Strain Rate Effects on the Microstructure of ECAE Processed Pure Mg

Magnesium and its alloys have drawn significant research attention for potential structural applications owing to their remarkable properties like high specific strength. However, industrial applications are often limited by their lack of formability. For instance, a combination of strong tension-comprehension anisotropy and the absence of primary slip systems during conventional thermomechanical processing enhances shear localization while reducing ductility. On the other hand, warm-forming, especially during severe plastic deformation (SPD), has demonstrated the ability to process magnesium and refine its microstructure effectively. Among the many methods of SDP, Equal Channel Angular Extrusion (ECAE) has been identified as an effective approach for enhancing the properties of magnesium.

A typical ECAE process comprises extruding or pressing the material through a tool with intersecting channels. While the effects of process parameters on the microstructure of ECAE-processed materials have been extensively investigated, most of the previous studies focused on route design and extrusion temperature. Generally, the resulting grain morphology and texture of the extruded materials are often associated with route design and tool geometry. Despite this level of information, there is still only a limited understanding of the influence of process parameters on the microstructural evolution of hexagonal close-packed metals, such as magnesium and its alloys. Moreover, the effects of extrusion rate have attracted less research attention despite their potential impact in enhancing the properties of ECAE-processed materials.

Interestingly, previous studies involving other materials like aluminum suggested the possibility of modifying the texture of the materials by twinning while at the same time improving or maintaining the extent of grain size reduction or Hall-Petch strengthening. A similar phenomenon is envisioned for magnesium and its alloys. Equipped with this knowledge, Dr. Nicholas Krywopusk, Professor Laszlo Kecskes, and Professor Timothy Weihs from The Johns Hopkins University studied the effects of extrusion rate on the microstructural evolution of pure magnesium during the ECAE process. The aim was to describe and provide insights into the underlying mechanism behind the microstructural refinement. The work is currently published in the journal, Materials Characterization.

In their approach, the research team extruded pure as-cast magnesium samples by ECAE at three different rates: 0.127, 0.381 and 0.762 mm/s, via the B_C route using a total of four passes. All the extrusions were carried out under the same processing conditions at a temperature of 250 °C. The effects of the extrusion rate on the refinement process of the resulting magnesium were investigated. Also, the magnesium samples were characterized by Electron Backscatter Diffraction (EBSD) to quantify and examine how their textures and microstructures evolved with the increasing number of extrusion passes.

The authors showed that for all the three extrusion rates, a significant reduction in the average grain size of about 30 µm was recorded after the second pass. However, the grain size varied depending on the extrusion rate following the third and fourth passes. For example, the extrusion rates of 0.127 and 0.381 mm/s increased the grain size by 30-40%, while that of 0.762 mm/s maintained a constant grain size. Furthermore, the extrusion rate also affected the presence of twins in the product microstructure, with the intermediate extrusion rate yielding a lower volume fraction of twins compared to those of the slowest and fastest rates. This was attributed to the ability of the microstructure to maintain orientations favorable for twinning at lower rates and greater strain accommodation at higher rates.

In summary, a detailed investigation of strain rate effects on the texture and microstructural modification of pure magnesium during ECAE processing by characterizing the samples after every pass was reported. Significant grain refinement was achieved at the highest rate after the fourth (4B_C) pass. Additionally, multiple dynamic recrystallization (DRX) mechanisms, including nucleation at low and intermediate angle grain boundaries, were observed. Notably, it was possible to enhance DRX and suppress twining by controlling the basal plane orientation to favor dislocation slip-induced deformation. In a statement to Advances in Engineering, the authors explained that their findings will advance the creation of new textures and enhance grain size refinement, leading to the design of higher-performance magnesium alloys.