Model behaviour of ceramic and intermetallic alloys
Physical modelling to bridge the knowledge gaps between the atomistic and
macroscopic levels of intermetallic alloys and ceramic coatings is under development.
The work, led by the University of Cambridge in the UK, will help to improve thermal barrier coatings, as well as provide a better understanding of what causes failures in interconnects and microelectromechanical devices.
‘The importance of size effects in materials engineering is increasing due to the development of new fine-structure metallic alloys and ceramic coatings on length scales in the range 10nm to 10µm,’ explains Professor Norman Fleck from the University of Cambridge, one of seven organisations working on the project, which also include Corus UK, and the Max Planck Institute for Metals Research in Germany.
Fundamental materials problems at the nanoscale involve interactions between microstructure, for example grain boundaries, and defects, such as dislocations and cracks. ‘Conventional continuum descriptions fail to predict the dependence of strength upon microstructural size, and the associated evolution of microstructure with deformation,’ says Fleck.
Gaps in the modelling space map therefore reflect gaps in materials understanding, such as the strengthening mechanisms of grain boundaries leading to the Hall-Petch and inverse Hall-Petch effects (methods of strengthening materials by changing their average grain size).
‘The main objective of this project is to develop a hierarchy of methods involving information transfer from one level to the next,’ says Fleck. ‘For example, thermal barrier coatings in gas turbines comprise thin ceramic coatings bonded to an underlying superalloy. These new modelling techniques will help us understand the effect of interfaces [within ceramic or metallic crystals, or between disparate phases] upon mechanical properties such as creep strength.’
In order to do this, the team established a series of models focused on molecular dynamics, discrete dislocation and diffusion calculations, to predict strength, creep resistance and fracture toughness. They used homogenisation techniques to identify a set of macroscopic equations for the relevant internal variables and to develop robust macroscopic constitutive laws that discriminate between the competing phenomenological theories. Finally, room and high temperature torsion and bending experiments were conducted to validate the modelling.
‘Our experiments have led to discrete dislocation techniques for creep of engineering alloys, thus the rate of high temperature deformation in turbine blades and similar materials can be more accurately predicted,’ says Fleck.
The origins of enhanced creep resistence and fracture toughness in crystalline silicon has also been determined using initio-molecular dynamics calculations. Atomic rearrangement mechanisms were discovered at the crack tip, explaining observed crack propagation instabilities.
Fleck says that the project could also help develop new nanoscale and micro-machined components.