Min Young Na

Effect of minor addition of Zr on the oxidation behavior of Ti-Cu metallic glasses

The oxidation behavior of (TiCu)100-XZrX (x=3, 5, 7 at%) amorphous alloy during continuous heating up to 1073 K has been investigated. The weight-gain due to oxidation occurs in a two-step mode. The first-step oxidation (630~730 K) is associated with supercooled liquid region. The oxidation layer consists of intermixed TiO2 + ZrO2 layer at outer side and amorphous oxide layer at inner side. The oxidation rate at the second-step above 880 K becomes significantly different depending on the Zr content. In x=3 alloy, the oxidation layer consists of TiO2 layer at outer side, intermixed TiO2 + ZrO2 layer, and Ti3Cu3O layer at inner side. As the fraction of ZrO2 increases in x=5 and 7 alloys, the growth of oxide layer is accelerated due to provision of easier diffusion path in the intermixed oxide layer, forming intermixed multi-phase (TiO2 + ZrO2 + Ti3Cu3O + Cu51Zr14) oxide layer.

Crystallization Behavior of Al-Ni-Y Amorphous Alloys

The crystallization behavior in the and amorphous alloys has been investigated. As-quenched amorphous phase decomposes by simultaneous formation of Al and intermetallic phase at the first crystallization step, while as-quenched amorphous phase decomposes by forming Al nanocrystals in the amorphous matrix. The density of Al nanocrystals is extremely high and the size distribution is homogeneous. Such a microstructure can result from rapid explosion of the nucleation event in the amorphous matrix or growth of the preexisting nuclei embedded in the as-quenched amorphous matrix. The final equilibrium crystalline phases and their distribution at 873 K are exactly same in both and alloys.

Optimum Combination of Thermoplastic Formability and Electrical Conductivity in Al–Ni–Y Metallic Glass

Both thermoplastic formability and electrical conductivity of Al–Ni–Y metallic glass with 12 different compositions have been investigated in the present study with an aim to apply as a functional material, i.e. as a binder of Ag powders in Ag paste for silicon solar cell. The thermoplastic formability is basically influenced by thermal stability and fragility of supercooled liquid which can be reflected by the temperature range for the supercooled liquid region (ΔTx) and the difference in specific heat between the frozen glass state and the supercooled liquid state (ΔCp). The measured ΔTx and ΔCp values show a strong composition dependence. However, the composition showing the highest ΔTx and ΔCp does not correspond to the composition with the highest amount of Ni and Y. It is considered that higher ΔTx and ΔCp may be related to enhancement of icosahedral SRO near Tg during cooling. On the other hand, electrical resistivity varies with the change of Al contents as well as with the change of the volume fraction of each phase after crystallization. The composition range with the optimum combination of thermoplastic formability and electrical conductivity in Al–Ni–Y system located inside the composition triangle whose vertices compositions are Al87Ni3Y10, Al85Ni5Y10, and Al86Ni5Y9.

Formation of crystalline phase in the glass matrix of Zr-Co-Al glass-matrix composites and its effect on their mechanical properties

The microstructural evolution and mechanical properties of Zr-Co-Al alloys, with compositions of (Zr50Co50)x (Zr56Co26Al18)1-x (x = 1/6, 2/6, 3/6, 4/6, 5/6, 1) and Zr54Co35Al11, (referred to as Z1, Z2, Z3, Z4, Z5, Z6, and Z4.5), were investigated. Alloys Z1-Z3 consisted of crystalline phases, while alloys Z4 and Z4.5 consisted of crystalline phase particles (~3 vol% and ~35 vol%, respectively) embedded within the glassy matrix. Alloys Z5 and Z6 consisted of a monolithic glass phase. The crystalline phase of alloys Z1-Z4.5 consisted of primary B2-ZrCo dendrite and an interdendritic B2-ZrCo/Zr6CoAl2 eutectic phase. The B2-ZrCo dendritic phase exhibited a high work-hardening rate, which originated from the deformation-induced B2-to-B33 martensitic transformation. However, when the brittle interdendritic B2-ZrCo/Zr6CoAl2 eutectic phase fraction increased, the work-hardening rate significantly decreased. The ductility of the glass-matrix composites was significantly impaired by the presence of the interdendritic eutectic phase in the crystalline phase. The results indicate that the design of the crystalline particle microstructure is important with regard to enhancing the plasticity of glass-matrix composites.

Effect of Co addition on deformation-induced martensitic transformation in Cu-Zr alloy

Thermal stability, phase transformation after deformation and mechanical properties of B2 phase alloy with the compositions of Cu50-xCoxZr50 (x = 2, 4, 6, 10, 15, 20, 25, 30, 40, 45 and 50) have been investigated in the present study. The addition of Co in Cu50Zr50 alloy effectively improves the stability of B2 phase. The Ms and As temperatures generally decrease with increasing Co content. Before deformation, the alloys with x = 10, 15, 20, 25, 30, 40, 45 and 50 consist of B2 phase. After deformation, B2-B19´ martensitic transition occurs in x = 10, 15, 20 and 25 alloys, and B2-B33 martensitic transition occurs in x = 50 alloys. No martensitic transformation occurs in x = 40 and 45 alloys. The alloys with x = 10, 15, 20 and 25 exhibit double yielding phenomenon and higher strain-hardening rate, owing to B2-B19´ martensitic transformation during deformation. The alloys with x = 30, 40, 45 and 50 exhibit lower strain-hardening rate, in spite of conventional dislocation slip or B2-B33 martensitic transformation.

High thermal stability of the amorphous oxide in Ti44.5Cu44.5Zr7Be4 metallic glass

The oxidation behavior of Ti44.5Cu44.5Zr7Be4 metallic glass has been investigated. The oxide layer with a fully amorphous structure forms when heated up to the SCL temperature region, indicating that the presence of Be in the oxide layer improves the thermal stability of the amorphous oxide. The amorphous oxide is stable even when heated above the crystallization onset temperature. The thickness of the amorphous oxide layer reaches to ∼160 nm when heated up to 773 K. The oxide layer grows in both inward and outward directions, leaving Cu-enriched crystalline particles at the middle section of the oxide layer.

Synthesis of Amorphous Matrix Nano-composite in Al-Cu-Mg Alloy

The microstructure of as-quenched alloy has been investigated in detail using transmission electron microscopy. Al nano-crystals about 5 nm with a high density are distributed in the amorphous matrix, indicating amorphous matrix nano-composite can be synthesized in Al-Cu-Mg alloy. The high density of Al nano-crystals indicates very high nucleation rate and sluggish growth rate during crystallization possibly due to limited diffusion rate of solute atoms of Cu and Mg during solute partitioning. The result of hardness measurement shows that the mechanical properties can be improved by designing a nano-composite structure where nanometer scale crystals are embedded in the amorphous matrix.