NiTiHf high-temperature shape memory alloy fabricated via laser powder bed fusion showing fully recoverable actuation under tensile stress

Abstract

NiTiHf high-temperature shape memory alloys (HTSMAs) fabricated via laser powder bed fusion (LPBF) suffer from poor tensile strength preventing their successful use in engineering applications. We managed to fabricate NiTiHf HTSMA showing tensile actuation under stresses up to 500 MPa in the temperature range 250–350 °C. A tensile strength of 821 MPa, an actuation strain 2.34 % under 300 MPa tensile stress with negligible irrecoverable plastic strain, and fracture upon cooling under 600 MPa tensile stress are the best values reported so far for LPBF-fabricated NiTiHf HTSMAs in the literature. The enhanced tensile strength reaching half of the strength of the conventional cast and thermomechanically processed NiTiHf HTSMAs was attributed to the lack of micropores (porosity 0.02 %). The tensile actuation performance is claimed to be facilitated by the strengthening effect from homogeneously distributed oxide nanoparticles introduced naturally by the LPBF fabrication. The oxide nanoparticles decrease the size of domains of (001) compound twinned martensite created by the forward martensitic transformation upon cooling under stress, which reduces actuation strain but also suppresses dislocation slip in martensite. This dimensional strengthening mechanism lowers actuation strain but increases cyclic stability of the actuation response of the LPBF NiTiHf HTSMA.