Effect of post-processing heat treatments on microstructure and mechanical properties of LPBF-fabricated NiTiHf alloy capable of tensile actuation

• LPBF-fabricated and heat treated NiTiHf alloys display tensile actuation under 600 MPa stress • LPBF introduces fine oxide nanoparticles and chemical heterogeneity affecting actuation performance • Reconstruction of martensite variant microstructures in grains of LPBF-fabricated NiTiHf • Combined EBSD and nanoscale orientation mapping of martensitic microstructures in TEM • Effects of heat treatments on actuation performance of LPBF NiTiHf alloys revealed In order to explore whether the tensile actuation performance of Ni 50.13 Ti 29.22 Hf 20.65 (at.%) high temperature shape memory alloy (HTSMA) fabricated by Laser powder bed fusion (LPBF) can be further improved, three post-processing heat treatments commonly used in NiTiHf research were applied (650°C/3 h; 1100°C/3 h; 1100°C/3 h+650°C/3 h). Microstructures, textures, and secondary phase oxide particles in LPBF-fabricated and heat-treated alloys were characterized, and functional thermomechanical properties were evaluated. It was found that post-processing heat treatments modify: (i) the austenitic grain size, shape and texture, (ii) the size, morphology, surface area fractions of fine homogeneously dispersed oxide nanoparticles created during the LPBF fabrication, (iii) the chemical composition of the matrix and oxide particles, (iv) increase the size of martensite domains, (v) the transformation temperatures, (vi) the stability of transformation response upon thermal cycling, (vii) do not affect significantly the strength but improve ductility of LPBF-fabricated alloy in isothermal tensile and compression tests, and (viii) increase actuation strain and decrease cyclic stability in tensile actuation tests. It is concluded that transformation temperatures and tensile actuation performance of LPBF-fabricated NiTiHf alloy containing finely dispersed oxide nanoparticles can be manipulated by post-processing heat treatments, which promote diffusion across oxide/matrix interfaces and affect the chemical composition of the matrix, oxide nanoparticles, and ultimately the size of domains in martensite variant microstructures.