Hot deformation behavior of Zr-l%Nb alloy: Flow curve analysis and microstructure observations

摘要

The hot deformation behavior of Zr-l%Nb alloy has been studied at temperatures of 770-850 ℃ and strain rates of 10~(-2) to 1 s~(-1). To minimize friction during the tests at high temperature, the ends of the specimens were coated with graphite powder and a thin layer of mica was placed between the face of the specimen and the anvils. In order to investigate the warm working behavior of Zr-l%Nb alloy, all data were analysed stepwise and the step-by-step procedure was provided. The occurrence of the dynamic recrystallization (DRX) phenomena at different working conditions was analysed by the work hardening rate analysis. The stress-strain curve of many samples was identical to the typical DRX stress-strain curve which consists of a single peak stress followed by a gradual decrement toward the steady-state stress. Microstructural observations showed that at low temperatures and high strain rates the flow curves can be explained by localized deformation as a consequence of shear band formation. The transmission electron microscopy (TEM) analysis showed that the DRX has taken places through a continuous mechanism. At high Zener-Hollomon (Z) parameter a drop in flow stress was detected as a consequence of adiabatic deformation heating. The warm working constants of this alloy were determined through the general constitutive equations. Furthermore, after analyzing the work hardening behavior of this alloy, the activation energy for deformation of Zr-l%Nb alloy was calculated as 514 kJ/mol.