Martensite Strain Memory in the Shape Memory Alloy Nickel-Titanium Under Mechanical Cycling

Abstract

This paper describes an experimental study of stress-induced martensitic phase transformation in the SMA Nickel-Titanium. The rich local thermo-mechanical interactions that underlie transformation are examined using three-dimensional Digital Image Correlation (strain fields) and infrared imaging (thermal fields). We quantify the complex local interactions between released/absorbed latent heat and the extent of transformation, and explore the characteristics of the phase fronts and the evolution of martensitic volume fraction. We also quantify a strong strain memory in the martensite that forms in the wake of the phase transformation front. The accommodated strain in the martensite will remain constant during loading, even as the existing phase front propagates. There also exists a remarkable strain memory in the martensite that persists from cycle to cycle, indicating that the local elastic stress fields in the martensite are driven by a dislocation structure and martensitic nuclei that largely stabilize during the first loading cycle.

Role of grain size on the martensitic transformation and ultra-fast superelasticity in shape memory alloys

“…We see that some grains fully transform while others remain in the austenite phase. This supports the idea that favorable grains transform first, and that local stresses can severely hinder transformation in certain regions [10,11].…”

“…The different crystallographic orientation between neighboring grains mechanically constrains their deformation and transformation, increasing the stress or undercooling that is required for the transformation to progress [10]. Latent heat also results in the suppression of the transformation [10,11]. These effects are more dominant in fine-grain alloys and experimental studies have shown that reducing the grain size of polycrystalline SMAs to the nanoscale hinders martensitic transformation, both reducing the martensite start temperature (Ms) and the amount transformed [12].…”

Failure of Fiber Reinforced Composite Archwires

“…The plateaus and hysteresis regarding the superelastic NiTi were not observed once these properties are mainly associated with phase transformation (Fernandes et al, 2011;Frenzel et al, 2010;Kim and Daly, 2011;Hou et al, 2011) which does not occur in FRC material. The figure 2 and table 1 demonstrated that FRC wires could cover the range of strength corresponding to the Ni-Ti wires under a deflection of 1.5 mm.…”

Microscale Repeatability of the Shape-Memory Effect in Fine NiTi Wires

“…It is known that several factors can influence the behavior of SE nickel-titanium under cyclic loads, including Ni content, heat treatment of the sample prior to testing, temperature, and strain rate at which the test is conducted, and initial microstructure, among others. Several aspects of the response of SE NiTi under mechanical cycling can be related to the development of dislocations and/or retention of martensitic nuclei, and the interplay between plastic slip and propensity to transform based on crystallographic texture [1][2][3][4][5][6][7][8][9][10][11][12]. In comparison, the relative lack of information on the SME response under thermo-mechanical cycling is due to, among other factors, experimental difficulties in obtaining the microstructure of martensitic NiTi and in characterizing the self-accommodation process that underlies the shapememory effect.…”

“…High-resolution microscale strains were captured using an emerging experimental approach using self-assembled nanoparticles, and used to infer the extent of transformation and examine its spatial similarity with thermo-mechanical cycling. It has been shown in previous work [12][13][14] that strain can be successfully used to determine transformation location and extent at both the macroscale and the microscale in superelastic Nitinol. At the microscale, full-field strain maps and their relationship to the underlying microstructure were successfully analyzed in superelastic Nitinol in order to determine active variants and the connection between detwinning and residual strain accumulation [14].…”