In-Situ Tem Analysis of Nanometre-Sized Oxide Precipitates in a Metal Matrix
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IN-SITU TEM ANALYSIS OF NANOMETRE-SIZED OXIDE PRECIPITATES IN A METAL MATRIX
BART J. KOOI AND JEFF Th.M. DE HOSSON Laboratory of Applied Physics, Materials Science Centre and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
ABSTRACT The objective of the present work is the in-situ study of the transformation of small oxide precipitates in a metal matrix by conventional and high-resolution transmission electron microscopy (HRTEM). As an example the reduction of Mn3O4 into MnO for nano-sized oxide precipitates in a silver matrix was studied in detail. A convenient method for monitoring the reduction process is shown for a large number of precipitates simultaneously. It is based on two-beam dark-field images showing distinct Moiré patterns for the MnO and the various types of Mn3O4 precipitates embedded within an Ag matrix. A controlling factor of the transformation kinetics appeared to be the rate in which the system can relax the strains due to the accompanying volume reduction of the precipitates. Other interesting aspects of the Mn3O4 to MnO transformation scrutinized and explained were the shape change of the precipitates upon reduction and the fact that mixed Mn3O4/MnO precipitates were only detected within a small temperature/time interval. Ostwald ripening of the MnO precipitates was observed as well.
INTRODUCTION The equilibrium phase of nano-sized particles may deviate from that of the bulk owing to the contributions of the surface energy and surface stress. For small inclusions also of the strain energy exerted by the matrix may become appreciable, in addition to the contribution of the interfacial energy. Striking examples of the effect of residual strain is the occurrence of ‘magic’ sizes for Pb inclusions in an Al matrix [1] and the presence of solid noble gas inclusions at temperatures substantially above the triple point of these gases [2,3]. Another example showing the strong influence of interfacial and strain energy on small inclusions can be found in the more classical precipitation sequence(s) in Al alloys e.g. GP zones, θ’’, θ’ and θ [4]. Knowledge and understanding of the behaviour of small particles and inclusions is of importance for many engineering materials, particularly with the ongoing trends towards materials with a controlled nano-structure. In this context the study of phase transitions of small particles and inclusions deserves special attention, since it will probably play a key-role in the improvement of the understanding and in the control of the microstructure. Transmission electron microscopy (TEM) is particularly suited for in-situ studies of phase transitions of small particles and inclusions. Melting/solidification of particles and inclusions have particularly experienced considerable attention, e.g. [5-14]. Nevertheless, insitu TEM studies of solid-state structural phase transformations in particles and inclusions have received only little systematic attention, an exception of which is the study of the
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