Grain refinement of coarse grained gold by combined thermo-mechanical process of severe plastic deformation and low temperature annealing

A high purity gold composed of coarse grains (d = 1.7 mm) was repeatedly thermo-mechanically processed by multi directional forging (MDF) at room temperature and annealing at warm temperature range. The effects of small addition of Ca (180 at. ppm) to Au on the static recrystallization (SRX) behavior and ultrafine grain evolution were investigated. Extensive SRX was observed in the MDFed Au sample to cumulative strain of ΣΔε = 1.2 even at 453 K. After 2 cycles of the repeated TMP, fine grains of about d = 10 μm was obtained in the Au sample. The Au-Ca alloy MDFed to ΣΔε = 1.2, however, was hardly SRXed even by annealing at 543 K. Subsequently, the fine-grained Au and Au-Ca alloy were further MDFed to ΣΔε = 8.0 at maximum at room temperature. By the prolonged MDF, average (sub)grain size of about 200 nm was achieved in the Au sample. The hardness increased with increasing cumulative strain. Although the tensile strength was also raised with cumulative strain, large loss of ductility did not appear.