Microstructure and Properties of Al Alloys: A Personal View

The mantra ‘Microstructure determines properties’ is widely accepted as a truism, but is here critically assessed and found wanting. While bifilms from the casting process are typically invisible in the microstructure, they are often just as important, if not far more important, because they are often represented as a dense population of cracks throughout the metal. The morphology of many characteristics of cast and wrought structures is controlled by the bifilm population. Bifilm control of pore morphology and Si morphology in Al-Si alloys, as well as dendritic arm spacing, are explored for cast alloys (DAS). The benefits of grain refinement in terms of tensile properties appear to be mostly bifilm controlled. Bifilm composition appears to have a strong influence on ductility and fatigue, as well as invasive corrosion processes such pitting, intergranular corrosion, hydrogen blistering, and cracking. The notion of bifilm control is offered as a new way to improve and control metallurgical properties.