Project A1: Design of Al-base structures for enhanced strength
	Target: Improving strength without diminishing ductility
•	Project Chief Investigators: Simon Ringer (Project Leader), Arne Dahle, Christopher Hutchinson, Barry Muddle
•	Senior Researchers/Research Associates: UQ: C.M. Gourlay; Monash: Y. Estrin, J. da Costa Teixeira, L.N. Bourgeois, F. De Geuser; Sydney: G. Sha.
•	Research Students: UQ: B. Meylan; Monash: S. Knight, S. Katsoulotos, J. Rosalie; Sydney: R.K.W. Marceau, P.D. Liddicoat,
•	Research Assistants Visiting Scholars, Hons Students etc: UQ: Mylene Brondolin, Monash: D. LaCreuse, Shengdan Liu, Zhang Yong, D. Cram, K. Huynh, P. H. Loo.; Sydney: Hamidreza Mohseni, Alex La Fontaine, Thomas Burdsall.
•	External Collaborators: S.P. Lynch (DSTO), J.D. Embury (McMaster University), Jiang Na (Southwest Aluminum Group Co. Ltd.), Guo Fuan (Suzhou Institute for Nonferrous Metals Processing Technology), Zhang Xinming (Central South University), C. Sinclair (University of British Columbia).
Project Summary
This project is concerned with the metallurgical science and technology of developing novel property-parameter space for structural Al alloys. The emphasis is on extending strength and maintaining good elongation, and the project connects with other Centre projects related to the development of other Al alloys’ engineering properties and the stability thereof. There are three inter-related streams of research: (i) Melt Processes, Processing and Solidification, (ii) Early Stages of Ageing and Clustering Processes and (iii) Nucleation and Precipitation Processes.
Project Progress: Technical Details, Targets and Research Outcomes
A1 - Stream 1 (S1) focuses on the flow and deformation of partially solid Al alloys with a view to understanding and ultimately controlling the mechanical behaviour during solidification processing. This year the project has concentrated on the development of mechanical strength during equiaxed dendritic solidification.
The measurement capabilities of the rheometer at UQ have been upgraded significantly. Data acquisition improvements now enable us to simultaneously log rheological and thermal data in LABVIEW. The incorporation of a linear variable differential transformer (LVDT) into our rheometer, has markedly improved our ability to measure volumetric strain during semi-solid deformation (Figure A1).
Figure A1 - Mechanical data acquired using the upgraded rheological measurement capabilities at UQ.  Part (a) shows that Al-10Cu deformed in the semi-solid stated expands as it is sheared.  Part (b) shows that this rheological response is very similar to the classical response of a saturated granular material (wet compacted sand in this case).
Research has focussed on high Cu-containing binary Al-Cu alloys because primary dendrite size and morphology can be well-controlled by inoculation in these alloys. The improved instrument has been used to confirm that Reynolds dilatancy occurs when Al alloys are deformed during equiaxed solidification at solid fractions after crystal impingement. Dilatant shear bands, a form semi-solid strain localisation, have been shown to occur in an Al-10Cu alloy and have been compared with those that form in Mg-based alloys. Vane rheometry and quenching experiments have revealed that the deformation mechanisms that operate in partially solid Al-alloys are similar to those in Mg-based alloys.
We are also in the early stages of research exploring novel methods for detecting the point of crystal impingement during equiaxed solidification of Al- alloys.
A1 - Stream 2 (S2) includes sub-projects A. Design with Cluster Strengthening for Enhanced Strength and Elongation, B. Design of a Novel Al Alloy for High Toughness and C. Processing for Enhanced Strength and Toughness.
The work in S2A is geared to cover medium strength-high elongation alloys, such as those used for beverage cans (e.g. 3xxx and 5xxx) and car panels (e.g. 6xxx). Our research has developed new insights into the relationships between the microstructure and yield strength (σy) & uniform elongation (εu) and paves the way for the exploitation of strategies to potentially improve both properties simultaneously. Our emphasis is on what we consider are 'engineered' solid solutions that contain solute clusters which affect both σy & εu. A first order model based on bond-breaking previously proposed by Ringer has been developed. This requires more work but the results suggest that already we are on the correct track. Similarly, we have made new advances in understanding the link between the strain hardening behaviour and the microstructure in solid solutions through experiments in model Al-Cu alloys which suggests new directions for controlling the uniform elongation of engineering alloys. We have demonstrated that the initial work hardening rate in Al-Cu solid solutions depends strongly on the bulk Cu content and less strongly, though still significantly, on the applied strain rate. The dynamic recovery behaviour is shown to be independent of Cu content and strain-rate. This is opposite to the usual teaching in this field. We have developed a model based on the effect of solute on dislocation-dislocation junction strength and the efficiency of dislocation trapping is being developed.
