Dynamic precipitation during high-pressure torsion of a magnesium-manganese alloy

Abstract

An ultrafine grained magnesium alloy has been produced through room temperature high-pressure torsion (HPT) of solutionised Mg-1.35 wt.%Mn. Dynamic precipitation of nanometer-scale Mn particles occurred during deformation. These particles populated the grain boundaries, acting as pinning sites which allowed the alloy to develop a grain size of 140 nm after 0.5 rotations. Further HPT deformation resulted in a gradual increase in grain size with no increase in precipitate size. Despite the extensive deformation applied, the alloy did not develop a bimodal grain structure and retained a grain size of 230 nm after 10 complete rotations, demonstrating the stability and effectiveness of these pinning particles.

Figures (9)

• Figure 1: A schematic of the process of sample preparation and analysis.
• Figure 2: Vickers indentation hardness of -deformed Mg-Mn versus applied strain. The line is included as a guide for the eye, only.
• Figure 3: The microstructure of the as-received alloy, prior to solution-treatment and deformation. (a) back-scattered electron image showing coarse rod-like, and occasional spheroidal, particles of $\alpha$-Mn. spectra from (b) the particle marked "A" in the image, and (c) the matrix, showing a higher concentration of Mn at the particle, and the absence of additional elements. (d) image. The inscribed circle shows the aperture position for the inset , which indicates this grain is oriented along the $11\overline{2}0$ direction, and which shows weak reflections assigned to MgO but no evidence for the existence of Mn-containing phases.
• Figure 4: Microstructures present in the as-compressed sample. The figure shows - images of the (a) matrix, (b) twin and (c) ultrafine-grained regions, with the corresponding inset in the image. While MgO reflections were often observed (and are visible in (a) and (b)), the Mn 411 and 330 reflections appear only in the regions such as (c).
• Figure 5: (a) image showing the microstructure of M1 after 1 rotation of deformation. (b) The obtained with the aperture centred over the area circled in (a). In addition to Mg reflections the diffractogram shows a weak reflection attributable to the Mn (411) and (330) planes.