Relationship between the superelasticity and strain field around Ni4Ti3 nano-precipitates in NiTi shape memory alloy via laser powder bed fusion

Abstract

Although the simulation results had demonstrated that the strain field introduced by Ni₄Ti₃ nano-precipitates in NiTi shape memory alloys (SMAs) was related with their superelasticity inherently, the corresponding experimental result was rarely documented heretofore, especially in additive manufactured NiTi SMAs. In this work, we tailor the morphologies and resultant strain field of Ni₄Ti₃ nano-precipitates by heat treatment of a NiTi SMA subjected to laser powder bed fusion (LPBF), and further authenticate relationship between the superelasticity and the strain field in the LPBF NiTi samples. When holding times were 1 h, 3 h, and 5 h at aging temperature of 350 °C after solution treatment, the Ni₄Ti₃ nano-precipitates in the LPBF NiTi samples exhibit spherical, ellipsoidal, and lenticular morphologies, respectively. Accordingly, the strain field around Ni₄Ti₃ nano-precipitates in B2 matrix decrease from 0.15 % to 0.13 % and 0.10 %, respectively. The LPBF and aged NiTi samples present large superelasticity, which exceeds 6 % recovery strain together with high recovery rate of ˃99 % during 10-times cyclic compression loading. Interestingly, the LPBF and aged sample with the spherical Ni₄Ti₃ and highest strain field displays the worst superelasticity stability, while the one with the lenticular Ni₄Ti₃ and smallest strain field exhibits the relatively stable and biggest superelasticity of 6.36 %. Basically, this is attributed to different mechanisms between the Ni₄Ti₃ nano-precipitates and dislocations generated during cyclic loading, which is induced by different interfaces between the Ni₄Ti₃ and B2 matrix in the three types of the NiTi samples. For the sample with the highest strain field, its spherical Ni₄Ti₃ was cut through by generated dislocations due to coherent interface between the spherical Ni₄Ti₃ and B2 matrix. In contrast, the one with the smallest strain field, its lenticular Ni₄Ti₃ can impede effectively generated dislocations because of semi-coherent or non-coherent interface between the lenticular Ni₄Ti₃ and B2 matrix. Therefore, these results can provide meaningful insights into tailoring the nano-precipitates and thereby obtaining excellent superelasticity of NiTi SMAs by LPBF.