Hot isostatic pressing effects on multiscale mechanical properties and chromium-dependent strength retention in LPBF Fe-based shape memory alloys

Abstract

This work investigates the effect of Hot Isostatic Pressing (HIP) on FeMnAlNi and FeMnAlNiCr shape memory alloys (SMAs) fabricated by Laser Powder Bed Fusion (LPBF). Specimens with and without in-situ laser remelting were HIP-treated at 1150 °C and 150 MPa for 4 h, thereby eliminating most internal porosity and increasing the relative density from ~ 97% to 99.98%. This densification substantially improved ductility, with the Cr-free alloy reaching elongations up to 24%, while the strength and hardness decreased. The addition of ~ 4 at% Cr mitigated these softening effects, enabling higher retention of strength and hardness after HIP. Notably, a remelted Cr-containing sample achieved an excellent property balance with post-HIP indentation-derived equivalent ultimate tensile strength of ~ 1085 MPa, hardness nearly 295 HV, and a modest modulus increase to 161.8 GPa. In contrast, the Cr-free alloy exhibited a larger drop in strength but greater gains in ductility, underscoring a composition-dependent trade-off between strength and plasticity. Nanoindentation testing confirmed these trends, showing reduced nano-hardness and elastic recovery after HIP in both alloys. While Cr additions preserved more of the reversible transformation capacity, pop-out events observed during unloading suggest localized transformation-related deformation during indentation, and that HIP homogenization likely does not suppress pseudoelasticity. Overall, this study bridges a research gap in combining Cr alloying with post-HIP treatment. The results highlight the synergistic role of LPBF parameter control, Cr alloying, and HIP treatment in tailoring Fe-SMAs with near-full density, high strength retention, and reliable ductility.