Eunjoo Shin

Effect of wavelength-dependent attenuation on neutron diffraction stress measurements at depth in steels

The wavelength dependence of the maximum feasible penetration depth was studied for neutron diffraction stress measurements in ferritic and austenitic steels. This property was examined with wavelengths from the close vicinity of the Bragg edges, where the neutron total cross section has its local minimum and for which the scattering angles are convenient for stress measurements. These wavelengths (e.g. 2.39 and 2.19 A) are longer than those commonly used in stress measurements (∼1.6 A). By using such wavelengths, configured by a focusing bent perfect crystal Si(111) monochromator, it was observed that the available total beam path length is about 85 mm in both ferritic and austenitic steels. This study provides specific information for choosing the instrument configuration suitable for most strain-scanning experimental tasks.

Development of Abnormal Grain Growth in Cold Rolled and Recrystallized AA 5182 Sheet

Grain growth in the cold rolled and subsequently recrystallized AA 5182 sheets was investigated by means of microstructure observations and texture measurements. Grain growth behavior strongly depends on the annealing temperatures. Grain growth hardly took place at temperatures lower than 470°C, which is attributed to a low mobility of grain boundaries. Abnormal grain growth occurred at temperatures ranging from 480 to 530°C. Annealing above 560°C gave rise to the dissolution of inhibitor precipitates, which led to normal grain growth.

Textures and precipitates in Ti-stabilized interstitial-free steel

Samples of titanium-stabilized interstitial-free steel were coiled at 600 and 700 °C. The effect of the coiling temperature on the formation of textures and precipitates during coiling, cold rolling and recrystallization annealing was studied by means of neutron texture measurements, small angle neutron scattering analysis and transmission electron microscopy observations. Whereas coiling at the lower temperature resulted in an insufficient scavenging of carbon in solid solution, coiling at the higher temperature promoted scavenging of carbon in solid solution, and consequently the formation of a large number of precipitates. The large amount of carbon in solid solution caused the retardation of softening in the cold-rolled sample during annealing below 700 °C. Upon annealing above 700 °C the impact of the coiling temperature on grain size and precipitates was not observed. The samples coiled at different temperatures exhibited a nearly identical cold rolling texture. However, the cold-rolled sample having a large amount of carbon in solid solution displayed a weak recrystallization texture.

Effect of precipitate size and dispersion on recrystallization behavior in Ti-added ultra low carbon steels

The effect of coiling temperature on precipitates and solid solution was investigated in P-free and P-added Ti-stabilized ultra low carbon steels. The volume fractions of the fine precipitates smaller than 60 nm were evaluated by using small-angle neutron scattering technique. The solute P was quantified from the lattice parameters obtained from neutron diffraction patterns. In the P-free steels, TiC and Ti4C2S2 with various sizes ranging from 5~60 nm were observed. In addition to these precipitates, FeTiP precipitates of a size exceeding 50 nm were also observed in the P-added steel. The amount of fine precipitates smaller than 10 nm and the concentration of solute P was higher in the samples coiled at low temperature. The recrystallization temperature increases if the coiling temperature decreases in both, P-free and P-added steels. The recrystallization temperature of P-free steels is lower than that of P-added steels. In the P-free steels, the pinning effect of fine precipitates played a key role for the retardation of the recrystallization. In the P-added steels, the retardation of recrystallization is due to both, the pinning effect of fine precipitates and the solute drag effect of P in solid solution.

Influence of multiple small-angle neutron scattering on diffraction peak broadening in ferritic steel

Peak broadening of neutron diffraction was studied at various neutron wavelengths (1.24–2.61 A). As the neutron beam path through a specimen increased, significant peak broadening was observed in ferritic steel, but not in austenitic steel. The peak broadening was reduced under a magnetic field applied perpendicular to the beam direction. Small-angle neutron scattering results showed significant reduction in scattering intensities under a magnetic field of 1.2 T. It is suggested that the peak broadening can be attributed to multiple small-angle neutron scattering by magnetic domains. Thus, a sufficiently strong magnetic field could enhance the deep penetration capability of neutron diffraction by reducing the peak broadening.

