Reinforcement of metallic matrix with second phase ceramic particles can lead to significant increase in creep strength. Some of these composites also exhibit high strain rate superplasticity. The diffusional relaxation of stresses during creep and superplastic deformation is examined. Below a critical creep rate intergranular fracture is observed, whereas above the critical creep rate transgranular fracture is observed. This transition in creep fracture behavior can not be explained on the basis of diffusional relaxation rate. However, these observations can be explained on the basis of metal/ceramic interface decohesion. On the other hand, the change in high strain rate superplasticity mechanism with the size of reinforcement phase can be explained using the diffusional relaxation models.
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