Optical Microscopic Study on NiCoMnAl Metamagnetic Shape Memory Alloy by In Situ Observation under a Pulsed High Magnetic Field

Abstract

In situ microscopic study was carried out on Ni_44.3Co_5.1Mn_31.4Al_19.2 metamagnetic shape memory alloy under a pulsed magnetic field up to 35.5 T. Magnetic field-induced reverse martensitic transformation was directly observed. The critical magnetic fields were determined from the contrast change of the microstructure, where good agreement had been found by magnetization measurement in a previous study. Comparison of the micrographs during thermal and magnetic field cycles between Ni₄₅Co₅Mn_36.7In_13.3 and Ni_44.3Co_5.1Mn_31.4Al_19.2 was conducted. It is concluded that though the energy barrier related to thermal activation strongly effects the martensitic transformation of Ni₄₅Co₅Mn_36.7In_13.3, its effect is very small on Ni_44.3Co_5.1Mn_31.4Al_19.2. Qualitative discussion yielded a reasonable understanding of the results of microstructure observation.