Atomic-scale clustering in a high-strength Al-Mg-Si-Cu alloy

Abstract

Effects of pre-aging and natural aging on the cluster distribution and mechanical properties of an Al-Mg-Si-Cu alloy before and after artificial aging are reported . Using atom probe tomography, a difference in clustering behavior was characterized between the naturally aged and the pre-aged heat treatments. In the naturally aged then artificially aged state, the formation of Si-rich clusters that did not readily grow into Guinier-Preston zones and β’’ precipitates(the primary strengthening phase for this alloy) resulted in a strength of 216 MPa. If the alloy was pre-aged and immediately artificially aged, a high density of β’’ precipitates formed, and the yield strength increased to 289 MPa. Copper was found in all strengthening particles approximately 1 nm or larger and is deemed essential for the faster precipitation kinetics to enable growth of β’’ precipitates. In addition, pre-aging followed by an extended secondary natural aging treatment for 30 days then subsequently arfiticially aged revealed a modestly lower yield strength (282 MPa). This drop in strength is linked to the formation of more detrimental Si-rich clusters that evolve over the extended naturally aging period suggesting potential time limitations when these artifically aging conditions can achieve optimal strengthening outcomes.