Dislocation arrangements in tantalum single crystals deformed in tension at 373°K

Abstract

The dislocation structure in tantalum single crystals deformed in tension to various strains at 373°K has been investigated by transmission electron microscopy. Foils were prepared by a disc technique and various sections were analyzed at each strain. In stage I of the stress-strain curve tangles of dislocations are observed lying parallel to the slip plane; these tangles contain many straight screw dislocations and a high density of dislocation loops. In stage II the tangles tend to become linked together to form walls of dislocations approximately parallel to the slip plane. Misorientations across the walls become increasingly apparent as the crystals are deformed further into stage II and stage III. It is suggested that the walls are small-angle boundaries formed when short-range secondary slip occurs to reduce the long-range stresses at the ends of slip lines. Accordingly a high density of secondary dislocations is observed. The flow stress is found to be proportional to the square root of the total dislocation density. Dislocation reactions in b.c.c. metals are analyzed and a quantitative theory of stage II hardening is developed based on the cell formation theory of Hirsch.