• nanoscale microstructural engineering

A unique facility for the production of nanostructured materials by vapour condensation within an ultra-clean inert gas atmosphere has recently been commissioned within the Institute for Materials Research. This system is capable of operating either in natural or forced convection to produce monolithic nanocrystalline or nanocomposite materials by in-situ compaction of nanosized powders or nanosized coated powders respectively. The forced gas synthesis technique can be used to produce metal-metal nanocomposites or metal-ceramic nanocomposites, the latter having either metallic or ceramic dispersed phases. Furthermore, the application of photocorrelation spectroscopy enables the particle size evolution to be followed continuously, in-situ and non-invasively.

(a) Silver nanosized powders produced by condensation of Ag vapour in high purity He at 40 mbar pressure; (b) Ag nanoparticles coated with a thin layer of SnO by two-source condensation in flowing inert gas.

A group of University of Leeds researchers is currently seeking to develop a research programme in the production and properties of polymer matrix nanocomposite materials by the dispersion of nanosized metallic and ceramic powders in thermosetting polymer matrices.These materials potentially offer exciting optical, electronic, magnetic and mechanical properties.

• nanostructured materials and composites

Nanocrystalline and nanostructured materials can also be produced by high-energy ball-milling of ductile metallic powders. This technique has been used to produce an ultra-fine, controlled dispersion of ferromagnetic/superparamagnetic clusters in a paramagnetic, metallic matrix by controlled decomposition of highly metastable supersaturated solid solutions, for example of Co in Cu.

These materials have been compacted and their transport properties studied in a collaborative programme with the Condensed Matter Physics Group in the Department of Physics. When the microstructure is carefully engineered, these materials show giant magnetoresistance comparable to that shown by similar 3D nanostructured alloys produced by sputtering and melt spinning.

• synthesis from solution
• gas-phase pyrolysis of metal-organic precursors

We are currently seeking to expand our research to encompass the production of metallic, ceramic and coated nanosized partices by techniques such as solution chemistry and gas-phase pyrolysis which are amenable to scaling up to an industrial production technique.These nanosized powders will form the precursor to the production of bulk nanocrystalline and nanocomposite materials by compaction and sintering or dispersion in a polymeric matrix.

(click on diagram above to enlarge)

Electromagnetic levitation/fluxing system developed in the Institute for Materials Research by Dr A.M. Mullis and Dr R.F. Cochrane with financial support from EPSRC.

Grain refinement is a spontaneous transition observed in many pure metals and alloys above a critical undercooling, DT*. The coarse columnar grain structure evident below DT* is replaced by a fine equiaxed structure. The transition is accompanied by a break in the growth velocity-undercooling curve and a change in the shape of the solid-liquid interface from angular to smooth.

At Leeds we have developed a model which explains grain refinement by a kinetically induced instability at the dendrite tip. Above DT* repeated multiple tip splitting results in a highly unstable structure which is prone to remelting in the presence of non-equilibrium solute concentrations to give the observed grain refined structure. Recent experiments on ultra-high purity Cu have shown that this material also displays a break in the velocity undercooling curve above a critical undercooling DT*.

However, when the as-solidified samples are examined we observe the remnants of dendritic seaweed, a microstructure produced by dendrite tip-splitting, providing strong evidence for our proposed model. In this particular case we believe remelting has been inhibited by the very high purity of the starting material.

Comparison of simulated 'dendritic seaweed', middle, (after Akamutsu, Faivre & Ihle, 1995) with the as-solidified microstructure Cu undercooled by 280 K, lower image. In both cases the dendrite growth direction is from the lower left towards the upper right.

Many pure metals and alloys solidify by the growth of complex crystals called dendrites. Unlike most inorganic crystal which display a faceted interface, the low entropy of fusion of most metals means that dendrites exhibit a smooth, continuously variable interface, which displays multiple branching. This leads to the formation highly complex growth patterns, the morphology of which can influence many aspects of the materials performance such as mechanical strength, corrosion resistance and surface finish. Modelling research into solidification is concentrating on developing phase-field models of dendritic growth. By assuming the solid-liquid interface is diffuse a continuous variable that describes the phase of the material may be defined. In phase-field the evolution of the phase variable is solved for using standard differential techniques, without the need to explicitly track for the solid liquid interface.

Phase-field simulation of dendritic growth showing extensive side-branching leading to the development of a highly complex geometry.

or click here to watch the crystal growing (avi movie)

Modelling of rapid solidification processes has focused on the phenomenon of spontaneous grain refinement. Using phase-field modelling we have been able to demonstrate that above a critical undercooling, DT*, dendrites may undergo a kinetically induced instability which results in repeated multiple tip splitting, or doublon formation. In our simulations these doublon structures are formed at crystal growth velocities comparable to those associated with spontaneous grain refinement, typically around 20-40 m s-1. These structures are highly unstable and, in the presence of a solute (i.e. in alloy systems), are prone to rapid remelting. This gives rise to the small, equiaxed grains observed following spontaneous grain refinement.

Key Reference: Mullis A. M. & Cochrane R. F., 2001. A phase field model for spontaneous grain refinement in deeply undercooled metallic melts. Acta Materialia, 49, 2205-2214.

Phase-field simulation of kinetically induced dendrite tip-splitting at high growth velocity.

or click here to watch the crystal growing (avi movie)