Papers by Author: Yong Bum Park

Abstract: Cu/Ni composite electrodeposit was fabricated by electroplating nickel on the both sides of an electroplated copper sheet. In order to lower interfacial stresses between copper and nickel, the microstructure of nickel was controlled to consist of grains with a mean size of 15 nanometers. The different parts of the composite electrodeposit underwent different evolution of textures and microstructures during annealing. In the Cu electrodeposit, the as-deposited texture characterized by a relatively high <100>//ND and twin components transformed to be diffuse due to grain growth during annealing above 300°C. This is attributed to a large number of twins conducting the as-deposited microstructure. On the other hand, in the Ni electrodeposit, grain growth that takes place during annealing above 250°C corresponds to abnormal grain growth in terms of the scale change of the grain size. This grain growth also transformed the as-deposited texture of strong <100>//ND into a diffuse texture. In the interface between copper and nickel, the atomic diffusion was generated by excessive vacancies resulting from the grain growth during the annealing of nanostructured Ni electrodeposit. An 'interface texture' began to developed in the previous Cu region above 500°C, and the microtexture development was similar to the growth texture of the annealed Ni electrodeposit.

Abstract: Within the framework of the lattice-statics and static fluctuation-waves’ methods, the available energiesof strain-induced interaction of interstitial–interstitial, interstitial–substitutional and substitutional–substitutional impurity atomic pairs are collected and analysed for f.c.c.-(Ni,Fe)–C solutionsallowing for discrete atomic structure of the host-crystal lattice. The lattice spacings, elasticity moduliand/or quasi-elastic force parameters of the host-crystal lattice, and concentration coefficients of thedilatation of solid-solution lattice due to the respective solutes are selected as the input numerical experimentaldata used. The above-mentioned interaction energies prove to have non-monotonically decreasing(‘quasi-oscillating’) and anisotropic dependences on discrete interatomic radius-vector, andthemselves are strong and long-range. In all f.c.c.-(Ni,Fe)-base solutions, there is strain-induced attractionin many co-ordination shells. In general, the strain-induced interaction between impurity atomsin γ-Fe is weaker than in α-Ni (but in some solid solutions, it may prove to be of the same order).The verification of applicability of the approximation of strain-induced interaction of impurities forf.c.c.-(Ni,Fe)–C alloys (by means of analysis of thermodynamic C activity and ‘short-range order’ parametersof C-atoms’ distribution revealed by Mössbauer spectroscopy) showed that it must be supplementedwith additional short-range (‘electrochemical’) repulsion in the first co-ordination shell.Nevertheless, in any case, the strain-induced interaction of impurity atoms must be taken into accountfor analysis of structure and properties of f.c.c.-(Ni,Fe)-base solutions.

Abstract: The texture evolution due to grain growth that takes place during annealing was investigated in nanocrystalline Fe-Ni alloys fabricated by using an electroforming method. In the current materials, the as-deposited textures were of fibre-type characterized by strong <100>//ND and weak <111>//ND components, and the occurrence of grain growth during annealing resulted in the strong development of the <111>//ND components with a significant decrease of the <100>//ND components. It was clarified that abnormal grain growth plays an important role on the evolution of the microstructures and textures. The abnormally grown grains were observed using orientation imaging microscopy in the early stages of grain growth, and their morphological features have been discussed.

Abstract: Abnormal grain growth (AGG) proceeds in case that normal grain growth is inhibited. It has long been known that the inhibition involves finely dispersed particles and/or the development of specific textures. There is another strong obstacle against the grain boundary (GB) motion; the solute atoms can reduce their energy by moving from the bulk into a GB. Resultant interaction between the solute atoms and a GB makes the GB motion more difficult. However the role of the GB segregation effect on AGG has not been clarified. In this study we simulate the 2D and 3D grain growth accompanying boundary segregation of solute atoms by using a phase-field model. It is shown that the segregation plays an important role on the occurrence of AGG. The boundary-segregation-induced AGG can take place when the average driving force of grain growth approaches a critical condition for pinning-depinning transition in solute-drag atmosphere.

