The Influence of Aging on Critical Transformation Stress Levels and Martensite Start Temperatures in NiTi: Part I—Aged Microstructure and Micro-Mechanical Modeling

Abstract

Transmission electron microscopy is used to determine the microstructures of a Ti-50.8 at% Ni alloy given different aging treatments. Two different peak-aging treatments are shown to result in disk shaped semi-coherent Ti3Ni4 precipitates with a diameter ranging from 50 nm to 200 nm depending on the aging temperature. In the peak-aged materials, strong strain fields are clearly visible on TEM micrographs. An Eshelby based model is used to predict the local stress fields due to the differences in the lattice parameters of the precipitates and surrounding matrix. The position dependent local stress fields are then resolved onto the 24 different martensite correspondence variant pairs (CVP’s). It is further demonstrated that due to the unique orientation relationship that exists between the precipitate variants and the martensite CVP’s, the local resolved shear stresses are extremely large on some CVP’s and negligible on others. When the Ni rich NiTi is over-aged, it is found that the precipitates coarsen to approximately 1000nm, they become in-coherent, and the local stress fields disappear. It is also determined that after over-aging the average composition of the matrix drops from 50.8 at% Ni to approximately 50.4 at% Ni. In a subsequent paper (part II) the results here are used to explain the dependence of the critical transformation stress levels and martensite start temperatures on the aging treatment.

Influence of precipitation and dislocation substructure on phase transformation temperatures in a Ni-rich NiTi-shape memory alloy

“…Evaluation of Equation 2 yields decreases of 0.22 and 0.27 at.-% depending on the type of particles (TinNi|4 or Ti 3 Ni 4 ). A direct comparison of these results suggests that shifts in PTTs are associated with the precipitation of T^Ni^ Our results show that the method described in [13] can be experimentally validated. There is an interaction between martensitic phases and subgrain boundaries as has already been observed [9] and described [9,11] in the literature and which was again identified in the present study, Figure 4b.…”

Tensile deformation of NiTi wires

“…The introduction of Ti 3 Ni 4 precipitates in NiTi is known to favor the martensitic transformation and suppress dislocation motion. 47,48 Despite this knowledge, the effect of the precipitates on the overall stress-strain response of NiTi wire has not been systematically examined in prior work. In the present study, we show that precipitates have a marked influence on the overall shape of the stress-strain response in addition to causing material strengthening.…”

Reduction of environmentally induced cracking of laser-welded shape memory NiTi wires via post-weld heat-treatment

“…It has been reported by Gall et al [34,35] that the precipitation of coherent Ni4Ti3 particles after heat treatment can greatly influence the critical stress level required for martensitic transformation by the generation of strong local stress fields between the precipitates and the NiTi matrix. Such local stress fields as a consequence of the lattice parameter mismatch between the precipitates and the matrix create preferential nucleation sites for martensitic transformation, and hence lower the critical stress level to induce the transformation process [36].…”

“…However, it is worthwhile to note that size and coherency of the Ni4Ti3 precipitates are critical in affecting the mechanical and functional properties of the weldment [34][35][36].…”