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

Abstract

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 β 1-Mg 3 Nd and β-Mg 12 Nd phases but without any evidence for the metastable β ''' (undifferentiated β ''-DO19 and β'-Mg 7 Nd) or equilibrium β e-Mg 41 Nd 5 phases. The β 1-Mg 3 Nd and β-Mg 12 Nd phases appeared rapidly after HPT processing and their amounts were relatively large compared to the nondeformed sample. The Avrami time exponent of the β-Mg 12 Nd 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 .

Thermal Stability of an Mg–Nd Alloy Processed by High‐Pressure Torsion

“…Ageing above 250 °C causes a rapid disappearance of GP‐zones and the β ′ ‐Mg 7 Nd phase together with the formation of coarse precipitates of the stable β 1 ‐Mg 3 Nd and β‐Mg 12 Nd phases . Furthermore, HPT processing increases the volume fraction of these precipitate phases compared with the nondeformed sample . The average diameter of the β‐Mg 12 Nd phase was reported to increase from ≈120 to ≈300 nm when ageing at 250 and 350 °C, respectively .…”

“…Several studies were conducted earlier on the binary Mg–RE processed by HPT in an attempt to extend the fundamental understanding of the effect of RE elements and SPD processing on the precipitation, recrystallization, and gain growth phenomena . First, the effect of HPT processing on the microstructural and textural evolution in Mg–1.43Nd, Mg–1.44Ce, and Mg–1.44Dy (wt%) alloys was investigated .…”

“…First, the effect of HPT processing on the microstructural and textural evolution in Mg–1.43Nd, Mg–1.44Ce, and Mg–1.44Dy (wt%) alloys was investigated . Second, the sequence and kinetics of pre‐precipitation in a Mg–1.43Nd alloy were investigated after HPT processing using in situ synchrotron X‐ray diffraction and differential scanning calorimetry (DSC) . Third, the recrystallization process and thermal stability after HPT processing were characterized by DSC and electron backscatter diffraction (EBSD) in Mg–1.44Ce and Mg–1.44Dy alloys, respectively .…”

An investigation of the thermal stability of an Mg Dy alloy after processing by high-pressure torsion

“…One major advantage of HPT processing is that the grain refinement of Mg-based alloys may be achieved at room temperature without sample cracking and/or segmentation due to the high imposed hydrostatic pressure which effectively prevents propagation of fracture during torsional straining [13]. Moreover, HPT processing may significantly affect the kinetics, size and distribution of precipitates within the crystalline matrix [14,15].…”

An investigation by EXAFS of local atomic structure in an Mg-Nd alloy after processing by high-pressure torsion and ageing

“…In addition, the application of severe plastic deformation (SPD) processing, as in high-pressure torsion (HPT), has been used to achieve good mechanical properties through grain refinement and the introduction of high dislocation densities [4]. Thus, alloying elements and deformation processing may influence the sequence and kinetics of phase transformations such as recrystallization and precipitation [5].…”