Microstructural Evolution and Electrochemical Properties of HRDSR AZ61- ( = Ca, Ti) Alloys.

The microstructure and corrosion properties of as-cast AZ61 (Mg-6% Al-1% Zn) and AZ61 alloys doped with titanium and calcium and subjected to high ratio differential speed rolling were investigated. Addition of the alloying elements to the AZ61 alloy resulted in remarkable modification of the morphology and the amount of continuous beta (Mg17Al12)-phase. Addition of Ti to the as-cast AZ61 alloy causes a decrease in the volume fraction (or discontinuity of the beta-phase), leading to strong anodic dissolution. In contrast, addition of Ca to the as-cast AZ61 alloy is rather effective for preventing pitting corrosion. This is attributed to the formation of a semi-continuous network beta-structure. The (Mg, Al) 4Ca phases dispersed between the beta (Mg17Al12)-phases led to continuity in the AZ61 alloy with Ca. The AZ61 and AZ61-X(Ca, Ti) alloys subjected to severe plastic deformation via high-ratio differential speed rolling possessed a nano-composite-like microstructure in which the alpha-Mg matrix with an ultra-fine grain was surrounded by a large number of fine beta particles. These particles were either dynamically precipitated or broken at the grain boundaries, as well as in the grain interiors, by the high ratio differential speed rolling process. The corrosion resistance of the AZ61 and AZ61-X (X = Ca, Ti) alloys subjected to high ratio differential speed rolling was largely improved by the microstructural modification. The high ratio differential speed rolling process greatly influenced the texture of the Mg alloys, which significantly affected their corrosion behavior.