Unleashing the elastocaloric cooling potential of 3D-printed NiTi alloy with heterogeneous microstructures and Ni4Ti3 nanoparticles

Abstract

Toward the development of elastocaloric (eC) refrigeration applications, this study examines how Ni₄Ti₃ nanoparticles improve the eC properties of laser powder bed-fused (LPBF) NiTi alloys, with a focus on the formation of a NiTi/Ni₄Ti₃ nanocomposite. The LPBF-produced microstructure offers significant advantages: i) a heterogeneous grain structure that provides complementary benefits—fine grains offer higher deformation resistance while coarse grains initiate phase transformation (PT) earlier, releasing more latent heat; ii) a strong <001>//building direction texture that enhances recoverability. However, these benefits are partially limited by grain boundary slip during PT, leading to lower cooling efficiency and increased energy dissipation in as-built NiTi alloys. To address these issues, Ni₄Ti₃ nanoparticles are introduced through aging treatment, forming a composite structure that strengthens grain boundaries. Given the challenges of applying severe plastic deformation to 3D-printed components, this approach may offer a more practical solution. The study also reveals that Ni₄Ti₃ nanoparticles contribute to: 1) reducing Ni content in the matrix, increasing lattice size and enthalpy, which enhances the temperature drop (ΔT_ad); and 2) promoting R phase formation, which hinders the B2↔B19’ PT, reducing energy dissipation and improving the coefficient of performance (COP_mat). The balance of these effects depends on nanoparticle size, with smaller particles (∼5–12 nm) amplifying the second effect, while larger particles (∼130 nm) increase the first effect. At 350 °C aging, the optimized nanocomposite exhibits a maximum COP_mat of 15 and ΔT_ad of 14K, representing a 163 % improvement over the as-built alloy. This work highlights the potential of NiTi composites in 3D-printed eC components.