Institute of Metals Division - Discussion: Role of Grain Boundaries in the Ductile-Brittle Transition Behavior of Bcc Refractory Metals

H. F. Ryan and J. Suiter (CSIRO)—In this paper the authors have presented "a physical model which has as its central hypothesis the solution strengthening of regions along grain boundaries in the order of tens of angstroms thick", and their calculations have used 40Å as the zhalf-width of this boundary region. Selection of this value by the authors suggests that they were unaware of a number of recent field-ion microscope observations of grain boundary structures (e.g., Refs. 29 and 30). The accompanying figure, Fig. 9, shows a portion of an ion micrograph of a tungsten specimen containing a grain boundary, and the regular arrangement of atoms in various crystal planes in each grain can be seen, except in the boundary zone which is not more than two atom diameters in width. Since it is expected that solute atoms will segregate only to the region where the lattice is disturbed, it would seem that a value of 4Å for the half-width of the boundary region would be more appropriate than the value of 40Å assumed by the authors. If the former value is used in Eq. [8] of their paper to obtain curves showing solute concentration in the grain boundary as a function of temperature, the resulting curves differ significantly from those obtained by the authors. Although there are only relatively slight changes in the regions corresponding to higher temperatures (i.e., grain boundary concentrations of approximately 1 at. pet), at lower temperatures the curves are no longer realistic. For example, the curve corresponding to u = 7000 cal per mole gives a solute grain boundary concentration in excess of 100 pet at a temperature of approximately 360°K, and "flattens out" at a grain boundary concentration of about 300 pet! Similar results are obtained with each of the other curves. Ion microscopy of dilute solid-solution alloys has shown that solute atoms give rise to image points which in most cases are of higher intensity than those of the solvent atoms.31 If the same behavior occurs when the solute atoms are present in much lower concentrations then ion microscopy should provide a sensitive tool for studying the segregation of impurities to the grain boundary regions. Although many grain boundaries in tungsten have been examined after different thermal and mechanical treatments, there has not been, as yet, any systematic study of such segregation. In general the micrographs show no clear indications of marked grain boundary segregation, but occasionally areas are found where the grain boundary contains an appreciable number of image points of high intensity in a very narrow zone, see Fig. 9. If it is assumed that these correspond to impurity atoms then it is possible to estimate the impurity concentration both in the grain boundary and within the body of the grains. Application of this method (to a larger area