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 0.4mass% 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 (tmax) was shortened by deformation just prior to aging for all alloys.The maximum hardness value (Hmax) 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 Hmax, 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 tmax 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 Hmax to a higher value equivalent to that obtained by the conventional aging at 423 K.