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Resonant ultrasound spectroscopy measurements and density functional theory calculations were used to analyze the effect of Co and Cu doping (3-6 at. %) on elastic constants of non-modulated martensite of the Ni-Mn-Ga ferromagnetic shape memory alloy. Due to the doping, the studied alloys exhibited decreased tetragonal ratios c/a≈1.14. Both the experiments and the calculations revealed that the lowering of the c/a ratio resulted in a change of the orientation of the softest shearing modes of the tetragonal lattice. The newly appearing softest shearing modes for the doped materials have approximatelyorientations and indicate a lattice instability directly related to the particularly low twinning stress for compound á ñ 101 101 { } ¯twins, needed for the magnetically induced reorientation.

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In the presented paper, a sample of polycrystalline shape-memory NiTi alloy is studied under compression up to 5% by the means of laser-excited and laser-detected ultrasound waves. The evolution of a propagation velocity of the surface acoustic wave is measured in situ during mechanical loading. An inverse method based on the Ritz-Rayleigh numerical approach is then used to obtain the development of elastic properties of the sample. This process enables an analysis of the evolution of stress-induced transformation from the austenitic to the martensitic phase with the possibility to describe several stages of such transformation, i.e., the transformation to full R-phase, its reorientation causing strong anisotropy of the polycrystalline sample, and consecutive gradual transition to martensite.