CONSTITUTIVE EQUATION OF A SUPERPLASTIC AL-ZN-MG COMPOSITE REINFORCED WITH SI3N4 WHISKER

A constitutive equation of a superplastic Al-Zn-Mg alloy composite reinforced with 20 vol.%Si3N4 whisker has been analyzed based on the mechanical properties tested by a constant strain rate tension test mainly at strain rates between 10(-4) and 10(2) s(-1) in a temperature range from 798 to 833 K, which includes above and below the partial melting point (828 K) of the composite measured by a differential scanning calorimeter (DSC). After incorporation of the temperature dependence of threshold stress, shear modulus, and grain size, the activation energy for superplastic flow in a temperature range from 798 to 818 K is 148 kJ mol(-1), which is similar to or slightly above that for lattice self-diffusion in aluminum (=142 kJ mol(-1)). This suggests that the dominant process of superplastic deformation in the composite in solid state is gain boundary sliding accommodated by dislocation motion controlled by lattice diffusion. On the other hand, the activation energy for superplastic how above the partial melting point is much higher than that for lattice self-diffusion in aluminum, indicating that the deformation mechanisms of the composite in liquid and solid state are clearly different from those in solid state only.