Revisiting Grain Boundary Precipitation in 7xxx Aluminium Alloys

Abstract

7xxx series aluminium alloys are widely applied in the aerospace and automotive industries due to their outstanding mechanical properties. In the past decades, a large amount of research has been carried out on the effect of alloy composition and ageing treatments on their material performance, but the quench sensitivity problem caused by the large thickness of commercial alloy plate has often been underesti- mated. This study has investigated the quench sensitivity of 7xxx aluminium alloy thick plates, focusing on their through-thickness variation of grain boundary η-phase precipitate (GBP) distribution and its impact on the mechanical and electrochemical properties of 7xxx alloys, which is a key factor in controlling their fracture tough- ness and stress corrosion performance. Novel material characterization methods and CALPHAD-informed phase-field simulations, were developed and used in this work to obtain a better understanding of this phenomenon. To study the quench sensitivity behaviour the Jominy end quench test was applied to previous generation (AA7050) and new generation (AA7085, AA7449) thick plate alloys, to generate samples with a range of controlled cooling rates and compare their behaviour. A Cold intergranular fracture (CIF) method was devel- oped and employed to fracture open the GBs and allow more rigorous observation and quantification of the GBP distributions and morphologies formed on the GB plane. Quantification was performed by Weka segmentation of the different classes of precipitates. Several CALPHAD-informed phase-field models, that incorporated a simplified particle geometry and nucleation model, were also used to simulate the nucleation and growth of GBPs. A CALPHAD database was adopted to describe the thermodynamics of the high order quaternary Al-Zn-Mg-Cu alloying system. In general, the simulations matched well with the experimental observations, and replicated the greater quench sensitivity of AA7449 and AA7050 compared to that of AA7085. Two families of GPs were discovered with very different characteristics, in terms of their size, morphology and compositions depending on whether they formed dur- ing quenching from the solution treatment temperature (Q-GBPs) or subsequently during artificial ageing (A-GBPS). The precipitate free areas seen surrounding the Q-GBPs where sufficient solute had been consumed to prevent GBPs forming during ageing were also quantified. After Weka segmentation and quantification, it is shown that the area fractions of the A-GBPs varied in a non-intuitive way with increasing cooling rate. As expected, increased cooling rates were observed to decrease the av- erage size and GB coverage of the Q-GBPs formed during quenching. The Q-GBPs were found to have a complex-branched dendrite-like appearance at slower cooling rates and reduced in aspect ratio and complexity and high cooling rates. How- ever, the density of Q-GBPs also increased dramatically at cooling rates close to the rate at which their nucleation became completely prevented, which dramatically reduced the area coverage of A-GBPs, leading to a minima in their area fraction. The GB area coverage of A-GBPs then increased again at lower cooling rates as the spacing between the Q-GBPs increased. Advanced scanning electron transmis- sion microscopy (STEM) and focused ion beam techniques were adopted to perform accurate composition measurements of the Q-GBPs. Through performing the P- FIB TEM foil preparation along the growth direction of Q-GBP particle branch, considerable chemical gradients were observed in the STEM-EDS measurements. Simulations using CALPHAD-informed phase-field model and two-sublattice model gave results consistent with the experimental observations, in terms of predicting the precipitate chemistries with bulk alloy composition and cooling rate, but were less successful in predicting the size distributions and precipitate area fractions because of the statistical limitations caused by the computa

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Pratheek Shanthraj (Supervisor) & Philip Prangnell (Supervisor)