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.
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.
The mechanism of thermally driven shape recovery from a single variant of 2Hmartensite to the parent austenitic phase is experimentally studied on a specimen of Cu-Al-Ni shape memory alloy (SMA). The formation and motion of the martensite-to-austenite transient interfaces is controlled by a thermal gradient, and recorded by a CCD camera. Independently, the moving boundaries are observed by an infrared camera to capture the temperature evolution accompanying the propagation. Both the velocity profiles of the propagation and the thermal images indicate that the shape recovery of SMAs is a complex dynamic mechanism, which cannot be described by a classical Stefan's model of phase transitions, known from the thermal conductivity problem.
Resonant ultrasound spectroscopy was applied to analyze the temperature evolution of shear modulus of thick aluminum, nickel, copper and titanium coatings prepared by cold spraying in the temperature range 20 • C→560 • C→20 • C. The results show that the thermal cycle leads to a significant increase in the shear modulus of nickel, while for copper, titanium and aluminum the shear modulus remains nearly unchanged or exhibits a weak increase only. Evolution of the internal friction parameter shows a significant decrease after the thermal cycle, being the most pronounced for nickel again. The decrease of the internal friction observed in the studied coatings indicates grain coarsening and consequent lower influence of grain boundary sliding.
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