Abstract
The martensitic transformations in In-(18∼30 at%)Tl and In-(4.0∼5.5 at%)Cd alloys (fcc→fct:c⁄a>1), In-(28∼35 at%)Pb alloys (fcc→fct:c⁄a<1), In-(12∼16 at%)Pb alloys (fct:c⁄a<1→fco→fct:c⁄a>1) and In-(13∼15 at%)Sn alloys (fct:c⁄a<1→fct:c⁄a>1) have been studied by means of X-ray and electron diffraction. Intensive thermal diffuse scattering is observed on the {110} reciprocal lattice planes in all the indium-rich solid solutions. The mechanism of the phase transformations is discussed on the basis of the observed diffuse scattering due to the lattice vibration. An octahedral cell model composed of ⟨110⟩ atomic chains is useful to explain the atomic movements of the phase transformations. The displacement mode of atoms, which is expressed in terms of linear combination of the strain tensor components, corresponds to the basis belonging to the two-dimensional represnetation Eg of the point group Oh. Actual atomic movements of the phase transformations in indium-rich solid solutions can be given as a result of coupling of two predominant phonons, on the assumption that one phonon mode corresponds to one cooperative atomic movement. The shape memory effect, which is associated with a reverse transformation from a low temperature phase to a high temperature one, is well explained in terms of crystallographical reversibility between the two phases. It is shown that the crystallographical reversibility is related with the change in crystal symmetry during the phase transformation.
