6. Time Dependent Deformation—Creep in Silicides

Andrade was the first to formulate creep phenomena indicating its stress, time and temperature dependence. Basic conditions for β and κ creeps are presented. Disregarding instant elongation, creep is known to occur at three stages: transient creep, steady state creep and accelerated creep. Limited information on creep in silicides exist and therefore only MoSi₂, MoSi₂-WSi₂ and TiSi₂ are considered. Grain size has an important effect on the creep behavior and in polycrystals large grain size is essential, but single crystalline components are preferential. Basic relations for creep are presented for the creep rate showing the importance of d the grain size, p the grain size exponent and n the stress exponent. There is a transition from Newtonian viscous flow which is self diffusion dependent to a power-law creep associated with dislocation climb. Dislocation creep involves glide and climb and both are associated with diffusion and the slowest of them controls the creep rate. A threshold stress and temperature exist below which creep will not occur.