Abstract
PROF. W. L. BRAGG, in the twenty-fifth annual May Lecture before the Institute of Metals on May 8, dealt with the inner structure, or atomic arrangement, of metals and alloys. In general, when one metal is alloyed into another a series of phases appears. Metal A dissolves a certain amount of metal B with a gradual alteration in properties as the proportion of B increases. At a certain composition, a limit is reached, and for greater amounts of metal B a new phase appears as separate crystals of quite different properties mixed with the first phase. Regions of single and double phase alternate as the composition varies from pure A to pure B. These phases are the nearest approach in an alloy system to the chemical compounds formed by combining elements. X-ray analysis has shown that each phase has its own definite pattern, such as a cubical array with atoms at corners and centres, or at corners and centres of faces. The pattern changes from phase to phase. One of the most striking generalisations about alloy patterns to which X-ray analysis has led us is the empirical Hume-Rothery rule, which states that the ratio of free electrons to atoms in a structure is the same for alloys with the same pattern. H. Jones has recently shown how the alloy pattern affects the binding energy of these free electrons, and so has given a reason for this rule. Another point brought out by the X-ray analysis is that the method of arrangement of the atom amongst the positions of the phase pattern can be varied widely. The phase pattern is an entity apart from the way the atoms are distributed, in marked contrast to ordinary chemical compounds. The study of the movements of the atoms amongst the positions, as affected by heat treatment, can be made the basis of a very interesting theory; at high temperatures the atoms are shuffled up in a random way, while at low temperatures they sort themselves out into a regular alternation. The importance of this work is that it provides a basis for the chemistry of compounds formed between metals.
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Atomic Arrangement in Metals and Alloys. Nature 135, 784 (1935). https://doi.org/10.1038/135784b0
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DOI: https://doi.org/10.1038/135784b0