Development of Lightweight, Corrosion-Resistant Magnesium Alloys Formable at Room Temperature

FINAL REPORT Background and Motivation There is a need for lightweight structural materials for transportation to improve the performance and energy efficiency. Because magnesium (Mg) is 36% less dense than aluminum (Al) (1.74 g/cm for Mg vs. 2.70 g/cm for Al), Mg-based alloys have received increasing attention lately. The limiting property of Mg and its alloys at the present time is poor ductility (and formability) at room temperature; therefore the Mg alloys are used primarily in the cast or extruded condition. Development of Mg alloys with sufficient strength and enhanced ductility at ambient temperature will lead to significant energy savings (a) by replacing Al alloys with lighter Mg alloys in vehicles and (b) by being able to be formed into complex shapes at room temperature instead of 150-200°C (required for forming) or 650°C (required for casting or extrusion). The goal of the project was to provide the basis for development of a new class of magnesium alloys that are formable at room temperature. The poor ductility of Mg and its alloys at room temperature is due to its HCP (hexagonal close-packed) crystal structure, which provides only two independent slip systems for easy plastic deformation. These are on the basal plane. Homogenous deformation of polycrystalline metals requires five independent slip systems. Slip on the basal plane requires only a small stress to move a dislocation from one energy valley to the next (the Pieirls stress). In contrast slip on the pyramidal or prismatic planes require a slip component in the c direction. These dislocations have very high Pieirls stresses, the source of the very poor formability. Dislocations with high Pieirls stress move by nucleating a double kink along their length followed by expansion of the double kink. Nucleation of a double kink requires high activation energy when the Pieirls stress is high. The interaction between a dislocation and a small misfitting nanosize size precipitate locally reduces the activation energy needed to form a double kink increasing the dislocation’s mobility. A lower stress is then needed to nucleate a double kink increasing the formability at room temperature. Our approach to find and develop a ductile Mg alloy has been to seek a precipitation hardening systems where such nano-sized coherent precipitates would form. We used this approach successfully in the past for design of fracture-tough steel at cryogenic temperature.[1] We have also demonstrated recently the applicability of this concept to Mg alloys by examining a specific region of the Mg-Li-Zn phase diagram, specifically Mg-2.4Li5.1Zn (in wt.%; density 1.71 g/cm). The alloy was cast for us by Sophisticated Alloys, Inc., followed by solution treatment and aging we devised. This alloy was shown to achieve remarkable formability; this alloy did not form cracks even when bent on itself 180° around a mandrel at room temperature. In the present project we investigated two systems: (a) Mg-Ca-Zn ternary and (b) Mg-Li-Zn-Ca quaternary. We selected these systems for several reasons. First, Li is known to improve some ductility of Mg. Second, the two lower-order systems (Mg-Zn and Mg-Ca-Zn) are known to form nanometer-sized precipitates (GP zones) as result of solution treatment and aging at appropriate temperatures. These precipitates were expected to increase strength and enhance room-temperature ductility. Further, there is literature evidence that Ca improves the corrosion resistance of Mg alloys. In addition, the reduction of the amount of Zn in the alloy and substituting it with Ca should lead to further weight reduction and to improved corrosion resistance.