A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 5 7 Shape Memory and Superelasticity: Advances in Science andTechnology (SMJ) is theofficial journal of the International Organization on Shape Memory and Superelastic Technologies (SMST), an affiliate society of ASM International. The journal publishes original peer- reviewed papers that focus on shape memory materials research with contributions from materials science, experimental and theoretical mechanics, physicswith cognizance of the chemistry, underlying phases, and crystallography. It also provides a forum for researchers, scientists, and engineers of varied disciplines to access information about shape memory materials. Three articles were taken from our December 2021 and March 2022 issues and were selected by Shape Memory Editor-in-Chief Huseyin Sehitoglu. SMJ is available through springerlink.com. For more information, visit asminternational.org/web/smst. December 2021 EFFECT OF Ni/Ti RATIO AND Ta CONTENT ON NiTiTa ALLOYS S. Cai, J.E. Schaffer, and Y. Ren Influences of Ta and Ni/Ti ratio on transformation temperatures, microstructures, and superelastic properties of NiTiTa alloys were investigated. It is found that: (1) increasing Ni/Ti ratio increases the solubility limit of Ta in NiTi matrix and suppresses the formation of Ta-rich secondary phases. 4 at.% Ta can be dissolved in alloys with Ni/Ti ratio of ~1.1; (2) high Ni/Ti ratio promotes the formation of Ti2Ni3 phase, which is suppressed by increasing Ta additions; (3) phase transformation temperatures increase with Ta content at a rate of ~70–80°C/at.% Ta, but rapidly decrease with increasing Ni/Ti ratio; (4) super-elastic properties of NiTiTa alloys can be optimized by adjusting residual cold work, Ta content, and Ni/Ti ratio; and finally, (5) phase transformation temperatures of NiTiTa alloys decrease with increasing valence electron concentration in general (Fig. 1). December 2021 COHERENT PRECIPITATES AS A CONDITION FOR ULTRA-LOW FATIGUE IN CU-RICH TI53.7NI24.7CU21.6 SHAPE MEMORY ALLOYS L. Bumke, E. Quandt, N. Wolff, C. Chluba, T. Dankwort, and L. Kienle Sputtered Ti–rich TiNiCu alloys are known to show excellent cyclic stability. Reversibility is mostly influenced by grain size, crystallographic compatibility, and precipitates. Isolating their impact on cyclic stability is difficult. Ti2Cu precipitates for instance are believed to enhance reversibility by showing a dual epitaxy with the B2 and B19 lattice. Their influence on the functional fatigue, if they partly lose the coherency, is still unknown. In this study, sputtered Ti53.7Ni24.7Cu21.6 filmshavebeenannealedatdifferent temperatures leading to a similar compatibility (λ2 ~0.99), grain size and thermal cyclic stability. Films annealed at 550°C (referred to as LT) exhibit a superior superelastic fatigue resistance but with reduced transformation temperatures and enthalpies. TEM investigations suggest the formation of Guinier– Preston (GP) zone-like plate precipitates and point towards a coherency relation of the B2 phase and finely distributed Ti2Cu precipitates (~60 nm). Films annealed at 700°C (referred to as HT) result in the growth of Ti2Cu precipitates SMJ HIGHLIGHTS 4 Fig. 1 — Comparison between experimental results and modeling results of alloy A4 confirms the presence of Ti2Ni3 precipitates. The upper half of the figure shows modeling results, and the lower half figure is experimental data. Fig. 2 — STEM-HAADF images of the LT (a) and the HT sample (b) at room temperature reveal a similar grain size of TiNiCu for both samples, but a large difference in the sizes and arrangement of Ti2Cu precipitates (dark contrast). The sizes of the precipitates is approx. 60 nm for the LT sample and 280 nm for the HT sample, respectively. 1 1
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