Shape memory response of ni2mnga and nimncoin magnetic shape memory alloys under compression

Abstract

In this study, the shape memory response of Ni2MnGa and NiMnCoIn magnetic shape memory alloys was observed under compressive stresses. Ni2MnGa is a magnetic shape memory alloy (MSMA) that has been shown to exhibit fully reversible, stressassisted magnetic field induced phase transformation (MFIPT) in the I X-phase transformation because of a large magnetostress of 7 MPa and small stress hysteresis. The X-phase is a recently discovered phase that is mechanically induced, however, the crystal structure is unknown. To better understand the transformation behavior of Ni2MnGa single crystal with [100] orientation, thermal cycling and pseudoelasticity tests were conducted with the goal of determining the Clausius-Clapeyron relationships for the various phase transformations. This information was then used to construct a stresstemperature phase diagram that illustrates the stress and temperature ranges where MFIPT is possible, as well as where the X-phase may be found. NiMnCoIn is a recently discovered meta-magnetic shape memory alloy (MMSMA) that exhibits unique magnetic properties. The ferromagnetic parent phase and the paramagnetic martensite phase allow the exploitation of the Zeeman energy. To gain a better understanding of the transformation behavior of NiMnCoIn, thermal cycling and pseudoelasticity tests were conducted on single crystals from two different batches with crystallographic orientations along the [100](011), [087], and [25 7 15] directions. A stress-temperature phase diagram was created that illustrates the Clausius- Clapeyron relationships for each orientation and batch. SQUID tests revealed the magnetic response of the alloy as well as the suppression of the martensite start temperature with increasing magnetic field. Pseudoelasticity experiments with and without magnetic field were conducted to experimentally quantify the magnetostress as a function of magnetic field. For the first time, it has been shown that NiMnCoIn is capable of exhibiting magnetostress levels of 18-36 MPa depending upon orientation, as well as nearly 6.5% transformation strain in the [100] direction. The results of this study reveal increased actuation stress levels in NiMnCoIn, which is the main limitation in most MSMAs. With this increased blocking stress, NiMnCoIn is a strong candidate for MFIPT.