Y.I. Chumlyakov

Energy harvesting using martensite variant reorientation mechanism in a NiMnGa magnetic shape memory alloy

Magnetic shape memory alloys demonstrate significant potential for harvesting waste mechanical energy utilizing the Villari effect. In this study, a few milliwatts of power output are achieved taking advantage of martensite variant reorientation mechanism in Ni51.1Mn24Ga24.9 single crystals under slowly fluctuating loads (10Hz) without optimization in the power conversion unit. Effects of applied strain range, bias magnetic field, and loading frequency on the voltage output are revealed. Anticipated power outputs under moderate frequencies are predicted showing that the power outputs higher than 1W are feasible.

Stress-induced martensite to austenite phase transformation in Ni2MnGa magnetic shape memory alloys

The effects of temperature on the stress-induced variant reorientation process and the possibility to trigger stress-induced martensite to austenite phase transformation in Ni2MnGa single crystals were investigated under compression. It is revealed that, as the temperature decreases, both the critical stress (under 16?kG magnetic field) and the critical magnetic field levels (under ?1?MPa) for martensite variant reorientation increase. Magnetic-field-induced strain is also found to increase with temperature. Near the austenite start temperature, it is possible to attain stress-induced martensite to austenite transformation under constant magnetic field. A thermodynamical guideline is introduced to explain the conditions for ?stress-induced martensite to austenite transformation? in Ni2MnGa alloys. It is concluded that, when magnetocrystalline anisotropy energy is large enough, stress-induced martensite to austenite transformation can be achieved within a narrow temperature range below austenite start temperature.

Shape memory behavior in Fe3Al-modeling and experiments

The Fe3Al alloy with D03 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe3Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE – generalized stacking fault energy) for {1 1 0}〈1 1 1〉 and {1 1 2}〈1 1 1〉 slips in D03 Fe3Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement.

Temperature dependence of mechanical properties in [1¯11]-oriented single crystals of CoCrFeNiAl0.3 high entropy alloy

Some results concerning flow curves, the strain hardening coefficient, plasticity, mechanisms of deformation and fracture in the temperature range T = 77–573 K in tension in [1¯11]-oriented single crystals of fcc CoCrFeNiAl0.3 (atom percent) high entropy alloy are presented. It is shown that in [1¯11]-oriented single crystals after the strain 5% a planar dislocation structure develops in the temperature range T = 77–423 K. As the test temperature decreases from 573 to 77 K, the yield stress σ0.1 increases 3.6 times, the fracture stresses σfr grows 2.3 times, and the strain hardening coefficient increases from 1150 to 1800 MPa. Moreover, [1¯11]-oriented single crystals demonstrate good ductility and ductile fracture. The physical reason of good ductility and ductile fracture is associated with the development of a planar structure that shifts the conditions of neck formation to the region of high strains and stresses.

Microstructure and shape memory behavior of [111]-oriented NiTiHfPd alloys

The relationship between the microstructure and shape memory properties of [111]-oriented Ni45.3Ti29.7Hf20Pd5 (at%) single crystals was explored. In this precipitation-strengthened alloy, the size and volume fraction of precipitates and interparticle distances govern the martensite morphology and the ensuing shape memory responses. Aging of the solution-treated material, leading to a microstructure of fine, closely spaced precipitates, resulted in a material capable of a shape memory strain of 2.15% at 1000 MPa in compression. Larger precipitates formed after aging the as-grown single crystals (without a prior solution treatment) resulting in a shape memory strain of 2.5% at this same stress level in constant-stress thermal cycling experiments. Superelastic strains of 4% in compression without any residual strain were possible under various microstructural conditions and the stress hysteresis could be varied between nearly 500 and 1000 MPa depending on the microstructure.