Professor Andrew Mullis

Profile

My career has been dedicated to research into advanced materials, particularly with regard to the solidification processing of metals far from equilibrium (rapid solidification). This research has been pursued through both experimental studies and numerical simulation, with these two approaches being seen as complimentary, the experiments acting as validation for the models and the models informing which experiments would be most valuable. My research has been supported by a range of sponsors including EPSRC,   European   Commission,   European Space Agency, Wolfson Foundation and The Royal Society. I have communicated the outcomes of the research through authoring approximately 110 scientific publications and delivering over 50 conference presentations, including invited/keynote presentations at a number of major international meetings.

Research interests

My research is focused around the study of crystallisation in metallic materials, which I am investigating by a variety of experimental and computational techniques to gain both a fundamental understanding of the processes involved and to solve industrially related problems.

I have two main areas of interest in the field of experimental solidification studies, namely semi-solid processing and solidification processes occurring far from equilibrium (rapid solidification). The latter of these I have investigated via a range of containerless processing routes, utilising techniques such as high vacuum electromagnetic levitation, wherein the solidification of a molten metal is studied in an environment in which it is held without being in physical contact with a containing vessel.  This is necessary as the contact between the liquid and the solid container initiates the nucleation of unwanted solidification events that complicate the analysis of the as-solidified material.   In addition, a 6m, high vacuum drop tube for the study of solidification under free-fall conditions has recently been commissioned.  This facility, funded by a major European Framework VI Integrated Project, is currently being used to study novel binary alloys with good glass forming ability and high strength, rapidly quenched eutectics. I am also an investigator on the European Space Agency's NEQUISOL project, in which a number of microgravity solidification experiments are being flown on the International Space Station.  

My theoretical work is based around the development of computational models capable of simulating the highly complex crystal morphologies that result from the solidification of low entropy materials such as metals.  The problem may be framed as a free boundary problem on a set of coupled, highly non-linear, partial differential equations.  My own contribution is concerned with the application of the phase-field method, a technique that permits the solution of the free boundary problem without the need to explicitly track the moving free interface, via the use of non-conserved order parameters.  This approach has proved to be far more flexible in the simulation of complex morphologies than traditional front tracking methods that have been used for free boundary problems in areas such a fluid dynamics.  This is particularly true where the topology of the crystal may change with time such as during the coalescence of growing crystals or the detachment of side-branches under the influence of an imposed flow.  Most recently I have been collaborating with colleagues in Computer Science to develop solvers which utilise spatially adaptive meshing and a fully implicit non-linear multigrid solver to solve the phase-field problem over widely disparate time and length scales.  This model is now being used to study the fully coupled thermo-solutal solidification problem which is appropriate at solidification velocities of 1-5 m s-1 (typical of process such as laser welding), where diffusion of both heat and solute are equally important to the development of microstructure. 

Qualifications

  • BSc (Durham)
  • DPhil (York)

Professional memberships

  • Fellow Institute of Materials, Minerals and Mining (IOM3)
  • Chartered Physicist

Student education

I am passionate about Materials education at all levels and have designed and delivered a wide range of Materials related courses as part of various Leeds degree programmes. In order to ensure my teaching is delivered effectively I have undertaken Professional Development via the University of Leeds 40 credit Postgraduate Teaching Certificate. My teaching has included specialist courses in materials selection in the aerospace and consumer products sectors, a course in radiation mediated degradation mechanisms in civil nuclear systems, advanced courses in materials modelling and simulation and various courses in solidification theory and solidification microstructures.

Research groups and institutes

  • Complex Systems and Processes
  • Functional Materials, Products and Devices
  • Waste Management and Resource Recovery

Publications:

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