In situ neutron diffraction study of magnetic field induced martensite reorientation inNi–Mn–Ga under constant stress

Abstract

The magnetic field induced reorientation of martensite twins as a function of acompressive stress and crystallographic direction was studied on a single-crystallineNi49.7Mn29.3Ga21 magnetic shape memory alloy by the in situ neutron diffraction technique. The compressivestress 0–3 MPa and magnetic field 0–3.5 T were applied perpendicularly to each other alongthe [001] and [100] ([010]) crystallographic directions of tetragonal martensite, respectively.The neutron diffraction method provides integrated information about the presence andvolume fractions of individual martensite variants involved in magnetic actuation. It hasbeen found that (i) martensite variant reorientation is not completed by applying magneticfield as high as 2.5 T, (ii) recoverable strain of about 3% due to switching between twovariant microstructures is observed upon cyclic application of magnetic field underexternal compression stress 0.9 MPa, (iii) the magnetic field induced reorientation wassuppressed by the bias external stress of 3 MPa and (iv) mechanical training bysuccessive compression deformation on two different faces of the cuboid Ni–Mn–Gasingle crystal is essential to achieve the magnetic field induced cyclic reorientation.