Hae Jin Park

Micro-to-nano-scale deformation mechanisms of a bimodal ultrafine eutectic composite

The outstading mechanical properties of bimodal ultrafine eutectic composites (BUECs) containing length scale hierarchy in eutectic structure were demonstrated by using AFM observation of surface topography with quantitative height measurements and were interpreted in light of the details of the deformation mechanisms by three different interface modes. It is possible to develop a novel strain accommodated eutectic structure for triggering three different interface-controlled deformation modes; (I) rotational boundary mode, (II) accumulated interface mode and (III) individual interface mode. A strain accommodated microstructure characterized by the surface topology gives a hint to design a novel ultrafine eutectic alloys with excellent mechanical properties.

Phase evolution, microstructure and mechanical properties of equi-atomic substituted TiZrHfNiCu and TiZrHfNiCuM (M = Co, Nb) high-entropy alloys

In this study, alloys with composition of equi-atomic substituted TiZrHfNiCu, TiZrHfNiCuCo, and TiZrHfNiCuNb high-entropy alloys (HEAs) were produced by suction casting method. The effects of addition elements on phase composition, microstructure and mechanical behaviors of the HEA were studied. The suction casted Ti20Zr20Hf20Ni20Cu20 HEA exhibits single C14 Laves phase (MgZn2-type) with fine homogeneous microstructure. When Co or Nb elements are added, morphologies are slightly modulated toward well-developed dendritic microstructure, phase constitutions are significantly changed from single Laves phase to mixed multi-phases as well as mechanical properties are also altered with increased plasticity and high strength. It is believed that modulated mechanical properties are mainly ascribed to the change of phase constitution and crystalline structure, together with the microstructural characteristics. This clearly reveals that the selection and addition of supplementary elements based on the formation rule for HEAs play an important role on the evolution of phase, microstructural morphology and mechanical properties of Ti20Zr20Hf20Ni20Cu20 HEA.

Thermally-triggered Dual In-situ Self-healing Metallic Materials

The microstructural evolution and crack filling phenomena of (Al81Cu13Si6)100−x(Sn57Bi43)x (x = 0, 1, and 3 at.%) composites was investigated. The Sn and Bi elements were selected by considering the ability for liquid phase separation when combined with Al, Cu, and Si. Because of liquid phase separation, both Al-Cu-Si-rich L1 and Sn-Bi-rich L2 phases separately solidified at different temperatures yielding a trimodal eutectic structure in the cast alloys. The Sn and Bi elements have high mobilities due to the large interface of the eutectic microstructure and tend to strongly diffuse towards higher strained region during heat treatment. Furthermore, the mobile Sn and Bi elements in the Al-Cu-Si-based bimodal eutectic structure evidently fill cracks during warm rolling at 423 K. These results reveal that the developed alloy system has simultaneously dual self-healing characteristics, derived from the both precipitated Sn-Bi-rich particles and low melting agent, and the proposed alloy design based on liquid phase separation provides a novel strategy for creating self-crack filling metallic materials.

Influence of Nb on microstructure and mechanical properties of Ti-Sn ultrafine eutectic alloy

In this study, A series of the high strength (T82Sn18)100-xNbx (x=0, 1, 3, 5, and 9 at%) ultrafine eutectic alloys with large plasticity are developed by suction casting method. The Ti82Sn18 binary eutectic alloy consists of a mixture of a hcp Ti3Sn and a α-Ti phases having the plate-like lamellar type duplex structure with micro scaled eutectic colony. From the (T82Sn18)97Nb3, the alloy display structural heterogeneous distribution of ultrafine-scaled phases composed of β-Ti(Nb) solid solution surrounded by alternating plate-like shaped Ti3Sn and α-Ti phases. With increasing Nb content, the volume fraction of β-Ti is continuously increased, which induced improving mechanical properties both strength and plasticity. Especially, (Ti82Sn18)91Nb9 alloy has the outstanding combination of the high strength (σy≈1.1 GPa) and large plasticity (εp≈36%) at room temperature.

Phase Transformation and Work-hardening Behavior of Ti-based Bulk Metallic Glass Composite

In present work, work-hardening behavior of TiCu-based bulk metallic glass composite with B2 particles has been studied by systemic structural and mechanical investigations. After yield, pronounced work-hardening of the alloy was clearly exhibited, which was mainly related to the martensitic transformation as well as the deformation twinning in B2 particles during deformation. At the early plastic deformation stage (work-hardening stage), the stress-induced martensitic transformation from B2 phase to B19` phase and deformation-induced twinning of B19` phase was preferentially occurred in the around interface areas between B2 phase and amorphous matrix by stress concentration. The higher hardness value was observed in vicinity of interface within the B2 particles which are probably connected with martensitic transformation and deformation twinning. This reveals that the work-hardening phenomenon of this bulk metallic glass composite is a result of the hardening of B2 particles embedded in amorphous matrix.

Formation of Nano-oxides on Porous Metallic Glass Compacts using Hydrothermal Synthesis

Abstract Porous metallic glass compact (PMGC) are developed by electro-discharge sintering (EDS) process of gasatomized Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 metallic glass powder under of 0.2 kJ generated by a 450 µ F capacitor beingcharged to 0.94 kV. Functional iron-oxides are formed and growth on the surface of PMGCs via hydrothermal synthesis.It is carried out at 150 o C for 48hr with distilled water of 100 mL containing Fe ions of 0.18 g/L. Consequently, twotypes of iron oxides with different morphology which are disc-shaped Fe 2 O 3 and needle-shaped Fe 3 O 4 are successfullyformed on the surface of the PMGCs. This finding suggests that PMGC witih hydrothermal technique can be attractivefor the practical technology as a new area of structural and functional materials. And they provide a promising roadmap for using the metallic glasses as a potential functional application. Keywords: Porous metallic glass compact, Hydrothermal process ······························································································································· ·······························································································