An analysis of the effect of cavity nucleation rate and cavity coalescence on the tensile behavior of superplastic materials

A model utilizing a simple force-equilibrium approach was developed to establish the effect of the cavity nucleation rate and cavity coalescence on the uniaxial tensile behavior of superplastic metals. All cavities were assumed to be spherical and uniformly distributed within the material, irrespective of the degree of deformation. Material input parameters for the model comprised the cavity nucleation rate (N), the strain-rate sensitivity of the flow stress (m), and the growth parameter for individual cavities ( eta ), which was taken to be a function of m. The effect of cavity coalescence on average void size and volume fraction was treated using an empirical relation, which correlates an average void growth rate to the growth rate of individual, noninteracting cavities. Model predictions indicated that the macroscopic quantities often used to describe cavitation behavior, i.e., "initial cavity volume fraction" (C sub v0 ) and "apparent cavity growth rate" ( eta sub APP ) describe the combined influence of cavity nucleation, growth, and coalescence. With regard to the overall tensile behavior, simulation results revealed that increasing cavity nucleation rates reduce ductility in a manner analogous to the effect of decreases in the strain-rate sensitivity. In addition, the failure mode was established with regard to the relative magnitudes of the cavity nucleation rate and the strain-rate sensitivity. Model predictions of tensile elongation and cavity-size distributions were validated by comparison to measurements found in the literature for cavitating superplastic materials.