Shape memory alloys (SMAs) with high transformation temperatures can enable simplifications and improvements in operating efficiency of many mechanical components designed to operate at tem...

High temperature shape memory alloys

Finding new materials with targeted properties has traditionally been guided by intuition, and trial and error. With increasing chemical complexity, the combinatorial possibilities are too large for an Edisonian approach to be practical. Here we show how an adaptive design strategy, tightly coupled with experiments, can accelerate the discovery process by sequentially identifying the next experiments or calculations, to effectively navigate the complex search space. Our strategy uses inference and global optimization to balance the trade-off between exploitation and exploration of the search space. We demonstrate this by finding very low thermal hysteresis (ΔT) NiTi-based shape memory alloys, with Ti50.0Ni46.7Cu0.8Fe2.3Pd0.2 possessing the smallest ΔT (1.84 K). We synthesize and characterize 36 predicted compositions (9 feedback loops) from a potential space of ∼800,000 compositions. Of these, 14 had smaller ΔT than any of the 22 in the original data set.

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Accelerated search for materials with targeted properties by adaptive design.

Welding and Joining of NiTi Shape Memory Alloys: A Review

Effects of nanoprecipitation on the shape memory and material properties of an Ni-rich NiTiHf high temperature shape memory alloy

Load-biased shape-memory and superelastic properties of a precipitation strengthened high-temperature Ni50.3Ti29.7Hf20 alloy

Recent developments in China on TiNi-based shape memory alloys (SMAs), including TiNi binary SMAs, TiNiNb wide hysteresis SMAs, TiNiCu narrow hysteresis SMAs, TiNiHf high temperature SMAs and TiNiRE SMAs were concisely reviewed. The damping characteristics and corresponding mechanisms of TiNi and TiNiNb alloys were described and discussed. Some surface modifications of TiNi binary alloys were employed to improve the corrosion resistance and the biocompatibility, which are very useful for the medical applications. Shape memory effect and mechanical properties of TiNiHf alloys were enhanced by aging Ni-rich TiNiHf alloys, while the Ms still remain enough high. The authors found that the addition of RE to TiNi alloys increases the phase transformation temperatures and even changes the transformation sequence. Fundamental research on the TiNi-based alloys is still in the ascendant, leading to increasingly extending of the applications.

Recent development of TiNi-based shape memory alloys

Effect of aging on martensitic transformation and microstructure in Ni-rich TiNiHf shape memory alloy

Shape-memory behaviors in an aged Ni-rich TiNiHf high temperature shape-memory alloy

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

More attention has been paid to ternary Ti–Ni–Hf high-temperature shape memory alloys (SMAs) due to their high phase transformation temperatures, good thermal stability and low cost. However, the Ti–Ni–Hf alloys have been found to have low ductility and only about 3% shape memory effect and these have hampered their applications. It is well known that there are three methods to improve the shape memory properties of high-temperature SMAs: (a) cold rolling + annealing; (b) adding another element to the alloy; (c) aging. These methods are not suitable to improve the properties of Ti–Ni–Hf alloys. In this paper, a method of conditioning Ni-rich Ti–Ni–Hf alloys as high-temperature SMAs by aging is presented. For Ni-rich Ti80−xNixHf20 alloys (numbers indicate at.%) the phase transformation temperatures are on average increased by more than 100 K by aging at 823 K for 2 h. Especially for those alloys with Ni contents less than 50.6 at.%, the martensitic transformation start temperatures (Ms) are higher than 473 K after aging. Transmission electron microscopy shows the presence of (Ti + Hf)3Ni4 precipitates after aging. Compared with the precipitation of Ti3Ni4 particles in Ni-rich Ti–Ni alloys, the precipitation of (Ti + Hf)3Ni4 particles in Ni-rich Ti–Ni–Hf alloys needs higher temperatures and longer times.

X.L. Meng

Papers

Recent development of TiNi-based shape memory alloys

Effect of aging on martensitic transformation and microstructure in Ni-rich TiNiHf shape memory alloy

Shape-memory behaviors in an aged Ni-rich TiNiHf high temperature shape-memory alloy

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

Phase transformation and precipitation in aged Ti–Ni–Hf high-temperature shape memory alloys