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

Irene Ferretto, Christian Leinenbach

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.

2022

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Theory-guided design of high-strength, high-melting point, ductile, low-density, single-phase BCC high entropy alloys

Carolina Baruffi, William Curtin, Christian Leinenbach, You Rao

The search for new high-temperature alloys that can enable higher-efficiency/lower-emissions power generation has accelerated with the discovery of body-centered cubic (bcc) refractory High Entropy Alloys (HEAs). These many-component, non-dilute alloys in the Cr-Mo-W-V-Nb-Ta-Ti-Zr-Hf-Al family hold the potential for combining high strength and thermodynamic stability at high temperature with low density and room-temperature ductility, but searching the immense compositional space is daunting. Here, very recent theories and expanded thermodynamic tools are used to guide the discovery of new alloys satisfying the required suite of properties. The search and discovery method is first demonstrated for 5-component equicomposition alloys, identifying HfMoNbTaTi as the one alloy satisfying many constraints, with predicted properties agreeing with experiments. The design process then discovers new quinary and quarternary alloys in the Hf-Mo-Nb-Ta-Ti space having even better overall properties. One new quinary alloy is fabricated and shown to be single phase with high room temperature hardness, high melting point, and low density. More broadly, the new design process can further be used to explore millions of alloys with other desired multi-dimensional performance requirements. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

PERGAMON-ELSEVIER SCIENCE LTD2022

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Laser-powder bed fusion (L-PBF) has gained significant research interest, not only for its profound advantage of producing near-net shape complex geometries of metallic parts, but also for the possibility of producing tailored microstructures. Here we exploit the capability of manipulating the crystallographic texture by L-PBF to tailor the deformation behavior of austenitic stainless steels. In specific, by adjusting the laser power and the laser scanning speed, tailored crystallographic textures can be obtained, along the uniaxial loading direction in 304L stainless steel samples produced by L-PBF. In situ neutron diffraction and uniaxial tension and compression tests are undertaken to investigate the extent of the transformation induced plasticity effect and to correlate it with the tailored macrostructures. The influence of the initial and the evolving crystallographic texture on the deformation behavior is demonstrated and elaborated accordingly. The observed asymmetry in the deformation behavior between tension and compression is also discussed in detail.

ELSEVIER SCI LTD2022

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