Stress-state dependence of cavitation and flow behavior in superplastic aluminum alloys

A detailed and quantitative investigation of the stress-state dependence of superplastic cavitation in fine-grained aluminum alloys has been carried out to develop clear evidentiary support to build future models. Several stress states, such as uniaxial tension, plane-strain tension, plane-strain compression, shear, and equibiaxial tension have been examined. Tests were carried out to large strain in an interrupted manner under a constant effective strain rate (ε_e) in the range of 10^-4 to 10^-2 s^-1. Measurements of volume fraction, population density, and size distribution of cavities, made by image analysis via optical microscopy, show continuous emergence of new cavities as well as growth of cavities during superplastic straining. The total cavity volume fraction (V) increases exponentially with strain. The cavity growth rate, represented by η (equal to d ln V/ dε_e), as well as the cavity population evolution rate with strain (dN_c/dε_e, where N_c is the cavity number/unit area) are found to increase with normalized mean hydrostatic tensile stress (σ_m/σ_e). An empirical equation for the biaxial forming limit in terms of the principal surface strains (ε₁ and ε₂) has been defined for a fixed cavity volume, as given by ε₁ = a V^b - α ε₂, where a and b are constants determined from ε₁ values for plane strain (ε₂ = 0). The value of b is found to be 0.2 to 0.3, and α is 0.4 to 1.0.