The Centre for Mass and Thermal Transport has a very strong research capability in the theory and simulation of mass and thermal transport in a wide variety

Research

These materials are of great interest as light-weight heat sinks. This research is concerned with steady-state and transient thermal transport in cellular metallic matrices. Using a Lattice Monte Carlo method developed in the Centre and finite element analysis we are investigating the effective thermal conductivity and transient temperature profiles in various models of cellular metallic matrices (see Figure 1) and related materials.

a) Open-cell M-Pore aluminium foam, b) Portion of the corresponding simulated structure

Figure 1 -  a) Open-cell M-Pore ® aluminium foam, b) Portion of the corresponding simulated structure.

In this research, new ways to tailor the mechanical properties of 3rd generation biomaterials are being investigated. Scalability of properties to match surrounding tissue for mechanical compatibility and change of properties during maturation in a physiological environment is of the central interest.

A biomaterial is 'a (non-living) material intended to interact with biological systems'. Biomaterials are of essential importance for modern medicine, improving the quality of life and constituting a multi-billion dollar market. Due to intense research and their outstanding medical and economic potential, 1st and 2nd generation biomaterials have been manufactured to a very high standard. However, further improvements are restricted by the simple fact that synthetic materials are unable to respond to changing physiological loads or biochemical stimuli. This puts a strong limitation on the lifetime of artificial body parts.

a)
 a) micro-structure of PLGA reinforced  with TiO2 particles [20]
b)
b) cellular meso-structure

Fig. 1  Third generation biomaterial: a) micro-structure of PLGA reinforced with TiO2 particles [20] b) cellular meso-structure.

The solution to this was a paradigm shift towards biologically-based methods that focus now on the repair and regeneration of tissue instead of substitution. These 3rd generation biomaterials combine the benefits of 2nd generation materials being simultaneously resorbable and bio-active but aim to support the body's self-healing mechanisms. Two major challenges remain to be mastered: biological and mechanical compatibility. Recent research addresses the biological compatibility by attempting to position signalling molecules on surfaces in order to control the protein interaction.

The second major challenge is the focus of this research: mechanical compatibility between biomaterial and surrounding tissue. This requires tailoring the stiffness and strength of biomaterials as well as ensuring structural integrity during resorption until the formation of new tissue is completed. Due to its great complexity, this task is best achieved using computational modelling.

Many materials depend on interdiffusion for their formation. In service, the components of many materials segregate as a result of the high temperatures and large driving forces present. This can lead to a degradation of designed properties. In this research program we have been investigating all aspects of interdiffusion and segregation processes in technologically important materials.

Segregation of atomic species to surfaces and grain boundaries is of special interest in the Centre. At present, we are examining cation segregation processes in the solid electrolytes: yttria-stabilized zirconia and magnesium and strontium doped lanthanum gallate as well as interdiffusion in a number of multicomponent alloys including austenitic stainless steel.

Recently we have been awarded research funding for work on the diffusion kinetics in high entropy alloys (HEAs). HEAs are a special class of multicomponent alloys with roughly equi-atomic elements. With between 5 and 13 components they have extraordinary properties including high levels of creep, oxidation and corrosion resistance at high temperatures, outstanding wear resistance, exceptionally high temperature strength, high hardness, superior thermal and chemical stability and outstanding magnetic properties

Thermodynamics tells us that because of the high configurational entropy, HEAs are stable at high temperatures but probably not at lower temperatures. Nonetheless, the high temperature state remains metastable at lower temperatures because of the sluggish kinetics of mass transport. Mass transport is thus the key which determines the longevity of HEAs by determining the kinetics of unmixing, intermetallic compound formation, grain growth and creep. The aims of this part of the Project are to:

  1. Focus on HEAs with an intensive development of a new atomistic theory that has input from experimental thermodynamic data and density functional theory. This theory will have the capacity to predict mass transport behaviour such as unmixing, intermetallic formation, grain growth and creep in HEAs.
  2. Extensively develop and extend their phenomenological mass transport formalism to target the very rapid acquisition of experimental mass transport data in extreme multicomponent (5 or more) alloy systems.
  3. Concurrently engage in an integrated experimental program for mass transport data acquisition in key HEA systems and theory validation.

Diffusion-bonding is a very attractive process for the strong bonding of dissimilar engineering materials in order to form engineering devices and structures. However, because of the differing diffusion rates of the components, porosity often forms in the bonding zone during fabrication or in-service conditions. This results in a very substantial loss of strength of the bonds that is greatly limiting the uptake of this bonding process to other technologies.

Based on new research into the fundamental principles of inter-diffusion and with the assistance of computer simulation we are developing a robust and versatile theory that will predict the onset and extent of porosity formed during diffusion-bonding and will guide methods for its control.