In Situ Neutron Diffraction Studies of Increasing Tension Strains of Superelastic Nitinol

Abstract

A micromechanical study of the effect of varying amounts of tensile strains on the microstructures and subsequent mechanical behaviors of superelastic Nitinol rods is presented. It is found that strains up to *8-9 % develop microstructures that assist both forward and reverse transformation relative to un-strained material. This superelastic phenomenon is explained to be analogous to two-way shape memory effect in Nitinol actuation materials. These results provide understanding as to why such ''pre-strains'' may lead to improvements in subsequent superelastic fatigue life. Beyond 9 %, a drastic change is observed, as large amounts of martensite (75 % and more) are retained in unloaded samples. Thus, a competition between transformation, plasticity, and reorientation is found to give rise to microstructures that inhibit complete transformation. Furthermore, even though similar inelastic strain magnitudes are observed in loading and unloading plateaus, micromechanical mechanisms differ substantially from samples with less pre-strain. For example, in highly pre-strained samples at least half of the plateau strains are due to martensite reorientation, whereas, in low and moderately pre-strained samples nearly the entirety of the plateau strain is due to transformation. We also find that latent heat of plastic flow is larger than latent heat of transformation.

Specific forward/reverse latent heat and martensite fraction measurement during superelastic deformation of nanostructured NiTi wires

“…Furthermore, the path described by the martensite fraction in Fig. 15, where its value is higher during unloading than during loading, was also reported in the study of [47].…”

In Situ Neutron Diffraction Studies of Large Monotonic Deformations of Superelastic Nitinol

“…Furthermore, building upon a previous in situ study of shapesetting Nitinol [14], the current work provides some fundamental insight into another important processing issue: the effect of pre-strain and plasticity on Nitinol microstructures, and why pre-straining a device into Regime iii or iv may lead to improvements in fatigue lives of components [27,28]. We further explore this issue in a parallel work investigating the effects of different amounts of tension pre-strain on Nitinol microstructures and mechanics [29].…”

“…They also support the conclusion that in bending a wire, the detrimental deformation would occur in compression due to the large strain asymmetry found in this mechanistic transition. These factors will be explored in more detail in the companion paper [29].…”

Anisotropy and Clausius-Clapeyron relation for forward and reverse stress-induced martensitic transformations in polycrystalline NiTi thin walled tubes

“…There are other evidences that martensitic transformation completion does not coincide with the end of stress plateau [30,31,40]. One work in particular, by Pelton et al [28], shows that at the end of macroscopic stress plateau the martensite fraction has reached around 85%. These measurements were performed with neutron diffraction technique, which allows to compute the behaviour of the bulk material.…”

“…In this paper, the increment dε tr is assumed proportional to the infinitesimal step df m [28]: dε tr = df m ∆ε tr , where ∆ε tr is the tensile transformation strain for a complete transformation. Thus, W tr mech = 1 ρ σ∆ε tr .…”