Project A2: Design of Mg-base structures for enhanced strength
	Target: Matching Al in strength-ductility space
•	Project Chief Investigators: Jian-Feng Nie (Project Leader), Arne Dahle
•	Senior Researchers/Research Associates: Senior Researchers/Research Associates: UQ: Chris Gourlay; Monash: Allan Morton, Laure Bourgeois, Jie Geng, Sam Gao, Colleen Bettles, Rimma Lapovok
•	Research Students: UQ: Bastian Meylan; Monash: Yuman Zhu, Matthew Moss
•	Research Assistants Visiting Scholars, Hons Students etc: Zheng Ma
•	External Collaborators: Kazuhiro Hono (NIMS), Zhengyan Zhang (Shanghai Jiao Tong University), Penghuai Fu (Shanghai Jiao Tong University), Xi-Ya Fang (Central South University), Milo Kral (University of Canterbury)
Project Summary
This project includes three major streams, namely (1) rheology of solidifying magnesium alloys, (2) structures for enhanced strength in magnesium casting alloys, and (3) structures for enhanced strength in magnesium wrought alloys.
Stream 1 focuses on the development of mechanical behaviour in solidifying Mg-based alloys with the aim of producing high-integrity material in the first step of alloy processing. It also aims to develop a quantitative understanding of an important phenomenon that allows further improvement of alloys and casting processes. This phenomenon was recently discovered by Prof. Dahle and it shows that solidifying metals behave as dilatant granular media during solidification. This discovery can explain the formation of common defects in castings, particularly in high-pressure die castings, semi-solid castings and squeeze castings.
Stream 2 aims to (i) develop a group of high strength magnesium casting alloys via proper control of alloying additions and thermal processing conditions, and (ii) examine the stability and evolution of nano-scale aggregates (and solute contents in the matrix) in magnesium alloys that have been subjected to systematic change in alloying additions and thermal processing conditions.
Stream 3 aims to (i) develop a group of high strength magnesium wrought alloys via proper control of intermetallic dispersoids, precipitation and thermomechanical processing conditions, and (ii) characterise the microstructures and deformation behaviours of the thermomechancially processed alloys.
Project Progress: Technical Details, Targets and Research Outcomes
A2 - Stream 1: Improvements in data acquisition and measurement capabilities to the rheometer at UQ have enabled semi-solid deformation mechanisms to be studied. A particular focus has been given to transitions in rheological behaviour during semi-solid microstructure evolution. Three deformation regimes have been identified in equiaxed solidifying AZ91. Regime I: between nucleation and the point of crystal impingement, the material behaves as a dilute suspension and can be described as a notional fluid by Herschel Bulkley-type equations. Regime II: shortly after crystal impingement the material deforms primarily by crystal rearrangement and Reynolds’ dilatancy occurs (the material expands in response to shear). In Regime II, we have shown that the material can be considered a saturated granular material similar to water-saturated sand. Regime III: when deformation is initiated at higher solid fraction a transition to shear cracking occurs which we propose is due to insufficient liquid flow to accommodate dilatant deformation.
Quenching during semi-solid deformation has clearly demonstrated that, during Regime II, deformation readily localises into dilatant shear bands, regions of significantly lower crystal packing-density than the surroundings in which intense crystal rearrangement occurs (Figure A2).
Figure A2 - Strain localisation in semi-solid alloys. A dilatant shear band in AZ91 deformed during equiaxed solidification at 36% solid. This sample was deformed by vane rheometry for one vane revolution at 0.5 rad.s-1 and then quenched. Black is quenched liquid; white was solid at the time of the quench. Vane rotation was clockwise.
A2 - Streams 2 & 3: Alloy development activities have been focused on two major alloy groups: Mg-RE (rare-earth) system, and the Mg-Zn system. The selection of these two systems is mainly based on the potential for precipitation hardening. In 2007, a number of alloying elements, either individually or in combination, have been added to Mg-RE and Mg-Zn alloys. A promising alloying element has been identified for each of the two alloy systems, and considerable efforts have been made to evaluate the age hardening response and to characterise the precipitate microstructures of the resultant alloys (Figure A3). It is now possible to achieve hardness values that are equivalent to tensile yield strengths in the range of 250-300 MPa in the Mg-RE based alloys. This level of strength is 50% higher than existing commercial magnesium alloys. One of the targets of this project has been achieved.
Figure A3 - Precipitates in a Mg-8wt%Zn alloy (Scripta Materialia, vol. 56, pp. 645-8, 2007).
Efforts are now being focused on identification of lower-cost alloying elements that can substantially enhance the precipitation hardening response. Preliminary work on the Mg-Zn based alloys indicates that additions of a particular alloying element lead to a noticeable increase in age hardening response. A number of additional alloying elements have been tried, but these additional elements do not seem to promote further hardening effect.
The tensile properties and microstructures of the newly-developed alloys will be characterised in 2008. Two precipitation hardenable alloys have been identified and hot extruded at Deakin. These extruded alloys will also be tensile tested.