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

A model utilizing a simple force-equilibrium approach was developed to esta blish the effect of the cavity nucleation rate and cavity coalescence on th e uniaxial tensile behavior of superplastic metals. All cavities were assum ed to be spherical and uniformly distributed within the material, irrespect ive of the degree of deformation, Material input parameters for the model c omprised the cavity nucleation rate (N), the strain-rate sensitivity of the flow stress (m), and the growth parameter for individual cavities (eta), w hich was taken to be a function of rn. The effect of cavity coalescence on average void size and volume fraction was treated using an empirical relati on, which correlates an average void growth rate to the growth rate of indi vidual, noninteracting cavities. Model predictions indicated that the macro scopic quantities often used to describe cavitation behavior, i.e,, "initia l cavity volume fraction" (C-nu 0) and "apparent cavity growth rate"(eta(AP P)) describe the combined influence of cavity nucleation, growth, and coale scence, With regard to the overall tensile behavior, simulation results rev ealed that increasing cavity nucleation rates reduce ductility in a manner analogous to the effect of decreases in the strain-rate sensitivity. In add ition, the failure mode was established with regard to the relative magnitu des of the cavity nucleation rate and the strain-rate sensitivity. Model pr edictions of tensile elongation and cavity-size distributions were validate d by comparison to measurements found in the literature for cavitating supe rplastic materials.