This dissertation studies the deformation behavior of high temperature alloys with an aim to understand creep damage and fracture mechanics of these materials. First, we study the creep fatigue deformation of a unified viscoplastic material subjected to uniaxial cyclic loading using a dynamical system approach. We find oscillation of back stress significantly increases the inelastic strain accumulation in a cyclic test. The accumulated inelastic strain at long times are sensitive to the initial condition (e.g. whether one starts with tension or compression). We define a ratcheting ratio to quantify the interaction of creep and cyclic plasticity on the accumulated inelastic strain per cycle. The second part of the dissertation focuses on solving the asymptotic stress and strain field near the tip of a plane strain Mode I stationary crack in a viscoplastic material. For small scale creep where the region of inelasticity is small in comparison with typical specimen dimensions, our asymptotic and finite element analysis show that the near tip stress field has the same singularity as elastic power law creeping materials with a time dependent amplitude. This amplitude is found to vanish at long times and the elastic K field dominates. For the case of cyclic loading, we study the effect of stress ratio on inelastic strain and find that the strain accumulated per cycle decreases with stress ratio. iii The third part of the dissertation carries out finite element simulations on the planar deformation of random sized power law creeping grains with sliding and cavitating boundaries. Grain boundary sliding and grain boundary separation due to cavity nucleation and growth are incorporated into a cohesive zone model. Finite element simulation of a relaxation test shows that more grain boundary separation occurs in a microstructure with sliding resistant grain boundaries than in a microstructure with more freely sliding grain boundaries. The overall inelastic strain rate of the microstructure in uniaxial tension test is found to be greatly enhanced by grain boundary sliding and grain boundary cavitation. Finally, we extend the cohesive zone model in the third part of the dissertation to account for interface embrittlement caused by grain boundary impurities. Finite element simulation of an uniaxial creep test using a two dimensional random grain structure shows that grain boundary cavitation and interface embrittlement are two competing mechanisms for grain boundary separation. The occurrence of one grain boundary separation mode would slow down or even inhibit the other. iv