Understanding Preferential Nucleation During Recrystallization of Highly Textured Magnesium Alloys
Discipline: Nanoscience or Materials Science
Subcategory: Materials Science
Session: 2
Room: Gallery Place
Rogine Gomez - The Ohio State University
Co-Author(s): Aeriel Leonard, The Ohio State University, Columbus, OH
Magnesium (Mg) alloys show great promise in vehicle weight reduction due to their light weight, high specific strength, and good castability. However, the full adoption of Mg alloys in the automotive industry is limited by low ductility and poor formability, and as a result, Mg accounts for less than 1% of the average vehicle weight. This can be attributed to the formation of a strong basal texture during thermomechanical processing (TMP). Recent studies have shown that the texture can be weakened by alloying with Ca and Zn and therefore, can improve the ductility and formability of these alloys. In many cases, recrystallization (RX) or the nucleation of strain-free grains is used to alter grain behavior, enabling changes in properties and overall crystallographic texture. The hypothesis of this work is that RX is driven by 1) high strain localization and incompatibilities near and across twin boundaries (TB) and grain boundaries (GB) and 2) changes in interface boundary mobility due to the preferential co-segregation of Ca and Zn to grain boundaries during TMP which promotes the nucleation and growth of RXed grains with randomized orientations.In this study, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and in-situ heating was used to understand and quantify the changes in crystallographic texture throughout the RX process and identify preferential nucleation sites in Mg-2Zn and Mg-2Zn-0.5Ca. Furthermore, regions of interest were investigated using bright field (BF), diffraction contrast imaging (DCI), scanning transmission electron microscopy (S/TEM), energy dispersive spectroscopy (EDS), and high angle annular dark field (HAADF) to understand dislocation structure and interactions as well as segregation behavior of Ca and Zn along TB and GB. To understand static RX behavior, samples were compressed to 20% strain and annealed at temperatures ranging from 250ºC to 350ºC for 0.5 to 240 minutes. After compression, the dominant deformation modes were different between the two alloys. In Mg-2Zn, in addition to dislocation slip, deformation twins were found throughout the microstructure, occupying approximately 60% of the total scan area. In Mg-2Zn-0.5Ca, only 3% of the total area was occupied by twins, suggesting that dislocation slip is the dominant deformation mode in this alloy. After RX, the texture obtained upon deformation remained in Mg-2Zn but was greatly weakened in the Mg-2Zn-0.5Ca alloy. EDS revealed the co-segregation of Ca and Zn along the twin and grain boundaries during annealing of the Mg-2Zn-0.5Ca alloy, which can contribute to grain refinement and precipitation strengthening. The results suggest that the combination of Ca and Zn in Mg alloys show potential in weakening the strong basal texture during RX and reducing deformation twinning during processing. These findings can lead to optimization of Mg alloy design and manufacturing processes to meet strength and ductility requirements.
Funder Acknowledgement(s): The research is funded through the National Science Foundation (NSF) Graduate Research Fellowship Program
Faculty Advisor: Aeriel Leonard, leonard.649@osu.edu
Role: All of the research mentioned in the abstract was performed by me.