Grain boundary precipitation of i-phase in 8090 alloy sheet

Abstract

The formation of I-phase at grain boundary sites in 8090 alloy is believed to have a detrimental effect on the material toughness, most notably in thick product forms, e.g., plate and forgings. The formation of a phase having an icosahedral crystallographic symmetry and termed the 1 phase has been studied during extended annealing at 300--350 C, but its structure and its proposed composition of Al[sub 6]Cu(Li,Mg)[sub 3] have not been determined unambiguously. There is evidence that it forms during quenching from the solution treatment temperature in thick product forms, but can be suppressed by high quench rates in alloy sheet. Other workers have observed these particles to nuclear and grow during isothermal heat treatment in the range 170--520 C. The present work set out to ascertain the factors controlling the formation of this phase. Experiments have been designed to investigate the effect of solution treatment temperature and time, the effect of quench rate, and the effect of aging temperature and time on the density and distribution of the particles.