Hahn Choo

Orientation-dependent grain growth in a bulk nanocrystalline alloy during the uniaxial compressive deformation.

The microstructural evolution during the uniaxial compression of an as-deposited bulk nanocrystalline (nc) Ni–Fe (average grain size d≈23nm) at ambient temperature was investigated by the high-energy x-ray diffraction (HEXRD) and the transmission-electron microscopy (TEM). HEXRD measurements indicated that the grain growth occurred in the nc Ni–Fe alloy during the uniaxial compression tests and that the grain growth shows orientation dependence, i.e., the grains preferentially grow perpendicular to the loading direction. This preferred grain growth was further confirmed by the TEM observations, indicating that the grains were elongated after the compressive plastic deformation.

Transformation-induced plasticity in an ultrafine-grained steel: An in situ neutron diffraction study

An ultrafine-grained steel with an average grain size of about 350nm was developed. The tensile testing at ambient temperature shows a threefold increase in the yield strength compared to its coarse-grained counterpart. Moreover, the increase in the strength was achieved without the sacrifice of the ductility due to strain-induced martensitic transformation. The evolution of lattice strains and phase fractions of the austenite and martensite phases during the deformation was investigated using in situ neutron diffraction to provide a micromechanical understanding of the transformation-induced plasticity responsible for the combination of high strength and ductility.

In situ neutron diffraction measurements of temperature and stresses during friction stir welding of 6061-T6 aluminium alloy

Abstract The evolution of temperature and thermal stresses during friction stir welding of Al6061-T6 was investigated by means of in situ, time resolved neutron diffraction technique. A method was developed to deconvolute the temperature and stress from the lattice spacing changes measured by neutron diffraction. The deep penetration capability of neutrons made it possible for the first time to obtain the temperature and thermal stresses inside a friction stir weld.

Electrochemical corrosion behavior of a Zr-based bulk-metallic glass

Using cyclic-anodic-polarization tests, the corrosion behaviors of the as-cast, deformed, and relaxed Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 bulk-metallic glasses were investigated. Their corrosion behaviors are related to the amount of free volume. As the free volume decreased, the glasses became more corrosion resistant. The as-cast and the deformed glasses are less resistant to pit propagation, while the relaxed one is more vulnerable to pit nucleation. It is thought that an increase in free volume favors the pit propagation, and the chemical ordering caused by the relaxation may promote the pit nucleation.

Shear strain in a shear band of a bulk-metallic glass in compression

Using an infrared camera, the authors observe in situ the dynamic shear-banding operations in the geometrically constrained specimens of a bulk-metallic glass during compression at various strain rates. Based on the observed number of shear bands in a collection of simultaneous shear-banding operations that cause a serration, the authors calculate the shear strains in individual shear bands. The results demonstrate that the shear strain in a shear band is up to 103%–104% and dependent on strain rates. The higher the strain rates, the larger the strain in a shear band.

Thermophysical and elastic properties of Cu50Zr50 and (Cu50Zr50)95Al5 bulk-metallic-glass-forming alloys

By employing a containerless high-temperature high-vacuum electrostatic levitation technique, the thermophysical properties, including the ratio between the specific heat capacity and the hemispherical total emissivity, the specific volume, and the viscosity, of Cu50Zr50 and (Cu50Zr50)95Al5 bulk-metallic-glass (BMG)-forming liquids have been measured. Compared with Cu50Zr50, the improved glass-forming ability of (Cu50Zr50)95Al5 can be attributed to its dense liquid structure and its high value of viscosity. Additionally, the relationship between the viscosity of various BMG forming liquids at the melting temperature and the elastic properties of the corresponding glasses at room temperature will be compared.

Structural transition of ferromagnetic Ni2MnGa nanoparticles

We report here that the ball-milling process induces the phase transformation from the tetragonal structure to the disordered face-centered-cubic structure in Ni2MnGa ferromagnetic shape-memory alloys. The in situ high-energy x-ray diffraction analyses reveal that an intermediate phase, which is characterized by amorphous structure, controls the transformation kinetics during the postannealing process. Completely different from their coarse-grained counterparts, the ferromagnetic Ni2MnGa nanoparticles undergo various sequences of structural transitions that are tailored by the crystallite size, atomic order, and intrinsic magnetic structure.

Uniaxial tensile plastic deformation of a bulk nanocrystalline alloy studied by a high-energy x-ray diffraction technique.

By employing a high-energy x-ray diffraction technique, the authors report that uniaxial tensile plastic deformation induced the grain growth and texture development in a bulk nanocrystalline Ni–Fe alloy. The effects become more pronounced with increasing the plastic strain (closer to the fracture surface). The texture development accompanying the grain rotation indicates that dislocation motion contributed to the observed plasticity in the nanocrystalline Ni–Fe alloy. The quantitative experimental data suggest that the dislocation storage was absent in the uniformly deforming region; whereas the dislocation storage was present in the necking region, where the grain growth was substantial.

Effect of temperature on mechanical behavior of Zr-based bulk metallic glasses

The compressive tests and Vickers microhardness measurements were conducted on the as-cast Zr-based bulk metallic glasses at different temperatures. The results show that the strength is proportional to the temperature. Furthermore, at cryogenic temperatures, more shear bands were observed near the fracture surface and surrounding the indentation marks. The analysis suggests that both the formation and propagation of the shear bands are thermally activated processes.

Structural model for bulk amorphous alloys

A structural model is proposed for bulk amorphous alloys based on the pair distribution functions (PDFs) measured using neutron scattering at ambient and cryogenic temperatures and different structural states. Reverse Monte Carlo (RMC) simulations were performed, in which icosahedral and cubic structures were used as the initial structures for the PDF refinement. The combined PDF and RMC studies show that strongly bonded clusters, with atomic-bond lengths shorter than their crystalline counterpart structures, are randomly distributed and strongly connected in the amorphous matrix. An attempt has also been made to identify the relationship between amorphous structures and their mechanical properties.