Quantification of local lattice distortions in refractory high-entropy alloys by scattering-based techniques

Licentiate thesis, 2024

The increasing demand for advanced structural materials for use in extreme conditions, particularly including the aerospace applications, has promoted the development of refractory high-entropy alloys (RHEAs). A deeper understanding of the interaction between microstructure and properties is essential to overcome the scientific challenges associated with these alloys. One notable characteristic of high entropy alloys (HEAs), including RHEAs, is local lattice distortion (LLD) caused by the solid solutioning of multiple principal elements with varying atomic sizes. LLDs generate strain fields that hinder dislocation movement, thus enhancing strength via the solid solution strengthening mechanism.

However, quantifying LLDs in RHEAs poses a challenge due to the subtle nature of these structural changes and a lack of consensus on appropriate methods. The approach of determining average structures of materials through the analysis of X-ray diffraction patterns has been well-recognised. Meanwhile, total scattering technique, along with pair distribution function (PDF), has also shown significant potential in uncovering information at local structure. Both Bragg diffraction and PDF have previously proven effective for the quantitative assessment of LLDs.

In this thesis, it is demonstrated that LLDs in body-centered cubic (BCC) structured RHEAs can be accurately measured through both reciprocal-space Rietveld method and real-space small-box PDF analysis. The obtained LLDs are significantly larger than previously reported values for face-centered cubic (FCC) structured HEAs. Additionally, a comprehensive simulation study explored the effects of chemical segregation, confirming the feasibility of precise LLD determination in RHEAs under specific levels of segregation and LLD. Robust determination of LLDs through small-box PDF analysis is essential for future time-resolved measurements.