Non-basal slip systems in HCP metals and alloys: source mechanisms

A possible source mechanism for non-basal 〈c+a〉 slip dislocations is proposed based on the formation of an attractive junction between glissile 〈a〉 and sessile c dislocations from the prism plane into a pyramidal plane. The driving force for the junction formation, which comes from the long-range elastic interaction between c and 〈a〉 dislocations, is relatively large in most hexagonal close-packed (hcp) metals. Beryllium, which has an unusually low Poisson's ratio, is an exception to this rule. The cross-slip process is energetically unfavorable in Mg and Ti, from a viewpoint of the change in anisotropic elastic line tension, but it becomes favorable in Ti at elevated temperatures above 300°C. Discussion is given on intrinsic stacking fault energies and kinetic aspects of the cross slip.