Effect of low-temperature aging on phase transformation and superelasticity of niti alloys with different grain sizes

Abstract

Nanocrystalline (NC) and Ultrafine grain (UFG) NiTi shape memory alloys possess superior functional and structural mechanical properties than the traditional coarse grain (CG) ones. The superelasticity adjustment of the NC and UFG NiTi alloys is a significant issue in this field. Although grain size alteration can be used to adjust their functional properties, the superior structural mechanical properties (e.g., high strength, high hardness, high fatigue resistance, and high corrosion resistance, etc.) brought about by the special NC and UFG grain sizes may be deteriorated. Thus, how to realize the functional property adjustment of NC and UFG NiTi alloys without losing the original fine grain size becomes a frontier research issue. It was recently found that the low-temperature aging (LTA) can introduce Ni4Ti3 nanoprecipitates into the matrix of Ni-riched NiTi shape memory alloys without changing their grain size, which is an effective solution to address this issue. Therefore, to understand the influence of LTA on the phase transformation and superelasticity of NiTi alloys regarding different grain sizes is a fundamental precondition for the superelasticity adjustment of NC and UFG NiTi alloys via LTA. In this study, the effect of LTA on NC and CG NiTi alloys was comparatively investigated, and the feasibility of altering the phase transformation and superelasticity via LTA without changing grain size was verified. The different responses of NC and CG towards LTA were obtained. To obtain the NiTi alloy with different grain sizes, the influence of direct electric resistance heat treatment on NiTi alloy with different grain sizes and aging states was studied. It was found that the nanograin size suppresses Ni4Ti3 precipitation during the direct electric resistance heat treatment. Based on the logistic function, grain size adjusting models were established by altering direct current magnitude or heating time, respectively. By using this method, NiTi alloys with grain sizes ranging from 34 to 8021 nm were rapidly obtained. LTA at 573 K from 0 to 48 h was applied to the afore-obtained NiTi alloys, and its influence on their phase transformation characteristics was investigated. The variations of phase transformation sequence, temperature, enthalpy, thermal hysteresis, etc. with the alteration of grain size and the LTA time were obtained. The mechanisms of the different responses of the NiTi alloys with different grain sizes towards LTA were further investigated based on the microstructural evolutions and hardness variations. According to the experimental results, a B2 ->R phase transformation temperature model was established, which can calculate the B2 ->R phase transformation temperature at the given grain size and LTA time. At which deformation stage that the unloading happens greatly affects the superelastic stress-strain path. To understand the effect of unloading at different deformation stage on superelasticity, the superelasticity that unloads at the maximum applied strain of 3% and 9% are respectively studied, which represents the unloading within and beyond the plateau deformation stage respectively. The variations of the loading and unloading plateau stress and strain, stress hysteresis, residual strain, etc. with the alteration of grain size and the LTA time were obtained. Based on the microstructural evolution, hardness variation, and the corresponding phase transformation characteristics, the underlying mechanisms of the superelasticity variation were revealed. According to the experimental results, the function of the stress hysteresis increment and the post-plateau stress increment was proposed, and a stress hysteresis model considering the effect of grain size and LTA time was established. A superelastic stress model considered the effect of grain size and LTA time was established based on Graesser model and the superelasticity experimental results. The parameters of the model, which can alter the shape of stress-strain curves, were treated as a function of grain size and LTA time so that the whole model can reflect the effect of these two factors. The model can be used to calculate the superelastic stress-strain curves of NiTi alloy at a certain condition (within the maximum applied strain from 3% to 9%, grain size from 34 to 8021 nm, and LTA time from 0 to 48 h at 573 K). This study primarily investigated the influence of LTA on the phase transformation and superelasticity of the NiTi alloys with different grain sizes. It provides the experimental basis and theoretical reference to determine the processes and methods of the superelasticity adjustment of NC and UFG NiTi alloys by using LTA.