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

Abstract

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 °C are not sufficient to completely undergo phase transformation from bcc-δ to fcc-γ, whereas temperatures higher than 800 °C lead to grain growth and thickening of hcp-ε phases. The latter has a severely negative effect on shape memory. Furthermore, heat treatment at 800 °C for more than 3 h leads to σ-phase formation, which also negatively affects the shape memory effect (SME) by changing the chemical composition of the fcc-γ 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-ε 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.