Influence of the Surface Condition of Specimens on the Size of Coherently Diffracting Domains

Abstract

Metallic sample preparation process from cutting to polishing introduces defects into the subsurface layers by plastic deformation. Methods of bulk sample characterization are usually sensitive to the microstructure and presence of defects e.g. dislocations, grain boundaries, etc. near the surface. The X-ray diffraction technique (XRD) is a unique method used primarily for phase identification and quantification of crystalline materials. The extending of the method by the whole pattern fitting using Rietveld refinement analysis enables also utilisation for other purposes such as determination of the size of coherently diffracting domains (CDD). This property quantifies the degree of disorder in material and is usually correlated with mean crystallite size, i.e. subgrain size. However, since the penetration depth of the laboratory X-ray radiation is limited to the order of tens of micrometers (depending on the material), the observed result is highly affected by the density of defects such as dislocations in the surface area of the specimen and thus by their surface condition. The effect of the different final steps of grinding and polishing on the observed size of CDD was studied and discussed for two materials - aluminum Al7075 alloy and high entropy alloy HfNbTaTiZr. The reasonable finishing for the optimal results was found for both materials.