Effect of the degree of plastic deformation on the thermal electromotive force of cu-x5crni1810 steel thermocouple

Ivan Milicevic,Aleksa Maricic,Radomir Slavkovic,Milica Spasojevic,Miroslav Spasojevic

doi:10.2298/sos1804421m

Abstract

The thermal electromotive force (TEMF) and the thermal electromotive force coefficient (TEMFC) of the thermocouple consisting of a copper wire and an (X5CrNi1810) steel wire plastically deformed under tension or bending conditions were found to increase with increasing degree of plastic deformation. The increase in the degree of deformation disturbs the microstructure of steel due to increases in the density of chaotically distributed dislocations and internal microstress, resulting in a decrease in the electron density of states near the Fermi level. Through the effect of thermal energy, annealing at elevated temperatures up to 300?C leads to microstructural ordering along with simultaneous increases in the free electron density of states, TEMF and TEMFC. Based on the temporal change of the TEMF, the kinetics of microstructural ordering was determined. During the initial time interval, the process is a kinetically controlled first-order reaction. In the second time interval, the process is controlled by the diffusion of reactant species.

Highlights

When subjected to mechanical stress, metals undergo deformation simultaneously with a change in their microstructure
The objective of this study was to assess the effect of relative strain and heating temperature on the thermal electromotive force (TEMF) and the thermal electromotive force coefficient (TEMFC), and use this effect to establish the kinetics of microstructural ordering in deformed steel wires
Transmission electron micrographs of deformed samples of steel before and after annealing showed neither new phase formation nor notable precipitation. This indicated that the change in the TEMF is a plausible consequence of shortrange ordering rather than phase or precipitate formation. This is the result of structural rearrangement in the material during annealing, with specimens undergoing microstructural ordering as the density of chaotically distributed dislocations and the internal microstress decrease, resulting in a better overlap of 3d and 4s orbitals, which increases the electron density of states in the conduction band near the Fermi level

Summary

Introduction

When subjected to mechanical stress, metals undergo deformation simultaneously with a change in their microstructure. The metastable state of most deformed metals as well as of steel does not transform into a stable state at an appreciable rate since thermal energy is not large enough to overcome the activation energy for microstructural ordering [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Thermal energy reaches the value of the activation energy. This leads to microstructural ordering, causing an increase in the free electron density of states and changes

Objectives

Results

Conclusion