Project C2: Micro- and nano-scale composites (Dispersed hybrid structures)
C2.1: Pseudo micro truss structures in HPDC Mg-Al alloys
•	Key researchers: C. Caceres, A.V. Nagasekhar
Micro-truss structures (Fig. C5) are characterised by extreme stiffness and high mechanical efficiency (i.e., high stiffness/mass ratio) as well as by an increasing strength as the cell size of the structure is reduced. The latter effect opens the possibility of using the scale of the micro-truss structure as a design parameter.
Figure C5: Microtruss structure				Figure C6: Percolating β-phase intermetallic structure
The yield strength of hpdc Mg-Al alloys seems to be determined by the percolating β-phase intermetallic structure (Fig. C6) which is thought to behave like a pseudo micro-truss structure. (The link across the cells body converting the compliant foam into a stiff micro-truss structure is provided by the matrix alloy, hence the prefix “pseudo”). Experiments show that as the intermetallic structure becomes finer, e.g, for thinner castings, the yield strength of the casting increases, as expected for a micro-truss structure with increasingly smaller cell size. This study will use a sequential sectioning technique to determine the 3-D morphology of the intermetallic structure in alloys with predetermined contents of Al in order to characterise the micro-truss structure. At a later stage high resolution x-ray tomography will also be used with the same objective. In situ tensile testing will also be used to correlate the plastic behaviour of the alloys with the cracking of the intermetallic structure. This information should enable understanding the origin of the yield strength, the skin effect, the yielding behaviour, the strain hardening behaviour at low strains, the development of damage and the bound imposed by the cracking of the intermetallic structure to the alloy’s ductility.
Progress Report
Tensile testing on hpdc Mg-Al alloys with different contents of Al has been carried out. Some relevant results are shown in Fig. C7. The compositions studied range from 0.5%Al to 12% Al. The aim of this part of the study is to isolate the effect of solute in solution from that of the intermetallic and grain size on the strength of the alloys. The use of similarly cast hpdc specimens ensures a similar grain size and distribution in all alloys. Figure C8 shows the intermetallic structure at the core and edge regions of a 12% Al casting. The work has been extended to measuring the grain size for each composition using EBSD (Fig. C9). A phenomenological model has been developed to account for the observed results (presented at the Sixth Pacific Rim International Conference on Advanced Materials and Processing (PRICM 6), 6-9th November 2007, Korea).
Figure C7 - (a) The (0.2%) proof stress of the alloys studied, as a function of the Al content, (b) The tensile elongation to fracture of the alloys studied, as a function of the Al content.
Currently the work concentrates in the development of a 3D image of the intermetallic structure using a sequential sectioning technique. The 3D image will be generated using a dedicated software package. Future work will involve the mapping of cross sections with microhardness in order to assess the extent of the so called skin effect.
Figure C8 - SEM-BSE images of glycol etched HPDC Mg Al alloys with 12 mass% Al. (a) the centre of the cross section (core), (b) near the specimen’s surface (skin region). The α-Mg phase appears dark, whilst the β-Mg17Al12 intermetallic phase appears bright.
Figure C9 - EBSD images of the grain structure in HPDC Mg Al alloys with 0.47 and 8.77 mass% Al, respectively. (a) the centre of the cross section (core), (b) near the specimen’s surface (skin region).
Related work Progress has been made in related spin-off research dealing with the hardening caused by twinning in pure Mg (Fig. C10), mechanical behaviour of Mg-Zn and Mg-Al solid solutions, and environmental issues re light alloy applications (Fig. C11).				Figure C10: Kocks-Mecking plots for random and textured polycrystals of Mg. The shaded area near the origin indicates the region affected by the quasi Hall-Petch hardening due to twinning. Cáceres, C.H., P. Lukác and A. Blake, "Strain Hardening due to Twinning in Pure Magnesium ", Philosophical Magazine A, 2008, In press.
Figure C11 - The CO2 penalty correlated with the driving distance to break-even, for light alloys substitutions for ferrous components in automotive applications. [Caceres, C.H., "Economical and environmental factors in light alloys automotive applications", Metall. Mater. Trans., 2007, 38A, 1649-1662.]
