Dynamic grain growth and ductility enhancement at intermediate temperature

Abstract

Dynamic grain growth has been observed during hot tensile tests of copper at intermediate temperatures from 0.50 to 0.75 T m over the strain rate range of about 3 × 10 −5 3 × 10 −3 s −1. Microstructural examination revealed that at strain rates < 3 × 10 −5 s −1, dynamic recovery dominates and the grain sizes of the specimens after the test are comparable to that of the original material. When the strain rate is increased above 3 × 10 −5 s −1, dynamic grain growth occurs in that the sparsely nucleated grains are able to glow freely during the test, resulting in the formation of exceedingly large grains. As the strain rate is increased further above 3 × 10 −3 s −1, dynamic recrystallization sets in and the final grain size drops to below the original value. When dynamic grain growth occurs, the stress-strain curves become serrated with a few coarse ripples of gradually decreasing peaks. Interrupted tests further revealed that the stress drop during the test is associated with the occurrence of rapid grain growth in the material. Boundary serration is also a common feature under the test conditions when dynamic grain growth is observed. Detailed metallographic and fractographic studies further showed that the occurrence of dynamic grain growth is responsible for the upturn of the hot tensile ductility of copper with increasing temperature, during which the fracture mechanism changes from profuse grain boundary cavitation with low ductility to sparse wedge cracking with considerable ductility.