70-30黄銅の673 Kにおける延性に及ぼす結晶粒径およびひずみ速度の影響

Abstract

The purpose of this paper is to consider the effects of initial grain size and strain rate on the ductile behavior of a Cu-30 mass%Zn alloy under high temperature deformation, and the related microstructural changes associated with it. Salt bath tensile tests on specimens with initial grain sizes of 12, 22 and 45 μm were studied under various strain rates from 3.3×10−5 s−1 to 2.0×10−2 s−1 at 673 K. The elongation and the fractured optical microstructures of the specimens were investigated. Maximum elongation of the specimens occurred at an initial grain size of 12 μm under a strain rate of 6.7×10−4 s−1. In this case most of the microstructure of the specimens changed to a fine dynamic recrystallized structure. The possibility that the principal deformation mechanism of the fine structure is grain boundary sliding, similar to superplasticity should be considered. As a consequence, the specimens exhibit high elongation. Under the lower strain rates, dynamic recrystallization occurred, i.e. dynamic recrystallized fine grains were observed. The dynamic recrystallized grains grew after the deformation, and thus the elongation of the specimens decreased.By increasing the strain rate or the initial grain size, the slight dynamic recrystallized structure occurred around the initial grain boundary. The dynamic recrystallized structure zones are geometrically softer than the initial grains, and thus basal slip is likely to be the dominant mechanism. Therefore the specimens show low elongation and shear fracture. Furthermore, the extent of ductility depends on the shape of the cavity which is induced by the differences of the initial grain size and the strain rate.