The lattice dynamics in the IV-VI compounds GeTe, SnTe and PbTe were studied by 125 Te and 119 Sn nuclear inelastic scattering and the obtained partial density of phonon states were compared with published theoretical calculations. The phase purity and structure were characterized by high energy X-ray diffraction. The effect of the atomic arrangement, rhombo-hedral for GeTe and cubic for SnTe and PbTe, is visible in the density of phonon states. Vibrational properties are found to be in good agreement with available calculated data and the softer character of the NaCl-type structures in comparison with the rhombohedral GeTe is confirmed.

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Abstract

As-grown Co 49 Ni 21 Ga 30 [001]-and [123]-oriented single crystals were subjected to cyclic compression loading at room temperature above the austenite finish temperature of 15°C. Straincontrolled experiments were performed using both incremental strain steps and constant strain amplitudes. Cyclic deformation with a maximum strain amplitude of 2.5 pct resulted in rapid accumulation of irrecoverable strains in the [123]-oriented crystals. However, after a few cycles, the samples demonstrated cyclic stability with fully recoverable transformation. By contrast, the [001]-oriented crystals displayed excellent cyclic stability with hardly any change in stress-strain characteristics. In-situ optical microscopy and electron backscattered diffraction analysis were employed to clarify the events that take place at different stages of a typical loading-unloading history. The in-situ observations also revealed that the initiation and growth characteristics of stress-induced martensite (SIM) are heterogeneous on the microscopic scale in CoNiGa alloys. In addition, theoretical transformation and detwinning strains, and resolved shear stress factors (RSSFs), were calculated based on the energy minimization theory and are compared to the experimentally obtained orientation-dependent transformation stress and strain levels. It is shown that the selection of an appropriate orientation is one of the key criteria to optimize the pseudoelastic (PE) response and cyclic stability of CoNiGa alloys.