Comparative study of the β″‐phase in a 6xxx Al alloy by 3DAP and HRTEM

The metastable β″‐hardening phase in aged 6xxx‐series Al alloys has been investigated by three‐dimensional atom‐probe in order to evaluate the composition (Mg 5 Si 6 ) of the structure, previously determined by high‐resolution transmission electron microscopy (HRTEM) and electron diffraction. For β″, a significant number of Mg and Si atoms are found to be lost during detection in atom‐probe analysis. The reason of this is non‐resolved events, in which the correlation between the time‐of‐flight (TOF) and the detected position of the field‐evaporated ion is lost. The problem is attributed to uneven evaporation and can be avoided by the use of an advanced delay‐line detector (ADLD), which is capable of discriminating extremely closely spaced detection events. Preliminary analysis of a β″‐particle using this detector gave a Mg/Si ratio of 0.9 and a composition of 15 at.% Al. The Al level is significantly lower than those previously measured by other atom‐probe detectors, because of the higher efficiency of the ADLD in detecting Mg and Si in particles. Atom‐probe analysis with the ADLD detector is consistent with the β″ Mg/Si ratio derived from electron microscopy analysis. The measurements indicate that Al atoms may substitute both Si and Mg positions. Copyright © 2007 John Wiley & Sons, Ltd.