Coarsening of dispersed intermetallic Phases in Al-4.5 Pct Cu-2 Pct Mn Alloy

Abstract

Dendritic monocrystals of Al-4.5 wt pct Cu-2 wt pct Mn were directionally solidified at 0.20 m/h under a thermal gradient of 3 × 103 K/m. Crystal pulling was stopped for various lengths of time prior to quenching the remaining liquid, thus making it possible to evaluate the transformation and coarsening kinetics of dispersed intermetallic phases, Al6Mn and Al20Cu2Mn3, as a function of temperature. Coarsening of Al6Mn intermetallic particles surrounded by liquid (Type II) follows an average particle size\(\bar d \sim t^{1/3} \) relationship much closer than it does a\(\bar d \sim t^{1/2} \) relationship. This suggests that convection plays no important role in coarsening. For coarsening of Al6Mn particles in a solid matrix (Type I) the relationship\(\bar d \sim t^{1/3} \) fits the experimental measurements reasonably well. Coarsening kinetics studies were extended to intermetallic particle sizes an order of magnitude finer than those occurring in directionally solidified alloy, in order to derive information required by an on-going project on the effect of intermetallic phase geometry on corrosion behavior. Jt was found that coarsening of Al20Cu2Mn3 particles contained in a melt-spun ribbon follows a\(\bar d \sim t^{1/4} \) relationship, as predicted by Kirchner for grain-boundary diffusion-controlled ripening. Finally, coarsening of Al6Mn particles surrounded by liquid indium film and contained in a plastically deformed matrix follows a\(\bar d \sim t^{1/3} \) relationship, as predicted by LSW theory for liquid diffusion-controlled ripening. Shortening the time required to obtain coarse intermetallic particles during a homogenization treatment is important in deep drawing.