Effect of Aging Condition on Microstructure, Mechanical Properties, and Shape Memory Behavior of Solution-Treated Fe-17Mn-5Si-5Cr-4Ni-0.2Ti-0.1C Shape Memory Alloy

In the present study, the effect of aging conditions on the microstructure, mechanical properties, and shape memory behavior of a solution-treated Fe-17Mn-5Si-5Cr-4Ni-0.2Ti-0.1C shape memory alloy was investigated. In addition, a revised mechanism for the precipitation-induced improvement of the shape memory effect (SME) is proposed. The effect of increasing the aging temperature from 500 to 900 °C was evaluated with a fixed time of 0.5 h, finding that the SME was initially improved upon increasing the temperature up to 600 °C, whereas further increasing the aging temperature exerted a detrimental effect on the SME. This was attributed to the change in the fractions of the ε and γ phases. The same increasing and decreasing trends were observed when the aging time was increased while maintaining the temperature constant at 600 °C. In this case, no change in the fractions of the ε and γ phases was detected. Instead, a change in the size and distribution of TiC precipitates might be the main reason for changes in properties. The presence of finer and densely distributed precipitates in the sample aged for 3.0 h could lead to a higher integrated pulling force, back stress, stacking faults, or ε phase cutting through the precipitates, resulting in a more effective back-transformation from ε to γ, compared with the sample aged for 6.0 h, which exhibits larger and sparsely distributed precipitates. Continuously decreased and increased 0.2% proof and tensile stress indicated an easier martensitic transformation in the early stage of deformation and precipitation hardening. However, according to the different trend in the SME and 0.2% proof stress, not only an easier martensitic transformation from γ to ε but also an integrated pulling force, which can be altered by tuning the size and distribution of precipitates, should be considered to achieve high SME induced by carbide precipitation.