C Research Program C: Hybrid Materials
Program Leader: Professor Mark Hoffman – University of New South Wales
The hybrid structures program has as its goal the design of unique combinations of light metal-based materials to develop means of making significant leaps in the engineering material property space, in particular, structural design.
Multiple approaches are being considered to examine, looking at the synergies necessary to develop optimised composites, relating structural mechanics with nanostructural design. A key aspect of this program is to develop design codes and novel processing methods which may be used for the further development and application of the unique hybrid structures.
The program is divided into two sections; projects which explore laminated structures to develop optimised directional properties, and dispersed structures which have mechanical properties of an isotropic nature.
• Project C1: Macro- and meso-scale composites (Laminated and layered structures)
  The targets of this project are the development of fundamental understanding and then optimised hybrid structures at multiple link scales, ranging from millimetre to nanometre.
On the largest scale, the project is investigating metallic foams and seeking to develop optimised structures with extremely high strength and stiffness to weight ratios not usually attained through the use of metallic materials. The project assesses the effect of intrinsic mechanical properties upon the deformation behaviour of metallic foams, particularly under contact and impact loading. Subsequently, it assesses the effect of particular surface laminates upon a) the damage tolerance of these structures, and b) the remnant strength following mechanical damage. This knowledge is critical for the development of these materials in structural applications. The project will subsequently seek to develop new metallic foam materials.
Examples of structures developed to date include carbon fibre clad aluminium foams, compared with those with high-strength aluminium skins. Design codes for the optimisation of skin and foam mechanical properties, linking these with localised contact and impact damage, have led to unique findings in terms of deformation mechanisms. In particular, the significance of skin buckling in ascertaining the strength of the component is being studied. Furthermore, design codes have been developed to predict the deformation of foams under a broad range of contact geometries.
The lamination of light metal-based materials is being assessed on both a nanometre level and micrometre level. Based upon theoretical developments, unique nanolayered structures of aluminium-palladium have been created with layer thicknesses range from 5 to 80 nm. It has been found that yield strengths well in excess of either of the constituents, determined through the use of nanoindentation, can be attained by using very thin layers of palladium. The significance of this material combination is the effect of varying lattice constants combined with differing shear moduli of the constituents, as predicted by the model.
This information will then be applied to the development of roll-bonded aluminium-titanium structures. A key feature of this project is the development of means to assess the hardness, and subsequently fracture toughness, of the structures, both of which are predicted to increase due to the unique layered combination of high and low yield strength materials. Hence, a project is underway to use surface profiles following indentation to de-convolute not only a hardness number but also yield strength and work hardening behaviour through the use of numerical calculations.
• Project C2: Micro- and nano-scale composites (Dispersed hybrid structures)
  This activity is seeking to develop homogeneously dispersed combinations of light metal-based materials to obtain enhanced strength and ductility. Approaches being undertaken include the development of HPDC magnesium-aluminium alloys which form a micro truss type structure of highly ductile magnesium within an intermetallic magnesium-aluminium phase. Distinct transitions in yield stress are observed when the intermetallic phase transitions from dispersed islands to an interconnected network resulting in significantly enhanced strength. This changes further when the intermetallic phase grows into thick islands as a part of the network.
Another approach is the development of dispersed phase in situ composites through the development of aluminium-zinc-lamellar alloys which have attained strength in the order of 200 MPa and ductility greater than 20% through the creation of a hard discontinuous lamellar structure. Targets include the determination of methods of processing the unique fine scaled lamellar and the processing routes used to attain this. This work links closely with the project looking at nanometre sized layered structures above.
A third approach is to create nanostructured multiphase alloys by the use of equal channel angular processing of nanostructured aluminium and titanium powder combinations, including starting powders of varying particulate size. Unique multiphase structures of aluminium and alumina have enabled high ductilities in the order of 14% to be attained with a compressive yield strength of 270 MPa; strains of 5% and strengths of 700 MPa have also been attained.