Direct measurement of large reversible magnetic-field-induced strain in Ni-Co-Mn-In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni45Co5Mn36.5In13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125 MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.