The sequence and kinetics of pre-precipitation in Mg-Nd alloys after HPT processing: A synchrotron and DSC study

The sequence and kinetics of pre-precipitation in an Mg-1.43Nd (wt.%) alloy was investigated after severe plastic deformation by high-pressure torsion (HPT) at room temperature using in situ synchrotron X-ray diffraction and differential scanning calorimetry (DSC). In situ aging at 250 °C up to 5 h led to precipitation of the β1-Mg3Nd and β-Mg12Nd phases but without any evidence for the metastable β‴ (undifferentiated β''-DO19 and β′-Mg7Nd) or equilibrium βe-Mg41Nd5 phases. The β1-Mg3Nd and β-Mg12Nd phases appeared rapidly after HPT processing and their amounts were relatively large compared to the non-deformed sample. The Avrami time exponent of the β-Mg12Nd phase had a value near unity indicating a mechanism of nucleation after saturation and growth of the particles in 2 dimensions. DSC analysis revealed all metastable phases as well as the equilibrium phase. The activation energies associated with the pre-precipitation phases ranged between ∼126 and ∼235 kJ mol−1.