INTERNAL STRESS SUPERPLASTICITY IN METALS AND METAL-MATRIX COMPOSITES

Superplasticity in polycrystalline metallic materials refers to their ability of achieving unusually high tensile deformation without failure. High strain rate sensitivity is a prerequisite for superplastic behavior. Internal stress superplasticity is associated with high internal stresses, which enhance plastic flow in materials. The high internal stresses arise either from phase transformations involving internal volume change from one phase to another phase, or from internal expansion mismatch during temperature change due to anisotropy of thermal expansion coefficients. This type of superplasticity has remained a scientific curiosity since it was first observed sixty years ago. In this investigation, the mechanical behavior of materials is evaluated as influenced by internal expansion mismatch. The experimental method used is thermal cycling with a concurrent external stress. A pure metal (zinc) as well as several metal-matrix composites were studied. The composites investigated include two particulate composites (zinc plus aluminum oxide particles) and one fiber-reinforced composite (aluminum plus silicon carbide whiskers). It has been shown that the composites behave superplastically under thermal cycling. To the best of our knowledge, this is the first successful attempt in making metal-matrix composites superplastic. High strain rate sensitivity is observed, typically equal to unity at low stresses. A proposed phenomenological equation is used to describe the behavior of the materials under thermal cycling. The equation contains a contribution of internal stress to the applied stress. The equation can describe the experimental data remarkably well. A further extension of the concept of internal stress contribution to plastic flow has been made to unify the creep predictions of Harper-Dorn and power law creep.