Diffraction and Crystallographic Texture Characterization Based on Large Atomic Configurations

Author:E Jun Cheng

Supervisor:luo sheng nian

Database:Doctor

Degree Year:2019

Download:20

Pages:131

Size:11041K

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Observing transient changes of material structure at the microscopic scale under ul-trafast extreme conditions or metastable states is at the frontier of modern science.How-ever,due to the fact that many of these problems are extremely fast,highly transient,difficult to repeat,and depend on the characteristics of the microstructure,the method-ology development targeted at such problems is of great challenge.One way to tackle this challenge is to simulate texture evolution of a polycrystalline material under shock loading.On the other hand,in-situ real-time diffraction measurements are performed using more advanced characterization tools such as a third-generation synchrotron ra-diation source or an X-ray free electron laser facility.In order to help the design and interpretation of related dynamic experiments,large-scale parallelized diffraction simula-tion methods are in great demand.We thus develop a texture characterization method,and a polychromatic diffraction simulation method in this thesis,both with ultralarge atomic configurations as inputThe diffraction simulation code is used to investigate the grain size effects on recip-rocal space nodes and 2D diffraction pattern.The diffraction peak broadening and peak shift of single crystals and polycrystalline solids with small grain sizes are studied for a polychromatir synchrotron undulator source.For polycrystalline solids,a diffraction peak is broadened by a polycrhomatic beam,and its shape follows that of the beam spec-trum.Asymmetric spectra induce both peak broadening and shift.Combining the texture characterization method and the diffraction simulation code,the effect of nanocrystalline texture on the grain size analysis based on x-ray diffraction is also investigated,and it is found that for textured nanocrystalline systems with quasi-equiaxed grains,the error increases with increasing percentage of preferred orientationAs application examples,the microstructures of materials,including the evolution of textures,during crystallization nueleation and shock loading,are investigated with the two methods developed above,combined with molecular dynamics(MD)simulationsFor homogeneous nudeation during crystallization process of supercooled liquid Cu we study the process with MD simulations and obtain 2D x-ray diffraction patterns.Through the statistics of 100 independent MD simulation runs,the analyses with the mean first-passage time method and survival probability method yield a nucleation rate of 4.2 × 1032 m-3s-1.The effect of loading strength and substrate orientation on the crys-tallization of Lennard-Jones liquids under shock or quasi-isentropic compression is studied Heterogeneous nucleation occurs in all cases,but it is followed by homogeneous nucleation for quasi-isentropic loading of sufficient strength.Crystallization rate is anisotropic,in-versely proportional to the cosine of the dihedral angle between the substrate plane and a main {111} growth plane.Finally,we explore the texture changes under shock loading of nanocrystalline Ta without initial texture(random orientation)and found that the shock compression induces the 〈001〉+〈111〉 double fiber texture.