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Heterophase and homophase interfaces: fundamentals and applications

This route focuses on the structural aspect of interfaces as a central theme; i.e. it considers mainly the atomic scale with HRTEM and the effects of segregation on in-situ fracture using UHV small-spot SEM/SAM. We have concentrated on the question whether HRTEM image information can be interpreted in terms of the atomic structure and whether the latter will provide information of the bond-strength along the (segregated) heterophase interface between dissimilar materials, like a metal and a ceramic material. It is accurate to say that important properties of materials in high-technology structural and functional applications are strongly affected, even controlled, by the presence of solid interfaces.

interfaces_1In collaboration with the NCEM center at LBNL-Berkeley-USA the influence of impurities on the mobility of grain-boundaries in Al-Mg alloys was examined using in-situ nanoindentation in a transmission electron microscope (TEM). It was found that Mg solutes effectively pin only high-angle grain boundaries in the Al-Mg alloy films, not low-angle grain boundaries. In situ TEM displacement-controlled indentations indicate that many dislocations are nucleated prior to the initial yield point and relatively few during the yield event itself. A striking new result found in these alloys is that the macroscopic yield event itself is associated with a rearrangement of dislocations and not with the nucleation/propagation of dislocations. Interfaces play a dominant role in the quantitative understanding of the behavior of nanocrystalline metals.

 


ACTA MATERIALIA Volume: 55 Issue: 12 Pages: 4041-4065 Published: JUL 2007

Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents an overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure metals with nanoscale growth twins.

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ACTA MATERIALIA Volume: 54 Issue: 14 Pages: 3827-3833 Published: AUG 2006

 

movie1 (In-situ TEM indentation - Adobe Flash movie)  

ACTA MATERIALIA Volume: 52 Issue: 20 Pages: 5783-5790 Published: DEC 1 2004