발간논문

Home > KJMM 논문 > 발간논문

Behavior of recrystallization of 1.8wt.% thoriated tungsten was studied compared with pure tungsten and 0.3wt.% NaCl-1.8wt.% ThO₂-tungsten. In the hardness test the pure and the alloyed tungstens showed almost the same start recrystallization temperature of 1400℃, but the hardness and recrystallization temperature of the thoriated tungstens were higher than those of the pure tungsten. The hardness of the alloyed tungsten decreased slowly from 470 to below 350 VHN with increasing temperature. This result of hardness tests was consistent with that of the microstructure observation : At 1400 ℃ the pure tungsten was already recrystallized about 40% in the specimen, but the alloyed was not recrystallized. Moreover the grain growth of the alloyed was inhibited in spite of temperature increase up to 2520℃.

keyword :

Cu-Cr alloys have been used as precipitation hardened alloys, due to low solubility of Cr in Cu. The strength of copper alloys is known to decrease as precipitates and grain size get coarser as service temperature is increased. Meanwhile, using MA process, higher strengths could be obtained through uniform distribution of fine Cr-particles. In this study, Cu-Cr powder mixtures were mechanically alloyed in order to develop high strength Cu alloys. The properties of extruded bars were investigated using various analytical techniques. The ratio of pre-alloyed powders to steel balls was 1 : 20 and the mechanical alloying was performed for 10hours in Ar atmosphere. TEM observations showed that the Cr-particles were distributed in Cu-matrix, and grain size of the extruded bars was about 100㎚. The tensile strength of extruded bars was found to increase with the volume fraction of Cr and its maximum value was 500㎫ in Cu-4vol%Cr-alloy. The tensile strength was slightly decreased after annealing at 800℃ far 1hours, but greatly decreased after annealing at 900℃ for 1hour. As the result of this study, the strength of Cu-Cr alloys was dependent on the volume fraction of Cr and grain size.

keyword :

He ion bombardment behaviors of Mo-0.5V alloy scavenged by addition of 0.2wt% Sc was investigated by hardness measurement and transmission electron microscopy. The ion bombardment was carried out at two temperatures, 373K with 50MeV He ions and 1073K with 10MeV He ions. He ion bombardment-induced hardening was increased with increasing fluence in the range of 5.0×10^(20) to 1.8×10^(21) He^(++)/㎡ in the both of hardness plateau region and hardness peak region. No cavity was observed in the specimen bombarded at 1073K to a fluence of 1.2×10^(21) He^+/㎡, however a number of cavities were observed in the specimen post-bombardment annealed at 1473K for 1 hour.)

keyword :

Microstructural changes in STB2 bearing steel caused by rolling contact fatigue test, were investigated using optical and scanning electron microscopy. Dark area, butterfly structures and plate-like carbides were found in the area below the flaking. Rolling contact caused the formation of Dark area in the subsurface area of contact in the region of maximun Hertzian stress. Most subsurface cracks were initiated at the region of Dark area. Butterflies found only at the oxide inclusions, were frequently associated with cracks. Plate-like carbides acted as the potential crack initiation site responsible for the flaking. The growth of plate-like carbides was caused by the carbon diffusion by rolling stress between not only oxide inclusions but also fine residual carbides.

keyword :

The main objective of this study is to establish fatigue crack initiation and growth behavior of a powder metallurgy 2124 aluminum alloy-SiC whisker composite. Cyclic compressive loading was adopted for the fatigue experiments using compact tension type specimens. The effect of compressive overload in terms of ΔK variation was also examined. Fatigue crack initiation was found to occur easily for a given ΔK, but its crack length was saturated quickly. Following an overload, the saturation crack length increased again with increasing ΔK. The results provide a basis for the development of rational method to control fatigue crack length following a compressive overload.

keyword :

