Inter-martensitic transitions in Ni-Fe-Ga single crystals

The strain-temperature response of Ni-Fe-Ga single crystals underscores the role of the inter-martensitic transformation in creating intersecting heating and cooling segments; the separation of these segments occurs due to irreversibilities at high stresses and at high temperatures. An ultra-narrow tensile (1 degrees C) and compressive (<10 degrees C) thermal hysteresis are observed for the I OM;::: 14M case, accompanied by a small stress hysteresis (< 30 MPa) in compressive and tensile stress-strain responses. The hysteresis levels increase and the intersecting segments disappear at high stresses and at high temperatures. This paper reports the use of a thermo-mechanical formulation to rationalize the role of inter-martensitic transformations. Plotting the transformation stress as a function of temperature indicates that inter-martensitic transformations enable a very wide pseudoelastic temperature range, as high as 425 degrees C. The measured Clausius-Clapeyron curve slope in compression (2.75 MPa degrees C-1) is eight times the tensile slope (0.36 MPa degrees C-1); the higher slope is attributed to the predominance of A; = L1(0) at high temperatures. (c) 2007 Published by Elsevier Ltd on behalf of Acta Materialia Inc.