Yasumasa Koyama1, Toshiyasu Ukena1 and Osamu Nittono2
1Graduate School, Tokyo Institute of Technology, Tokyo
The α1(fct, c/a>1)\rightleftarrowsα2(fct, c/a<1) and α2(fct, c/a<1)\rightleftarrowsβ(fcc) phase transformations and the shape memory effect in indium-rich lead alloys have been studied by means of X-ray diffractometry supplemented by metallographic observations. The alloys containing 12∼15 at% lead transform from the α2(fct) phase to the α1(fct) phase by way of an intermediate phase (m phase) on cooling. The results of X-ray diffraction show that the metastable intermediate phase is observed both on cooling and heating, and has a face-centered orthorhombic (fco) structure. It is concluded that the α1\rightleftarrowsα2 transformation is expressed by the α1\rightleftharpoonsm\rightleftharpoonsα2 transformation both on usual cooling and heating with the rate more than 8×10-3 K/s. The alloys containing 30∼36 at% lead transform from the β(fcc) phase to the α2(fct) phase on cooling. Surfaces of these m- and α2-phase alloys show a banded structure (surface relief) due to 110 transformation twinning, and the banded structure disappears above the transformation temperature. The m\rightleftharpoonsα2 and α2\rightleftharpoonsβ transformations take place with a mechanism involving macroscopic shear and are of diffusionless (martensitic) type. The temperature hysteresis in the two transformations is 10∼13 K between the heating and cooling transformations. The mechanism of these phase transformations is discussed in terms of the soft phonon mode, considering the double shear mechanism. A remarkable shape memory effect is observed in the reverse transformation on heating. The features of the shape memory behavior are presented and the mechanism of the shape memory effect is also discussed on the basis of the crystallographic reversibility of these transformations.
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