Influence of heat treatment on thermokinetic EMF during reverse phase transition in titanium nickelide

Abstract

The effect of duration and annealing temperature in the range of 400–800 °C on the thermokinetic EMF value in titanium nickelide, the composition of which is close to the equi-atomic one, at a reverse phase transition was investigated. Thermokinetic EMF was measured directly using a digital millivoltmeter MNIPI V7-72. The phase and elemental composition of the alloy and the kinetics of thermoelastic phase transformations have been checked by X-ray diffraction and calorimetric studies, and X-ray microanalysis. Annealing at temperatures of 500 and 800 °C leads to an increase in the thermokinetic EMF value from 0.22 to 0.25 mV. Removal of the oxide layer from the sample surface annealed at 700 °C for 0.5 h leads to an increase in the thermokinetic EMF value from 0.22 to 0.26 mV for the 1-st thermal cycle. It was found that thermal cycling causes a decrease in the thermokinetic EMF values down to 0.98 mV for the 20th thermal cycle for the samples without an oxide layer and to 0.3 mV for the samples with an oxide layer, respectively. With the increase in annealing time up to 20 h at 700 °C, the decrease in the thermokinetic emf value to 0.16 mV was observed. The thermokinetic EMF value after heat treatment is associated with changes in the physical and mechanical properties of the alloy and characterized by a shift of the characteristic temperatures of the phase transition. The research results are important for understanding the physics of thermoelectric phenomena in shape memory alloys during nonstationary heating and can be used both to control the homogeneity of their physical and mechanical properties and to design smart actuators and sensors, mechanisms of control systems.

Thermokinetic EMF during a reverse phase transition in titanium nickelide as a way of information recording

“…The thermokinetic EMF value depends on the temperature in heating zone: it arises at the temperature of the beginning of the reverse phase transformation and reaches a maximum at the temperature of the end of the reverse phase transformation. The temperature and duration of heat treatment, thermal cycling, oxide layer, preliminary deformation, loading influence on the functional properties of the TiNi alloy and, therefore, should influence on the thermokinetic EMF value [2][3][4][5][6].…”