Iron based shape memory alloys (SMAs) such as Fe-Ni and Fe-Mn based alloys have attracted considerable attention in recent years. In order to show a fully reversible pseudoelastic behaviour, these alloys have to meet several requirements including an only small volume change upon martensitic transformations and the presence of fine coherent precipitates .
Especially Fe-Mn-Al-Ni alloys, such as Fe43.5Mn34Al15Ni7.5, are of particular interest due to their excellent pseudoelastic behaviour over a wide temperature range, which is originated in the extremely low slope for the Clausius-Clapeyron equation .
The improvement of the mechanical and pseudoelastic properties of these SMAs requires adjustment of their microstructures by appropriate heat treatment. The planned optimisation of heat treatment procedures in alloys however, requires knowledge of the stable and the relevant metastable phase equilibria. Therefore, a thermodynamic description of the Fe-Mn-Al-Ni system using the CalPhaD method was developed, allowing the calculation of equilibria between the relevant phases. Apart from optimisation of the heat treatment procedures, the knowledge of the thermodynamics of the austenitic and martensitic phases allows the refinement of the alloy composition to adjust e.g. the transition temperature.
The development of the thermodynamic description started from the ternary Fe-Mn-Al and Mn-Al-Ni systems. Thermodynamic calculations were supported by the results of microstructural investigations using SEM, EPMA, XRD and TEM revealing the phase equilibria between the disordered alpha (A2), the ordered beta (B2) and the gamma phase (A1). The established preliminary thermodynamic description of the quaternary Fe-Mn-Al-Ni system was used to predict the phase equilibria, which were additionally investigated using (i) diffusion couples between the master alloy (Fe43.5Mn34Al15Ni7.5) and several different endmembers (Fe, Ni, Mn65Al35, … ) and (ii) alloys in the quaternary Fe-Mn-Al-Ni system which were heat treated at different temperatures. These predictions could be confirmed by the different experimental techniques. Moreover, a variety of differently structured martensitic microstructures, which formed during the heat treatment of diffusion couples, were identified and investigated by combined EBSD and TEM diffraction analyses.
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 T. Omori et al., Science, 333 (2011) 68-71.