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Dendritic evolution during coarsening of Mg-Zn alloys via 4D synchrotron tomography

DOI: 10.1016/j.actamat.2016.10.022 DOI Help

Authors: Enyu Guo (University of Manchester; Research Complex at Harwell) , A. B. Phillion (Research Complex at Harwell; Department of Materials Science and Engineering, McMaster University) , Biao Cai (University of Manchester; Research Complex at Harwell) , Sansan Shuai (Research Complex at Harwell; Tsinghua University, University of Manchester, Shanghai University) , Daniil Kazantsev (University of Manchester; Research Complex at Harwell) , Tao Jiang , Peter D. Lee (University of Manchester; Research Complex at Harwell)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acta Materialia , VOL 123 , PAGES 373 - 382

State: Published (Approved)
Published: November 2016
Diamond Proposal Number(s): 13302

Open Access Open Access

Abstract: The scale of solidification microstructures directly impacts micro-segregation, grain size, and other factors which control strength. Using in situ high speed synchrotron X-ray tomography we have directly quantified the evolution of dendritic microstructure length scales during the coarsening of Mg-Zn hcp alloys in three spatial dimensions plus time (4D). The influence of two key parameters, solute composition and cooling rate, was investigated. Key responses, including specific surface area, dendrite mean and Gauss curvatures, were quantified as a function of time and compared to existing analytic models. The 3D observations suggest that the coarsening of these hcp dendrites is dominated by both the re-melting of small branches and the coalescence of the neighbouring branches. The results show that solute concentration has a great impact on the resulting microstructural morphologies, leading to both dendritic and seaweed-type grains. It was found that the specific solid/liquid surface and its evolution can be reasonably scaled to time with a relationship of ∼ t−1/3. This term is path independent for the Mg-25 wt%Zn; that is, the initial cooling rate during solidification does not greatly influence the coarsening rate. However, path independence was not observed for the Mg-38 wt%Zn samples because of the seaweed microstructure. This led to large differences in the specific surface area (Ss) and its evolution both between the two alloy compositions and within the Mg-38 wt%Zn for the different cooling rates. These findings allow for microstructure models to be informed and validated to improve predictions of solidification microstructural length scales and hence strength.

Journal Keywords: Magnesium alloy; Semi-solid; X-ray tomography; Coarsening; Dendrite

Subject Areas: Biology and Bio-materials, Materials
Collaborations: Diamond Manchester

Instruments: I13-2-Diamond Manchester Imaging