Our first attempt has succeeded in reaching a design target for doubling of strength and uniform elongation of an alloy based on a ‘clustered’ solid solution. We have developed a solid solution Al-Cu-Mg alloy with a yield strength of 230 MPa and a uniform elongation of ~30%. This is twice as strong as existing medium strength-high elongation alloys of the same elongation. Or equivalently, it is twice the elongation for similar alloys of the same strength. Atom probe tomography and transmission electron microscopy experiments have been completed to provide key inputs to understand the phenomenology and to guide modelling efforts. A model describing the strain induced dissolution of the clusters and the resulting effects on the strength and work hardening has been developed. These dynamic effects are in the spirit of transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects that are exploited in certain classes of high strength-high elongation steels. This cluster strengthening phenomenon provides new materials design opportunities where both σy & εu may be increased significantly.
Our S2B work is a fundamental alloy design and development exercise and in the planning stages. Work will commence in 2008 when the alloy design principles in S2A are more complete.
The S2C work is based around high pressure torsion (HPT) methods. Again, we seek to explore what types of microstructures can develop ultra-high levels of strength in Al alloys and yet preserve reasonable levels of ductility. An aspirational design target of achieving 1 GPa yield strength in an Al alloy fabricated by ingot metallurgy has been achieved: yield strengths of ~ 1GPa have been achieved in a HPT 7075. Detailed atom probe experiments are in progress to understand the origins of this extraordinary strength. Results so far have demonstrated a remarkable dispersion of solute in this system that we attribute to the strengthening effect.
There are three aspects to stream 3 (S3): A. Design of High to Ultra-high Strength Alloys, B. Structural Integrity of High Strength Aluminium Alloys and C. Designing Multi-Scale Microstructures in Precipitation Hardened Al Alloys for Simultaneous Enhancements in Both Strength and Elongation. In S3A, new Monte Carlo simulations of the decomposition in model Al-Cu alloys.
In S3B, work is exploring two key aspects of structural integrity in Al alloys: environmental embrittlement and the structure and properties of interfaces in alloys. Specific work includes characterisation of environmental embrittlement of 7000 series alloys and Li-containing high-strength aluminium alloys, and the composition, structure and structural integrity of alloy interfaces.
In S3C, we are concerned with the underlying science controlling the strength-elongation combinations possible in microstructures containing precipitates. This approach is geared to cover high strength-medium elongation alloys used for aerospace applications (e.g. 2xxx and 7xxx). The work has required that we revisit the theory of precipitation strengthening, bearing in mind the non-spherical precipitate morphologies in age hardenable Al alloys. The analyses have produced a new model for precipitate strengthening from shear-resistant plate shaped precipitates with very some surprising and powerful insights into new strategies for strengthening by non-spherical precipitate distributions. The key conclusions are that completely different strategies from those developed for spherical precipitates should be used. In the case of precipitate plates, the maximum strengthening increment depends only on the bulk alloy content and the temperature of aging. This is a rather remarkable conclusion that is very different to previous treatments but which has been confirmed through carefully controlled experiments.
New strategies for microalloying to change precipitate morphology have been explored and it has been demonstrated that certain precipitate morphologies and orientation relationships can be controlled by microalloying. Significant new progress has been made: for the first time, we have observed and controlled the precipitation of new morphological variants of key strengthening precipitates such as Θ' and σ in Al-Cu-based systems. We have also observed the first example of precipitation-within-precipitation and now have new degrees of freedom in tailoring precipitate morphology.
We have also produced a new model for the work hardening of precipitation hardenable microstructures that explicitly describes the kinematic and isotropic hardening components as a function of the microstructure. By combining the two new models for strengthening and work hardening we have been able, for the first time, to successfully describe the strength-elongation combinations, as a function of precipitate state, in as model Al-Cu alloy. This has been used to identify two new strategies for increasing the strength of precipitation hardenable alloys without decreasing their elongation.
A first attempt to deploy these combined strategies has allowed us to increase the yield strength of these alloys by 35% without decreasing the uniform elongation. There are many possibilities for these alloys and doubling the strength and elongation simultaneously should not be considered out of reach.