Terence G. Langdon
Professor of Engineering
University of Southern California
Los Angeles, California
USA

and

University of Southampton, U.K.

Flow Mechanisms at High Temperatures: Current Status and New Opportunities

Over the last sixty years, corresponding to the life-time of Acta Metallurgica and Acta Materialia, much progress has been made in delineating and understanding the flow mechanisms occurring in crystalline materials when they are subjected to a load at elevated temperatures.  These analyses have revealed a clear transition from dislocation processes at intermediate and high stresses, including flow behavior dictated by the rates of dislocation climb or glide, to new mechanisms at low stresses including the stress-directed flow of vacancies in diffusion creep and grain boundary sliding.  These mechanisms are effective in explaining the flow characteristics for metals having grain sizes larger than about 1 mm.  The situation has changed over the last two decades as new procedures have been developed, based on the imposition of severe plastic deformation, for the fabrication of bulk metals having submicrometer or nanometer grain sizes.  The results to date suggest that these new ultrafine-grained materials exhibit both similarities and differences with respect to the more conventional coarse-grained metals

Anthony M. Glazer
Clarendon Laboratory
Parks Road
Oxford OX1 3PU
UK

Perovskite: a flexible friend

Materials based on the perovskite structure, general chemical formula ABX3 (A,B cations, X, anion) are some of the most widely abundant and important crystalline solids in the world. The fundamental crystal structure consists of corner-linked anion octahedra with the A cation occupying a position between the octahedra and the B cation at the centre of each octahedron. As has been known for many years, this simple crystal structure is capable of many subtle variations involving small atomic displacements, both polar and antipolar, together with tilting of the anion octahedra. In recent times, it has become increasingly apparent that these structures often have additional complexities that we were not aware of initially: in particular much attention these days is being paid to breaks in the long-range order that is normally a feature of crystals. Such flexibility of the structure results in different physical properties, many of which are of great importance in technology. In particular, they can lead to enhanced optical, electrical and mechanical effects that have applications in many industrial devices. For instance, certain polar perovskites in which the cations tend to be displaced all in one sense can be used as pyroelectric detectors (thermal detectors that can act as intruder alarms, heat imaging systems and counting of people passing through security gates and through supermarket check-outs) and as piezoelectric devices (where for example an oscillating electric field creates ultrasonic waves). While considerable progress has been made over the last 70 years or so in understanding the link between structure and physical properties, it is fair to say that much is still not fully understood. In this talk, I shall describe some of the crystal structural variations and how they impact on useful properties.

Jean Pol Vigneron
Physics Department
University of Namur
Belgium

Physics of structural color: photonic nanoarchitectures

Many living organisms have evolved hard photonic structures to produce colouration by light interference, as a result of a continuing natural selection that started million years ago. The physical origin of the spectral selective behaviour of this kind of structures - which only uses moderate refractive indexes - will be reviewed. Many examples from recent studies will be given, showing that one-, two- and three-dimensional photonic crystals with partial gaps are active in nature. The role of disorder and multiscale effects will be emphasized and the possible biomimetic transposition of these natural structure to actual artificial devices will also be briefly discussed.