Abstract: Uniaxial compression tests on hot-rolled AZ31 Mg alloy were carried out at a temperature of 300°C. In order to investigate work hardening and texture evolution during plastic deformation, cylindrical specimens were compressed to the rolling direction. Experimental investigation reveals that flow curves are strongly dependent on microstructure evolution such as deformation twinning and softening phenomenon. The occurrence of deformation twinning and softening phenomenon was revealed by the observation of microtexture using electron backscatter diffraction (EBSD). A visco-plastic self-consistent (VPSC) polycrystal model was used to simulate the work hardening, softening and texture evolution during the uniaxial compression. In order to calculate orientation of deformation twins, predominant twin reorientation (PTR) scheme was implemented into the polycrystal model. A softening scheme was also implemented in the polycrystal model to predict softening phenomenon and texture evolution after a peak stress.

Abstract: The crystallographic texture and mechanical properties were investigated in a nanocrystalline Fe-50%Ni alloy fabricated by using an electrodeposition method. The as-deposited texture was characterized by strong <100>//ND and weak <111>//ND fibre components, and the occurrence of grain growth resulted in the strong development of the <111>//ND fibre components with the minor <100>//ND fibre components. The elastic modulus and hardness were measured by means of a nanoindentation test. The annealing led the specimen to an increase in the elastic modulus and a strong decrease in the hardness. The elastic modului measured were compared with the theoretic predictions based on an elastic self-consistent (ESC) polycrystal model. The theoretical values of the elastic moduli through the thickness direction in the sample were in a good agreement with the experimental results.

Abstract: The texture development and the mechanical properties were investigated in a nanocrystalline Fe-50wt%Ni alloy fabricated by using an electrodeposition method. The as-deposited texture was characterized by a mixture of major <100>//ND and minor <111>//ND fibre components. Grain growth occurred in the specimen during annealing above 410oC, and resulted in the texture change that the <111>//ND fibre component developed strongly with decreasing <100>//ND fibre component. This texture evolution was attributed to the abnormally rapid growth of the <111>//ND grains, which became much coarser than the <100>//ND and other oriented grains in the fully annealed specimen.

Abstract: The textures and nanomechanical properties of nanocrystalline Fe-50wt%Ni foil fabricated by using an electroforming method were investigated. The as-deposited texture was characterized by major <100>//ND and minor <111>//ND fibre components. Annealing of the as-deposited specimen resulted in the texture change that the <111>//ND fibre texture developed strongly with decreasing <100>//ND intensity. The elastic modulus and hardness were investigated by nanoindentation test, and these experimental results were compared with the theoretical predictions based on an elastic self-consistent (ESC) polycrystal model. Annealing led to an increase in the elastic modulus and a strong decrease in the hardness. At the low ratio of indentation depth to the specimen thickness, the theoretical predictions of the elastic modulus in the sample thickness direction showed a good agreement with experimental results.

Abstract: In electroformed pure Ni and Fe-Ni alloys with nanometer-sized crystallites, grain growth that takes place during annealing results in a common texture change. With regard to the macrotextures, the as-deposited textures were of fibre-type characterized by strong <100>//ND and weak <111>//ND components, and the texture development due to grain growth was defined by strong <111>//ND fibre texture with the minor <100>//ND components. It was clarified by means of the microtexture analysis that abnormal growth of the <111>//ND grains occurs in the early stages of grain growth. The possible effects of the abnormal grain growth on the texture evolution have been discussed in terms of the orientation dependence of energy density.

Abstract: In general, thermal treatment at 500oC ~ 900oC ranges depending upon alloy composition of stainless steels can sensitize the steels and promote the intergranular cracking, and their intergranular corrosion resistance is decreased. These behaviors seem to be related to the change of microstructures. So, heat treatment at that temperature range should be avoided in fabrication, especially welding of stainless steels. In this work, it is focused on the effect of thermal treatment on caustic stress corrosion cracking of super austenitic stainless steel - S32050 The low temperature thermal treatment increased greatly the resistance to caustic SCC than those of annealed specimen. This enhancement might be closely related to the reduction of residual stress and slightly large grain, but its resistance was not affected by the anodic polarization behavior.