Ultralow-fatigue shape memory alloy films

Abstract

Functional shape memory alloys need to operate reversibly and repeatedly. Quantitative measures of reversibility include the relative volume change of the participating phases and compatibility matrices for twinning. But no similar argument is known for repeatability. This is especially crucial for many future applications, such as artificial heart valves or elastocaloric cooling, in which more than 10 million transformation cycles will be required. We report on the discovery of an ultralow-fatigue shape memory alloy film system based on TiNiCu that allows at least 10 million transformation cycles. We found that these films contain Ti2Cu precipitates embedded in the base alloy that serve as sentinels to ensure complete and reproducible transformation in the course of each memory cycle.

Cryogenic superelasticity with large elastocaloric effect

“…28 It has been reported that the near o1004-oriented single crystal exhibits very large ε SE approaching 10%. 31 If we assume ε SE = 10%, σ Ms = 50 MPa at 4.2 K, temperature-independent σ hys = 20 MPa, and critical slip stress = 500 MPa in the near o1004-oriented single crystal, operations of superelasticity at temperatures up to 366 K and of the EC effect at temperatures as low as T CO = 24 K may be feasible; one specimen can exhibit both superelasticity and the EC effect over a wide temperature window of~340 K. As previously mentioned, the crystallographic compatibility between the parent and martensite phases [10][11][12][13] was considered to be a factor determining the magnitude of the MT temperature hysteresis. Although experimental investigations have not been attempted, this guidance appears to be valid for suppressing σ hys and temperature hysteresis; thus, the existing COP mat metrics ( Figure 5) may be revised for such supercompatible superelastic alloys.…”

“…In contrast, σ hys in SMAs with large ε SE , for example, in FeNiCoAlTaB, 9 is generally large. Recently, an extremely small temperature hysteresis and thereby excellent fatigue properties were achieved in certain SMAs [10][11][12] because of the very small lattice mismatch between the parent and martensite phases 10,11 or among them and precipitates 12 arising from special relationships intrinsic to their lattice correspondence [10][11][12][13] (termed 'supercompatibility' by James 14 ). Another, more conventional approach involves tailoring the microstructure to reduce intergranular constraints; the fatigue properties of Cu-Al-Mn SMA in the forms of a single crystal, bamboo-like polycrystals, and columnar-oriented polycrystals have been successfully improved while realizing small σ hys and large ε SE .…”

Deformation concentration for martensitic microstructures in the limit of low volume fraction

“…It has been found in experiments that compatibility between the phases is closely related to the width of the hysteresis loop accompanying the phase transformation (see [29,20,43,42,37,40,2]). This theory of hysteresis predicts that the energy barrier inf I p ε,θ plays a major role for reversibility of the phase transformation (see [44,11]). …”

Length scales and pinning of interfaces

“…Recently, Lisfi et al [20] observed almost negligible ferromagnetic hysteresis in an annealed Co 65 Fe 35 alloy with heterogeneities (precipitates) at the scale of nanometres and ferromagnetic domain walls at the scale of 100 nm. The recently discovered shape-memory alloy [21] that shows remarkably low hysteresis and functional fatigue (stress-strain curve unchanged over 10 7 cycles) contains nano-precipitates. In dislocations, it has long been known that solution hardening is effective in body-centric cubic alloys with compact cores, but less so face-centred cubic alloys with an extended core [5].…”