Superelasticity and Shape Memory Effects in Polycrystalline Ni-Mn-Ga Microwires

M. F. Qian, Harbin Institute of Technology
X. X. Zhang, Harbin Institute of Technology
Cassie Witherspoon, Boise State University
J. F. Sun, Harbin Institute of Technology
Peter Müllner, Boise State University

Abstract

Non-stoichiometric Ni_49.9Mn_28.6Ga_21.5 microwires with a diameter of 30-80 μm and cellular grain size of 1-3 μm were developed by melt-extraction. The superelasticity and shape memory effect of the microwires were investigated on a Dynamic Mechanical Analyzer (DMA Q800). The onset critical stresses for austenite to martensite transformation increase linearly with temperature and can be described by Clausius-Clapeyron relationship. The martensite to austenite transformation occurs upon unloading occurs only at temperatures at least 11 °C higher than the martensite start temperature. The as-extracted microwire was loaded in its martensite state to a strain of 2.7% without breaking and 92% plastic deformation strain recovers upon heating.