Thermodynamics of martensite formation in Fe–Mn–Al–Ni shape memory alloys

Abstract Fe−Mn−Al−Ni based shape memory alloys are regarded suitable for large scale applications due to relatively low materials costs and a good cold workability. The austenitic state consists of an A2 structured matrix with coherent B2 precipitates which are required to ensure thermoelasticity, supposedly due to a strengthening effect on the matrix. The martensitic state is based on the A1 structure. For a series of alloys, the martensite start temperatures were determined to be below ambient temperature, whereas for the same alloy compositions T0 temperatures of about 1000 °C are predicted for the transformation of the A2 matrix into A1 by means of CalPhaD-based calculations. It is shown by thermodynamic considerations that the undercooling of about 1000 K is caused by the resistance of the B2 precipitates (~NiAl with some content of Fe and Mn) to undergo the transformation to the L10 structure enforced by the martensitic transformation of the matrix.