Real-time microstructure of shock-compressed single crystals from X-ray diffraction line profiles

Abstract

Methods to obtain and analyze high-resolution real-time X-ray diffraction (XRD) measurements from shock-compressed single crystals are presented. Procedures for extracting microstructural information – the focus of this work – from XRD line profiles are described. To obtain quantitative results, careful consideration of the experimental geometry is needed, including the single-crystal nature of the sample and the removal of instrumental broadening. These issues are discussed in detail. Williamson–Hall (WH) and profile synthesis (PS) analysis procedures are presented. More accurate than WH, the PS procedure relies on a forward calculation in which a line profile is synthesized by convoluting the instrumental line profile with a line profile determined from a diffraction simulation. The diffraction simulation uses the actual experimental geometry and a model microstructure for the shocked crystal. The shocked-crystal microstructural parameters were determined by optimizing the match between the synthesized and measured line profiles. XRD measurements on an Al crystal, shocked along [100] to 7.1 GPa using plate-impact loading, are used to demonstrate the WH and PS analysis methods. In the present analysis, the microstructural line broadening arises because of a distribution of longitudinal elastic microstrains. The WH analysis resulted in FWHM longitudinal microstrain distributions of 0.22 and 0.38% for Lorentzian and Gaussian line shape assumptions, respectively. The optimal FWHM longitudinal microstrain for the PS method was 0.35% with a pseudo-Voigt distribution (40% Lorentzian–60% Gaussian). The line profile measurements and PS analysis presented in this work provide new insight into the heterogeneous distribution of elastic strains in crystals undergoing elastic–plastic deformation during shock compression. Such microstrain distribution measurements are complementary to continuum measurements, which represent averages of the heterogeneous strains or stresses. The PS analysis is a general method capable of incorporating microstructural models more complex than the microstrain distribution model used in this work. As a next step, the PS method will be applied to line profiles of multiple diffraction peaks to separate strain- and size-broadening effects in shocked crystals.