Fractal Structures on Fe3O4 Ferrofluid: A Small-Angle Neutron Scattering Study

A small-angle neutron scattering (SANS) which is a powerful technique to reveal the large scale structures was applied to investigate the fractal structures of water-based Fe3O4ferrofluid, magnetic fluid. The natural magnetite Fe3O4 from iron sand of several rivers in East Java Province of Indonesia was extracted and purified using magnetic separator. Four different ferrofluid concentrations, i.e. 0.5, 1.0, 2.0 and 3.0 Molar (M) were synthesized through a co-precipitation method and then dispersed in tetramethyl ammonium hydroxide (TMAH) as surfactant. The fractal aggregates in ferrofluid samples were observed from their SANS scattering distributions confirming the correlations to their concentrations. The mass fractal dimension changed from about 3 to 2 as ferrofluid concentration increased showing a deviation slope at intermediate scattering vector q range. The size of primary magnetic particle as a building block was determined by fitting the scattering profiles with a log-normal sphere model calculation. The mean average size of those magnetic particles is about 60 – 100 A in diameter with a particle size distribution σ = 0.5.

Quantitative phase analysis of strongly textured alloy mixtures using neutron diffraction

Based on the Rietveld profile refinement of neutron diffraction patterns and a new texture analysis method, a quantitative phase analysis (QPA) method is proposed and examined in the case of artificially produced multiphase alloys having strong textures. The preferred-orientation factors (POFs) of the diffraction peaks were extracted from the inverse pole figure calculated from the orientation distribution function (ODF) that was computed from the averaged pole figures measured by the rotating sample stage, providing a faster and simpler texture measurement. The reliability of this method was examined using binary alloy mixtures of known fractions of zirconium and aluminium. In addition, the fraction of unknown phases in an artificially produced aluminium matrix composite was determined by introducing a standard zirconium sample. QPA of the binary mixtures successfully predicted the weight fraction of each component with an absolute error of less than 0.3 wt%. This method also provided appropriate results on the calculation of the weight fraction of unknown phases in the aluminium matrix composite.

Separate Evaluation of the Kinetics of Carbide Precipitation Occurring at the Interface of Preexisting Particles and Within the Austenitic Matrix in a Microalloyed Steel

The isothermal kinetics of carbide precipitation occurring at the interface of preexisting (Ti,Nb)(N,C) particles and within the deformed γ-austenite matrix were separately evaluated using a Nb-Ti-V microalloyed steel through small-angle neutron scattering and transmission electron microscopy. While the specimen was isothermally held after deformation at 1223 K (950 °C), (Nb,Ti)(C,N) particles were precipitated at the interface of coarse (Ti,Nb)(N,C) particles preexisting in the recrystallized γ matrix. This resulted in a single size distribution curve, which was converted from the measured magnetic scattering cross section. However, during isothermal holding after deformation at 1123 K (850 °C), fine (Nb,Ti,V)(C,N) particles formed mainly within the deformed γ matrix, although some of them were precipitated at the interface of preexisting coarse (Ti,Nb)(N,C) particles. Accordingly, the specimens held at 1123 K (850 °C) exhibited double size distribution curves. The separate evaluation between matrix and interface precipitation kinetics was successfully performed using the size distribution curves due to the difference in particle size according to the nucleation site. The reliability of carbide precipitation kinetics was confirmed by comparing the measured ratio between magnetic and nuclear scattering cross sections with the ratio calculated based on the measured chemical composition of precipitates.

Residual stress analysis of an aircraft landing gear part using neutron diffraction

The residual stress of a landing gear part of a fighter jet that has a frequent practice of takeoff and landing was evaluated for the safety. The sample was a cylindrical steel bar with a 22.2 mm diameter and 55 mm length used to fix the main landing gear to the aircraft body. For a deep measurement up to 6 mm, we used a neutron beam. From the measurements, the tensile and compressive strain in the axial direction were observed around one side of the pin hole which was across the steel bar vertically with an 8 mm diameter. The strain distribution along the length of the bar presented a similar tendency through the thickness and a larger value on the surface. The maximum value of the residual stress around the pin hole was about 100 MPa. However, there was no strain on the opposite side of the pin hole. From the results, it may be surmised that the steel bar received a steady force in one direction around the pin hole, however the force was weak and affected a small limited area and thus not influence on the steel bar on the whole.