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Phase transformations in MgTbNd alloy



Precipitation in the MgTbNd alloy with ∼4 wt.%Tb and ∼2 wt.%Nd solution treated at 500 °C/4 h was studied by means of differential scanning calorimetry (DSC) with linear heating rates 1–20 K/min. Electrical resistivity measurements at 78 K, positron annihilation spectroscopy (PAS), and transmission electron microscopy (TEM) were performed on the specimens isochronally annealed in steps 20 K/20 min or 30 K/30 min. Microhardness HV0.1 annealing curves for various isochronal heating programs corresponding to linear heating rates 0.5–10 K/min revealed hardening caused by individual precipitation processes. The precipitation sequence includes successively DO19 clusters, DO19 particles, extremely small particles of the cbco phase, β1 phase with fcc structure, β phase of Mg5(Tb,Nd) type, and β phase of Mg41(Tb,Nd)5 type. All the particles except the last one have a plate-shape oriented in the Mg matrix in a triangular arrangement. Activation energies of the individual precipitation processes were determined from calorimetric measurements. The results of the PAS and electrical measurements correspond well to the DSC results as well as to the hardening caused by the β1 phase particles.