METALLandOPHYSICS ADVANCED TECHNOLOGIES

The changes of strength properties of superplastic eutectic Bi-43% wt. Sn alloy, caused by repeated tension in the interval of microplastic deformation, are studied. Mechanical tests are carried out in the active loading mode. Relative elongations of the samples did not exceed 3 . 10 -3 . The experiments are carried out using the tensometric method of measurements at room temperature. The loading conditions are adapted to the conditions for the alloy to exhibit the superplasticity effect, which are established in separate experiments. The studied alloy is obtained in laboratory conditions by casting on a massive copper substrate. The ingots are compressed on a hydraulic press by approximate to 70% immediately after casting and then aged for 12 days. The dependence of the stress sigma on the relative elongation c indicates the weakening of the studied alloy as a result of repetitive microplastic deformation and subsequent exposure of the samples in the unloaded state. During exposure after unloading, inelastic compression of the samples is observed. This reveals the presence of internal stresses in the material, the value of which is sufficient for its plastic flow. It is assumed that the occurrence of such stresses is associated, in particular, with the presence of the supersaturated alpha (Sn)-phase in the structure of the alloy, which is formed under conditions of rapid crystallization. With the help of theoretical calculations, it is shown that the decomposition of this phase during the transition of the alloy to a phase state equilibrium at lower temperatures compared to the eutectic temperature is accompanied by an increase in the specific volume of the material. This transformation is prevented by the rigid framework of (3 (Bi)-phase grains, which occurs during crystallization. The action of external mechanical tensile stress causes the relaxation of internal stresses and the occurrence of the initial stages of decomposition of the supersaturated solid solution based on tin, accompanied by a decrease in the strength of the alloy.