Influence of Heat Treatment on Solidified Microstructure, Phase Transformation Behavior and Mechanical Properties of Thin NiTi Alloy Samples Fabricated by Laser Powder Bed Fusion

This work systematically investigates the effects of various heat treatment regimes, including solution treatment, solution treatment followed by aging at 623 K, 723 K and 823 K, and direct aging at the same temperatures, on the solidified microstructure, phase transformation behavior, and nanoindentation properties of thin NiTi samples fabricated by laser powder bed fusion (LPBF). The as-fabricated sample exhibits a strong {100}B2<001>B2 Cube texture (maximum texture index 25.77), a high dislocation density (2.70 × 1018 m−2), a single-step B19′↔B2 reversible transformation with Af = 308.17 ± 3.08 K, and a recovery ratio of 0.46 ± 0.02. Subsequently, solution treatment homogenizes the microstructure, resulting in a lower dislocation density and a partial transformation from the Cube texture to the Goss texture. Further aging at 623 K after solution treatment achieves the highest recovery of 0.52 ± 0.03 by introducing fine and inferred-coherent Ni4Ti3 precipitates while maintaining a higher fraction of B2 phase at room temperature. However, aging at 723 K after solution treatment leads to a Goss-dominated texture, mixed austenite/martensite phases, and the lowest recovery (0.34 ± 0.01). In contrast, direct aging at 623 K or 723 K also yields lower recovery ratios (0.40 ± 0.06 and 0.35 ± 0.01, respectively), due to retained compositional inhomogeneity and higher dislocation densities. For direct aging at 823 K, however, the recovery ratio significantly increases to 0.49 ± 0.06. It is therefore suggested that the enhanced recovery performance can be achieved by combining solution treatment with low-temperature aging, which synergistically combines coherent precipitates, a fully austenitic matrix, and a favorable texture.