Robert Cahn is the truest of physical metallurgists. For 50 years now his scientific interest has been totally concentrated on the structures of metals and alloys, what determines them, and what properties they in turn govern. It has never wavered from this and remains just as strong and focused today. It must have been inborn in him for he has come from a non-scientific family and the science teaching in his early schooling was, in his own words, terrible. The only serious intellectual rival to his metallurgical science has been his love of the English language and literature-which he shares with his wife, Patricia-and which has given his writing a most graceful and refreshing character.
He had a peripatetic upbringing, being taken by his mother in 1933 from Germany to live in Spain, as refugees from Hitler; and then in 1936 being sent to England to escape the Spanish civil war. After erratic early schooling hazarded by the London Blitz he finished up in Workington, Cumbria, where he was at last well taught in the sciences. From there he gained a place at Trinity College, University of Cambridge, and chose to specialize in metallurgy and crystallography. A good degree gained him entry into Egon Orowan's group in the Cavendish Laboratory, where he began research on the recrystallization of bent metal single crystals.
At Cambridge he met and married Patricia, then a student of English at Newnham College, and a daughter of Professor Daniel Hanson, the distinguished physical metallurgist. This coincidence of scientific interests was completely accidental, but it was also the happiest of accidents which helped confirm Robert in the correctness of his choice of subject.
Robert's work at Cambridge provided the first clear and recognized evidence for the existence of dislocations. The elongated Laue spots produced from bent crystals break up on annealing into short strings of discrete spots, indicating that the curved lattice converts-in the process called polygonization-into a mosaic of unbent sub-crystals set at slightly different angles to one another, angles which match the original curve of the bent crystal. At that time I was working in Hanson's Department, at Birmingham University, on the theory of dislocations; and, at his suggestion, Robert wrote to me about these observations. What he had discovered was that plastic curvatures tend to straighten up by concentrating all the lattice bending into small-angle boundaries. I knew that plastic bending could only be brought about by edge dislocations, all of the same sign, and that a counter-intuitive feature of the elastic interaction between such dislocations is that they are attracted into vertical walls, thereby forming small-angle boundaries. Putting our heads together on this we realized that his observations had provided, in 1946, the first clear experimental proof of the existence of dislocations in metal crystals. In retrospect., we ought perhaps to have made more of this.