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Physics and mechanics of plastic instabilities in novel materials

This program of research concerns experimental and theoretical investigations for the plastic flow and fracture of novel materials including amorphous metal alloys, nanophase metals and thin films.

Experimental investigations for the development of plastic instabilities were made by using metallography, electron microscopy, X-ray analysis, high-speed cinematography and mechanical testing. Theoretical methods were developed based on classical elasticity, gradient elasticity and defect kinetics in order to understand the stability and patterning of deformation in nanocrystals, small particles and thin films. Models of microscopic processes of plastic deformation of novel materials in the framework of a theory for the collective behaviour of defects, including investigations of hardening mechanisms, strain localisation and misfit dislocation generation in such bulk materials and films, were analysed.

First, the methods of both classical and gradient theories of elasticity were used to calculate elastic properties of defects. A special technique of virtual surface dislocations has been developed to find exact elastic solutions for defects in novel materials and thin films. The boundary value problem in the classical theory of elasticity for a screw dislocation near a triple junction of phases having different elastic moduli has been solved. Calculations of the elastic fields for both screw and edge dislocations in the framework of gradient theory of elasticity have also been performed resulting into the elimination of strain singularities at dislocation cores, as well as in the appearance of new characteristic distances in elastic theory: i.e., the core radius and the radius of short-range interaction between dislocations. Calculations were also performed for the shift of misfit dislocations to the stand-off position at the interface, the formation of nonuniform misfit dislocation distributions along the interface, as well as the temporal and spatial oscillations in dislocation densities near the interface. These calculations allow researchers to find stand-off positions of misfit dislocations which are in good agreement with experimental observations and permit the determination of the critical misfit strain and thickness of the film corresponding to the transition between uniform and nonuniform dislocation distributions.

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Reported by

Aristotle University of Thessaloniki
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