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Abstract:
Nickel–titanium (NiTi) shape memory alloys (SMAs) with a nominal composition of 50.8 at% Ni and 49.2 at% Ti were fabricated using the as-fabricated laser powder bed fusion (LPBF) technique. This study focuses on the impact of post-processing heat treatments—specifically solution annealing and aging at 500 °C for 1 and 20 h—on phase transformation behavior and functional performance. Phase analysis (XRD) was conducted at room temperature (~ 25 °C), while uniaxial tensile testing was performed at both room (~ 25 °C) and sub-zero (–20 °C) temperatures. Differential scanning calorimetry (DSC) was carried out over a wide temperature range to evaluate the thermal behavior of the material. The results indicate that heat treatment conditions significantly affect transformation temperatures, phase constitution, and mechanical response. Depending on the treatment and test temperature, the microstructure varied from fully austenitic to fully martensitic or mixed-phase states. These variations manifested as distinct features in the stress–strain behavior, particularly in terms of martensitic transformation and superelasticity. The study demonstrates the feasibility of fine-tuning functional properties in LPBF-produced NiTi SMAs through optimized thermal processing strategies.

Abstract:
This study evaluates how solution treatment and aging influence the deformation mechanisms, phase transformations and functional performance of NiTi alloys produced by laser powder bed fusion (LPBF). Tensile tests performed at room temperature (RT) and −20 °C (LT) were combined with Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) analyses to correlate mechanical response with transformation thermodynamics and microstructural evolution. In the as-fabricated (AF) condition, deformation is governed by twinning and martensitic plasticity due to suppressed stress-induced martensite (SIM). Solution treatment (ST) restores reversible SIM at RT and preserves partial recoverability at LT as a result of microstructural homogenization and internal stress relief. Aging at 500 °C (A1h, A20h) promotes Ni4Ti3 precipitation, increasing transformation temperatures and stabilizing martensite, which leads to entirely irreversible deformation at both temperatures. These findings establish a clear functional continuum—ranging from recoverable (ST) to dissipative (AF) and fully irreversible (A20h) behavior—and provide a mechanistic framework for tailoring LPBF NiTi components for actuators, energy-storage and energy-dissipation applications.

Keywords:
NiTi shape memory alloy, laser powder bed fusion (LPBF), stress-induced martensite (SIM), pseudoelasticity, aging heat treatment, Ni4Ti3 precipitation, microstructural homogenization, low-temperature mechanical behavior