Key role of high-Tc twinned martensitic materials to gain a magnetic actuation higher than 15%

Abstract

Twinning angle outcomes of the twinned martensitic (TM) and detwinned martensitic (DTM) structural transformations on the magnetic features of the austenite (A) parent phase are researched by using the effective field theory established by Kaneyoshi. The implementation of the effective field theory verifies that the shape memory mechanism occurs with phase transformations, A→TM→DTM→A. It also shows that the austenite parent phase has two types of actuations: one-dimensional actuation (in only y-axis) for TM, and two-dimensional actuation (in x and y-axes) for DTM. Magnetic field-induced actuation (strain) in the range of 5–15% at twinning angle θ ≥ 120.816° of TM and DTM is reported for some materials in the literature. On the other hand, in this work, it is estimated that a twinning angle lower than this twinning angle (i.e., θ < 120.816°) must be achieved to have a strain higher than 15%. We also suggest that materials with higher magnetization, Curie temperature, coercive field and remanence magnetization should be taken into account to get a strain higher than 15%, since all these features are directly affected by the twinning angle (θ). Our results on Curie temperature (Tc) match with the experimental results of Ni49.8Mn28.5Ga21.7 (achieved 6% strain) with Tc = 95 °C (368 K) by Murray et al., and Ni46Mn24Ga22Co4Cu4 with Tc = 393 K (achieved 12% strain) by Sozinov et al.