Microstructure and mechanisms of cyclic deformation of aluminum single crystals at 77 K |
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Authors: | M E Kassner M A Wall M A Delos-Reyes |
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Affiliation: | (1) the Department of Mechanical Engineering, Oregon State University, 97331-6001 Corvallis, OR;(2) M L Tech, 95267 Stockton, CA;(3) Present address: Materials Science Program, Oregon State University, is Engineer, Hewlett-Packard, Corvallis, OR |
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Abstract: | Aluminum single crystals were cyclically deformed in single slip at small strain amplitudes at 77 K to presaturation. The
observed mechanical behavior is consistent with recent work by other investigators. The dislocation substructure can be described
as consisting of dense bundles or veins of edge dislocation dipoles, of a single Burgers vector, separated by lower dislocation
density regions or channels where substantial debris is evident. This debris was determined as almost exclusively relatively
short edge-dipole segments. Screw dislocations with the same Burgers vector span the channels. In situ cyclic reverse (shear) deformation experiments in the high-voltage transmission electron microscope (HVEM) were successfully
performed using the X-Y technique where thin foils are stressed in alternating perpendicular directions. Our experiments indicate that loops frequently
expand from the dipole bundles into the channel and the edge component is absorbed by nearby bundles, leaving screw segments
behind. The screw dislocations that span the channel move easily and reverse direction with shear reversal. Screws may move
first with a strain reversal. A comparable fraction of the strain during each cycle appears to be provided by screw and edge
dislocations. Dipole “flipping” was not observed. There is no obvious evidence for internal backstresses that assist plastic
deformation on reversal of the applied shear. |
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