Research profiles - UK-based PhD students

Materials World magazine
,
1 Nov 2009
Dominc Henry recently completed a PhD on improved wrought magnesium alloys

What are PhD students in the UK focusing on in the light metals arena? Materials World finds out

Dominic Henry completed his PhD at the School of Materials at The University of Manchester, UK, in 2009. His thesis study focused on understanding how to develop improved wrought magnesium alloys by controlling microstructure and texture.

He says, ‘My work has been studying how both fine and coarse particles can be used to control grain size and orientation developed during thermomechanical processing of magnesium alloys. The idea is to use particles to help develop alloys which have a stable fine grain size, and desired texture after processes such as rolling and extrusion’.

Both fine and coarse particles are widely used in aluminium alloys to control grain structure and texture, but this is said to be the first study to systematically apply these methods to magnesium. Henry’s work aimed to gain the fundamental understanding required to make best use of particles in magnesium alloys, where the deformation and recrystallisation mechanisms are different from those seen in aluminium.

‘We have shown that coarse particles can stimulate recrystallisation and introduce new texture components, which has the potential to reduce anisotropy and asymmetry, two key weaknesses of current wrought magnesium alloys. I have also demonstrated how fine dispersoid particles can be used to suppress recrystallisation, although there are key differences between the effects of both fine and coarse particles on deformation and recrystallisation in magnesium compared with aluminium.’

Henry’s PhD was supported by funding from the EPSRC and international metals company Magnesium Elektron, and he has taken up a position in the development team at Magnesium Elektron in Manchester.

Like a shot

Shot peening is an important industrial process that improves the fatigue and tribological properties of an engineering component by introducing favourable residual stresses beneath the peened surface. However, the process can damage the mechanical integrity of titanium alloys.

Meurig Thomas, a PhD Student in the Department of Engineering Materials at the University of Sheffield, UK, is researching the precise affect of peening on 
titanium alloys following long-term thermal exposure. A second key aspect of the research seeks to identify the mechanisms that promote oxygen uptake in shot peened material.

To tackle this, advanced microstructural characterisation techniques such as electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) are being employed to understand the active deformation mode in titanium mode during peening and the resulting microstructural changes that accompany increased oxygen uptake during thermal exposure. It is anticipated that a greater understanding of shot peening from a metallurgical perspective may provide potential solutions to alleviate this and help further develop the peening process itself.

Thomas began his project in October 2007 and is well on the way to determining the effect of initial forging microstructure and texture on the degree of subsurface damage induced during shot peening.

Holy grail

Load bearing structures in nature, such as trees, bones and coral, use porous structural materials. Compared with bulk material, porous metals possess special properties resulting from the presence of pores, such as low weight, good strength, stiffness and compressibility. In addition, porous metals can absorb impact and be effectively applied to sound absorption, electromagnetic shielding and vibration damping. Porous titanium alloys were designed to achieve the best combination of the properties of porous structures and titanium alloys. Various processing techniques are available to produce structures using such materials, including sintering titanium powders with or without space holders, solid state foaming by superplastic expansion of argon-filled pores or polymeric foam replication. In practice, the space holder sintering method is preferred because it can provide adjustable porosity, in shape and size distribution. The space holders are carbamide and ammonium hydrogen carbonate particles due to their ability to decompose completely at relatively low temperatures so as to avoid the reaction with the host powders.

Wenjuan Niu, PhD student in the Department of Engineering at the University of Leicester began her project in 2008, she has successfully made adjustable porous titanium using the space holder technique and investigated the roles of the various parameters in mechanical properties. She now plans to model the mechanical performance of porous titanium and opimise its design. Finite element models will be developed to present real geometry of porous materials. Based on these models, a full field solution of displacement or stress will be predicted.

Shaping up

A scientific approach to casting design has only recently been applied. Casting technology has traditionally been ruled by iterative trial and error techniques, which have been largely down to ‘rule of thumb’ and the engineer’s experience. This method is still employed by many foundries and can lead to poor quality castings that have given the process an undeserved reputation for poor performance and high scrap rates.

However, a well designed casting process can result in precision near net shape products with good mechanical properties at a competitive price. This should make casting appealing to designers.

By preventing free surface turbulence and oxide entrainment, the alloy’s mechanical characteristics can be relied upon. Extensive research into casting of aluminium alloys at the University of Birmingham, UK, has highlighted the principal mechanisms of free-surface entrainment.

Carl Reilly’s research has developed techniques to allow modelling of free surface entrainment, giving quantitative assessment of casting systems. One of these techniques allows the entrainment, and advection within the bulk fluid, to be modelled and visualised by the foundry man, thus final oxide position and defect density within the casting to be defined. These techniques are targeted for use with optimisation tools such as MAGMASOFT Frontier or Opticast. Reilly intends to submit his thesis during December.