Influence of powder metallurgy route on precipitation processes in MgTbNd alloy

Solution treated MgTb3Nd2 alloy (nominal composition in wt.%) (ST) and the alloy prepared by hot extrusion of isostatically pressed powder (PM) were isochronally heat treated and studied by electrical resistivity and hardness measurements and by differential scanning calorimetry. Microstructure development was investigated in transmission electron microscopy. Successive precipitation of transient phases in the sequence β″ (D019 plates) → β′(cbco) → β1 (Mg3Gd type, fcc) → β (Mg5Gd type, fcc) known from the ST alloy was identified also in the PM alloy. The early precipitation stage (D019 clusters) revealed in the ST alloy as well as precipitation of equilibrium βe phase Mg41(Tb,Nd)5 manifest themselves only slightly in the PM alloy. Powder metallurgy route does not change the values of activation energies but shifts the temperature ranges of these processes. Vickers hardness of the as prepared state is higher in the PM alloy and is very resistant against the heat treatment up to 510 °C. Contrary to the ST alloy precipitation due to isochronal annealing does not lead to pronounced hardness changes in the PM alloy.