Data-Driven Analysis of Neutron Diffraction Line Profiles: Application to Plastically Deformed Ta

Non-destructive evaluation of plastically deformed metals, particularly diffraction line profile analysis (DLPA), is valuable both to estimate dislocation densities and arrangements and to validate microstructure-aware constitutive models. To date, the interpretation of whole line diffraction profiles relies on the use of semi-analytical models such as the extended convolutional multiple whole profile (eCMWP) method. This study introduces and validates two data-driven DLPA models to extract dislocation densities from experimentally gathered whole line diffraction profiles. Using two distinct virtual diffraction models accounting for both strain and instrument induced broadening, a database of virtual diffraction whole line profiles of Ta single crystals is generated using discrete dislocation dynamics. The databases are mined to create Gaussian process regression-based surrogate models, allowing dislocation densities to be extracted from experimental profiles. The method is validated against 11 experimentally gathered whole line diffraction profiles from plastically deformed Ta polycrystals. The newly proposed model predicts dislocation densities consistent with estimates from eCMWP. Advantageously, this data driven LPA model can distinguish broadening originating from the instrument and from the dislocation content even at low dislocation densities. Finally, the data-driven model is used to explore the effect of heterogeneous dislocation densities in microstructures containing grains, which may lead to more accurate data-driven predictions of dislocation density in plastically deformed polycrystals.

Diffraction line profiles from polydisperse crystalline systems. Corrigenda

Equation (16) and some entries in Table 1 in the article by Scardi & Leoni [(2001), Acta Cryst. A57, 604–613] are corrected.

Measurement of the X-ray Elastic Constants of Amorphous Polycarbonate

In polymer materials, residual stress introduced during injection molding affects yield reduction due to deformation during molding and delayed fracture during operation, so the establishment of nondestructive stress evaluation of polymer products is desirable. The X-ray elastic constants of polycarbonate were measured for the purpose of obtaining fundamental data for X-ray stress measurement of amorphous polymer materials. The structural function was obtained from the diffraction data, and the strain measured by X-ray was determined from the shift of the first peak by the Q-space method. The peak position was determined using the pseudo-Voigt function approximation method and the diffraction line width method. The Young’s modulus measured by X-ray obtained by the diffraction line width method was close to the mechanical value. Although these values varied widely, they changed depending on the peak ratio. A simple and practical measurement method directly using the raw profile data was also discussed. The Young’s modulus determined by the diffraction line width method decreased with increasing peak ratio. On the other hand, the values determined by the pseudo-Voigt method were almost constant, irrespective of the peak ratio. The strain calculated by the line width method was determined more accurately than that by the pseudo-Voigt method.

The Certification of Standard Reference Material 1979: Powder Diffraction Line Profile Standard for Crystallite Size Analysis

This rather long-standing project has resulted in a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) for the analysis of crystallite size from a consideration of powder diffraction line profile broadening. It consists of two zinc oxide powders, one with a crystallite size distribution centered at approximately 15 nm, and a second centered at about 60 nm. These materials display the effects of stacking faults that broaden specific hkl reflections and a slight amount of microstrain broadening. Certification data were collected on the high-resolution powder diffractometer located at beamline 11-BM of the Advanced Photon Source, and on a NIST-built laboratory diffractometer equipped with a Johansson incident beam monochromator and position sensitive detector. Fourier transforms were extracted from the raw data using a modified, two-step profile fitting procedure that addressed the issue of accurate background determination. The mean column lengths, (L)area and (L)vol, were then computed from the Fourier transforms of the specimen contribution for each reflection. Data were also analyzed with fundamental parameters approach refinements using broadening models to yield (L)area and (L)vol values. These values were consistent with the model-independent Fourier transform results; however, small discrepancies were noted for the (L)area values from both machines and both crystallite size ranges. The fundamental parameters approach fits to the laboratory data yielded the certified lattice parameters.

Experimental study of micro-deformation of structural steel under simulation of operating conditions

Diagnostics of the load bearing capacity of machines and structures, service properties of structural materials are carried out by destructive and non-destructive methods. In the system of non-destructive methods of diagnosing and monitoring of the service properties of materials and products, radiation technologies are of particular importance, since they have instrumental capabilities for diagnosing and controlling changes in the soundness of a solid body at different structural levels. The X-ray diffractometry method provided determination of the stability of the stress-strain state of structural steel samples subjected to prolonged (over 5 years) static loads not exceeding the yield strength of the material. The purpose of the study is to determine experimentally the effect of continuous (since 2013) elastic stress and climatic factors on the change in the half-width of the diffraction line profile. It is shown that the straight-line dependence of the half-width of the diffraction line profile is maintained in the range of elastic stresses not exceeding 0.5σt. The results of changes in the microstructural state (micro-strains) identified by the characteristics of the diffraction lines profile are presented and discussed. The results of the experimental study of the effect of small steady elastic stresses reveal that periodic annual fluctuation of temperatures (2013 – 2018) does not cause a significant change in the properties of the diffraction line profile of the 08ps structural steel samples. On the contrary, low climatic temperatures contributed to the elimination of individual instrumental errors attributed to design conditions of the experiment. A sharp change in the true half-width of the diffraction line profile at the stress values σ > 0.5σt, probably indicates the minimum margin of safety of 08ps structural steel when setting the permissible stress value (σ). The revealed changes in microplastic deformation observed in structural steels in the range of elastic stresses corresponding to real operational loads require further study and analysis.