Synchrotron X-Ray Diffraction Based Depth-Resolved Stress Characterization Method And Its Application

Internal stresses generated by plastic deformation during preparation,processing,and service can significantly impact the mechanical performance,dimensional stability,and service safety of metallic components.Casting,welding stresses can lead to material deformation and even cracking.Internal stresses can also lead to premature failure of parts in service.Nevertheless,surface strengthening techniques can introduce compressive stress layers,substantially enhancing fatigue performance.The presence of microscopic internal stress gradients plays a crucial role in the plastic deformation and damage behavior of materials.Therefore,there is a pressing need to develop non-destructive characterization techniques that can achieve high spatial and strain resolution over large depth ranges of stress and elastic strain gradients,which has significant engineering application value and scientific significance.Existing stress characterization techniques have limitations in terms of their ability to measure stress gradients in metallic materials.While laboratory X-ray diffraction can only measure surface stresses,neutron diffraction has a low spatial resolution despite its centimeter-scale penetration advantage.Synchrotron highenergy X-ray diffraction technology offers advantages in reciprocal space,real space,high time resolution,and large penetration depth,but is limited in its ability to achieve stress gradient analysis through conventional experimental setups.To address this,the present work proposes a new method that combines the advantages of the deep penetration of medium/high-energy X-rays in synchrotron radiation and the high spatial resolution of differential aperture technology for depth-resolved stress gradient characterization using differential aperture technology in the transmission geometry of synchrotron medium/high-energy X-ray diffraction.Research on simulation methods was carried out for synchrotron-based diffraction experiments,developed the device and data processing algorithms for the new method.In order to help accelerate the extension of differential aperture technology to the characterization of stress gradients over a large depth range in engineering materials,this research studied the three-dimensional stress gradients of a fatiguedeformed superalloy using differential aperture-based microdiffraction technology.1.A simulation method for synchrotron-based X-ray diffraction experiments was developed by employing the principles of X-ray diffraction kinematics and the finite element method.The calculation method utilized the Ewald method as diffraction criteria and the random discretization method for reciprocal domains to approximate the actual diffraction process.Moreover,a crystal structure import program was designed to read crystal structures of any space group and perform crystallographic calculations,diffraction spectra,and absorption calculations.Furthermore,the simulation investigated the influence of grain size and mosaic misorientation on diffraction in polycrystalline powder diffraction and verified that the experimental method based on slits can measure strain gradients.The results of the simulation demonstrate the reliability of the developed program in reflecting the laws of X-ray diffraction experiments.2.The newly proposed method for depth-resolved stress characterization using differential aperture technology in the monochromatic X-ray transmission geometry was investigated.The experimental device was developed,and a data reconstruction algorithm was established.Based on the developed simulation program,the depth stress resolution ability of the new method was simulated.The results showed that the new method has a spatial resolution of 20 μm in the depth direction and a elastic strain resolution of less than 200 με.Synchrotron medium/high-energy(30-60 keV)X-rays can penetrate 1 mm of aluminum and 0.5 mm of nickel,which satisfies the measurement requirements for significant depth range stress/elastic strain gradients in engineering materials.This research provides a promising approach for the non-destructive,three-dimensional characterization of stress distribution in materials and components.3.This thesis studied the area detector data processing method in synchrotronbased X-ray diffraction experiments and developed an automatic calibration method for the detector geometry parameters using image processing algorithms and the RANSAC algorithm.The developed calibration method can handle automatic calibration tasks for complete diffraction rings,segmented diffraction rings,and even speckle-like data in a robust manner.Moreover,a two-dimensional diffraction image integration algorithm based on weighted histogram and pixelsplitting algorithms was developed.The pixel-splitting algorithm offers better smoothness,but it requires more computing time,which we addressed by using LUT technology to speed up the process.The integration results were compared with those of similar foreign software,validating the accuracy of the algorithm.To provide a user-friendly interface,these data processing methods were compiled as executable software with concise interfaces for fast batch processing and visualization functions.4.Microdiffraction characterization technology with high depth resolution was conducted to study the long-range elastic strain gradient and local crystal rotation of a poly crystalline nickel-based superalloy subjected to high-temperature lowcycle fatigue.The relationship between microstructure damage and fatigue failure was established,revealing that the enormous elastic strain gradient at the PLBmatrix interface and the defect and stress concentration at the junction of PLBs are the primary intrinsic micro-mechanisms of fatigue damage in this alloy.This study shows that depth-resolved elastic strain gradient characterization technology has great potential for revealing micro-mechanisms of fatigue damage in metallic materials.This thesis presents a novel approach for characterizing depth-resolved stress gradients using the differential aperture technique in synchrotron/high-energy Xray diffraction transmission geometry.The core innovation of this work lies in the proposal and implementation of this method,which offers significant advantages over existing techniques.We have developed the necessary experimental equipment and verified the depth resolution using a developed X-ray diffraction experimental simulation program.This method represents a major advancement in the field of stress gradient characterization in metallic materials,offering critical technical support over an extended depth range.