Abstract
The discontinuous precipitation process of Mg–Al alloys during aging has been investigated by metallographic observation. In Mg–Al alloys, the initial stage of discontinuous precipitation cell formation had preferential precipitation and local instability of the grain boundary. With continued aging, the local migration of the original grain boundary rose into the adjacent grain. Consequently a solute depleted region was formed behind the advancing grain boundary. After a while, the preproposed S-mechanism required for the formation of double seam morphology appeared. The occurrence of both single seam and double seam morphologies were conformed at both aging temperatures lower and higher than Tm/2 (Tm: melting temperature). These facts do not coincide with Baumann's assumption that the highest dependence is on aging temperature. After the solution treatment, a Mg–9.5Al specimen was cold compressed by 3.2∼15.3%. The cold work prior to aging does introduce additional nucleation sites for general precipitation, with the age hardening somewhat accelerated. The fractional area of discontinuous precipitation cells was suppressed by the deformation twins, only onset of cell formation increased the rate.
