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Submitted on 1 Jan 1990
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INTERGRANULAR AND INTERPHASE
BOUNDARIES IN MATERIALS : CONCLUDING REMARKS
J. Levy
To cite this version:
J. Levy. INTERGRANULAR AND INTERPHASE BOUNDARIES IN MATERIALS : CON- CLUDING REMARKS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-1067-C1-1069.
�10.1051/jphyscol:19901166�. �jpa-00230272�
SEPTEMBRE 1989
INTERGRANULAR AND INTERPHASE BOUNDARIES IN MATERIALS : CONCLUDING REMARKS
J.B. LEVY
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ECOLE DES MINES DE PARISINTRODUCTION
First of all, I am grateful to the Organizing Committee for having given to me the opportunity of speaking today. I am glad that this meeting could take place at Ecole des Mines since many distinguished metallurgists have been trained in this institution. Besides people more famous in industry like MARTIN, HEROULT, PERRIN, CHARPY, PORTEVIN, I want to mention HENRI LECHATELIER and GEORGES FRIEDEL, who was the first to publish a general theory of twinning and discovered the concept of coincidence sites lattice.
This meeting has shown, once more, that the field of interfaces is a very active one : 234 papers were presented, among which 45 communications and 189 posters. At the symposium held in Japan (Minakami) on the same subject in 1985, the closing session was a workshop entitled "what is really significant ? " : the question arises also today. After some historical considerations, I shall try to summarize very shortly the main results obtained since that time.
Finally, I shall try to indicate what in my sence is still needed and in what directions research could be performed.
SOME HISTORICAL CONSIDERATIONS
When I first got involved into grain-boundary studies in 1962, the main macroscopic problems where this type of interface plays a crucial role, were identified, at least in metallic alloys and semi-conductors.
Table I shows a tentative list of these problems, with examples of industrial applications. A lot of work had been made using macroscopic means of observations : but, it was difficult to go further on. The main ideas were the following :
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Although the twin concept emerged early in mineralogy, it took some time to transfer it to metallurgy and to accept that the structure of at least some boundaries was ordered. More precisely, three types of boundaries were identified : low angle, "special" or twin, "general" or somehow amorphous grainboundaries.-
The effect of some elements in very small quantities, like phosphorus and sulfur in iron, was well known : it was generally. . .. . - - .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901166
Cl-1068 COLLOQUE DE PHYSIQUE
Most researchers all over the world then decided that, in order to'get more precise data, it was necessary to examine very well defined samples :
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From a crystallographic point of view : use oriented bicrystals,-
From a chemical point of view : use high purity materials (concentrations known with a precision of the order of several PPM).This approach, although very heavy (time and money-consuming), has been adopted all over the world and has proven very fruitful in conjonction with the development of precise experimental methods of investigation (H.R.E.M., F.I.M., Auger Spectrometry, etc . . . ) as well as Of powerful computers for theoretical developments and simulation.
Simultaneously, knowledge about heterophase boundaries has increased considerably, due to the development of thin film. Techniques in connection with the development of new devices for microelectronics.
What were the main conclusions of the Minakami workshop ? basic concepts about the structure and behaviour of ordered interfaces in metallic materials are well developed. Three main wishes have been expressed
l " ) Extend grain-boundaries studies to other types of interfaces : semi-conductors, ionic and covalent materials (E.G. Ceramics), and heterophase interfaces.
2 " ) Extend studies on special orientations to "amorphous"
grain-boundaries,
3 " ) Get significant results about polycrystals, which may yield texture control methods, "grain-boundary engineering".
THIS MEETING
There is a great interest, both experimental and theoretical, in INTERFACE STRUCTURE and this was the subject of the FIRST SESSION. It is recognised that having direct access to the atomic structure through H.R.E.M. is a great advantage. For example, two years only after the start of excitement for high-temperature superconductor ceramics, it has been demonstrated that one key of the basic mechanisms of superconductivity lies in the grain-boundary structure : a lot of experimental direct observations have already yielded very interesting results. The crystallography of such samples is more and more complex, since not only the lattices must be taken into account, but also the motif of each adjacent crystal. Let us also emphasize a strong interest for intermetallic compounds (E.G. Ni3 Al, Ni Al, Ti Al) as possible high-temperature materials. This session includes about 1/3 of the communications.
The SECOND SESSION, of about the same volume was devoted to DYNAMICAL PROPERTIES OF BOUNDARIES. Special interest for diffusion and migration appears, besides many other phenomena including the role of grain-boundaries in plastic deformation (interaction with dislocations, etc...). A lot of effort is put in this very difficult area of obvious engineering interest.
The THIRD SESSION, devoted to HETEROPHASE INTERFACES STRUCTURES, is again largely influenced by T.E.M. observations. The majority of the materials observed were metal-ceramics interfaces due to their clear industrial importance : but others, like precipitates in metallic alloys and electronic devices (As Ga, Si, ,..) were also examined. The main problem to be solved appears to be a good evaluation of the resistance (both chemical and mechanical) of the bonds.
Finally, the FOURTH SESSION was especially focussed on ELECTROCERAMICS AND SUPERCONDUCTORS. Results have been presented both on structure observations and electrical and mechanical properties. For example, it
through doping grain-boundaries.
CONCLUSION
Compared to the Minakami recommandations, a much greater interest than before appears for complex systems including non-metallic materials.
This is probably due to many factors, especially the interest forr new materials for high tech applications. Great prog.ress have been made in atomic observations and understanding of basic mechanisms.
One difficulty is common to all materials problems : due to the complexity of the systems, most observations are rather specific to the sample under consideration.
But the main problem lies in making quantitative predictions of the macroscopic behaviour through extrapolation of microscopic observations. There is not much progress about the understanding of the general disordered grain-boundary or interface and grain-boundary engineering (texture control). Some very convincing correlations have been made : but wa still lack good tools, both experimental and theoretical, for macroscopic predictions and modelling.
Finally, last but not least, I do not want to avoid one obstacle, which is to convince young smart students that this type of work is intellectually fascinating, practically useful and (possibly ! )
rewarding.
