Improvements in the metallography of as-cast AZ91 alloy

Evaluation of the grain structure and grain size of cast magnesium–aluminum alloys is usually performed after grain-boundary-enhancing treatments which involve tedious heat treatment at high temperature. In this work, a rapid and simple etching method is described for metallographic preparation of AZ91 alloy, revealing grains without altering the as-cast dendritic microstructure and the presence of secondary phases. The etching reagent, composed of acetic acid, alcohol and water, has not been reported previously. After etching and drying, a crackled film containing fine stripes is found at the surface of the substrate. The optical contrast between grains under polarized light is attributed to form birefringence induced by the stripes, preferentially aligned according to the orientation of underlying grains. The composition of the crackled film was evaluated and its characteristics are presented. It was found that a minimum concentration of aluminum is necessary in the magnesium-rich matrix for the formation of the crackled film. Likewise, beyond a maximum concentration of aluminum, the surface film does not crack.