Alloy Design and Processing Design of Magnesium Alloys Using 2nd Phases

Two phases dominate the performance of commercial Mg alloys: (1) beta Mg17Al12 and (2) porosity. Alloy design and process design to optimize the morphology of the first and to minimize the second are discussed. Second phase beta can be designed to improve tensile strength, fatigue strength, toughness, texture, formability and corrosion resistance of Mg alloys. Processing is applied to refine the grain size, to array this phase in micron sizes at grain boundaries, and to further precipitate this phase in nanometer arrays within the grains. Therein, the 2nd phase beta plays the following roles: (1) retarding grain growth, (2) randomizing texture, (3) Hall-Petch hardening, (4) Orowan hardening, and (5) moderating corrosion. Porosity is a detrimental 2nd phase common to cast Mg alloys in the form of gas or voids; its control being essential to engineering applications. Porosity can be diminished by sub- liquidus molding with high-velocity/high-pressure injection and then eliminated by subsequent hot deformation.