Yeqing Wang (1,2), Paul Chao (2), Saman Moniri (3), Jianrong Gao (1), Tobias Volkenandt (4), Vincent De Andrade (5), Ashwin J. Shahani (2)
Materials Characterization, 170, October 2020. DOI: 10.1016/j.matchar.2020.110685

Keywords

Isothermal annealing, Synchrotron X-ray nanotomography, 3D EBSD, Reactive diffusion, Topology

Abstract

Microstructural instabilities associated with a metastable lamellar eutectic when exposed to an elevated temperature were analyzed in a Zn􀀀 3Mg􀀀 4Al alloy as a model system. X-ray diffraction and scanning electron microscopy showed that annealing at a temperature 43 K below the equilibrium eutectic temperature converted the metastable ternary MgZn2/η-Zn/β-ZnAl eutectic to a stable mixture of Mg2Zn11/η-Zn/α-Al. 3D EBSD suggested that supersaturation of η-Zn with Mg and Al eased difficulties in nucleation of Mg2Zn11 and α-Al at the MgZn2/η-Zn phase interfaces. Quasi-in-situ X-ray nanotomography revealed that a reactive diffusion process prevailed in an early stage of annealing leading to rapid consumption of MgZn2 for growth of Mg2Zn11 and concomitant thinning and pinch-off of η-Zn lamellae. The reactive diffusion process also brought about significant and quantifiable changes in the topological characteristics of the η-Zn phase at remarkably shorter time scales compared to Rayleigh instability and Ostwald ripening mechanisms. The present results provide fresh insights on the short-term annealing effects on metastable eutectics, with relevance to structural evolution in additive manufacturing.

How Our Software Was Used

Dragonfly was used for the 3D visualization of X-ray reconstructed data.

Author Affiliation

(1) Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China.
(2) Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States.
(3) Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United States.
(4) Carl Zeiss Microscopy GmbH, Oberkochen 73447, Germany.
(5) Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.