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Measuring the dimensions and number density of nanoparticles dispersed in a solid matrix is usually accomplished via transmission electron microscopy (TEM) which suffers from high cost, low throughput, and small analytical volume. In comparison, scanning backscattered electron microscopy is inexpensive, requires little sample preparation, and allows for the analysis of large sample areas. However, the information depth is usually not known precisely and depends on several factors such as the composition of the nanoparticles and the matrix as well as the size of the nanoparticles, hindering the reconstruction of the actual size distribution and three-dimensional number density. Here we present a method to estimate the information depth for spherical nanoparticles of different sizes in order to accurately determine size distribution and number density. The approach is based on Monte Carlo simulation of electron trajectories in the material and analysis of the obtained backscattered electron signal-to-noise-ratio. Our experimental results are compared to those obtained via TEM and good agreement is demonstrated; this shows that TEM can be replaced by scanning electron microscopy for studying nanocomposites in many cases.

In the automotive industry Al-Mg-Si alloys are often chosen for high strength extruded profiles. However, the production of such profiles can be challenging for high alloy contents. Among several processing steps, the homogenization heat treatment before extrusion is readily accessible to modifications to improve high temperature formability. In this work, the influence of five different homogenization variants on the flow stress of AA6082 at elevated temperatures was assessed by compression testing in a deformation dilatometer at 450 °C, 480 °C, and 510 °C. The observed differences in flow stress were interpreted with regards to the microstructure. Samples from billets homogenized at higher soak temperatures exhibited lower hot flow stresses in subsequent dilatometer testing, indicating better processability. Since the decrease in flow stress is approximately constant at all test temperatures, it is suggested that it is due to a lower number density of relatively temperature-stable α-Al(Mn,Fe)Si dispersoids formed during homogenization at higher temperatures; these dispersoids are stable at all three test temperatures. In contrast, the influence of a cooling rate variation was relatively minor and diminishes at higher test temperatures since Mg-Si precipitates – whose size and number density is affected by changes of the cooling rate – are less stable at the higher test temperatures.

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 electron microscopy imaging, most common methods lead to dealloying of Mg-Si precipitates. In this article we systematically evaluate different sample preparation methods: 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 preparation method is investigated in depth and the obtained scanning electron microscopy images are compared with transmission electron micrographs: the shape and size of Mg-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.

SiC particles are effective grain refiners in Mg-Al alloys. Several investigations, from different researchers, into their effect on a range of alloys with different Al contents has been undertaken and it has been found that the greatest reduction in grain size occurs in alloys having low Al contents. Performing grain refinement studies on a range of alloy contents also allows for further investigation into the mechanisms of grain refinement. It was found that the size of the SiC particles is also important in magnesium grain refinement. However, the presence of Mg2Si in the microstructure and the consideration of phase equilibria suggest that SiC can transform into other binary or ternary carbides. If such carbides are formed, they may also act as an effective grain refiner for Mg-Al alloys. In this study, the possibility of formation of new carbides (Al4C3, Al2MgC2, Mn7C3, Mg2C, Mg2C3, Al2CO etc.) and their ability to be good grain refiners for Mg-Al alloys is discussed.

The addition of SiC particles effectively grain refined a range of Mg-Al alloys. The greatest reductions in grain size were found for the alloys with lower Al contents. The presence of Mg2Si in the microstructure after that SiC addition, and consideration of phase equilibria suggested that the SiC transforms to Al4C3, and this is the actual nucleant. The addition of Mn poisoned the grain refining effect of the SiC addition, probably due to the formation of less potent Al-Mn-carbides. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.