Precipitation Process in a Mg–Ce Alloy

Abstract

The aging characteristics of quenched Mg-1.3 wt%Ce alloy were studied by electron microscopy and hardness measurements. On isothermal aging at 150°C, precipitates occurred first on the grain boundaries and dislocations, followed by precipitation within grains. At an early stage of aging at temperatures higher than 200°C, precipitation on the grain boundaries and dislocations, and within grains took place simultaneously. The diffraction effect observed in the present experiment suggests that the precipitates formed and dispersed uniformly within the matrix at an early stage of aging were to be of an intermediate phase in which cerium atoms might be enriched on (10\bar10) planes of the matrix. In a later stage of aging, the precipitates transformed into the equilibrium hexagonal phase of β, ribbon shaped, with the following orientation relationship with the matrix:(0001)M〈11\bar20〉M⁄⁄(0001)P〈10\bar10〉P, cM≈cP, 3d{11\bar20}M\lesssimd{10\bar10}P.The ribbon shaped precipitates grew predominantly in the direction of 〈0001〉, and, to a much lesser extent, in the direction of 〈11\bar20〉 of the matrix. Significant age hardening observed in the present alloy should be associated with the precipitation of fine particles dispersed uniformly in the matrix. A certain amount of the intermediate precipitates appeared to remain even in a stage where the maximum hardness was attained. An activation energy for the age hardening process was determined to be 30 kcal/mol.