Observations of Strain Induced Precipitation during the Thermomechanical Processing of AA6111 Alloy

Abstract

The effect of interpass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield thermomechanical compression (TMC) facility, using the hit-hold-hit-quench approach. Following solution treatment at 560°C for 1200s, samples were water mist quenched to 320°C and deformed at a constant strain rate of 85s-1 to an initial strain of 0.5, unloaded and held for delay times of 0.019, 6, 60, 600 and 6000s and then given a second deformation for a further strain of 0.5, followed by a water quench to room temperature. Hardening of the alloy was observed, the extent of which was dependent on the hold time. The microstructure of the samples was quantified by TEM in order to determine the extent of strain induced precipitation. TEM identified precipitation, predominantly β and Q phases, on dislocation lines, the size and volume fraction of which were a function of the hold time. The coarsening rate during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. The implication of these observations on the thermomechanical processing of 6xxx alloys is discussed.