The project team has made good progress toward targets set for project A1. This project has led to important publications in internationally peer-reviewed journals, including articles in Acta Materialia, Metallurgical and Materials Transactions A, and Fundamentals of Al Metallurgy.

A new casting unit that will be installed at The University of Sydney will significantly extend the composition space of alloys to be explored.

Diffusion-couple experiments have been conducted successfully and achieved a more general understanding of rapid-hardening phenomenon in a wide composition space in the Al-Cu-Mg system. Total concentration of solutes has been found to be an important factor associated with the rapidly-hardening response of the Al alloys. Further investigation will be undertaken to pinpoint the precise nature of particular strengthening components responsible for these unique hardening effects.

Evaluations of the rollability of Al-1.1Cu-1.7Mg (at%) alloy has been conducted and found that a reduction of 10 mm to 0.2 mm could be achieved without serious cracking problem. Importantly, the initial rolling tests on the ‘cluster-strengthening’ alloy indicated that the alloy was highly formable during both hot and cold rolling. However, edge cracking was observed in a thicker block of 30 mm that contained micro-cavities formed during casting, demonstrating that casting quality is important for the rollability of the alloy. Further full evaluations on formability of cluster-strengthening alloys have been planned for after large cast ingots of 50 kg are available.

We have made the first observation of shear-induced dissolution of clusters/GP zones firstly observed in Al-Cu binary alloys. A bimodal distribution of precipitate microstructures has been identified as a key structural factor responsible for the alloys demonstrating high strength and high elongation rates. This research finding potentially is important for engineering ideal microstructure in order to obtain an excellent combination of properties in service.

We have made good progress in the experimental investigation on the effect of microalloying on rapidly-hardening response in Al-Cu-Mg alloys. In particular, the alloy with an addition of trace Sn was observed showing a complicated hardening response. Detailed understanding of the mechanism by which trace elements affect precipitate-microstructure evolution will be valuable for design and development of new alloys with effective addition of trace elements.

In order to design new-generation 2xxx series Al alloys with improved mechanical properties, the first stage of the expedition has a focus on understanding strengthening mechanisms of commercial AA2024 and AA2524 alloys, given that there is no consensus yet about the precise strengthening mechanisms of commercial 2xxx Al alloys in the scientific literature. This research effort is a close collaboration between the University of Sydney and Monash University, combining state-of-the-art aberration-corrected HRTEM and local electrode atom probe (LEAP). The new atomistic information about precipitate microstructures of the two alloys will help to identify main strengthening phases and to establish the strengthening effect of the Q-phase on these two alloys.

In addition to AA2024 and AA2524, AA6060 and AA6111 alloys have been included in the investigation because these other alloys have been considered to be strengthened by different types of precipitate phases. Currently, APT characterisation has been conducted on the two 6xxx Al alloys and has revealed that significant Cu partitioning takes place during early-stage precipitation in AA6111.

The overall aim of this project is to develop new mechanical property-performance space in Al alloys to increase their breadth of applications, to introduce new engineering-design possibilities and to make Al alloys more competitive in the face of the evolving properties of other materials. The project has two streams, namely A1.1 Understanding the properties and performance of a cluster-strengthened Al-Cu-Mg alloy, A1.2 Towards a New Medium or High Strength 2xxx Alloy.

A1.1 Fundamental Research: The Enabling Science
A cluster-strengthened Al- alloy can exhibit a medium strength of ~230 MPa and excellent total elongation of ~23%. These properties are superior to most of the 5xxx alloys. This is an attractive property baseline upon which to design in other effects. In this stream, we aim to achieve a thorough understanding of the properties and performance of a cluster-strengthened Al-1.1Cu-1.7Mg (at%) alloy with the view towards developing these properties for commercial application. We aim to produce a white paper from such work, which could be used as a negotiating tool to begin conversations with a possible industrial partner.

A1.2 Applied Research: Alloy Technology
A final objective of this stream is to develop new alloys with properties compatible to commercial AA2024, and AA2524 alloys.  The first stage of this research aims to develop a thorough understanding of strengthening mechanisms in the commercial Al alloys by careful microstructure characterisation using advanced electron microscopy and atom probe tomography, and to explore how microalloying modifies precipitation microstructure. This research also aims to unveil the potential coupled effects of cluster-strengthening on a precipitation microstructure. The second stage of this research aims to developing new alloys by successfully engineering alloy microstructure.

A1.1 Fundamental Research: The Enabling Science

Diffusion couples have been fabricated from Al-Cu and Al-Cu-Mg alloys in order to explore rapid-hardening effects within a broad compositional range. Both nuclear magnetic resonance (NMR) spectroscopy and atom probe tomography have been employed to evaluate microstructural evolution of the materials and gain insights into the microstructural characteristics and to develop better understanding on the hardening behaviour of the materials.

Initial rolling experiments show positive results about rollabilty of cluster strengthening Al-Cu-Mg alloy. When a large high-quality cast ingot is available, more comprehensive formability texts are planned in order to generate a full picture about properties of the cluster alloys.

A1.2 Applied Research: Alloy Technology

We have advanced our investigations of strength-elongation combinations in Al-Cu alloy microstructures containing a bimodal distribution of precipitates (theta prime + GP zones) by completing further tensile testing at a very low strain-rate of 10-6 s-1. Further experimental investigation is planned to extend this work to commercial 6061 Al alloy by employing a bimodal distribution of precipitate microstructures to gain a combined improvement of strength and elongation rate. In addition, new experiments have been designed to investigate the effect of strain-induced dissolution of GP zones in these Al alloys.

Through a combination of cold rolling and ageing treatments, we have demonstrated a very different hardening response with the present of trace Sn in a ‘clustering’ alloy. Further careful microstructural characterisation will be conducted by using TEM and APT to unveil the mechanisms responsible for the effect observed.

The initial hardness response of commercial 2xxx series Al alloys such as AA 2024 and AA2524 during ageing at 150 °C and 170 °C is a rapid hardness increase within 6 min of ageing.  We intend to do further microstructural characterisation to unveil the detailed strengthening precipitation phases in the alloys and the solute partitioning behaviour during ageing process. This will be a joint effort between Monash University and the University of Sydney that will draw on the advanced characterisation facilities in the two nodes. The new knowledge obtained about commercial Al alloys will be valuable for design and development of a new generation alloy, in which novel microalloying effects will be employed.