Characterisation and Modelling of Novel 5xxx Series Aluminium Alloys Containing L12 Dispersoid Forming Additions

Abstract

The use of aluminium alloys in armour plating of land and sea based combat vehicles can reduce the overall weight of the vehicles and improve manoeuvrability. Aluminium alloys from the 7xxx series (Al-Zn-Mg-Cu) and the 5xxx series (Al-Mg) are most commonly used. Of these, the 5xxx series present an attractive combination of properties including weldability and corrosion resistance. These alloys, however, are lower in strength than their 7xxx series counterparts, and can suffer from issues of sensitisation causing inter-granular corrosion and stress corrosion cracking. Additions of elements such as Zirconium (Zr) and Scandium (Sc) to the alloys can precipitate the L12 dispersoid phase during heat treatment. This phase helps to strengthen the alloys and can improve recrystallisation resistance. It is important to develop a suitable distribution of dispersoid particles within the microstructure of the alloys such that their effect on the material properties is desirable. The effect on material properties from dispersoids can vary widely depending on the treatment undergone by the alloys and the resulting distribution of particles. In this work three aluminium-magnesium (∼4wt% Mg) alloys were studied which contained varying concentrations of L12-forming additions. It was found that on casting a severely heterogeneous dendritic microstructure was formed in the alloys, with T-phase (Al-Mg-Zn-Cu) at grain boundaries, Zr concentrated in dendrite cores, and Sc concentrated in dendrite edges. This heterogeneity of L12-forming Zr and Sc could not be alleviated by homogenisation treatments and resulted in a complex distribution of dispersoids. Zr-enriched dendrite cores formed many small L12 dispersoids, with larger dispersoids found in the Sc-enriched dendrite edges after treatment. Electron probe micro-analysis revealed that additions of Erbium (Er) and Yttrium (Y), which were intended as L12-forming additions, instead formed a grain boundary eutectic phase on casting. The Er and Y content was not distributed back into the matrix during homogenisation and was therefore unavailable for secondary L12 dispersoid precipitation. It was also shown that recrystallisation resistance can be maintained to high temperatures with minor additions of L12-forming elements. The experimental work was used to inform a microstructure-property model capable of predicting the effect of changing process conditions on the dispersoid distributions. Coupled to a strengthening model, the strength and work hardening behaviour was calculated. The model demonstrated the synergistic effect that dispersoids have on enhancing work hardening, as well as their direct strengthening effect, and can be used as a predictive tool for alloy and process design.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Prangnell (Supervisor) & Joseph Robson (Supervisor)