C2.2 – Dispersed phase in-situ composites
•	Key researchers: H.P. Ng, C. Bettles, B.C. Muddle
Overview
Discontinuous precipitation, a solid-state transformation which involves the isothermal decomposition of a supersaturated solid solution (α') into a two-phase (α+β) aggregate, has been employed to develop in-situ nano-scale composite structures in a light-weight Al-14.6at%Zn alloy (Figure C12). Ultra-fine-scale two-phase composite materials are expected to demonstrate extraordinary mechanical properties due to the effect of co-deformation among phases with distinct crystal structures and to the presence of large interfacial area that interacts with defects in an anomalous way. The major aim of this project is to investigate the possibility of enhancing the formability and strength of the alloy by controlling the characteristic length scale of the α+β lamellar structure grown by the discontinuous precipitation process. The project is lead by Prof. Barry Muddle with a team of two research fellows (Colleen Bettles and HP Ng) and one final year project student (Changlin Zheng).
Figure C12 - Polarized light optical micrograph showing discontinuous precipitation (DP) colonies in Al-Zn alloy (left). TEM images showing the lamellar form (middle) and the fibrous form (right) of the nano-scale composite structures developed by DP.
Research Progress and Outcomes
The research effort up-to-date focuses on the synthesis of DP structure and the approaches to tailoring the composite microstructures. The project team has looked into several areas which concern the factors governing the transformation kinetics of discontinuous precipitation in the Al-Zn alloy. The relationships among the precipitation temperatures, interlamellar spacing and the mechanical behaviour of the alloy were studied. Besides, the feasibility to manipulate the initiation characteristics and the lamellae growth by various thermo-mechanical techniques was explored. Outlines of the technical details and outcomes are as follows:
C2.2.1: The Role of Grain Size on Transformation Kinetics
Preliminary metallographic study indicated that the DP transformation in the Al-Zn alloy is comparatively more sluggish in larger grain than in the smaller ones. The intended composite alloy relies on an effective initiation and growth of DP in order to take full advantages of the desirable properties at a reduced ageing/processing cost. Whilst the influence of grain size on the macroscopic transformation of DP is not well documented in the literature, a study is carried out to establish the relationship between grain size and the transformation kinetics of DP in the Al-Zn alloy. Samples with grain sizes ranging from 10 to 1200 μm were produced by equal channel angular pressing (ECAP) and conventional cold rolling, and quantitative metallography was performed to assess the volumetric transformation rate of DP during isothermal ageing. It is found that the rate of DP growth increases drastically with a decreasing grain size (d) until a critical d is met, below which the rate descends. This study manifests that, firstly, the heterogeneous nucleation occurring at grain boundary is the primary rate-determining process for the DP transformation kinetics at a given temperature. Secondly, an optimal grain size exists that maximizes the DP transformation rate in the present Al-Zn alloy.
C2.2.2: Lamellar Spacing and Mechanical Properties
Ageing temperature is by far the sole processing parameter adopted to control the length scale of the DP lamellar structure. The steady-state interlamellar spacing () in the present alloy could literally be varied from 150 nm to 380 nm approximately as the ageing temperature is raised from 65°C to 150°C. A major change in mechanical behavior is brought about by the refinement of interlamellar spacing in a way that the yield strength of the alloy is increased significantly at the expense of ductility – a phenomena which is typical of structure refinement for conventional ultra-fine grained materials (Figure C13). This is ascribable to the increase in α/β interphase interfaces that impede the motions of dislocations, resulting in a loss of ductility. In principle, a combination of high hardness and high ductility could be achieved when the dominant deformation mechanism of the current DP composite deviates from conventional dislocation glide or mechanical twinning. Upcoming research effort will be devoted to further downscaling of the current DP lamellar composite by specific techniques and to the modification of slip systems operating in the α/β aggregates by suitable alloying.
Figure C13 - Stress-strain curves of Al-Zn specimens (left) and micro-hardness of DP colonies (right)aged at different temperatures and lengths of time.
C2.2.3: Manipulation of DP Growth by in-situ Loading and Prior Deformation
Texture and microstructural anisotropy are of substantial influence to the formability of engineering materials. Attempts have been made to align the growth orientation of DP lamellar structure by applying a uniaxial stress in the course of isothermal ageing. The results revealed that the DP colonies exhibited a characteristic saw-tooth morphology with apexes pointing towards the directions of the applied stress, suggesting a stress-induced alignment of the microstructure has occurred. Nevertheless, individual α+β lamellae within the DP colonies appeared to be relatively insensitive to the stress alignment. And more importantly, the growth of DP colonies is likely to be hindered by the formation of dislocation substructures, which are found to be in favour of continuous precipitation, at the regions of strain-localization (Figure C14)
Figure C14 - (a) The morphology of DP colonies under in-situ stress alignment. (b) The ultra-fine composite structure developed by DP process with preceding plastic deformation.
