Mok-Soon Kim

Influence of chlorine on the oxidation behavior of TiAlMn intermetallic compound

Abstract Excellent oxidation resistance of reactive-sintered titanium aluminides was reviewed and the mechanism of forming protective Al 2 O 3 film was studied. TiAlMn containing more than 200 mass ppm chlorine from raw titanium powder showed very little mass gain after oxidation from 1173 to 1398 K in air. The chlorine effect was found obviously in composition of α 2 + γ and γ regions in the TiAl phase diagram. The chlorine effect was found even in a plasma arcmelted TiAlMn alloy with chloride covering or oxidized in air containing a small amount of chlorine gas. Also, other halogen elements as fluorine and bromine had the same effect as chlorine. It was clear, in particular by surface analyses that chlorine existed in TiO 2 near the oxide/metal interface in the early stage of oxidation and decreased oxygen ion vacancies in TiO 2 . Then TiO 2 growth was interrupted and protective Al 2 O 3 scale was formed on the titanium aluminide.

Effect of V addition on microstructure and mechanical property in the Nb–Si alloy system

Abstract Microstructure and mechanical property of vanadium-added two-phase alloys consisting of α-Nb 5 Si 3 phase and Nb solid solution (Nb ss ) were investigated in terms of a variation of chemical composition and compression test at room temperature and at 1773 K. The equilibrium phase field of α-Nb 5 Si 3 and Nb ss in the Nb–Si–V ternary alloy system was examined at 1773 K using scanning electron microscopy equipped with wavelength dispersive X-ray spectroscopy and X-ray diffraction. The α-Nb 5 Si 3 phase formed in the Nb–Si–V system was extended along the Nb 5 Si 3 –V 5 Si 3 pseudo-binary line without a solubility range of Si at 1773 K, while vanadium is soluble continuously along the Nb–V phase boundary with Si solubility of about 0.7 at%. It is observed that volume fraction ratio of the present alloy is predominantly dependent on Si content but exhibited no dependence on vanadium content. With increasing vanadium content, primary α-Nb 5 Si 3 and Nb ss phases became finer through the whole composition studied. The 0.2% offset yield strength at room temperature increased with increasing volume fraction of α-Nb 5 Si 3 phase and with increasing vanadium content in Nb ss . In contrast, the 0.2% offset yield strength at 1773 K decreased with increasing vanadium content in Nb ss . Details will be discussed in correlation with microstructure, phase relation and mechanical property.

Fabrication of unidirectional porous TiAl–Mn intermetallic compounds by reactive sintering using extruded powder mixtures

Abstract The unidirectional porous Ti–45at.%Al–1.6at.%Mn intermetallic compounds having fully lamellar structure were fabricated by reactive sintering method using the extruded mixtures consisting of elemental titanium and aluminum-manganese powders. The average porosity of the specimens is measured to have a distribution in the range from 25% to 35% in volume fraction within the experimental conditions investigated. The porosity of the specimen produced decreased with increasing heating rate in the reactive sintering process. The maximum porosity of 35% can be obtained when the specimen was reactively sintered under the heating rate of 0.17 K/s. The porosity of the reactively sintered specimen can be controlled by a variation of heating rate. From the microstructural observation, we found a directionality of pores oriented parallel to the extrusion direction and suggested that an aligned-structure of pores of the present alloy is closely associated with the extrusion microstructure consisting of elongated feature of constituent mixtures.

The effect of the addition of in on the reaction and mechanical properties of Sn−1.0Ag−0.5Cu solder alloy

The effects of the reduction of the Ag content and the addition of In on the mechanical properties, soldering characteristics and reaction behavior at solder/package interfaces are investigated in this study. It was found that the addition of In significantly improved the wettability of the solder at reflow temperatures ranging from 230°C to 240°C. Moreover, the addition also improves the elongation of the solder alloy, thereby increasing the toughness of the alloy. With the optimization of the In and Ag contents, a Sn−Ag−Cu−In quaternary alloy would be a strong candidate to replace the Sn-3.0Ag-0.5Cu composition.

High temperature strength and room temperature fracture toughness of Nb Mo-W refractory alloys with and without carbide dispersoids

Abstract Room temperature fracture toughness and high temperature strength at 1773 K of Nb–Mo–W based alloys with and without carbides were investigated by three-point bending and compression tests. W addition to Nb–5Mo solid solution gives rise to an increase in yield stress at 1773 K but to a decrease in fracture toughness at room temperature. On the basis of the experimental results obtained, it is suggested that solid solution hardening and cleavage fracture propensity are responsible for the increase of yield stress at 1773 K and for the decrease of fracture toughness at room temperature, respectively. The microstructure of Nb–5Mo–15W–x(Hf+C) alloys consists of Nb-rich bcc solid solution and (Nb,Hf,Mo)C carbide with B1-structure. Yield stress at 1773 K and fracture toughness at room temperature increase concurrently with increasing content of (Hf+C) in the Nb–5Mo–15W–x(Hf+C) alloys. The increase of yield stress at 1773 K due to the presence of (Nb,Hf,Mo)C phase is explained by the mechanism based on dispersion hardening, and the increase of fracture toughness at room temperature is attributable to the transition of fracture mode from cleavage to quasi-cleavage accompanied by decohesion between carbide and matrix phase, crack branching and deflection. Details will be discussed in relation to microstructural characteristics.

