High-temperature deformation of Al-Li-Cu-Mg-Zr alloys 8090 and 8091

Abstract

Specimens were prestretched in the range 0 to 7% plastic deformation prior to artificial ageing, or were duplex aged, to investigate the effect of dislocation substructure and of S(Al₂CuMg) particles on plastic flow during room and elevated temperature tensile tests. The yield stresses increased in Al-Li-Cu-Mg-Zr alloys 8090 and 8091 after stretching, due to the dislocation cells introduced by the stretch and also to the nucleation and growth of the S-phase particles on these dislocations. Up to 400 K the modulus-normalized proof stresses for the variously treated materials were constant, but they fell at higher temperatures, as dislocation climb mechanisms operated. The proof stresses of alloy 8090 fell at a temperature 50 K lower than alloy 8091, and this difference is considered to arise from the differing volume fraction of S-phase in the two alloys. The work hardening rate (ϑ) at a strain of 0.2% was measured between 300 and 500 K. In alloy 8090, the temperature at which dynamic recovery occurs was influenced by the degree of stretch, but this was not so in alloy 8091. In alloy 8090, the substructure introduced by stretching was able to act as a dislocation sink during dynamic recovery, whereas in alloy 8091 the higher copper content brought about more S-phase precipitation which was sufficient to inhibit this effect. The athermal hardening rate (ϑ_o) was obtained by extrapolation of the ϑ-σ_t curves, giving ϑ_o=10000 GPa, for alloy 8090 and ϑ_o=8500 G Pa, for alloy 8091. This difference may reflect a higher mobile dislocation density in alloy 8090 due to the lower volume fraction of S-phase particles in that material.