Disruptive Microstructures via Thermoelectric Control

Lead Research Organisation: Diamond Light Source
Department Name: Science Division

Abstract

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.

Publications

10 25 50

 
Description We are developing methods of refining the microstructure of alloys to help improve their properties.
Exploitation Route The methods could be used by metallurgical industries in the UK to improve their product quality.
Sectors Manufacturing, including Industrial Biotechology

 
Description Diamond, UoM, Greenwich collaboration 
Organisation University of Greenwich
Country United Kingdom 
Sector Academic/University 
PI Contribution This project includes both experimental and modelling and simulation ( M&S) tasks. Diamond and University of Manchester are the leads for experimental and the University of Greenwich, is the lead for M & S simulation works. The code for calculating metallic alloy growth under influence of an external magnetic field has been developed University of Greenwich and the results from the January DLS beamtime will be used to validate Greenwich models.
Collaborator Contribution Project is ongoing
Impact Project still ongoing
Start Year 2013
 
Description 2016 Royal Society Summer Exhibition: 4D Science 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This exhibition on "4D Science" highlighted how we've used 4D synchrotron X-ray tomography to solve a range of challenges from materials science to geology to food stuffs. Highlights included replicating the flow of magma to better understand volcanic eruptions, to the coarsening of ice crystals to understand why some ice cream tastes better. 16,000 visitors came to the week long exhibition. The Lee group led the work, together with Diamond Light Source, the Lavallee group at Liverpool, the Research Complex at Harwell, and Unilever.
Year(s) Of Engagement Activity 2016
URL https://royalsociety.org/science-events-and-lectures/summer-science-exhibition/exhibits/4d-science/
 
Description Diamond Open Days 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Research sparked questions from the open days attendees

N/a
Year(s) Of Engagement Activity 2014
 
Description Popular science talk to the Richmond Scientific Society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact I gave a talk to 40 members of the Richmond Scientific Society and general public on "4D Science: how we use synchrotron x-ray glasses to see inside a volcano (and other materials).
Year(s) Of Engagement Activity 2017
 
Description Royal Society Satelitte Exhibition 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact This exhibit showed how we've used 4D synchrotron X-ray tomography of magma to better understand volcanic eruptions, and of ice crystals, to work out why some ice cream tastes better! This was a repeat of the summer exhibition held outside London as part of Manchester Science Festival, nearly 13, 000 visitors came over the 5 day exhibition
Year(s) Of Engagement Activity 2016
URL https://royalsociety.org/science-events-and-lectures/science-exhibition-manchester/