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An Experimental and Modeling Investigation of Solidification and Microstructural Evolution in Magnesium Alloys using in situ Techniques

EMSL Project ID


The goal of this proposal is to understand the microstructural evolution during non-equilibrium, rapid solidification of a molten magnesium-aluminum-zinc (AZ91) alloy, and during its subsequent heat-treatment. Therefore, the objectives of this work are: (1) Understand the solidification kinetics of AZ91 melt at high cooling-rates and, (2) understand the kinetics of phase evolution of Mg17Al12 and alpha-Mg during heat-treatment. These objectives will be achieved through a combination of in situ experiments and atomistic simulations with the help of EMSL’s experimental and computational resources, respectively.

We propose to use EMSL’s dynamic transmission electron microscope (DTEM) to provide insights into the kinetics of solidification of molten AZ91 alloy under high cooling rates. We have developed the sputtering procedures to deposit electron-transparent (50-100 nm) thin films of Mg-9 wt.% Al suitable for DTEM experiments. Further, we have previously determined that 0.2-0.35 mJ energy laser pulse (Nd:YAG, 532 nm), an energy range achievable within PNNL's DTEM, is required for locally melting such Mg-Al films without causing any ablation damage. The results from in situ DTEM experiments will provide unique solidification kinetics data that is otherwise not achievable by any other technique.

We propose to use EMSL’s STEM/TEM capabilities to study the kinetics of phase evolution and grain-growth in Mg alloy thin films, during heat-treatment, at high spatial resolution. In FY15, binary Mg-Al as-sputtered films were heat-treated inside EMSL's TEM between 150 degrees C-300 degrees C for times between 5-190 min. Growth of alpha-Mg grains and formation of beta-Mg17Al12 precipitates were analyzed via TEM and EELS to determine the exponent and activation energy for grain-growth. For FY16, the same approach will be extended to the heat-treatment of the Mg-Al-Zn ternary films (corresponding to AZ91 composition) to elucidate the role of Zn in phase evolution and activation energy relative to binary Mg-Al films.

Finally, we propose to use atomistic simulations to model the kinetics of Al segregation and Mg17Al12 precipitation in Mg-Al alloys. In FY15, EMSL’s supercomputing resources were used to model the vacancy-mediated diffusion of an Al atom in pure Mg lattice using the atomistic on-lattice self-learning kinetic Monte Carlo (SLKMC) method. Activation barriers for vacancy-atom exchange processes were determined using the climbing image nudged-elastic band method (CI-NEB). In FY16, we will determine Al diffusivity in Mg-Al solid-solution as a function of Al concentration and temperature. We will also model the formation, size distribution and stability of Al-rich clusters in the Mg lattice that may act as nuclei for eventual Mg17Al12 precipitates. Such data will enable us to estimate the incubation time for the precipitation of the Mg17Al12 and these predictions will be compared against the heat-treatment experiment results.

Thus, this research is expected to provide valuable experimental and modeling data that is generally not available to date for the processing of Mg alloys. Development of such fundamental understanding of Mg alloys will enable their use as high-strength low-density structural components in light-weight cars and trucks to reduce fossil-fuel consumption and reduce green-house gas emissions.

Project Details

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Principal Investigator

Aashish Rohatgi
Pacific Northwest National Laboratory

Team Members

Karen Kruska
Pacific Northwest National Laboratory

Nigel Browning
University of Liverpool

James Evans
Environmental Molecular Sciences Laboratory

Libor Kovarik
Pacific Northwest National Laboratory

Venkata Rama Vemuri
Environmental Molecular Sciences Laboratory

Niranjan Govind
Pacific Northwest National Laboratory

Related Publications

Kruska K, A Rohatgi, VRS Vemuri, L Kovarik, TH Moser, JE Evans, and ND Browning. 2017. "Grain growth in nanocrystalline Mg-Al thin films." Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science 48A(12):6118-6125. doi:10.1007/s11661-017-4350-0
Nandipati G, N Govind, A Andersen, and A Rohatgi. 2016. "Self-Learning Kinetic Monte Carlo Simulations of Al Diffusion in Mg." Journal of Physics: Condensed Matter 28(15):Article No. 155001. doi:10. 1088/0953-8984/28/15/155001