Microstructural Characterization, Functional Behavior and Deformation Mechanism of Ni>sub>54Mn <sub>25</sub>Ga <sub>21-x</sub>Dy <sub>x</sub> High Temperature Shape Memory Alloys with Low Thermal Hysteresis

Abstract

Potential applications at elevated temperatures require shape memory alloys (SMAs) to possess high transformation temperature, excellent shape recovery ability and thermal/functional stability. The present study investigated the influences of Dy addition on the microstructural characterization, functional behavior and possible deformation mechanisms in high temperature Ni54Mn25Ga21 SMAs. The results indicated that adequate Dy addition contributed to a desirable tradeoff among compressive strength, ductility and functional performance. Ni54Mn25Ga20.9Dy0.1 alloy showed the highest shape memory recoverable strain due to the improvement of mechanical properties and formation of small Dy-rich precipitates. Simultaneously, an obvious pseudoelasticity was observed over a wide temperature range with a maximum recoverable strain of 3.74% at 320°C in this alloy. All the Dy-containing alloys exhibited high thermal cyclic stability and quite low thermal hysteresis. However, it was revealed that a complicated and abnormal relationship between thermal hysteresis and the middle eigenvalue λ2 existed for the investigated alloys with a lattice transition from cubic to tetragonal. Calculated results based on energy minimization theory revealed that both the transformation strain and critical stress for inducing martensitic transformation varied with crystallographic orientations. The detwinning of pre-existing (112) compound twins and formation of (112) deformation twins were the dominant deformation mechanisms of martensite, which were highly compatible with the macroscopical recoverable strain. The present study provided valuable insights for developing high-performance multifunctional shape memory alloys for high temperature applications.