Recent Development of the Magnetic Shape Memory Materials Research in Finland

Abstract

AbstractNi-Mn-Ga based magnetic shape memory (MSM) materials have been studied since 1998 in Finland at the Helsinki University of Technology (TKK, previously HUT). The large HUT-MSM-project resulted in MSM-alloys with high service temperature, 10 % field-induced-strain, as well as circumstances when and how a Ni-Mn-Ga alloy exhibits this phenomenon. The understanding of the structure and behavior of twin boundaries, and their role, for example, in the vibration damping and long-term actuation has been enhanced in the recent projects. Twin boundaries have been studied by XRD, by high-resolution transmission electron microscopy (HRTEM), and by in-situ straining in TEM, the last one in co-operation with the Institute of Physics in Prague (ASCR-IP), Czech Republic. The results obtained by neutron diffraction in co-operation with Hahn-Meitner-Institut Berlin, Institute for Metal Physics (IMP), Kiev, and Institut Laue-Langevin (ILL), Grenoble, have given new crystallographic information. Damping of Ni-Mn-Ga polymer composites has been proved to be excellent at high stiffness levels with the loss factor = 0.6 at E ≈ 1 GPa. This research was carried out in co-operation with the University of California Los Angeles (UCLA), USA. In the long-term actuation, a fatigue life of 2×109 has been recorded for a five-layered modulated Ni-Mn-Ga structure in mechanical cycling. The evolution of the MSM parameters during the long-term use is recorded and used as an input data for the models developed in the European MAFESMA co-operation. The search for alloys with wide stable thermal property range showing MSM effect has continued and alloys that are stable down to 4 K have been established. Modeling based on Ginsburg-Landau theory has been applied to evaluate aging and thermal fluctuations in the modulated Ni-Mn-Ga structures. As a commercial target, AdaptaMat Ltd. develops technology to produce Ni-Mn-Ga magnetic shape memory material with improved quality, lower twinning stress, longer fatigue life as well as lower cost and better availability for use in research and development.