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

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 T-0 temperatures of about 1000 degrees C are predicted for the transformation of the A2 matrix into Al 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 (similar to NiAl with some content of Fe and Mn) to undergo the transformation to the L1(0) structure enforced by the martensitic transformation of the matrix. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Fe-Mn-Al-Ni based shape memory alloys are preferred for large scale applications due to their low materials costs and good cold workability. The alloys consist of an A2 structured matrix with B2 precipitates, necessary for thermoelasticity. Thermodynamic considerations show that the undercooling of about 1000 K is caused by the resistance of the B2 precipitates to undergo transformation to the L1(0) structure enforced by the martensitic transformation of the matrix.