The influence of polycrystal grain size upon the creep ductility of copper
A study of the relationship between grain size and creep ductility has been made on OFHC copper when the fracture process involves intergranular cavitation. Tests have been carried out at a constant nominal strain-rate at 10−2 hr−1 at temperatures of 350°, 425°, and 500°C. At all temperatures a peak in the grain size vs ductility plots is obtained at (approximately) 30 μm for 350°C, 60 μm for 435°C, and 150μm for 500°C. The work involved a metallographic study of the fracture process and it is deduced that the final stage of fracture (cavity linkage) controls ductility, rather than either the rate of nucleation or individual growth of cavities. At coarse grain sizes and low temperature the crack length seems to be critical and increasing grain size decreases ductility. At fine grain sizes and high temperature, failure is by a ductile void-sheet process and the volume fraction of cavities is controlling so that decreasing grain size decreases ductility.
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