NiTi Alloy

Alloy name: NiTi
Diagram No.: 791
Type of diagram: TTT
Chemical composition in weight %: 52%, 54%, 56% Ni, balance is Ti
Alloy group: Nickel-based alloys
Note: TTT diagram computed after the aging behavior of Ti-(52, 54, 56 at.%) Ni (adapted and redrawn from Kainuma et al. [71]).

Nishida et al. performed aging heat-treatment studies on Ni-rich Ti-52Ni alloy to investigate the diffusion-controlled transformations using metallographic, TEM, SEM and EDS techniques. Their observations suggested that three phases, viz., Ti3Ni4, Ti2Ni3 and TiNi3, precipitate in sequence, depending on the aging temperature and time. In other words, at low temperature and shorter aging time, Ti3Ni4 phase precipitates, while at higher aging temperature and longer aging times TiNi3 phase appears and at intermediary aging temperatures and time, Ti2Ni3 phase precipitates. Additionally, upon continual aging, the prior-formed Ti3Ni4 dissolves into the matrix, and the Ti2Ni3 phase increases in both percentage and the of the precipitates. Continuing the aging further, Ti2Ni3 is reabsorbed by the matrix and is replaced by TiNi3 precipitates. These observations debunk many of the earlier observations or suggestions regarding the nature of various phases identified in the Ni-Ti alloy system, and indicate that Ti3Ni4 and Ti2Ni3 are only intermediary phases with TiNi3 being the equilibrium phase: NiTi B2 parent phase -> Alpha1 + Ti3Ni4 -> Alpha2 + Ti2Ni3 -> Alph3 + TiNi3, where Alpha1, Alpha2, Alpha3 are matrices with different Ni concentration. Nishida et al. summarized the aging studies using a time-temperature-transformation (TTT) diagram, as reproduced in Figure. Further corroborative work was accomplished by Kainuma et al. [71] who provided the TTT diagrams for Ti-(52, 54, 56)Ni, these are recomputed and plotted in Figure.

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