An Experimental and Modeling Investigation of Solidification and Microstructural Evolution in Magnesium Alloys using in situ Techniques
EMSL Project ID
49548
Abstract
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 resources. 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 prior years, binary Mg-Al as-sputtered films were heat-treated inside EMSL’s TEM between 150 ?C-300 ?C for times between 5-190 min. Growth of ?-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 FY17, we will analyze the grain growth using existing models (e.g. Burke-Turnbull model) and 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.
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
Start Date
2016-10-10
End Date
2017-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
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