Min-Ha Lee

Effect of crystallization on the surface area of porous Ni-based metallic glass foams

The change of the specific surface area in porous Ni59Zr20Ti16Si2Sn3 metallic glass (MG) upon partial crystallization was investigated. The observed increase in the surface area of the annealed Ni-based MG foams is due to the formation of homogeneously distributed Ni10(Zr,Ti)7 rod-shape intermetallic phases with nominal diameters around 250u2009nm and ∼800u2009nm length on the surface of MG struts during the crystallization. For longer annealing, the specific surface area decreases again due to a change of the morphology of the crystals from rod-like to disc-like appearance, thus suggesting an optimum regime for increasing the specific surface area upon isothermal annealing at a given temperature.

High strength nanostructured Al-based alloys through optimized processing of rapidly quenched amorphous precursors

We report the methods increasing both strength and ductility of aluminum alloys transformed from amorphous precursor. The mechanical properties of bulk samples produced by spark-plasma sintering (SPS) of amorphous Al-Ni-Co-Dy powders at temperatures above 673u2009K are significantly enhanced by in-situ crystallization of nano-scale intermetallic compounds during the SPS process. The spark plasma sintered Al84Ni7Co3Dy6 bulk specimens exhibit 1433u2009MPa compressive yield strength and 1773 MPa maximum strength together with 5.6% plastic strain, respectively. The addition of Dy enhances the thermal stability of primary fcc Al in the amorphous Al-TM -RE alloy. The precipitation of intermetallic phases by crystallization of the remaining amorphous matrix plays important role to restrict the growth of the fcc Al phase and contributes to the improvement of the mechanical properties. Such fully crystalline nano- or ultrafine-scale Al-Ni-Co-Dy systems are considered promising for industrial application because their superior mechanical properties in terms of a combination of very high room temperature strength combined with good ductility.

Effect of geometrical constraint condition on the formation of nanoscale twins in the Ni-based metallic glass composite

We investigated the effect of geometrically constrained stress–strain conditions on the formation of nanotwins in α-brass phase reinforced Ni59Zr20Ti16Si2Sn3 metallic glass (MG) matrix deformed under macroscopic uniaxial compression. The specific geometrically constrained conditions in the samples lead to a deviation from a simple uniaxial state to a multi-axial stress state, for which nanocrystallization in the MG matrix together with nanoscale twinning of the brass reinforcement is observed in localized regions during plastic flow. The nanocrystals in the MG matrix and the appearance of the twinned structure in the reinforcements indicate that the strain energy is highly confined and the local stress reaches a very high level upon yielding. Both the effective distribution of reinforcements on the strain enhancement of composite and the effects of the complicated stress states on the development of nanotwins in the second-phase brass particles are discussed.

Production of Porous Metallic Glass Granule by Optimizing Chemical Processing

Abstract In this study, we optimized dissolution the dissolution conditions of porous amorphous powder to havehigh specific surface area. Porous metallic glass(MG) granules were fabricated by selective phase dissolution, in whichbrass is removed from a composite powder consisting of MG and 40 vol.% brass. Dissolution was achieved throughvarious concentrations of H 2 SO 4 and HNO 3 , with HNO 3 proving to have the faster reaction kinetics. Porous powderswere analyzed by differential scanning calorimetry to observe crystallization behavior. The Microstructure of milledpowder and dissolved powder was analyzed by scanning electron microscope. To check for residual in the dissolvedpowder after dissolution, energy dispersive X-ray spectroscory and elemental mapping was conducted. It was confirmedthat the MG/brass composite powder dissolved in 10% HNO 3 produced a porous MG granule with a relatively high spe-cific surface area of 19.60 m 2 /g. This proved to be the optimum dissolution condition in which both a porous internalgranule structure and amorphous phase were maintained. Consequently, porous MG granules were effectively fabricatedand applications of such structures can be expanded.Keywords: Porous granule, Selective phase dissolution, High specific surface area

Strain dependence of diffusion in Zr-based bulk amorphous alloy

We evaluated the effect of strain on the diffusion process of amorphous alloys related to the strain mode in both as-cast and plastically deformed Zr-based bulk amorphous alloy. Experimental investigations of the diffusion process and the elemental distributions in the amorphous alloy were performed by secondary ion mass spectrometry after annealing at 523u2009K following 30% plastic deformation by multiple cold rolling. Mathematical modeling analyses show an inhomogeneous sequence of the diffusion process by effective strain. The compressive strain near the surface of the deformed amorphous alloy retards ∼13.7% of the diffusion process compared to the as-cast amorphous alloy. The combined experimental and mathematical analyses results reveal that the diffusion process of bulk amorphous alloys can be changed by strain variation and can be adjusted by controlling the effective strain during deformation.

Influence of Stress-strain on the Microstructural Change in the Metallic Glass and Metallic Glass Matrix Composite

At room temperature, metallic glasses deform inhomogeneously by strain localization into narrow bands as a result of yielding due to an external force. When shear bands are generated during deformation, often nanocrystals form at the shear bands. Experimental results on the deformation of bulk metallic glass in the current study suggest that the occurrence of nanocrystallization at a shear band implies the loading condition that induces deformation is more triaxial in nature than uniaxial. Under a compressive stress state, the geometrical constraint strain imposed by the stress triaxiality plays a crucial role in the deformation-induced nanocrystallization at the shear bands.

Microstructure and Characteristics of Ag-SnO 2 Contact Materials Prepared by Magnetic Pulsed Compaction

In this study, we reported the microstructure and properties of Ag- contact materials fabricated by a controlled milling process with subsequent consolidation. The milled powders were consolidated to bulk samples using a magnetic pulsed compaction process. The nano-scale phases were distributed homogeneously in the Ag matrix after the consolidation. The relative density and hardness of the Ag- contact materials were 95~96% and 89~131 Hv, respectively.

Phase Transformation of Ti-Ni-Zr Icosahedral Phase and Fabrication of Porous Ti and W Compacts using Electro-Discharge Sintering

Electro-Discharge Sintering (EDS) employs a high-voltage/high-current-density pulse of electrical energy, discharged from a capacitor bank, to instantaneously consolidate powders. In the present study, a single pulse of 0.57-1.1 kJ/0.45 g-atomized spherical powders in size range of 10~30 and consisting of -(Ti, Zr) and icosahedral phases were applied to examine the structural evolution of icosahedral phase during EDS. Structural investigation reveals that high electrical input energy facilitates complete decomposition of icosahedral phase into C14 laves and -(Ti, Zr) phases. Moreover, critical input energy inducing decomposition of the icosahedral phase during EDS depends on the size of the powder. Porous Ti and W compacts have been fabricated by EDS using rectangular and spherical powders upon various input energy at a constant capacitance of in order to verify influence of powder shape on microstructure of porous compacts. Besides, generated heat () during EDS, which is measured by an oscilloscope, is closely correlated with powder size.