Stress-strain response properties of the KhN55MVTs nickel alloy under uniaxial compression and their mathematical modeling

Abstract

The rheology of the KhN55MVTs alloy has been studied at finite quasi-static strains and at operational and elevated temperatures to develop methods for calculating the emergency conditions of heat-stressed equipment. Some basic features of the stress-strain response of the alloy and the physical mechanisms responsible for them were revealed. Constitutive models are proposed to describe the most significant and crucial effects in assessing the structural integrity. The rheological properties of the alloy were studied under isothermal uniaxial compression within a temperature range of 24 – 1150°C with log-strains up to 1.0 and deformation rate of 0.001 – 0.125 sec–1. The main deformation mechanisms have been revealed via optical microscopy. A constitutive model predicting the shape of stress-strain curves (a modification of the Bergström dislocation-based model of rigid-plasticity) is proposed which make it possible to obtain the rate and temperature dependencies of the mechanical properties. The dependence of maxima (peak values) of stresses on the deformation rate and temperature exhibits a non-monotonic character, while the yield stresses are weakly rate-dependent, and the linear slope is almost rate and temperature independent. The microstructure tests revealed the absence of a correlation between the softening stage and the onset of dynamic recrystallization process. No microcracks were found. Serrated flow and acoustic emission were observed within a temperature interval of 24 – 900°C probably attributed to dynamic aging (above 500°C), deformation twinning, and autowave effects during localization of the plastic strain. The proposed models of rheological effects differ from the existing dislocation models in a wider range of application — in terms of strain rates (10–8 – 1,0 sec–1) and temperatures (0 – 80% of the melting point). The rheological effects revealed in the experiments, analysis of their physical nature and constitutive description can be used in assessing failures of heat-stressed equipment and improving the methods for calculating emergency situations.