A Large Elastic Strain in Fe-Pd and Fe-Pt Shape Memory Alloys

In this presentation we report that a very large elastic-like strain of more than 6% appears in two iron-based shape memory alloys. One is a disordered Fe-31.2Pd (at.%) alloy, and the other is L1₂-type ordered Fe-25Pt (at.%, Fe₃Pt) alloy with degree of order 0.75; they are both known as Invar alloys. Each alloy exhibits a second-order-like martensitic transformation from a cubic structure to a tetragonal structure. The transformation temperature is about 230K for the Fe-31.2Pd alloy and about 90 K for the Fe₃Pt alloy. These martensitic transformations are associated with significant softening in elastic constant C’ because of the band Jahn-Teller effect. Therefore, extremely large elastic-like strain with extremely small stress hysteresis appears when compressed in the [001] direction near the transformation temperature. We also show that a large elastocaloric effect with very small hysteresis loss appears in these alloys.