Investigation of size effect on flow stress of Cu-3wt.%Ag-0.5 wt%Zr thin sheets at room and cryogenic temperature based on geometrically necessary dislocation evolution behavior

Abstract

The influence of the grain sizes ranging from 23.4 to 69.2 μm on mechanical properties of Cu-3wt.%Ag-0.5 wt%Zr thin sheets was investigated by conducting uniaxial tensile experiments at room temperature (RT 298 K) and cryogenic temperature (CT 77 K). At RT, the strength and ductility of the sheet decreased as the grain size increased, and the size effect on flow stress was observed when the ratio of thickness to grain size (t/d) was less than 4.5. At CT, the sheet exhibited excellent strength and ducility, accompanied by a reduction in the size effect on flow stress. GND distribution maps showed that the average GND density of grains in the sample surface (surface grains) was lower than that of grains in the core of the sample (core grains). As the distance from the grain boundary increased, the GND density decreased. A constitutive model was developed for Cu-3wt.%Ag-0.5 wt%Zr thin sheets based on the dislocation accumulation and recovery process, which was able to predict the effects of grain size and deformation temperature on the flow stress and GND evolution in Cu-3wt.%Ag-0.5 wt%Zr thin sheets. This constitutive model indicated that CT not only inhibited the GND recovery process but likewise promoted the GND accumulation process. The established constitutive model revealed that the ratio of SSD density to GND density increased with increasing strain. The total dislocation density and GND density of samples deformed at CT was higher than those of samples deformed at RT.