Abstract
Early stages of precipitation were investigated in solution treated binary Mg-Tb and Mg- Gd alloys. The supersaturated solid solution of Gd (or Tb) in Mg was formed by fast quenching of the alloys from solution treatment temperature.
Decomposition of the supersaturated solid solution and precipitation effects were investigated by positron lifetime spectroscopy combined with microhardness testing. During solution treatment at elevated temperature some thermal vacancies form pairs with solute atoms.
In quenched samples free vacancies are quickly annealed out, while more stable vacancies bound to solute atoms remain in the sample and enhance the diffusivity of solutes. The hardness of solution treated Mg-Tb and Mg-Gd alloys aged at ambient temperature rises due to formation of small clusters of Tb and Gd atoms.
Isochronal annealing of Mg-Tb and Mg-Gd alloys leads to precipitation of coherent β’’ phase, semicoherent β’ phase and incoherent β phase particles. It was found that natural aging of Mg-Tb alloy at ambient temperature has beneficial effect on subsequent hardening by β’’ phase particles formed during annealing.