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 °C) and compressive (<10 °C) thermal hysteresis are observed for the A ⇌ 10M ⇌ 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 °C. The measured Clausius-Clapeyron curve slope in compression (2.75 MPa °C^-1) is eight times the tensile slope (0.36 MPa °C^-1); the higher slope is attributed to the predominance of A ⇌ L1₀ at high temperatures.