Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

Abstract

An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9 at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading. Both macroscopic stress-strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19 0 phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys.

Challenges During Microstructural Analysis and Mechanical Testing of Small-Scale Pseudoelastic NiTi Structures

“…Using similar initial microstructures, we compare two different types of samples during tensile testing, and we discuss how different surface morphologies, clamping conditions, and strain measurement techniques affect the thermo-mechanical response. We also compare the material's tendency for localized deformation (i.e., formation and growth of fully transformed martensite in distinct bands) in small-scale samples to the well-known bulk behavior of NiTi [8,9].…”

Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys

“…Within Stage I, samples deform linear-elastically as strain increases up~0.5 pct strain for both quasi-static and dynamic compression. While both samples exhibited an elastic modulus of about 32 GPa which is within the range of elastic moduli values reported in the literature, [15,52,55] there does appear to be some slight deviation from linearity in the dynamic compression curve in Stage I. After about 0.8 pct, the onset of the stress plateau was observed (Stage II), where the stress in the specimen increases less prominently with increasing strain.…”

“…[12][13][14][15][16][17][18] The characteristic transformation temperatures for the austenite-to-martensite phase transformation as well as the mechanical response can be modified to meet application requirements through (i) thermo-mechanical processing, [19][20][21] (ii) slight variation from the equi-atomic NiTi chemical composition, [22,23] or (iii) addition of alloying elements. [24][25][26][27][28] This phase transformation generally involves an austenitic cubic B2 structure transforming to and from either a martensitic monoclinic B19 or orthorhombic B19¢ structure.…”

Shape Memory Alloy Actuator Design: CASMART Collaborative Best Practices

“…Representative responses are shown in Figure 4. Traditionally, effective Poisson's ratios (ν * A , ν * M ) have been assumed [8,9], though recent measurements of Poisson's ratios have been reported for NiTi [6,10,11].…”