Effect of chemical segregation on accuracy of local lattice distortions determination by pair distribution functions

Local lattice distortion (LLD) is a salient feature of bcc-structured refractory high-entropy alloys (RHEAs), closely associated with their mechanical properties. To quantify the extent of LLDs in RHEAs, the pair distribution function (PDF) analysis has been identified as a promising approach. However, the commonly observed chemical segregation within these alloys introduces challenges in accurately determining LLDs. In this study, the effect of chemical segregation on LLD quantification was investigated through fitting simulated two-phase composite PDFs, representing segregated microstructures, with a single-phase model and evaluating the errors to assess the accuracy and reliability of small-box analysis in this context. The results show that the errors introduced by chemical segregation increase with increasing lattice parameter difference, and the fitting quality gradually deteriorates to a point where it no longer adequately describes the data. We found that the lattice parameter difference should be below 1% for precise and reliable LLD measurements in bcc-structured RHEAs. Additionally, we observed that while the scattering length variation due to segregation does affect LLD quantification, its effect is comparatively minor.