High strain rate superplasticity in powder metallurgy processed Al-16Si-5Fe alloy

In the present paper, new additional experimental results for these aluminum alloys with a very fine grain structure are demonstrated over a strain range of 10{sup {minus}4} {approximately} 10{sup {minus}1} s{sup {minus}1} in the temperature range of 673 {approximately} 793 K, to analyze the possible mechanisms of superplasticity at high strain rates in very fine grained aluminum alloys produced by powder metallurgy method.

Improved reliability of Sn-Ag-Cu-In solder alloy by the addition of minor elements

Among the various Pb-free solder alloys, Sn-3.0Ag-0.5Cu has been an industrial standard in consumer electronics due to its moderate wetting behavior and reliability in thermal fatigue. Recently, however, its high material cost and low reliability in drop impact condition resulted in the use of Sn-0.3Ag-0.7Cu and the development of Sn-1.2Ag-0.7Cu-0.4In solder alloys. Authors have reported that the Sn-1.2Ag-0.7Cu-0.4In showed as good wettability as Sn-3.0Ag-0.5Cu and load drop reliability as Sn-1.0Ag-0.5Cu. It was believed that the small addition of In could make up for the large reduction of Ag with the material cost benefit of about 20%. It was also noteworthy that the load drop reliability of Sn-1.2Ag-0.7Cu-0.4In could be improved beyond Sn-1.0Ag-0.5Cu by the small addition of some elements. In this study, effects of the minor elements, on the reaction, mechanical properties and reliability were investigated.

Distribution behavior of vanadium and phosphorus between slag and molten steel

The correlation of the equilibrium behaviors of phosphorus and vanadium between slag and low carbon molten steel in inert atmosphere was investigated with respect to the experimental variables of slag basicity, the (P2O5) and (V2O5) content, and the reaction temperature. The distribution ratios of phosphorus and vanadium increased with an increase in the slag basicity. The logarithms of the vanadium distribution ratio were greater by a factor of about two than those of phosphorus in the range of low slag basicity, but the difference diminished with an increase in the slag basicity. The logarithms of the vanadium distribution ratio increased linearly with an increase in the (P2O5) and (V2O5) content in the slag, while those of phosphorus remained nearly constant. The logarithms of the phosphorus and vanadium distribution ratio decreased with an increase in temperature, and the dependence on temperature was greater for the phosphorus than for the vanadium. For both the maximization of the vanadium yield and the minimization of the rephosphorization of molten steel in the steelmaking process, the ratio of N(V2O5)/N(P2O5), the slag basicity, the ratio of f[P]/f[V], and the temperature should be maximized, and the (FeO) content in the slag should be minimized.

Effects of Cold Rolling Parameters on Sagging Behavior for Three Layer Al-Si/Al-Mn(Zn)/Al-Si Brazing Sheets

The effects of intermediate annealing (IA) and the final cold rolling (CR) condition on the microstructure and sagging resistance during brazing were investigated using three layer clad sheets composed of the Al−7.5 wt.%Si alloy (filler, thickness: 10 μm)/Al−1.3 wt.%Mn based alloy (core, 80 μm)/Al−7.5 wt.%Si alloy (filler, 10 μm). Also, the effect of 1.2≈2 wt.% Zn addition into the core on the sagging resistance of the clad sheets was determined. It was revealed that all the clad sheets fabricated by the optimum condition (IA at 690 K and CR to 20≈45%) show excellent sagging resistance with a limited erosion due to the formation of a coarsely recrystallized grain structure in the core during brazing. It was also revealed that the recrystallization behavior of the Al−1.3 wt.%Mn based alloy is hardly affected by the addition of 1.2≈2 wt.%Zn during the brazing cycle. Therefore, the sagging resistance of the clad sheets is found to be governed not by the Zn content added in the Al−1.3wt.%Mn based core, but by the intermediate annealing and final cold rolling condition.

Microstructure control and ductility in Ni3Al polycrystals

Abstract Equiaxed Ni 3 Al polycrystals at different impurity levels were prepared via isothermal forging and ductility at ambient temperature was measured in oxygen as a function of alloy composition and impurity content. Microstructures of single phase Ni 3 Al with stoichiometric and off-stoichiometric compositions were controlled by dynamic recrystallization occurring during isothermal forging. Dynamically recrystallized grain size increases with increasing deformation temperature and with decreasing strain rate. Grain boundary sliding takes place between dynamically recrystallized grains with a small grain size. Binary Ni 3 Al polycrystals exhibit appreciable ductilities even at the stoichiometric composition. Ductility is further increased by a deviation from the stoichiometric composition to the Ni-rich compositions. Ductilities of the Ni-rich Ni 3 Al polycrystals are increased by zone-refining starting material in UHV. It is concluded that polycrystalline Ni 3 Al is intrinsically ductile and its ductility is increased at Ni-rich compositions.