Abstract
The ageing characteristics of quenched Mg-1.3 wt%Ce alloy were studied by electron microscopy and hardness measurement. On isothermal ageing at 150°C, precipitates appeared first on the grain boundaries and dislocations at an early stage and precipitation within grains followed a little later. On ageing at temperatures higher than 200°C, precipitation on the grain boundaries, dislocations and within grains took place simultaneously at an early stage. The diffraction effect observed in the present experiment suggests that the precipitates formed and dispersed uniformly within the matrix at an early stage of ageing were to be of an intermediate phase in which cerium atoms might be enriched on (10\bar10) planes of the matrix. On a later stage of ageing, the precipitates transformed into the equilibrium hexagonal phase of β, ribbon shaped, with the following orientation relationship with the matrix;
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The ribbon shaped precipitates grew mainly in the direction of ⟨0001⟩ and, in much less 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 on a stage on which the maximum hardness was attained. An activation energy for age hardening process was determined to be 30 kcal/mol.
