Yield drop effects for basal and prismatic slip in Mg-Li alloy single crystals

Abstract

An initial yield drop ( Δ τ I ) and a strain aging yield increase ( Δτ SA ) were observed during both basal and prismatic slip in Mg-Li alloy single crystals containing 1.5–15 at. % Li. A solute atmosphere around the dislocations due to a size effect interaction with the edge components of dislocations is thought to be responsible for the strain aging yield point effects. A role was ascribed to vacancy-type defects in aiding diffusion of the solute to the dislocations. The maximum ( Δ τ SA ) for prismatic slip is five times larger than that for basal slip at 273°K (320 vs. 65 g/mm 2 for the 12 at.% Li). With basal slip ( Δ τ SA )(max) peaks near 100°K, the peak value gradually increases with Li content. With prismatic slip, ( Δ τ SA ) (max) peaks at about 400°K for 6 at. % Li and 275°K for 12 at. % Li and a strain aging yield effect is observed only in high Li alloys at low temperatures. Edge dislocations predominate after basal slip but for prismatic slip at low temperatures helical screw dislocations are observed in high Li alloys after deformation. The helical screw dislocations are difficult to move. It is for this reason that ( Δ τ SA ) (max) is greater for prismatic than for basal slip. In the low Li alloys the dislocation cores are thought to be relatively wide and the strain aging behavior was interpreted in terms of Friedel's cross slip theory.