Synchrontron Radiation X-ray Spectroscopy Study On The Microstructure Of The Substitutional Solid Solution

The present work address the structure-performance relationship of substitutional solid solution by synchrotron radiation X-ray spectroscopy.Substitutional solid solution exhibit special properties at certain compositions,which have been obtained through long-term and extensive experiment.Further expanding the relationship between the atomic and electronic structure and performance are of great significance in the field of materials.It is an urgent need to undertand the structural origin behind the special composition so that some scientific problems can be established to effectively comprehend.Synchrotron Radiation（SR）has many excellent features such as high energy resolution and wide wavelength.X-ray Spectroscopy based on SR has become a sharp tool for studying the electronic structure of different metal atoms and the occupation o f metal atoms in solid solution.Depending on the type of material,the substitutional solid solution mainly includes the metal solid solution and the metal oxide solid solution.In this thesis,the relationship between the structure and properties of the solid solution and the metal oxide solid solution is studied by taking the nickel-based alloy and the spinel solid solution as examples.As the key engineering materials,some nickel-based superalloy like Hastelloy N has intergranular embrittlement caused by the fission products Te in the fuel salts,mainly because some low-melting eutectic phases or some brittle compounds have formed at the grain boundary with the diffusion of Te element.Therefore,clarifying the speciation and atomic structure of these special solid solution in nickel based alloy could provide basic information for understanding the mechanism of intergranular embrittlement.Simultaneously,such spinel solid solution has the effects on the blue color of cobalt blue pigments.The spectroscopic determination of the oxidation states and speciation of Co,Mn,and Fe furthers understanding of the coloration of blue pigments in blue and white porcelain.In this work,pure nickel and Ni-Cr binary alloy with 18% Cr content were chosen as our research object.The Te corrosion source was uniformly obtained by electroplating method and thermal steam method,and then all samples were annealed at high temperature to simulate the Te corrosion behavior.The Te at GBs was identified by synchrotron radiation-based multi-technical methods including micro-X-ray fluorescence（μ-XRF）,micro-X-ray diffraction（μ-XRD）and micro-X-ray absorption near-edge structure（μ-XANES）combined with density function theory（DFT）calculations.The chemical speciation and structure feature of metal solid solution and intermetallic compounds were investigated at nanometer to sub-micrometer scale to explore the mechanism of intergranular embrittlement induced by Te.Two-dimensional synchrotron radiation induced grazing incidence X-ray diffraction（2D SRGI-XRD）,X-ray absorption fine structure（XAFS）as well as wavelet transform（WT）analysis method,were used to analyze the reaction products of Ni-Cr binary alloy.The primary purpose was to reveal the speciation and the atomic structure of tellurium corrosion products and confirm the anti-intergranular embrittlement mechanism.A series of cobalt blue pigments,which were synthesized based on the chemical compositions of the blue pigments in Kangxi blue and white porcelain,were investigated by Co,Mn,and Fe K-edge and L_2,3-edge X-ray absorption spectroscopy to determine the oxidation states and species of the elements and to discern their impact on the blue color.The spectroscopic determination of the oxidation states and speciation of Co,Mn,and Fe furthers understanding of the coloration of blue pigments in blue and white porcelain.The main results are summarized as follow:（1）The formation of these tellurides could probable lead to intergranular embrittlement.The annealed specimen was polished repeatedly to prepare apparent cross-sectional mapping of Te by a electron probe microanalysis（EPMA）.Then,a roughly 6μm-thick micron-scale corrosive sample at grain boundary was cut by a focused ion beam（FIB）machine.The distribution of Te and the structure of speciation in grain boundary were identified by synchrotron radiation-based multi-technical methods including micro-X-ray fluorescence（μ-XRF）,micro-X-ray diffraction（μ-XRD）and micro-X-ray absorption near-edge structure（μ-XANES）combined with density function theory（DFT）calculations.Results show that the concentration of Te concentration gradually decreases from the GB area to the grains.The subsequentμ-XRD analysis confirm the GB corrosion products are Ni₃Te₂ and Ni-Te solid solution.Particularly,μ-XANES results reveal a transfer of charge from Te p orbital to Ni sites and DFT calculations also exhibit that Ni-Te bonding in the Ni-Te solid solution are weak covalent bonds.The formation of these tellurides could probable lead to intergranular embrittlement.（2）Chromium seems effective in reducing the intergranular cracking.Samples of a Ni-Cr binary alloy were exposed to Te by vapor deposition and the high-temperature corrosion behavior was investigated at different annealing temperatures and different annealing time.The speciation and local atomic structure of the resulting films were identified by synchrotron radiation-based grazing incidence X-ray diffraction and X-ray absorption fine structure measurements.The corrosion process can be summarized as follows.Cr has the tendency to diffuse to the surface,react with Te in products,and continue forming CrTe when the reaction temperatures are in the range of 600 to 800℃.At the same time,Cr segregates in the GBs and grain crystals near the surface of the alloy,during which Te becomes tied up as CrTe and fails to diffuse into the alloy along grain boundaries.This is the reason that Cr plays a crucial role in controlling corrosion properties.However,when the reaction temperature is increased to 900℃,Te has the tendency to react with Ni and form Ni₃Te₂ due to the decomposition of CrTe.Meanwhile,Te atoms will propagate inward into the alloy along GBs forming apparent intergranular cracking（IGC）on the surface,which seriously deteriorates the mechanical properties of the structural material.The results at 1000℃ revealed that a majority of Te had penetrated into the lattice after a longer annealing time,and then a Ni-Cr-Te solid solution was formed.（3）The spectroscopic determination of the oxidation states and speciation of Co,Mn,and Fe furthers understanding of the coloration of blue pigments in blue and white porcelain.A series of cobalt blue pigments,which were synthesized based on the chemical compositions of the blue pigments in Kangxi blue and white porcelain,were investigated by Co,Mn,and Fe K-edge and L_2,3-edge X-ray absorption spectroscopy to determine the oxidation states and species of the elements and to discern their impact on the blue color.The results reveal that Co is bivalent and mainly located at tetrahedral sites,which is the main parameter controlling the blue color.Mn is mainly present as Mn²⁺,or Co_xZn_1-xAl₂O₄ and Fe is mainly present as Fe³⁺.In particular,Fe³⁺substitutes the Al in CoAl₂O₄ and occupies octahedral sites with a high Mn content.All the synthetic cobalt blue pigments can form a solid solution with various end-members or an intermediate solution spinel.