Dong Nyung Lee

Microstructures and crystallization of electroless Ni-P deposits

A study has been made of microstructures and crystallization of the electroless Ni-P deposits containing 11.3 to 23.0 at% P obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent by means of differential scanning calorimetry, X-ray diffractometry and hot stage transmission electron microscopy. The deposits containing low phosphorus content of 11.3 at% could be represented as an fcc Ni-P solid solution of 5 to 10 nm microcrystallites, whereas the deposits containing high phosphorus content were amorphous. The crystallization process of amorphous Ni-P solution involved more than one intermediate phases; precrystallized nickel or off-stoichiometric Ni3(P, Ni) or Ni5(P, Ni)2 phase in which some phosphorus sites are replaced by nickel atoms. The final equilibrium phases were bct Ni3P and fcc nickel crystals regardless of phosphorus content. The amorphous phase containing 20 to 22 at% phosphorus was the most stable among the amorphous Ni-P alloys.

A thermodynamic study on the AgSbSn system

Abstract A thermodynamic assessment for the AgSbSn solder alloy system has been attempted through the least squares optimization procedure taking available thermodynamic and phase equilibria information into account. The low-order AgSb and AgSn binary systems have been re-evaluated and thermodynamic properties of the ternary alloys have been extrapolated from these of the three binary systems using the Muggianu model. The calculated phase diagrams and thermodynamic quantities of the AgSb and AgSn binary systems show very good agreement with the experimental data, and liquidus projection and isoenthalpy curves in the ternary liquid alloys are well reproduced with the assessed thermodynamic model parameters.

A study on the microstructure and phase transformation of electroless nickel deposits

Electroless Ni-7.4 to 10% P deposits obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent were analysed by transmission electron microscopy, X-ray diffraction and thermal analysis. The deposits could be represented better by a microcrystalline structure composed of 4 to 5 nm fcc Ni-P solid-solution grains rather than an amorphous structure. The deposits also had the (111) texture, which persisted in nickel grains even after phase separation of nickel and Ni3P by heating in the case of the low nickel content, whereas the texture approached the random orientation with increasing phosphorus content. The phase transformation temperature was independent of the phosphorus content.

The effect of texture on ridging of ferritic stainless steel

Abstract Ferritic stainless steel sheets exhibit ridging parallel to the rolling direction when subjected to tension or deep drawing. The origin of ridging behavior has not been clearly explained yet. Many people agree that ridging originates from different plastic anisotropies of grains. In this study, 430 and 409L stainless steels having columnar and equiaxed structures were chosen as initial specimens to elucidate the role of microstructure and composition on ridging. The specimens initially having the columnar structure showed severe ridging and 409L stainless steel showed an inferior surface quality. The existence of band-like colonies of similar orientations was found in the center of the sheets by electron back-scattered diffraction measurement. In addition, the previous models suggested by other researchers were examined quantitatively by the crystal plasticity finite element method. In order to obtain a more realistic ridging simulation, the specimens containing variously oriented colonies in a textured matrix were also considered. The simulated results showed that the lower plastic strain ratio of {001} colonies and different shear deformations of {111} or {112} colonies resulted in ridging.

A model for development of orientation of vapour deposits

The texture of vapour deposits changes from the orientation that places the lowest energy crystal facets parallel to the substrate under the condition of low atom or ion concentration adjacent to the deposit, to the orientation that places the higher energy crystal facets parallel to the substrate as the atom or ion concentration adjacent to the deposit increases.

The evolution of recrystallization textures from deformation textures

Abstract A model for the evolution of recrystallization texture has been suggested. In the model, the most important driving force for recrystallization is assumed to be the stored energy due to dislocations whose arrangement is dependent on the deformation texture, which in turn gives rise to the absolute maximum principal stress direction. The maximum stress direction becomes the minimum elastic modulus direction on recrystallization. The model has been used to explain the evolution of the 〈100〉 recrystallization texture from axisymmetrically deformed fcc metals, the cube recrystallization texture from cold rolled fcc metals, the {111}〈112〉 recrystallization texture from cold rolled bcc iron alloys, and the recrystallization textures from axisymmetrically deformed bcc metals.

Analysis of deformation textures of asymmetrically rolled steel sheets

Asymmetric rolling has been studied to obtain the shear deformation texture of bcc steels through the thickness. Asymmetric rolling in which the circumferential velocities of two working rolls are different can be achieved by different roll radii at the same rotation rates, different roll speeds at the same roll radii and single roll drive. The deformation of steel sheets in the three different asymmetric rolling methods was analyzed by FEM. The deformation was used to calculate crystal rotation by full constraints Taylor model to predict the deformation textures. The texture evolution during the asymmetric rolling was measured and analyzed with the emphasis on the effect of shear reversions.

Strain energy release maximization model for evolution of recrystallization textures

Abstract In 1995, the author advanced a model for the evolution of recrystallization texture, in which the absolute maximum internal stress direction due to dislocations generated during deformation or fabrication in the deformed material is aligned with the minimum Youngs modulus direction in recrystallized grains, whereby the energy release during recrystallization can be maximized. This comes from the fact that the material concerned does not macroscopically change its shape and volume during recrystallization, and so the recrystallization is a displacement-controlled process. This strain energy release maximization model originates from the presumption that the stored energy due to dislocations is the major driving force for the recrystallization. The absolute maximum internal stress direction may be obtained from the operating slip systems, which are related to the deformation mode and texture. If one slip system is activated, the absolute maximum normal stress direction is parallel to the slip direction, or the Burgers vector direction. If more than one slip system is activated, the absolute maximum normal stress direction can be determined by the vector sum of related slip directions, by taking into account their contribution to slip. This paper reviews recrystallization textures of plastically deformed metals, based on the SERM model.

Effect of annealing on magnetic properties and microstructure of electroless nickel-copper-phosphorus alloy deposits

The relationship between magnetic properties and microstructure of the as-deposited and heat-treated Ni-Cu-P alloy deposits has been studied by means of vibrating sample magnetometry, differential scanning calorimetry. X-ray diffractometry and hot-stage transmission electron microscopy. The Ni-Cu-P deposits consist of low-P Ni-Cu solid solution crystallites and high-P-low-Ni-Cu amorphous phase. When the deposits are annealed at elevated temperatures, the Ni-Cu crystallites grow rejecting P from themselves and absorbing Cu from the neighbouring amorphous phase, while the P-rich amorphous phase transforms into Ni5P2 phase or into Ni5P2 and Ni3P phases. The metastable Ni5P2 phase finally transforms into the stable Ni3P phase. The non-magnetism observed in Ni-Cu-P deposits having a copper content above 28% in both as-deposited and annealed states, is attributed to their Cu-rich Ni-Cu solid solution crystallites. Nickel phosphides which crystallize from the amorphous phase existing mixed with the Ni-Cu crystallites in the as-deposited state, do not affect the saturated magnetic moment of the deposits.

Inoculated acicular ferrite microstructure and mechanical properties

Abstract Low carbon–manganese wrought steels with three different microstructures were prepared by different thermomechanical treatments without changing the chemical composition and their mechanical properties were compared. The microstructure of fine interlocking acicular ferrite nucleated intragranularly at Ti 2 O 3 inclusions shows higher strength and good toughness at low temperatures than those of coarser polygonal ferrite/pearlite and aligned ferrite with second phase.