The phenomena of appearance of thermo-emf at non-stationary heating in equiatomic titanium nickelide was examined. The peculiarities of kinetics of display of thermokinetic emf, also during thermocycling were determined. The possibility of generation of thermo-emf in amorphous-crystalline ribbon of Ti50Ni25Cu25 during the transition to crystalline state was determined.
Приведены результаты исследований электросопротивления никелида титана в условиях его нестационарного нагрева. Установлены особенности изменения электросопротивления, в том числе при термоциклировании в процессе многократного нестационарного нагрева. Ключевые слова: эффект памяти формы, никелид титана, электросопротивление, термоЭДС. The results of the electrical resistance investigation in titanium nickelide upon non-stationary heating are presented. The changes of the electrical resistance under thermal cycling have been determined.
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.
The behavior of the thermokinetic EMF and electrical resistance upon nonstationary heating of the elongated TiNi wire samples with a near-equiatomic composition having sections subjected to elastic and plastic deformation was studied. It was found that the thermokinetic EMF value sharply increases in the deformed zone in the 1 st thermal cycle with the movement of the heating zone along the sample. The increase in the relative deformation from 1 to 30% leads to a change in thermokinetic EMF (|ΔE|) from 0.01 to 0.37 mV. If the sample undergoes deformation up to 2%, the thermokinetic EMF value in the deformation area corresponds to the value on the nondeformed section during the 2 nd thermal cycle. The value of |ΔE| is increased by 0.1 mV during deformation from 2 to 10% and is not changed with an increase in deformation up to 30%. The behavior of the electrical resistance is similar to the behavior of the thermokinetic EMF when the heating zone moves along the length of the Ti-50 at.% Ni wire sample in deformed zone from 2 to 15%. The electrical resistance increases sharply upon the 1 st thermal cycle in the deformation zone. The electrical resistance increases by 25 μΩ • cm with an increase in the applied deformation up to 15%. The value of electrical resistance does not change in the zone of deformation up to 2% upon the 2 nd thermal cycle. When the value of applied deformation falls in the range from 5 to 15%, the electrical resistance falls by 5 ÷ 20 µΩ • cm. Deformation of the Ti-50 at.% Ni sample leads to a change in the properties of the alloy in the deformation zone, causing a shift in the characteristic temperatures of the phase transition and a change in the thermokinetic EMF and electrical resistance when the heating zone passes through the deformation zone. Changes in the thermokinetic EMF and electrical resistance as the heating region passes through the deformation zone are associated with a change in the characteristic temperatures of the phase transition. Based on the experimental data, a method and devices for determining inhomogeneous areas in elongated products made of shape memory alloys were developed. The method allows the value of thermokinetic EMF or electrical resistance to be continuously recorded during winding the wire when its section is heated above the temperature of the reverse phase transition. Tracking the change in the thermokinetic EMF or electrical resistance, it is possible to determine the sections of the material, which differ in physical properties from the predeterminated properties.
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