Growth crystallography and in-situ imaging of nucleation and growth dynamics of Al8Mn5 solidifying in AZ magnesium alloys
File(s)
Accepted version
Author(s)
Type
Journal Article
Abstract
Many magnesium-aluminium-based alloys contain a small manganese addition to improve corrosion resistance. However, this introduces Al-Mn intermetallics which add complexity to the phase transformations. Here we study the crystal growth of the most common Al-Mn phase, Al8Mn5, in Mg-Al-Zn-Mn-based magnesium alloy solidification by combining electron microscopy of the faceted growth crystallography with in-situ synchrotron X-ray imaging of the Al8Mn5 nucleation and growth dynamics. Three Al8Mn5 morphologies, equiaxed, rod and plate, are shown to all come from cyclic twinned growth associated with the pseudo-cubic symmetry of rhombohedral Al8Mn5. X-ray imaging revealed Al8Mn5 nucleated throughout the freezing range and grew with α-Mg dendrites over a wide temperature range by divorced eutectic solidification. This occurs because, in these alloys, Mn solute has little influence on the solute undercooling of growing α-Mg dendrites, although strongly affects the constitutional supercooling with respect to the Al8Mn5 liquidus. Rod/plate growth of Al8Mn5 is shown to be promoted by slow cooling rates and by divorced eutectic solidification. The findings provide new insights into the conditions that cause large, deleterious rods and plates, and explain why there is a large variation in Al8Mn5 growth morphology after solidification.
Date Issued
2025-09-01
Date Acceptance
2025-05-20
Citation
Acta Materialia, 2025, 296
ISSN
1359-6454
Publisher
Elsevier
Journal / Book Title
Acta Materialia
Volume
296
Copyright Statement
© 2025 Acta Materialia Inc. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies. This is the author’s accepted manuscript made available under a CC-BY licence in accordance with Imperial’s Research Publications Open Access policy (www.imperial.ac.uk/oa-policy)
License URL
Publication Status
Published
Article Number
121168
Date Publish Online
2025-05-23