Precipitate size evolution in an ultrafine-grained magnesium-manganese alloy

TL;DR

This study investigates how Mn precipitates evolve during room-temperature high-pressure torsion of Mg-1.35Mn. Using STEM, SAXS, and WAXS, it shows nanometer-scale Mn particles predominantly reside at grain boundaries, while most Mn remains in solution even after 10 rotations. Crucially, the mean precipitate size remains nearly constant, indicating interfacial rather than diffusional growth control, and grain growth is slowed by Mn particle pinning. The work highlights a dynamic balance between precipitation and grain refinement, suggesting a potential for further stabilization or tailored processing through heat treatment, with significant implications for Mg-based biomaterials and structural alloys. All observations are supported by a combined imaging and scattering approach, linking microstructural features to macroscopic grain stability.

Abstract

Precipitate size evolution during room temperature high-pressure torsion (HPT) of a Mg-1.35wt.%Mn alloy was studied using scanning transmission electron microscopy (STEM) and Small-/Wide-angle X-ray scattering (SAXS/WAXS). The volume fraction of the nm-scale $α$-Mn particles increased with applied strain, however small angle X-ray scattering (SAXS) indicated that the majority of manganese remained in solution even after 10 HPT rotations, indicating that the reaction progress is still limited by the diffusivity of Mn. Analysis of the precipitate size distribution determined that the mean particle size did not increase over the course of HPT. This, in combination with the precipitate size distribution suggested that precipitate growth was subject to interfacial rather than diffusional control.

Figures (10)

• Figure 1: A schematic of the disc geometry and sample preparation. SAXS measurements were made at radii between 0 and 3 mm. The centre of the foils corresponds to a radial distance of approximately 2 mm.
• Figure 2: - images post- Mg-Mn, for (a) 0 rotations (i.e. compression only), (b) 0.5 rotations and (c) 5 rotations. Nano-scale Mn particles are concentrated along the grain boundaries.
• Figure 3: Development of the Mn particle distribution (a) Mean particle radius. Volume-weighted values for each radial position are indicated by open symbols (blue in the colour version online). Number-weighted values from - are shown by filled symbols (red in the colour version online). The inset shows the data, converted to number-weighted radii. Curves are provided as a guide for the eye, only. (b) Violin plot showing the aspect ratio of the particles as measured by -. The horizontal lines indicate the maximum, mean and minimum values, respectively.
• Figure 4: (a)- image obtained in a sample after 1.0 rotations of . Regions of interest (A,red in the online version) in the grain, (B, blue in the online version) around a grain boundary particle and (C, blue) along the grain boundary are indicated. Histograms of the relative intensity for Mn and Mg for each region are shown in (b). The shaded band (purple in the colour version online) indicates a width of 1 standard deviation above and below the mean. The relative intensity in region $B$ is substantially higher than in the grain ($A$) or at the boundary itself ($C$).
• Figure 5: Particle size distributions from measurements. Darker histograms (blue in the colour version online) indicate the volume-weighted particle size distributions, while the lighter (red in the colour version) histograms indicate the number-weighted distributions. The standard deviation for each histogram is shown by solid lines, and dashed vertical line shows the mean particle radius. The volume fraction of Mn particles increases with applied strain, both (i) from left to right with increasing radial distance, from the disc centre (left in the figure) to the rim (right) and (ii) from top to bottom with increasing number of rotations. ($\overline{r}_v$).