Investigation of the martensite variant reorientation of the single crystal Ni-Mn-Ga alloy via training processes and a modification with a silicone rubber

Abstract

The shape changes of the Ni-Mn-Ga alloys, which could be driven via a manipulation of temperature, stress, and/or magnetic-field, are considered as promising materials for the application of future technologies, such as robots and smart medical devices. The single crystal (SC) Ni49Mn30Ga21 (at.%) alloy, which possessed a 7-modulated (7M) martensite phase, was prepared by a floating-zone (FZ) technique. The fundamental thermal analysis, phase identification, crystallographic orientation determination, magnetic properties, mechanical properties, and microstructure observations were carried out for the evaluations of the SC Ni-Mn-Ga alloy. Prior to the training processes via an introduction of certain compressive stress cycles, no martensite variant reorientation (MVR) could be observed in the magnetization-magnetic field (M–H) curves. During the training processes, it was obviously found that the stress for the commencement of the MVR was reduced gradually. An apparent MVR was thereafter realized after some specific training procedures. This could be attributed to the less pinning effect at the twin boundaries of the SC Ni-Mn-Ga alloy. In addition, obvious shape changes were revealed after the introduction of an external magnetic field to the SC Ni-Mn-Ga alloy. It was further found that the imposed MVR could be constrained by integrating the SC Ni-Mn-Ga alloy with a silicone rubber.