Texture and Strain Measurements from Bending of NiTi Shape Memory Alloy Wires

Abstract

Shape memory alloys (SMAs) are a new generation of materials that exhibit unique nonlinear deformations due to a phase transformation which allows the material to return to its original shape after removal of stress or a change in temperature. These unique properties are the result of a martensitic/austenitic phase transformation through the application of temperature changes or applied stress. Many technological applications of austenitic SMAs involve cyclical mechanical loading and unloading in order to take advantage of pseudoelasticity, but are limited due to poor fatigue life. In this paper, commercial pseudoelastic NiTi SMA wires (50.7 at.% Ni) were placed under different bending strains and examined using scanning electron microscopy and high-energy synchrotron radiation X-ray diffraction (SR-XRD). By observing the microstructure, phase transformation temperatures, surface texture and diffraction patterns along the wire, it is shown that the wire exhibits a strong anisotropic behavior whether on the tensile or compressive side of the bending axis and that the initiation of micro-cracks in the wires is localized on the compression side, but that crack propagation will still happen if the wire is reloaded in the opposite direction. In addition, lattice strains are examined for both the austenite and martensite phases.

The electron beam freeform fabrication of NiTi shape memory alloys. Part I: Microstructure and physical–chemical behavior

“…EDX data of the base materials as well as of the first, fifth, and tenth layer of the as-built NiTi-EBF3 specimens. The flatness of the wire's DSC profile may be due to the large plastic deformation resulting from the drawing process, which completely suppresses any transformation, 33 or may be the result of very low transformation temperature out of the range measured by the DSC. On the other hand, both SP and NiTi-EBF3 specimen show endothermic and exothermic peaks denoting the austenite $ martensite (M $ A) transformation, implying the occurrence of phase transformation upon heating and cooling.…”

High-energy synchrotron X-ray diffraction measurements of simple bending of pseudoelastic NiTi shape memory alloy wires

“…The volume has increased after phase transformation, the formation of martensite increase the local strains, which also induce the formation of microcarcks of NiTi wire. Further analysis of the data is presented in another publication (Carl et al , 2016), which addresses off-axis rotation of the bent wire and its effect on texture and strain measurements.

Figure 7. (Color online) Lattice strain maps for a heat-treated NiTi SMA wire in the (a) bending and (b) transverse to bending directions correspond to the same points in Figure 2(b).

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“…The volume has increased after phase transformation, the formation of martensite increase the local strains, which also induce the formation of microcarcks of NiTi wire. Further analysis of the data is presented in another publication (Carl et al, 2016), which addresses off-axis rotation of the bent wire and its effect on texture and strain measurements.…”

Experimental investigation of the pseudoelastic behavior on zirconium modified Cu–Al–Be shape memory alloys for seismic applications

“…Martensite phase (o) peaks of Zr 1 diffractogram, as depicted in figure 10, consists of several intersecting variants, viz., (2 0 2), (0 0 22), (2 0 12) and (2 4 8) reflections associated with different orientations of the stabilized β ′ 1 -phase, caused by mutual interactions of the martensite plates [62]. Besides, high intensities of the reflections indicated a more significant number of stabilized martensite plates remain permanently within the parent β 1phase [21,63].…”