Cluster Strengthening Contribution to Yield Strength and Behind Mechanism of Heat-Treatable 6xxx Aluminum Alloys

The present work investigated the precipitation behaviors and strengthening mechanisms of solute clusters in 6xxx Al alloys based on the classical precipitation theory and strengthening models. The cluster precipitation behaviors and strengthening effects are clarified from the perspective of precipitation thermo-kinetic synergy in terms of the Mg/Si ratio, natural aging (NA) time, and artificial aging (AA) temperature. It is found that the solute clusters precipitated during NA can strengthen the alloys directly, and the strengthening effects can be improved with increasing Mg/Si ratio, which provides the driving force for cluster precipitation. As exemplified, with increasing Mg/Si ratio from 1 to 2, the cluster precipitation driving force increases by 0.3 kJ/mol, and the strengthening contribution from clusters enhances by 10 MPa. These solute clusters can dissolve in subsequent AA stage, which can be accelerated by higher Mg/Si ratio, shorter NA time, and higher AA temperature. Such dissolution behaviors of solute clusters result in the increase of solute concentration, which provides the driving force for β″ precipitation. Consequently, the strengthening contribution from solute clusters decreases while that from β″ phase increases, thus the total strength enhances due to the superior strengthening effects of β″ phase. As exemplified, for the alloys with Mg/Si ratio = 2 aged at 453 K for 60 minutes, reducing NA time from 2 weeks to 2 days can enhance the strength by 43 MPa. Our investigation provides a novel insight into understanding cluster strengthening mechanisms of 6xxx Al alloys based on precipitation thermo-kinetic synergy, thus benefit to develop high-strength Al alloys.