Effect of post-heat treatment conditions on shape memory property in 4D printed Fe-17Mn-5Si-10Cr-4Ni shape memory alloy

The influence of post-heat treatment on the microstructure and properties of 4D printed Fe-17Mn-5Si-10Cr-4Ni (wt. %) shape memory alloy (SMA) produced via a laser powder bed fusion process is investigated in this study. It is shown that heat-treatment temperatures lower than 800 ? are not sufficient to completely undergo phase transformation from bcc-delta to fcc-gamma, whereas temperatures higher than 800 ? lead to grain growth and thickening of hcp-epsilon phases. The latter has a severely negative effect on shape memory. Furthermore, heat treatment at 800 C for more than 3 h leads to sigma-phase formation, which also negatively affects the shape memory effect (SME) by changing the chemical composition of the fcc-gamma matrix. The SME along the building direction (BD), is higher than that along the laser scanning direction (SD) and in 45 to the SD (45SD). The higher SME along the BD is attributed to the {110} texture in the plane normal to the BD, whereas almost random but weak {111} and {100} textures are observed in SD and 45SD. Furthermore, transmission electron microscopy revealed that a high stacking fault density and very thin hcp-epsilon phase are formed in the heat-treated sample, which explains the higher SME regardless of direction, compared with conventionally fabricated Fe-based SMAs with a similar composition.