In-Sn合金の相変化と形状記憶効果

Abstract

The structural change with increasing tin content and the shape memory effect in indium rich-(0∼20) at% tin alloy crystals have been studied by means of X-ray diffractometry and electron microscopy supplemented by metallographic methods. The alloys containing 0∼11 at% tin are α-phase solid solutions with a f.c. tetragonal structure (c⁄a>1) at room temperature, the axial ratio increasing continuously with tin content. The indium-(11∼15) at% tin alloys are mixtures of α and β phases, the β phase having a f.c. tetragonal structure (c⁄a<1). The alloys containing more than 15 at% tin are β-phase solid solutions. Etched surfaces of these tetragonal alloys do not reveal any banded structure. The indium-(12.9∼15.0) at% tin alloys show a shape memory effect only when quenched to the temperature of liquid nitrogen, although their effect becomes weak and finally disappears after keeping at room temperature for a long time. The structure of quenched alloys (α′ phase) is quite similar to that of the α phase. The quenched alloys show a banded structure due to transformation twinning parallel to {101} planes. The β→α′ phase transformation is of the diffusionless (martensitic) type, and takes place between 330 K at 12.9 at% tin and 150 K at 14.5 at% tin. The hysteresis of transformation temperatures on heating and cooling is considerably large (29∼40 K), depending on the composition. The features of the shape memory behavior are presented and compared with those of indium-thallium and indium-cadmium alloys which show a similar shape memory, effect, and the mechanism of the shape memory effect are also discussed briefly.