The inter-relationship between grain boundary sliding and cavitation during creep of polycrystalline copper
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I.
INTRODUCTION
T H E process of grain boundary sliding (GBS) involves the movement of individual grains which slide over each other with the movement taking place at, or in a zone immediately adjacent to, their common boundary, f~,2,31 Two distinct types of sliding may be identified,t41Lifshitz sliding refers to the sliding which accommodates diffusion creep and where the grains become elongated such that there is no net increase in the number of grains lying along the tensile axis,t~-8] whereas Rachinger sliding refers to the relative displacement of individual grains in high-temperature creep such that they retain essentially their original shape and there is an increase in the number of grains lying along the tensile axis. t9,~~ It is now well established that the occurrence of Rachinger sliding during high-temperature creep may lead to the nucleation, growth, and subsequent interlinkage of grain boundary cavities, thereby promoting the premature failure of the material51~-J6] However, there has been little or no attempt to investigate whether there is any inter-relationship between the rate and extent of GBS and the development of internal cavitation. The present investigation was motivated by very recent developments which make it possible to obtain, using a computer acquisition system, detailed quantitative information on the precise morphologies of any internal cavities.t~7] The experiments were conducted using polycrystalline copper,'because it is well known that copper and copper-based alloys often exhibit very extensive cavitation after creep testing at elevated temperatures.tl8.~9.2o]
AKWASI AYENSU, on leave from the Department of Physics, University of Cape Coast, Cape Coast, Ghana, is Fulbright Scholar, Department of Materials Science, University of Southern California, Los Angeles. TERENCE G. LANGDON, Professor, is with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. This article is based on a presentation made at the "High Temperature Fracture Mechanisms in Advanced Materials" sympsosium as a part of the 1994 Fall meeting of TMS, October 2 ~ , 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee. METALLURGICALAND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL MATERIAL AND PROCEDURE
The material used in this investigation was copper of commercial purity (ASTM No. 1 I0) supplied by the Olin Corporation, New Haven, CT, in the form of hot-rolled sheets. A chemical analysis of the material gave 0.001 wt pct Ag, 0.005 wt pct Fe, and 0.015 wt pct O with the balance (99.979 wt pct) as Cu. Flat tensile specimens were machined with gage lengths of 2.54 cm and with the tensile axes parallel to the rolling direction. The Cu samples were annealed in flowing argon at 1123 K for 2 hours and then mechanically polished down to 0.05 /zm alumina and etched in a mixture of HNO3 and HCI. The average spatial grain size, d (equivalent to 1.74 X L, where L is the mean linear intercept grain size), was dete
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