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Research

Yield criteria for small volumes of material

Principal investigator: Andy BUSHBY
Co-investigator(s): D.J. Dunstan, Cambridge University and PANalytical

TEM cross-section through a nanoindentation in an InGaAs strained layer superlatticeConventional theories of yielding are based on continuum mechanics and do not work at the small length scales of interest to micro and nanotechnology. In this project we use novel methods to investigate yield phenomena in nanostructures and strain gradients. Materials consisting of alternating coherently strained nano-layers that are routinely manufactured in the electronics industry (strained layer semiconductor superlattices). Such materials also provide ideal ‘model materials’ to study the fundamental physics of yield and plasticity behaviour. By systematically varying the internal lattice strain of the superlattice and the external applied strain, from nanoindentation experiments, we are able to control the plastic deformation behaviour of nominally brittle materials. We published a new theory of yielding that recognises that yield must initiate over a finite volume (Proc. Roy. Soc. A459, 2049 (2003)). More recently we have shown that the theory of critical thickness, used in strained layer crystal growth, can be applied to other geometries to explain strain gradient strengthening (Proc. Roy. Soc. A460, 2781 (2004)) and the indentation size effect (Phil Mag. submitted 2006).