Stability Of Two-way Memory Effect Research Articles

Thermomechanical stability of Ni–Mn–Ga single crystal

Thermomechanical behavior of Ni–Mn–Ga single crystal was investigated and the experimental results show that the stability of two-way memory effect (TWME) is very sensitive to the heating and cooling rate as well as external load. There were internal stresses accumulated during thermal cycling which affected the martensite variants distribution. Consequently, the strain versus temperature curve became inconsistent with martensite strain changed from contraction to elongation, or vice versa. The internal stress accumulated during thermal cycling was estimated based on the change in M s according to the modified Clausius–Clapeyron equation. Its magnitude is comparable to the critical stress needed to reorient the twinned variants. The irregularity in the strain versus temperature curve was explained as a result of the stress-affected redistribution of twinned variants during the forward transformation.

The two way memory effect in TiNi alloys

The two way memory effect (TWME) in shape memory alloys represents a reversible spontaneous shape changes during cooling and heating processes. This is a consequence of reversible phase transformations observed without application of any external stresses. The TWME is usually obtained after a thermomechanical treatment often called training. The different opinions were published concerning the influence of R-phase formation in TiNi based alloys on the stability of TWME. Stachowiak and McCormick have stated that the formation of R-phase diminishes the extent of obtained reversible strain as well as the stability of TWME. In contrast to this conclusion, the authors observed enhanced stability of TWME in the cases when the R-phase formation precedes the martensitic transformation B2 {yields} B19{prime}. Repeating the heating cycles at loading of the memory materials, the complex degradation mechanisms of TiNi alloys occurs. The dislocation generation in the matrix of TiNi alloys (work hardening) strongly influences the maximum level of generated stresses and the extent of reversible strain. As a contribution to the explanation of physical principles of TWME, the present paper is devoted to the study of substructure after so called hard training at which the specimens are deformed ({var_epsilon} = 4%) and cyclically heated andmore » cooled down at constrained conditions.« less