Scanning Electron Microscopy of Low-Contrast and Nanoscale Precipitates in Homogenized Aluminium AlloysThursday (09.11.2017) 11:15 - 11:35 Part of:
In the processing of Al wrought alloys, a multitude of interdependent precipitation processes occur. During casting, primary intermetallic particles and/or eutectic phases form. Some of these phases dissolve or transform during subsequent homogenization treatment and other phases such as nanoscale dispersoids are precipitated. In the case of Al-Mg-Si alloys, intragranular Mg-Si precipitates are also formed. Unlike the particles formed during casting, these different types of secondary precipitates cannot be studied in a satisfactory way by optical microscopy because of their small size. In fact, even scanning electron microscopy (SEM) of these particles is a challenge due to their low contrast, small size and the difficulty of adequate sample preparation. Therefore, these precipitates are usually studied in transmission electron microscopy (TEM), a method bringing about high cost and experimental effort and often not readily available to industrial users. Furthermore, the analytical volume in TEM is very small and the analysis might thus not be representative.
In comparison, SEM is inexpensive, allows for higher throughput and the analysis of large sample areas. In this talk, sample preparation methods for high resolution imaging of dispersoids and Mg-Si precipitates in the SEM are presented. Furthermore, a stereological method that allows for the accurate transformation of observed area densities and skewed size distributions of dispersoids imaged by backscattered SEM into three-dimensional number densities and undistorted size distributions is discussed. Examples are provided on how the acquired information on secondary precipitates can be used as input parameters for mechanistic models to predict processability in rolling and extrusion, or the mechanical properties of the final product.