E. E. Timofeeva

Peculiarities of high-temperature superelasticity in Ni–Fe–Ga single crystals in compression

The high-temperature superelasticity and temperature dependence of the yield stress of 14M and L10 martensite in [001]-oriented single crystals of Ni54Fe19Ga27 (at %) in compression alloy have been studied. As the temperature increases, the sequence of stress-induced martensitic transformations (MTs) changes from L21–14M to L21–L10. The yield stress of L10 martensite weakly depends on the temperature and is 1.7 times lower than that of 14M martensite. The temperature interval of superelasticity in [001]-oriented single crystals of Ni54Fe19Ga27 under compression is determined by the growth of critical stresses with increasing temperature, the coefficient of strain-hardening, and the yield stress of L10 martensite.

Stress-Induced Thermoelastic Martensitic Transformations and Functional Properties in [011]-oriented NiTiHfPd Single Crystals

The stress-induced martensitic transformation in the [011]-oriented Ni45.3Ti29.7Hf20Pd5 (at. %) single crystals in as-grown, homogenized and aged states were investigated in compression. It is experimentally shown that heat treatments of single crystals result in increase in martensitic transformation temperatures, two-fold decrease in reversible strain and increase in strain-hardening coefficient. As-grown single crystals demonstrate large temperature range of superelasticity (up to 140 K), large reversible strain (up to 4.3%) and large work output in comparison with homogenized and aged crystals.

Thermoelastic martensitic transformations in [011]-oriented Ni49Fe18Ga27Co6 single crystals under tension and compression loads

The effect of the stress state (tension/compression) on the thermoelastic L21-(10M,14M)-L10 martensitic transformations during heating/cooling cycles under constant tensile and compressive applied stress in [011]-oriented Ni49Fe18Ga27Co6 single crystals was determined. The martensitic transformation in cooling/heating occurs with a maximum reversible strain of 4.1% at a constant tensile stress of 50 MPa, and the value of the thermal hysteresis is independent of applied tensile stress. On the contrary, the value of thermal hysteresis increases by a factor of 4 with the growth of the external compressive stress level. The reversible strain 5.5% during cooling/heating cycles and a bursting character of reverse martensitic transformation at the compressive stress 60 MPa are observed. Such asymmetry of the properties in [011]-oriented Ni49Fe18Ga27Co6 single crystals at martensitic transformations is caused by the detwinning processes of L10-martensite crystals under compressive applied stress.

The Effect of Heat Treatment on the Shape Memory Effect and Superelasticity in NiMnGa Single Crystals

It was shown in Ni53Mn25Ga22 single crystals that the annealing at 1273 K for 1 h followed by slow cooling, at which the (L21+γ)-structure is formed, provides the increase of crystals plasticity as compared with the brittle initial L21-single crystals. The increase of critical stresses by more than 2 times (up to σ > 150 MPa), at which the cooling/heating cycles under stress are accompanied by sample destruction, is observed. Moreover, the value of thermal hysteresis is increased by 1.5 times and the perfect curve of superelasticity is observed at T=423 K.