XI. Bakerian lecture.–The crystalline structure of metals

Abstract

The microscopic study of metals was initiated by Sorby, and has been pursued by Arnold, Andrews, Behrens, Charpy, Chernoff, Howe, Martens, Osmond, Roberts-Austen, Sauveur, Stead, Wedding, Werth and others. The work of these authors has demonstrated the value of the microscope in metallurgy, not only as an aid to analysis, but as a means of observing structure. The structure of pure metals, of metals containing small quantities of foreign matter, and of alloys, has been made the subject of microscopic examination, and important conclusions have been reached. The work to be described in this paper proceeds on the same general lines. A large part of it deals with a branch of the subject which has not hitherto received much notice, namely, the effects of strain. The writers believe that they have established the fact that the structure of metals is crystalline even under conditions which might be supposed to destroy crystalline structure. They have found that the plastic yielding of metals when severely strained, occurs in such a manner that the crystalline structure is preserved. The observations to be described show how crystalline aggregates exhibit plasticity, and how, after straining, a metal continues to be a crystalline aggregate. The distinction which is often drawn between crystalline and non-crystalline states in metals appears to be unfounded. Except for a few simple innovations, the methods of experiment used in this research, especially as regards the preparation of specimens, do not differ materially from those of earlier workers. The specimens were first polished on commercial emery-paper which had been previously rubbed on a piece of hard steel in order to remove the coarser particles. They were finished on a rapidly revolving disc coated with fine wash-leather and charged with a thin paste of rouge and water. For most purposes the finest jeweller's rouge is suitable, but in special cases we resorted to the use of peroxide of iron obtained by precipitation from a solution of pure ferric chloride.