Research Area 11.1: HCP Mg Alloys Formable at Room Temperature

Magnesium (Mg) is an attractive material for many lightweight military, automotive, aerospace, and consumer applications because of its low density of 1.74 gm/cm3 vs 2.70 gm/cm3 for aluminum. However, since Mg has a hexagonal close-packed (HCP) crystal structure, it has two intrinsic problems in manufacturing and application namely poor room-temperature ductility and formability. Mg alloy components, therefore, are manufactured via casting or elevated-temperature forming. The disadvantage of these methods is slower processing and higher energy consumption, thus raising the manufacturing cost and limiting market penetration. Several approaches to improve the ductility of Mg alloys involve use of rare-earth elements. The major disadvantage of this approach is cost and strategical availability of rare-earth elements. In addition, many of these alloys are produced by rapid solidification and powder metallurgy. These manufacturing methods are not readily amenable to mass production of inexpensive and reliable Mg alloy components. The main objective of this exploritary project is development of a ductilization model with the ultimate goal to create new commercially useful Mg-based alloys with superior room-temperature ductility and formability, that could be produced by conventional methods and that are rollable and formable at room temperature without the use of rare-earth elements. In our preliminary investigation (a thermodynamic modeling and experimental work) it was shown that after certain heat-treatment precipitates are formed in Mg-Ca-Zn ternary system that lead to remarkable room-temperature ductility; plates did not form cracks when bent on themselves 180° around a mandrel at room temperature. To go further, the relationship among heat treatment, density and size of nano-sized precipitates, and mechanical properties (strength and ductility) should be quantified. Mg-alloy compositions and heat treatment conditions will be selected based on a thermodynamic modeling already performed. The microstructure and precipitates will be studied systematically through optical and electron microscopy, x-ray diffraction, and 3D atom probe tomography, along with mechanical property measurements. The above studies will allow us not only to determine composition and heat treatment resulting in Mg alloys with optimum combination of strength and ductility, but also result in fundamental scientific understanding of how precipitate structure and composition may be related to these improved properties. We anticipate, that this project will result in the development of a ductilization model with the ultimate goal to create new commercially useful Mg-based alloys that do not contain rare-earth elements with superior room-temperature ductility and formability, that could be produced by conventional methods and which are rollable and formable at room temperature. These lightweight alloys will contribute significantly to the Army’s mission.