Microstructure and mechanisms of cyclic deformation of aluminum single crystals at 77 K
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TRODUCTION
THE emphasis of this study is the examination of cyclic deformation of aluminum single crystals under single slip. There have been numerous studies of the cyclic deformation of single crystals, but they have often emphasized copper, as described in several more recent reviews of cyclic deformation.[1–5] Some work has been performed on other metals, including aluminum,[6,7,8] but these works are less detailed and have not fully determined the extent to which the copper results are applicable to other metals such as aluminum. Recent work[6,7,8] clearly shows that deformation of aluminum single crystals oriented for single slip at ambient temperature (0.32 Tm) results in a cellular dislocation substructure. This contrasts the ambient temperature observations in copper (lower stacking fault energy metal and lower homologous temperature at 0.22 Tm) oriented for single slip, where dislocations of a single Burgers vector are of a well-defined ‘‘vein’’ arrangement. In copper, deformed just prior to saturation (or before the observation of persistent slip bands (PSBs)), dislocations appear to be principally of edge character and these are arranged in dense bundles or veins of dipoles. The remaining volume consists of channels containing debris, suggested as small dipole segments and/or vacancy loops. Screw dislocations in copper, with the same Burgers vector as the dipoles, span the channels. The cellular substructure in aluminum single crystals oriM.E. KASSNER, Chevron Professor of Engineering and Director of Graduate Program in Materials Science, is with the Department of Mechanical Engineering, Oregon State University, Corvallis, OR 973316001. M.A. WALL, Consultant, is with M L Tech, Stockton, CA 95267. M.A. DELOS-REYES, formerly Graduate Student, Materials Science Program, Oregon State University, is Engineer, Hewlett-Packard, Corvallis, OR. Manuscript submitted May 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ented for single slip at ambient temperature is perhaps a consequence of cross-slip facilitated by a higher homologous temperature (T/Tm) and higher stacking fault energy. Two investigations[7,8] performed single-slip cyclic aluminum (single-crystal) experiments at 77 K. Although detailed transmission electron microscopy was not performed, it appears that slip was more planar and the dislocation substructure may have resembled the ambient-temperature copper substructure. Regardless of the material, the mechanism of cyclic deformation is not firmly understood. Several events may be responsible for plasticity on the primary slip system with deformation in either the forward or reverse direction. An early explanation by Feltner[9] was ‘‘dipole flipping,’’ which consists of a dipole in a vein (or a PSB wall) changing from one 45 deg equilibrium position to another. Others also consider the screw dislocations in the channels that ‘‘connect’’ edge segments, which belong to the dipole patches or veins[1,4,10,11] and suggest that at least some of the reversible strain is caused by reversed screw motion.
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