Dislocation Structure in Deformed Single Crystals of Magnesium

The dislocation structure produced by tensile deformation of magnesium single crystals at 77 K and at room temperature has been investigated by transmission electron microscopy techniques. Sections both parallel and perpendicular to the basal plane have been cut by spark machining from single crystals deformed into both stages of the hardening curve. In stage I, edge dipoles and braids consisting of edge dislocations and dipoles are the prominent features, and the low density of screw dislocations suggests that annihilation has occurred. Very few dislocations with non‐basal Burgers vectors have been observed and the density of forest dislocations is less than 10 7 cm −2 unless dislocations have been lost from the foils. There is no evidence for slip on non‐basal slip systems even at the highest deformation examined (0.8 shear). In the early stages of deformation large clear areas are seen between braids. As deformation proceeds the local spacing between braids decreases and in stage II the areas between braids become filled with dislocations. After deformation at 77 K the dislocation density is more uniform. Twinning was the only new feature found in crystals deformed into stage II. The formation of the braids and dipoles is discussed and strain hardening is considered in terms of the observed dislocation structure. The flow stress in stage I is believed to be determined by the number of excess dislocations of one sign present in the braids, and the stability of the braids is probably due to short edge dipoles which have been swept into them.