Structural Engineering with NiTi . I: Basic Materials Characterization

The overarching goal of this two-part paper is to provide a more unified understanding of NiTi shape memory alloys intended for use in structural engineering applications. Here, we present results from basic materials characterization of large diameter polycrystalline NiTi bars. Deformation processed bars with diameters of 12.7, 19.1, and 31.8 mm and various heat treatments were characterized at multiple length scales. Transmission electron microscopy revealed a nanometer scale precipitate structure present in the heat-treated, but not as-received bars. Spatial crystallographic texture measurements performed with electron backscatter diffraction, reveal a ⟨111⟩ texture along the longitudinal bar drawing axis in the majority of the bar, with a secondary longitudinal ⟨110⟩ component near the center of the bars. The prominence of the ⟨110⟩ texture increases with decreasing bar diameter or increasing percentage of deformation processing. Transformation temperatures and hardness were measured on samples extracted from the bars and are shown to depend strongly on bar heat treatment, but not bar diameter. The fine coherent precipitate structure induced during low temperature aging places transformation temperatures in the pseudoelastic range at room temperature and can be used to tailor material hardness.