Grain-resolved analysis of localized deformation in nickel-titanium wire under tensile load

Abstract

The stress-induced martensitic transformation in tensioned nickel-titanium shape-memory alloys proceeds by propagation of macroscopic fronts of localized deformation. We used three-dimensional synchrotron x-ray diffraction to image at micrometer-scale resolution the grain-resolved elastic strains and stresses in austenite around one such front in a prestrained nickel-titanium wire. We found that the local stresses in austenite grains are modified ahead of the nose cone-shaped buried interface where the martensitic transformation begins. Elevated shear stresses at the cone interface explain why the martensitic transformation proceeds in a localized manner. We established the crossover from stresses in individual grains to a continuum macroscopic internal stress field in the wire and rationalized the experimentally observed internal stress field and the topology of the macroscopic front by means of finite element simulations of the localized deformation.

Virtual Extensometer Analysis of Martensite Band Nucleation, Growth, and Strain Softening in Pseudoelastic NiTi Subjected to Different Load Cases

“…Only once these interfaces move apart (which occurs at constant stress as the same process, the passing of an interface across yet untransformed material, occurs repeatedly), the martensite band begins to grow and the fully transformed martensitic region in the center of the band can be observed. It is important to understand that, in agreement with the data presented in [43], material softening is likely to occur until the SIMT is completed, but the local reduction of stress is related to the complex stress state in the moving interfaces [44], whereas only the initial stress drop during band nucleation could be measured macroscopically and related to the intermediate strains observed in the interface regions in our experiments. …”

Tensile Deformation of Superelastic NiTi Wires in Wide Temperature and Microstructure Ranges

“…Stress-strain curves of wires selected for systematic tensile testing in wide temperature range are in bold and colored (pulse times (8,10,12,14,15,16,18,20) ms). It is well known, that tensile deformation of NiTi wires in the plateau range is localized in macroscopic interfaces-martensite band fronts propagating at constant force along the wire and converting the austenitic microstructure into the presumably martensitic microstructure within the martensite band [14]. It is much less known that these wires fail by strain localization in necks.…”

“…Stress-strain curves recorded in tensile tests until rupture on various NiTi wires (pulse times (8,10,12,14,15,16,18,20) ms) in temperature range from -100 up to 200°C are shown in Fig. 6.…”

Grain interaction mechanisms leading to intragranular orientation spread in tensile deformed bulk grains of interstitial-free steel

“…For completeness applications of 3DXRD to minerals (Borthwick et al, 2012;Hall and Wright, 2015;Sørensen et al, 2012a), deep earth science (Nisr et al, 2014(Nisr et al, , 2012Rosa et al, 2015), nuclear materials (Brown et al, 2014;X. Zhang et al, 2015), superalloys (Sedmák et al, 2016) and ferroelectrics Majkut et al, 2016;Varlioglu et al, 2010) should also be mentioned.…”