Disruptive Solidification Microstructures via Thermoelectric Control

Modern society has been transformed by the development of alloys that are lighter and stronger. We have invented a new method to potentially further enhance mechanical properties, reducing weight and increasing recyclability, whilst reducing the energy consumed during manufacturing. Many prior improvements were due to an understanding of how to control alloy microstructure during solidification. However, there are only two commonly utilised methods for control: cooling rate and composition (including things like grain refinement). We propose a novel additional tool to manipulate alloy microstructures as they grow. To have a third method of control could be transformative to the metals industry in the UK, enabling all new products/properties to be developed.

Alloys are a combination of many elements that commonly solidify as crystalline structures known as dendrites. The shape of these dendrites and their growth into linked grains produces the microstructure that ultimately determines overall material performance. Techniques for controlling the microstructure are therefore of paramount importance and via computational simulations performed by the proposers as part of an EPSRC funded PhD study, we have theoretically demonstrated that a new mechanism by which magnetic fields can be used to alter, or disrupt this microstructure is possible.

The new mechanism we propose utilises thermoelectricity, a relatively unexplored phenomenon in solidification. The fundamental principle relies on the fact that current is caused to circulate around the interface of two materials with different Seebeck coefficients, provided a thermal gradient exists along that interface. This effect has many practical applications in other fields: thermoelectric coolers in microelectronics; thermoelectric materials used to produce electricity from temperature differences within car engines. Both examples use semiconductor materials as these generally have larger Seebeck coefficients. Surprisingly this effect is also significant on the microscopic scale along the solid-liquid front of a solidifying alloy. It is in fact an inherent part of the system.

As a dendrite solidifies, latent heat is released creating temperature variations, and simultaneously some elements are partitioned more into either the liquid or solid phase. This compositional variation causes a discontinuity in the Seebeck coefficient, creating a potential across the interface resulting in thermoelectric currents between hot and cold regions. When solidification is subjected to an external magnetic field, these currents interact with it to create fluid motion. This phenomenon -named by the first researchers to observe it as Thermoelectric Magneto-hydrodynamics (TEMHD) - causes microscopic flow between dendrite arms and circulations around the dendrite. This flow can alter the dendrite shape, leading to further thermoelectric currents, and so on. Experimental evidence shows that external magnetic fields can lead to significant changes in microstructure, but so far a detailed analysis of how this occurs has not been conducted. Numerical simulations by the proposers have given some insight into the complex nature of the problem.

To harness this technique in real castings, systematic experimental and numerical studies are proposed. Real-time 3D observation of growing microstructure under various magnetic fields at Diamond light source will unequivocally prove our as yet only theoretical hypothesis. Numerical simulations are essential to design these experiments, optimising alloy and magnetic fields, maximising the impact on microstructure and hence properties. Key parameters (Seebeck coefficient, magnetic field and thermal gradient) will be examined through the use of a fully coupled 3D numerical model and experiments for a range of alloys.

The successful outcome of this research would have a significant scientific, economic and societal impact on the UK. From the scientific and engineering viewpoint, it would alter the way materials scientists and engineers understand the development and control of alloy microstructure. Direct control of the solidification front using external magnetic fields, will enable the design and development of tailor-made materials with improved mechanical, thermal, or electrical properties, important for new lightweight structural components for cars, or lightweight alloy conductors to replace copper in aircraft; both needed for greater fuel efficiency and reduced carbon footprint. These new ideas will stimulate further research, in measuring Seebeck coefficients, and developing new alloys. The ability to observe the dynamic progress of a solidifying front and the induced TE micro-convection will provide experimental analogues and benchmark data, for the development of new models and software for microstructure prediction.

From the economic outlook, successful control of microstructure, in particular refinement of dendrite arm spacing using pulse excitation will reduce the need for expensive rolling or heat treatment. Advances in TECalloys would give UK a lead in many important sectors: transport and aerospace, in filters and catalysts through the control of the porous structure of materials, in the design of scaffolds for implants, in the control of nanoparticles in MMNCs, in the search for new energy conversion materials, etc. In line with the government's call for advanced technology value-added manufacturing, the new techniques would encourage UK industry to innovate and produce specialised high value products ahead of its competitors.
This research is presently unique, in both concept and scope, so near-term benefits would be mostly academic. Primary long-term impact would be a paradigm shift in the way industry develops new alloys for specific applications, using external field influence as an extra control tool to cooling rate and alloy composition. Since no other group in the world is currently exploiting the ideas promoted here, the timing of the research is critical. The availability of the Diamond facility for fundamental real-time experiments, followed by detailed property measurements, will validate and extend the usability of the theoretical/ numerical models developed in parallel. As explained in "pathways to impact", the proposers have ongoing collaboration with many manufacturing companies in the UK (BAE, Rolls-Royce, Corus, Consark, Ford, Qinetiq, PSI, etc...) and are involved in knowledge transfer projects and networks (KTP ,TSB etc.). These links show readiness to collaborate with industry for exploitation of the knowledge, new processing techniques, or alloy compositions developed during the programme.
Academic Impact:
-Better understanding of thermoelectric effects and Lorentz-force-driven convections on crystal growth through a combination of experiments, analysis and numerical modelling
-A new dimension in the control of alloy solidification
-A new adaptive-grid microstructure code, that couples solidification front dynamics, fluid flow and external (AC,DC, pulsed) field interaction
-Starting a new area of research for the continuous and direct manipulation of flow at the solid front, irrespective of bulk flow conditions
-High definition real-time visualisation of solid front morphology for selected alloys, identifying micro-convections that influence solute and temperature distribution
-Determination of hitherto unknown properties of materials with altered crystalline morphology
-Contribution to MSc courses on multi-physics modelling at Greenwich (continuing a theme started with MTP funding)
-Training of young PDRAs at Greenwich and Manchester in theoretical and experimental multi-physics methods during the project