The objective of the present study is to investigate the adiabatic shear band of 2124-T6 Al-SiCw composites. The composites were highly deformed at high strain rate by ballistic impact and the adiabatic shear bands revealed white-etching bands with evidence of highly deformed flow on the edges of the cracks near the impacted region. Reaction products, microvoids and whisker rearrangement were observed within the shear band presumably due to the local temperature rise occurring during dynamic deformation. The bands had trends to change their propagation path along the extrusion direction because SiC whiskers effectively prevented the formation of adiabatic shear band by increasing the strength level and by blocking the propagation of the shear band. Ballistic performance of the Al-SiCw composites was also analyzed by comparing the shear band behavior with dynamic fracture toughness.

keyword :

Collected dust from LT system of converter was chosen as desiliconization agent and various tests were performed to find the possibility of the reuse of dust and the optimum conditions for the removal of silicon from pig iron. Also CaO was added to LD dust for the improvement of desiliconization efficiency and for the prevention of slag-foaming respectively. From this experiment, it was concluded that LD dust as dry state could be available for desiliconization and it showed many advantages during the reaction. CaO in the dust prevented slag foaming, the Mn-oxidation and phosphorization of pig iron. The optimum basicity of LD dust for desiliconization was 1.2-1.4. The only disadvantage of dust reuse was the existence of Zn and Pb. Pb was volatilized during the experiment and Zn remained as Zn-ferrite in the slag.

keyword :

TiAl intermetallic compound has the potential for high-temperature structural applications because of its positive temperature dependence of strength. In this study, the temperature dependence of cast TiAl intermetallic compound was investigated. The specimens of TiAl intermetallic compound were tasted by vaccuum induction melting system using calcia crucible. Compression test was carried out from room temperature to 1000℃. The positive temperature dependence of strength was not observed in the cast TiAl intermetallic compound. The ductile-brittle transition was occured at about 700℃. Strength of Ti-48at%Al alloy at room temperature was higher than that of Ti-50at%Al alloy, but the difference of strength between the two alloys was decreased with the increase of testing temperatures. The deformation structure at 800℃ consists of twins and cell structure of dislocation by cross-slip.

keyword :

In order to investigate the microstructures of Al-(0.8, 1.3, 3)wt%Fe alloys, solidification processes were performed with water-cooled mold, air-cooled mold and in furnace. The intermetallic particles of these alloys were analyzed by X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy. The eutectic structure between Al and Al_6Fe particles was dominant and a few Al₃Fe particles were observed as cooling rate increased. With the increasing Fe contents and cooling rate, the formation of microcellular structures of Al grains largely containing globular or rod-liked Al_6Fe intermetallic precipitates were predominant. The crystallographic orientation relationships between Al matrix and particles were identified as [2(33)￣]_(Al)//[(11)￣0]_(Al₃Fe), ((311)￣)_(Al)//(2(22)￣)_(Al_6Fe), (011￣)_(Al)//(001)_(Al_bFe) and [1￣12]_(Al)//[010]_(Al₃Fe). The Al-Fe alloy exibited goad thermal stability and appreciable strength up to 450℃. The results indicate that high solidification rate is necessary to obtain high temperature strength Al-Fe alloy.

keyword :

The oxidation kinetics and morphological changes of oxides in intermetallic compound TiAl containing 37wt% Al have been investigated in air over the temperature range of 973K to 1473K. The phases of oxidation products have been examined by X-ray diffractometer and scanning electron microscope. They are composed of four layers, that is, TiO₂, Al₂O₃, TiO₂+Al₂O₃, Ti₃Al from the outer layer. The mass gain during the oxidation increases rapidly at temperatures above 1173K, obeying the parabolic law. The oxidation resistance is related to the characteristics of a protective oxide layer(Al₂O₃, or TiO ₂). The existance of the Al₂O₃ layer has improved oxidation resistance of TiAl. However, this Al₂O₃ layer is not dense enough to prevent the oxidation above 1173K. The growth rate of TiO₂ is faster than that of Al₂O₃ at high temperature. Oxidation rate of TiAl is controlled by titania-forming kinetics above 1273K but controlled by alumina-forming kinetics below 1173K.

keyword :