Effect of excess Si on age-hardening in deformed Al-Mg2Si alloys

The effects of deformation and pre-aging on age-hardening of Al-Mg₂Si alloys containing 0.4 mass% excess silicon were investigated by Vickers micro-hardness test and transmission electron microscope observation. The time required to reach a maximum value of age-hardening curve (t_max) was shortened by deformation just prior to aging for all alloys. The maximum hardness value (H_max) increased with increasing the amount of deformation in an 0.4%Mg₂Si alloy aged at 473 K. The precipitates became finer by the deformation prior to aging. The H_max values were independent of the amount of deformation and were constant for alloys containing Mg₂Si ranged from 0.6 to 1.0%. For those alloys, the size of precipitates was larger and the number of precipitates was reduced by the addition of deformation. The t_max at 473 K was shortened by pre-aging at 423 K before deformation in an 0.6%Mg₂Si alloy. The pre-aging and deformation treatment has also the effect to increase the H_max to a higher value equivalent to that obtained by the conventional aging at 423 K.