Elena Panchenko

Tension - Compression Asymmetry in Co49Ni21Ga30 High-Temperature Shape Memory Alloy Single Crystals

This paper reports on the tension-compression asymmetry of [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. Maximum strains of -4.8 % and 8.6 % in compression and tension, respectively, were found. A linear Clausius-Clapeyron relationship was observed for both stress-states where the smaller slope in tension resulted in a significant increase of the phase transformation temperatures with stress, which reached 180 °C under a constant stress level of 150 MPa. In addition, the material demonstrated a large pseudoelastic temperature range of about 300 °C under both stress state conditions. The results in this study unequivocally indicate the potential of these alloys for applications where elevated temperatures and stress levels prevail.

Investigation of the two-way shape memory effect in [001]-oriented Co35Ni35Al30 single crystals

For the first time, a study of the two-way shape memory effect (the TWSME) for the quenched and aged at 673 K for 0.5 h [001]-oriented Co35Ni35Al30 single crystals in compression was carried out. In the quenched CoNiAl crystals, the TWSME with a reversible strain of (-3.1 ± 0.3) % was induced by training procedure in the superelasticity temperature range. The physical reason of the TWSME is the creation of internal stress fields due to the formation of dislocations next to γ-phase particles, and an interface between the B2-matrix and the γ-phase in the quenched crystals. It was experimentally shown that aging at 673 K for 0.5 h under a compressive stress of 100 MPa along the [110] direction creates the conditions necessary for observing the TWSME, without additional training with a reversible strain of (+2.2 ± 0.3) %. The conditions for observing the TWSME are determined by the action of the internal stress fields of 25 MPa created by the oriented arrangement of the dispersed particles in the stress-assis...

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.