A Personal Commentary on Crystal Structure of Solid Solutions

SINCE matter consists of atoms, any basic understanding of the properties and behaviors of materials ultimately progresses with the advancement of techniques that can reveal how atoms fit into structures and how they interact. If more than one atomic species are involved, as in a solid solution, then the questions to be answered grow in complexity. The studies involving X-ray diffraction by crystals, pioneered by the Braggs in England, Westgren and associates in Sweden, and others in Europe, in the early years of the last century, opened up a whole new area of research on alloys. Bain’s paper published in 1923 (from a young ‘‘metallurgical engineer,’’ working in the Cleveland wire division of General Electric), represents a striking example of how seminal contributions to the literature on materials often arise from the combination of a new technique coupled with inquisitive and imaginative thinking. To understand crystals in even greater detail, X-rays were soon to be assisted by electron diffraction and electron microscopy, then by atom probes, magnetic measurements, other physical properties, etc., and ultimately, also computer modeling. However, in 1922, Bain’s research tool was what was then referred to as an ‘‘X-ray spectrometer’’ with limited resolution. Bain elegantly points out that crystal planes act as three-dimensional gratings for diffraction. Instead of a constant grating splitting radiation into wavelengths, as in a prism revealing the spectrum of visible light, in X-ray spectroscopy we have a constant wavelength (k) and we measure the gratings, i.e., the planar spacings (d) of crystals by using the classic formula n k = 2d sin (h). At the time of his work in Cleveland, the crystal structures and lattice spacings of many pure metals had already been determined, so Bain concentrated his X-ray studies on a new area in materials—the metallic solid solutions. Unfortunately, the precision of the X-ray technique he had at hand was sometimes unable to reveal small changes of the lattice spacings on alloying. Nevertheless, his studies have led Bain to ponder some questions that would have been much easier to answer if he could resolve much smaller changes of d. It is also of interest to note that the terminology of this new field had not yet been rigorously established. Frequently, Bain uses the term ‘‘lattice’’ and ‘‘structure’’ interchangeably in a way that is sometimes confusing, as when he notes that ‘‘we may with no sense of vagueness refer to the copper lattice as an entity even though some points in the lattice are occupied by other elements.’’ It seems that the notion of an ‘‘alloy phase’’ had not yet been established sufficiently in the literature to be used when needed. Nevertheless, Bain demonstrated successfully the changes of lattice parameters of the solvent and solute elements taking place on alloying when these changes were sufficiently large, as in Cu-Ni. However, when the changes are small and the solubility is limited, as in some eutectics, Bain concluded that the X-ray patterns of the two component ‘‘lattices’’ merely change their respective intensities, as one changes the composition of the samples, so that ‘‘there is ... no inherent difference between a eutectic and amechanical mixture of the powders of the two metals.’’ When, however, some solid solubility is indicated by the displacement of the diffraction lines in the patterns, Bain proposes that ‘‘a solid solution has a crystal structure nearly identical with the structure of the (parent) solvent.’’ In the Cu-Zn series of alloys, even with 30 at. pct Zn dissolved in Cu, the calculated lattice parameter, 3.60 9 10 8 cm, appeared unchanged from that of Cu, and Bain concludes that the ‘‘copper atomic arrangement, or spacing, remains unchanged on alloying.’’ When additions of Zn reach nearly 50 at. pct and ‘‘...the bcc lattice pattern’’ is observed, Bain proposed that ‘‘a compound of copperzinc becomes the parent lattice for the bcc series of alloys.’’ He, thus, recognized the concept of ‘‘intermediate alloy phases’’ (without using the term) that may possess parent lattices (i.e., structures) that neither are those of the solvent nor of the solute, but are of an intermediate form. This field, of course, has been intensely pursued later in the middle of 1920s by Hume-Rothery and Westgren in Europe. Bain simply proposes that ‘‘we have a case of a solution of an element in a compound.’’ There is no indication in the literature that Bain has pursued these ideas further. It is surprising that these ideas about structures and solid solutions, and many others in this seminal article, T. B. MASSALSKI, Emeritus Professor, is with the Departments of Physics and Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 51213. Contact e-mail: massalski@cmu.edu *E.C. Bain: Trans. AIME, 1923, vol. 68, pp. 625–39.