C2.2.4: Impurity effects
The effect of trace amount of commonly occurring impurities such as Fe and Si has been explored in regard to the transformation characteristics of DP. Al-Zn castings with 0.8 wt.% of impurity Fe was found to exhibit a prominent segregation of the Fe species at grain boundaries and inhibit the mobility of the latter which is vital for the DP process. The incorporation of impurity Si shows a similar effect. Moreover, the impurities tend to shift the transformation solvus for DP in such a way that decomposition by continuous precipitation is favoured instead. These detrimental impurity effects represent a major technical challenge in the current project, provided that high purity raw materials are not employed. Precipitation of equilibrium Fe- and Si- rich phases in the matrix (at elevated temperatures) before the actual DP process may lessen the problem by minimising the impurity concentration at grain boundaries; further effort is required to corroborate the effectiveness of this approach.
C2.3 - Nanostructured multiphase alloys and composites
Key researchers: W. Xu, X. Wu, S. Bian, S. Goussous, E. Lui, K. Xia
Progress towards targets
This project aims to significantly enhance mechanical properties of existing light alloys including room temperature strength and ductility and high temperature creep resistance and to achieve a good combination of these properties. In particular, the microstructure will be designed using a selection of metallic, intermetallic, ceramic, amorphous and other structures. The designed microstructure will be synthesised from particles, in particular nano-structured particles, using the newly developed back-pressure equal channel angular consolidation process. This approach enables more flexible design of composition and microstructure and production of large volume of high integrity materials for reliable characterisation and study of their mechanical behaviour. The project will integrate design of microstructure, realisation of the designed microstructure through novel processing, characterisation of microstructure and mechanical behaviour, and modelling of the correlation between microstructure and mechanical behaviour.
In particular, the following progress has been made towards the overall targets:
•	A nanostructured Al-Al2O3 composite was successfully consolidated from nano Al particles to reach strength exceeding 700 MPa.
•	An Al-nano C composite was successfully consolidated from micro Al and nano C particles to reach strength exceeding 400 MPa.
•	An Al-Al/Cu/Fe composite was successfully consolidated from micro Al and ball milled Al-Fe-Cu particles to reach strength exceeding 500 MPa.
•	A nanostructured Ti was successfully consolidated from dehydrided Ti particles to reach strength exceeding 2400 MPa.
•	An ultrafine-structured Mg was successfully consolidated from micro Mg particles to reach strength of ~150 MPa.
Technical Report: There are two aspects to this research project.
Experimental methods:
Processing:
•	Back pressure equal channel angular consolidation (BP-ECAC) was used to consolidate the particles.
•	Mechanical milling was used to refine and disperse particles used.
Characterisation:
•	Grain, interface and deformation structures at nanoscale by electron microscopy and analysis, X-ray diffraction and atom probe.
Mechanical behaviour:
•	Microhardness and room temperature compressive and tensile testing were conducted.
Theoretical and computational methods:
Design:
•	Phase selection based on properties of individual phases, structure of interfaces, thermodynamic equilibrium/non-equilibrium and effects on mechanical behaviour.
•	Microstructure design using parameters such as the amount, size, shape and distribution of each phase selected.
Computational materials science:
•	Understanding of mechanical behaviour of nano and complex multi-phase materials based on examination and analysis of deformation structures and correlation with microstructure.
•	Mesoscale modelling of mechanical behaviour in relation to the designed microstructure.
Figure C15: Nanocrystalline Al and Al2O3 composite structure obtained by consolidation and severe plastic deformation of ultrafine Al particles. (Appl. Phys. Lett., 91 (2007) 031901)
Key items of infrastructure acquired during the project include:
•	A dedicated back pressure equal channel angular processing (BP-ECAP) facility has been established. The facility is designed to carry out consolidation of particles by severe plastic deformation as well as ECAP of bulk materials. The capacity of the press is 20 tonnes (both forward and back pressures) and multiple passes can be conducted efficiently at variable speeds and at temperatures up to 650°C. With the ongoing design of tooling, it will eventually be able to process materials at up to 1000°C.
•	A planetary ball milling machine capable of cryogenic processing has been established. It has a total capacity of 1000 ml and maximum rotation speed of 500 rpm.