Slip systems and dislocation densities in individual grains of polycrystalline aggregates of plastically deformed CoTi and CoZr alloys

Abstract

A novel X-ray diffraction-based technique for grain-by-grain assessment of dislocation density within polycrystals is applied. The technique discriminates dislocation densities of different slip modes, slip systems and dislocation character. Data which was formerly confined to the transmission electron microscope (TEM) is now available from X-ray diffraction (XRD). In addition, there is a profound statistical advantage of the XRD approach over the traditional TEM-based approach. Over 130 grains were analyzed for one sample of CoTi, which would be a Herculean task on the TEM, requiring dozens of samples as well as numerous hours on the microscope and analysis of the images generated for each sample. The present experiments were performed in about 30h per sample and the analysis is semi-automated, involving a Monte-Carlo-type algorithm to determine the dislocation structure best representing the single crystal diffraction peak profiles in a polycrystalline aggregate. Conclusive confirmation of a previously suggested explanation for the anomalous ductility of two CsCl structured intermetallic compounds, CoTi and CoZr, is provided: namely, that hard dislocation modes, with b=〈110〉 and 〈111〉, rarely observed in single crystal experiments, are active in nearly every grain. The results call into question the value of employing single crystal deformation experiments alone to understand the deformation behavior of polycrystalline materials. Further, the results re-emphasize that the uniform stress assumption implicit in Schmid factor analysis is a poor one for materials which are highly anisotropic at the single crystal level.