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Robert Cahn, F.R.S.
Intermetallics 8 (2000) 975
www.elsevier.com/locate/intermet
Correspondence
Robert Cahn, F.R.S. I am so glad to have this opportunity of writing an item for this book, honoring Professor Robert Cahn. Robert and I have been good friends for over 55 years now. We initially became acquainted through two distinct happenings: ®rst, of course, through our shared interest in trying to understand the structures and properties of metals and alloys in terms of fundamental principles of crystallography, thermodynamics and atomic theory. The second event was more personal. I was originally a student at the University of Birmingham, England, under the guidance of Professor Daniel Hanson, the distinguished physical metallurgist. He had two daughters, one of whom, Patricia, went to Newnham College, Cambridge, to study English. There she met Robert, who, at Trinity College, was beginning to build his career in crystallography and physical metallurgy, culminating in a research project in Egon Orowan's group in the Cavendish Laboratory. They were attracted to each other and soon embarked on a lifelong and happy marriage. My acquaintance with Robert began just after the end of the war. I had begun to work on the theory of dislocations, especially the elastic forces which they exerted on one another. Although dislocations of the same sign usually repel one another for an obvious reason, there was also a counter-intuitive feature of edge dislocations, which sprang from the dipolar character of their elastic ®elds. In certain positions, two parallel edges with the same Burgers vector, lying in dierent but parallel glide planes, attract each other and align, one above the other, in an elastically bound state. Hanson knew of my dislocation interests and also of the experimental work that Robert was doing on the structures of plastically bent metal crystals. He thought that there might be a useful link between these two researches and so suggested that Robert and I should get in touch. We did
and it produced a strikingly fruitful result. Robert's plastic bending must, for geometric reasons, have produced parallel edge dislocations of the same sign in parallel glide planes; and we realized that what he was observing, later known of course as polygonisation, was the direct result of elastic attraction between the edges, causing them to align into vertical walls, i.e. small-angle boundaries, as predicted by the theory. Robert's observations had thus provided the ®rst unambiguous evidence for the existence of dislocations. According to the self-eacing style of those days, it was quietly announced in a short note in a conference proceedings in 1947. Perhaps we should have trumpeted it more loudly. Although for many years afterwards our paths through the scienti®c world diverged, we always kept up a good correspondence; and it is a pleasure now, in these later days, that we have become close neighbours, both enjoying the hospitality of the Department of Materials Science in Cambridge. Here, I never cease to be amazed at Robert. His room is an almost impenetrable den of books and papers, stacked, column behind column, from ¯oor to ceiling. And he knows where every one is and can go to it immediately when wanting to look something up in his vast work of editing numerous journals and books, as well as pursuing his own original researches on intermetallics and writing many articles and books of his own in a characteristically clear and delightfully readable style. Patricia's great love of good English has surely found a matching response in Robert's own writing.
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00052-2
Alan Cottrell Department of Materials Science and Metallurgy University of Cambridge CB2 3QZ, UK
www.elsevier.com/locate/intermet
Correspondence
Robert Cahn, F.R.S. I am so glad to have this opportunity of writing an item for this book, honoring Professor Robert Cahn. Robert and I have been good friends for over 55 years now. We initially became acquainted through two distinct happenings: ®rst, of course, through our shared interest in trying to understand the structures and properties of metals and alloys in terms of fundamental principles of crystallography, thermodynamics and atomic theory. The second event was more personal. I was originally a student at the University of Birmingham, England, under the guidance of Professor Daniel Hanson, the distinguished physical metallurgist. He had two daughters, one of whom, Patricia, went to Newnham College, Cambridge, to study English. There she met Robert, who, at Trinity College, was beginning to build his career in crystallography and physical metallurgy, culminating in a research project in Egon Orowan's group in the Cavendish Laboratory. They were attracted to each other and soon embarked on a lifelong and happy marriage. My acquaintance with Robert began just after the end of the war. I had begun to work on the theory of dislocations, especially the elastic forces which they exerted on one another. Although dislocations of the same sign usually repel one another for an obvious reason, there was also a counter-intuitive feature of edge dislocations, which sprang from the dipolar character of their elastic ®elds. In certain positions, two parallel edges with the same Burgers vector, lying in dierent but parallel glide planes, attract each other and align, one above the other, in an elastically bound state. Hanson knew of my dislocation interests and also of the experimental work that Robert was doing on the structures of plastically bent metal crystals. He thought that there might be a useful link between these two researches and so suggested that Robert and I should get in touch. We did
and it produced a strikingly fruitful result. Robert's plastic bending must, for geometric reasons, have produced parallel edge dislocations of the same sign in parallel glide planes; and we realized that what he was observing, later known of course as polygonisation, was the direct result of elastic attraction between the edges, causing them to align into vertical walls, i.e. small-angle boundaries, as predicted by the theory. Robert's observations had thus provided the ®rst unambiguous evidence for the existence of dislocations. According to the self-eacing style of those days, it was quietly announced in a short note in a conference proceedings in 1947. Perhaps we should have trumpeted it more loudly. Although for many years afterwards our paths through the scienti®c world diverged, we always kept up a good correspondence; and it is a pleasure now, in these later days, that we have become close neighbours, both enjoying the hospitality of the Department of Materials Science in Cambridge. Here, I never cease to be amazed at Robert. His room is an almost impenetrable den of books and papers, stacked, column behind column, from ¯oor to ceiling. And he knows where every one is and can go to it immediately when wanting to look something up in his vast work of editing numerous journals and books, as well as pursuing his own original researches on intermetallics and writing many articles and books of his own in a characteristically clear and delightfully readable style. Patricia's great love of good English has surely found a matching response in Robert's own writing.
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00052-2
Alan Cottrell Department of Materials Science and Metallurgy University of Cambridge CB2 3QZ, UK