X-ray diffraction study of structural and phase states of a superplastic Sn-38 wt % Pb alloy and their variations under the effect of external mechanical stresses and aging

Structural and phase states of the eutectic alloy Sn-38 wt % Pb, which exhibits superplastic properties at room temperature, have been studied using samples that were cast, forged in a hydraulic press to similar to 73%, aged in the as-cast state, as well as preliminarily forged samples and samples deformed in the regime of superplasticity. It has been shown that the phase state of the alloy in as-cast condition does not correspond to the equilibrium tin-lead phase diagram. The superplastic flow of the alloy occurs against the background of processes of decomposition of the supersaturated solid solution. In the preliminarily forged samples, the aging is accompanied by an increase in the concentration of lead in the surface layers. This is related, in particular, to the existence of tensile stresses in the tin-based phase and to their relaxation in the process of aging. In the structure of cast and preliminarily forged samples, an amorphous-crystalline component has been revealed, which most likely is located at grain boundaries. The experimental results agree well with the previously obtained results that are indicative of the nonequilibrium character of the initial structural phase state of the Sn-38 wt % Pb alloy and of the occurrence of structural-phase transformations under conditions of superplastic deformation, and substantially supplement them.