Sample preparation methods for scanning electron microscopy of homogenized Al-Mg-Si billets: A comparative study

Abstract

Characterization of Mg-Si precipitates is crucial for optimizing the homogenization heat treatment of Al-Mg-Si alloys. Although sample preparation is key for high quality scanning electronmicroscopy imaging,most common methods lead to dealloying ofMg-Si precipitates. In this articlewe systematically evaluate different sample preparationmethods: mechanical polishing, etching with various reagents, and electropolishing using different electrolytes. We demonstrate that the use of a nitric acid and methanol electrolyte for electropolishing a homogenized Al-Mg-Si alloy prevents the dissolution of Mg-Si precipitates, resulting in micrographs of higher quality. This preparationmethod is investigated in depth and the obtained scanning electron microscopy images are comparedwith transmission electronmicrographs: the shape and size ofMg-Si precipitates appear very similar in either method. The scanning electron micrographs allow proper identification and measurement of the Mg-Si phases including needles with lengths of roughly 200 nm. These needles are β″ precipitates as confirmed by high resolution transmission electron microscopy