Quantification of nanoscale Al3Zr-dispersoids in 7xxx aluminium alloys using low energy backscattered electron microscopy

High strength 7xxx series aluminium alloys are commonly used in the aerospace industry and hold significant promise for advancing weight reduction efforts in automobiles. These alloys contain nanoscale phases, known as dispersoids, which precipitate during homogenization. Dispersoids have a major impact on processability and final product properties. Dispersoids are commonly analysed by transmission electron microscopy (TEM), especially in the case of Al3Zr-dispersoids with typical sizes < 50 nm. However, TEM foil preparation is rather time-consuming, and TEM operating costs are high. In contrast, scanning electron microscopy (SEM) is less expensive, and sample preparation is much easier and faster. However, resolution is limited in comparison to TEM.
The focus of this work is the SEM analysis of Al3Zr-dispersoid size distributions in aluminium alloys. A specialized low accelerating voltage (≤ 5 kV) backscattered electron detector is employed, greatly improving the attainable resolution compared to standard SEM conditions. The method is validated using two variants of alloy AA7108A with different dispersoid size distributions. The obtained size distributions are compared to those acquired using TEM, and good agreement is found.
Low-energy backscattered SEM using dedicated detectors allows for fast quantification of nanoscale Al3Zr-dispersoids in aluminium alloys, offering the potential to accelerate alloy and process development, particularly in contexts where optimizing dispersoid size distributions is critical.