JOURNAL OF MATERIALS SCIENCE 23 (1988) 2220-2224 The influence of stacking fault energy on ductile fracture micromorphology KENNETH S. VECCHIO, RICHARD W. HERTZBERG Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA The influence of stacking fault energy on microvoid coalescence in "pure" materials has been studied. It was shown that as a material's stacking fault energy (SFE) decreased, the extent of microvoid coalescence that occurred during ductile fracture also decreased. The decrease of microvoid coalescence in low SFE materials was attributed to a hindrance in the development of dislocation cells associated with the restricted motion of dislocations. In "pure" materials, microvoids are believed to initiate and grow along dislocation cell walls formed during deformation. As such, the absence or scarcity of cells in lower SFE materials limits the forma- tion of these voids during ductile fracture. 1. Introduction Microvoid coalescence is the common mode of duc- tile fracture in many engineering alloys and involves the nucleation, growth and eventual coalescence of many microvoids across the final fracture plane. The fracture surface resulting from these processes is characterized by the presence of nominally equiaxed (hemispherical) or elongated (parabolic) depressions. It is generally argued that microvoids initiate and grow from second phase particles in commercial materials. Void initiation can occur in these materials either by particle cracking [1] or by particle/matrix decohesion [2]. However, these mechanisms cannot explain the formation of microvoids in high-purity materials. In "pure" materials as well as in "clean" materials containing strongly bonded, ductile second phases, microvoids are believed to initiate and grow along the dislocation cell walls which form during the deformation process. Numerous studies during the past 15 years have used the high voltage electron microscope (HVEM) to directly observe the initiation of voids during in situ straining experiments (e.g. Wilsdorfand co-workers [3-6]). In each of these inves- tigations, microcracks (or voids) were seen to initiate and propagate along dislocation cell walls although none of these investigations attempted to correlate the dislocation substructures to the resulting fracture surface morphology. As microvoids in pure materials initiate and grow along dislocation cell walls, one might expect that the nature and extent of microvoid formation should be related in some manner to the dislocation cell size and the density of dislocations within the cell walls [7]. Specifically, it would be expected that with decreasing stacking fault energy (SFE) of a "pure" material, the size and extent of microvoids would decrease because SFE is a controlling parameter in the formation of dislocation cells. In low stacking fault energy materials, dislocation cross-slip is difficult because the partial dislocations are widely separated with their recombination onto the new slip plane requiring considerable energy. As a result, dislocations are restricted to planar arrays and dislocation cells do not readily form. With increasing SFE, the partial dislocations are closer together and may recombine readily to facilitate cross-slip. As such, dislocation cell formation is enhanced. Therefore, differences in microvoid morphology should be expected with changing SFE in association with void nucleation at dislocation cell walls. The objective of this study is to examine the effect of dislocation cell structures as influenced by SFE on the formation of microvoids. 2. Experimental procedure Three high-purity (> 99.999% pure) materials were selected for this study: aluminium (SFE ~- 200 mJ m-2), copper (SFE- 50mJm-2), and copper- 7wt% aluminium (SFE ~ 3mJm-2). The alloy was induction melted under vacuum and all three materials were suitably cold-worked and annealed to produce samples of the same grain size. Following the annealing treatments, round bar tensile speci- mens from each material were machined in accor- dance with ASTM E8 specification [8] for subsize tensile bars. Axial tensile tests were conducted at ambient temperature on these samples using an Instron tensile machine at a strain rate of 2 x 10 -3 cmsec -~ . Additional tensile tests were conducted at 400 ° C with copper and at - 150 ° C with aluminium samples using an environmental chamber. The purpose of the high- and low-temperature tests was to alter the dislocation cell sizes for a given material by the enhancement (at high T) or the restriction (at low T) of dislocation climb and thermally activated cross-slip. Therefore, if the premise of this research is true, then the size and character of the microvoids for a given material should change with test temperature. All fracture surfaces were examined in an Etec Autoscan scanning electron microscope (SEM) to determine the size and nature of the microvoids 2220 0022-2461/88 $03.00 + .12 © 1988 Chapman and Hall Ltd.