Papers by Author: Sung Hak Lee

Abstract: The fracture behavior of Mg-5Al-1Zn and Mg-5Al-1Zn-3Sn alloy was investigated by direct observation of microfracture process using an in-situ loading stage installed inside a scanning electron microscope chamber. Crack was initiated at the interface of Mg/second-phase particles or second-phase particles. Fracture of the alloys was predominantly dimple or/and quasi-cleavage failure. The improvement of what could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting and shear band branching.

Abstract: Powder injection molding (PIM) process was applied to Fe-based metamorphic alloy powders, and microstructure, hardness, and wear resistance of the PIM products were analyzed and compared with those of conventional PIM stainless steel products. When Fe-based metamorphic powders were injection-molded and then sintered at 1200 oC, completely densified products with almost no pores were obtained. They contained 34 vol.% of (Cr,Fe)2B borides dispersed in the austenitic matrix without amorphous phases. Since these (Cr,Fe)2B borides were very hard and thermally stable, hardness, and wear resistance of the PIM products of Fe-based metamorphic powders were twice as high as those of conventional PIM stainless steel products. Such property improvement suggested new applicability of the PIM products of Fe-based metamorphic powders to structures and parts requiring excellent mechanical properties.

Abstract: Sticking phenomenon occurring during hot rolling of ferritic stainless steels, STS 430J1L and STS 436L, was investigated in this study. The simulation test results at 900 oC and 1000 oC revealed that STS 430J1L had a smaller number of sticking nucleation sites than the STS 436L. When the test temperature was 1070 oC, the sticking hardly occurred in both stainless steels as Fe- Cr oxide layers were formed on the surface of the rolled materials. These findings suggested that the improvement of high-temperature properties of stainless steels and the appropriate rolling conditions for readily forming oxide layers on the rolled material surface were required in order to prevent or minimize the sticking.

Abstract: Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

Abstract: Wear resistance of Al2O3-8wt.%TiO2 coatings plasma-sprayed using nanopowders was investigated. Four types of nanostructured Al2O3-8wt.%TiO2 powders were plasma-sprayed on a low-carbon steel substrate by using different critical plasma spray parameters (CPSP). The coatings consisted of completely melted and partially melted regions. The hardness of the coatings increased with increasing CPSP, while the wear resistance was the highest for the coating sprayed with the lowest CPSP. The main wear mechanism was a delamination mode in the coating sprayed with the high CPSP, but was changed to an abrasive mode in the coating sprayed with the low CPSP. According to this change in the wear mechanism, the wear resistance was the best in the coating sprayed with lowest CPSP, while its hardness was lowest.

Abstract: The present study is concerned with the improvement of hardness and wear resistance in (Cr3C2,CrB)/carbon steel surface composites fabricated by high-energy electron beam irradiation. Two kinds of powder mixtures, 50Cr3C2-50STS304 and 50CrB-50STS304 (wt.%), were placed on a plain carbon steel substrate, which was then irradiated with electron beam. The surface composite layer of 1.0~1.3 mm in thickness was successfully formed without defects, and contained a large amount (up to 58 vol.%) of Cr7C3 or Cr1.65Fe0.35B0.9 particles in the austenite or martensite matrix. The hardness and wear resistance of the surface composites were 2~3 times higher than those of the steel substrate according to hard particles. Particularly, the surface composite fabricated with CrB powders showed excellent wear resistance because selective wear of the matrix was considerably reduced.

Abstract: The present study aims at investigating the effects of microstructure on fracture toughness of two A356 Al alloys. These A356 alloys were fabricated by casting processes such as rheo-casting and casting-forging, and their mechanical properties and fracture toughness were analyzed in relation with microfracture mechanisms. All the cast A356 alloys contained eutectic Si particles mainly segregated along solidification cells, and the distribution of Si particles was modified by the casting-forging process. Microfracture observation results revealed that eutectic Si particles segregated along cells were cracked first, but that Al matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the cast-forged alloys having homogeneous distribution of eutectic Si particles were superior to those of the rheo-cast alloy.

Abstract: Dynamic deformation and fracture behavior of Zr-based bulk metallic glass (BMG) and BMG composite containing dendritic β phases was investigated in this study. Dynamic compressive test results indicated that both maximum compressive stress and total strain of the BMG and BMG composite decreased with increasing test temperature because shear bands could propagate rapidly as the adiabatic heating effect was added at high temperatures. Above the glass transition temperature, total strain decreased more abruptly due to crystallization of amorphous phases. Maximum compressive stress and total strain of the BMG composite were higher than those of the BMG because β phases played a role in forming multiple shear bands. The BMG composite having more excellent dynamic properties than the BMG can be more reliably applied to the structures or parts requiring dynamic properties.

Abstract: Dynamic compressive tests were conducted on Zr-based amorphous alloys and amorphous matrix composite containing dendritic β phases. Dynamic compressive tests were conducted using a compressive Kolsky bar and then the test data were analyzed in relation to microstructure and fracture mode. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloy and composite were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. Deformation under dynamic loading lowered strain and compressive strength because of reduced fracture resistance, and the alloy containing dendritic β phases showed better compressive strength and ductility than the monolithic alloy.

Abstract: This study aims at correlating microstructure with hardness and corrosion resistance of surface alloyed materials fabricated with Fe-based metamorphic powders by an accelerated electron beam irradiation method. The surface alloyed materials contained 48 vol.% of hard Cr2B crystalline phases in the Cr0.19Fe0.7Ni0.11 matrix, and thus its hardness was 2.5 times greater than that of the steel substrate. The corrosion resistance of the surface alloyed materials was better than that of an STS304 stainless steel or coatings fabricated by high-velocity oxygen fuel spraying of Fe-based metamorphic powders because the Cr0.19Fe0.7Ni0.11 matrix of the surface alloyed layers and coating was selectively corroded, while Cr2B borides were retained inside pits. These findings suggested that the fabricated surface alloyed materials presented good application possibilities as excellent wear- and corrosion-resistant materials.