Quasi‐Linear Superelasticity with Ultralow Modulus in Tensile Cyclic Deformed TiNi Strain Glass

Abstract

Herein, a quasi‐linear superelasticity (≈2.3%) with the ultralow elastic modulus (≈18 GPa) over a wide temperature range (≈55 K) in tensile cyclic deformed TiNi shape memory alloys is reported. Differential scanning analysis (DSC) and dynamic mechanical analysis (DMA) show the suppression of normal martensitic transformation (MT) and the appearance of strain glass transition behavior. Electron backscatter diffraction (EBSD) analysis shows the existence of high‐density defects involving twin boundaries and dislocations under tensile cyclic deformation. Further transmission electron microscope (TEM) observation reveals stabilized B19' nanodomains after tensile cyclic deformation. The observed deformation defects and stabilized B19' nanodomains impose significant influence on the following transition behavior and mechanical property, yielding the detected quasi‐linear superelasticity with ultralow modulus. Herein, an effective way to design unique phase transitions with unprecedented properties by defect engineering may be suggested.

A polymer-like ultrahigh-strength metal alloy

“…1b) during loading/unloading even at a moderate stress of ~0.3 GPa. This is a well-observed behavior of Ti-Ni alloys when dislocation slip occurs during a stress-induced martensitic transformation 14 . Thus, the starting B2 alloy is neither a strong alloy nor a polymer-like alloy.…”

“…3) or a conventional stressinduced martensitic transformation (Extended data Fig. 1b ), where the inevitable nucleation leads to not only a much higher elastic modulus of E ~30-70 GPa but also a strongly nonlinear and hysteretic stressstrain behavior with a stress plateau 14,16 .…”

“…Over the past decades, signi cant efforts have been made to seek metal alloys with simultaneously high strength and low modulus [4][5][6][7][8][9][12][13][14][15][16][17] ; but an alloy showing both steel-like high yield strength (σ y >1 GPa) and polymer-like low modulus (E ~10 GPa) still remains unattainable. So far, several alloys based on shape memory alloys (SMA) 5,16,17 are reported to reveal 1 GPa-class high strength and a moderately low modulus E ~30 GPa, and a conventional Mg-Sc strain glass alloy 7 has recently been shown to possess a lower modulus E ~20 GPa but with a lower strength σ y ~0.3 GPa.…”

A mechanical criterion for Lüders-type deformation of polycrystalline NiTi

“…In fact, the suppression of Lüders-type deformation by such factors is also evident in the literature. One example is the "linear pseudoelastic behaviour" (total absence of Lüders deformation) of cold worked [52][53][54] or pseudoelastically cycled NiTi under tension [55,56]. This may be attributed to the effect of massive defects within the matrix introduced by the processes.…”