Hoon Kwon

Reaction products of Al–Mg/B4C composite fabricated by pressureless infiltration technique

Abstract The interfacial reaction products of the Al/B 4 C p composite fabricated by the pressureless infiltration method were analyzed by XRD, SEM, EDS, AES, and TEM. Since the spontaneous infiltration of molten Al–Mg alloys into the powder bed containing B 4 C particles occurred at 800°C for 1 h under a nitrogen atmosphere, it was possible to fabricate Al–Mg alloy matrix composites reinforced with B 4 C particles. During fabrication of composites various reaction products were formed in the Al matrix as well as on the surface of B 4 C particles by the severe interfacial reaction between the Al alloy and the B 4 C particles. From the SADP and CBED analysis, it could identify that AlB 2 , β-AlB 12 (Al 3 B 48 C 2 ), AlB 10 (AlB 24 C 4 ), and Al 3 BC were formed as interfacial reaction products.

Long-term clinical study and multiscale analysis of in vivo biodegradation mechanism of Mg alloy

Significance In the past decade, countless studies have been performed to control the mechanical and corrosion property of magnesium-based alloy, which degrades in the physiological environment, to overcome the flaws of the inert implant materials and shift the paradigm of conventional bone fixation devices. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study. There has been a tremendous amount of research in the past decade to optimize the mechanical properties and degradation behavior of the biodegradable Mg alloy for orthopedic implant. Despite the feasibility of degrading implant, the lack of fundamental understanding about biocompatibility and underlying bone formation mechanism is currently limiting the use in clinical applications. Herein, we report the result of long-term clinical study and systematic investigation of bone formation mechanism of the biodegradable Mg-5wt%Ca-1wt%Zn alloy implant through simultaneous observation of changes in element composition and crystallinity within degrading interface at hierarchical levels. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study.

Diffusion barrier and electrical characteristics of a self-aligned MgO layer obtained from a Cu(Mg) alloy film

Diffusion barrier characteristics and electrical properties of self-aligned MgO layers obtained from a Cu(Mg) alloy film have been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing a Cu(Mg) film in an oxygen ambient and prevented interdiffusion of Cu in SiO2 up to 700 °C. The thermal stability of a pure Cu/TiN/Si multilayer system has been significantly enhanced up to 800 °C by the MgO layers by forming a MgO/Cu/MgO/TiN/Si multilayer system. A combined structure of Si3N4(500 A)/MgO(100 A) increased the breakdown voltage up to 20 V from 15 V and reduced the leakage current density down to 3×10−9 A/cm2 from 1×10−8 A/cm2 compared to a pure copper system. Consequently, the deposition of Cu(Mg) alloy followed by annealing in an oxygen ambient gives rise to the formation of a self-aligned MgO layer with excellent diffusion barrier and electrical characteristics and the film can be applied as a gate electrode in thin-film transistor/liquid-crystal displays, resulting in a r...

Factors Affecting Passivation of Cu(Mg) Alloy Films

Variables affecting the passivation capability of Cu(Mg) alloy films, which were sputter deposited from a Cu (4.5 atom %) target, have been investigated. As-deposited Cu(Mg)/SiO 2 /Si multilayer samples were annealed for 30 min in various oxygen ambients at pressures ranging from 10 mTorr to 30 Torr and at various temperatures in the 200-800°C range. The results show that the passivation capability of a Cu(Mg) alloy film is a function of annealing temperature, O 2 pressure, and Mg content in the film. Increasing the annealing temperature favors formation of a dense MgO layer on the surface. Decreasing the O 2 pressure enhances the preferential oxidation of Mg over Cu. Furthermore, increasing the Mg content in the Cu(Mg) film promotes formation of a dense MgO layer. Vacuum preannealing before taking the as-deposited samples to O 2 annealings was found to be very effective in segregating Mg to the surface, facilitating the passivation capability of the Cu(Mg) alloy film even when the Mg content is low. In the current study, self-aligned MgO layers with low resistivity and an effective passivation capability over the Cu surface have been obtained by manipulating these factors when Cu(Mg) thin films are annealed.

Interfacial reactions in SiCp/Al composite fabricated by pressureless infiltration

Metal matrix composites (MMCs) reinforced by ceramic phases have been fabricated by various techniques including powder metallurgy, casting, etc. Recently Lanxide corporation developed the DIMOX and PRIMEX processes for fabricating ceramic- and metal-matrix composites, respectively. The PRIMEX process is an innovative technique for fabricating MMCs by the spontaneous infiltration of molten AL alloy containing Mg into a ceramic filler or preform under nitrogen atmosphere in pressureless state without the aid of vacuum or externally applied pressure. Although there were many patents on MMC fabrication by the above pressureless infiltration technique, however, few works on the resulting microstructures and their effects on mechanical properties were reported. Thus, in this study, the effects on interfacial reactions occurring during the fabrication of SiC{sub p}AL composite by the pressureless infiltration technique on microstructures and hardness have been investigated.

Tempered martensite embrittlement in Fe-Mo-C and Fe-W-C steel

A study on the phenomenon of tempered martensite embrittlement (TME) has been made in experimental Fe-Mo-C and Fe-W-C steel. Charpy impact testing was conducted to evaluate the impact toughness, sensitive to TME. Retained austenite was observed by an analytical transmission electron microscopy in both steels. Both steels represented TME. TME was correlated with the formation of the interlath cementite, resulting from the decomposition of interlath retained austenite. TME occurred in a limited range of test temperatures where the interlath cementite could act as a source of embrittling cracks. Therefore, both the interlath cementite resulting from the decomposition of the interlath retained austenite, and the level of matrix toughness, enabling the interlath cementite to act as an effective embrittler, are necessary to produce TME.

fracture behavior of intercritically treated complex structure in medium-carbon 6Ni steel

The fracture behavior of complex structure in 6Ni-0.3C steel in which the intercritical treatment in the range of 630 °C to 670 °C from the initial microstructure of coarse-grained martensite can control the amount and distribution of fibrous martensite has been investigated. The hardness increases, but the impact toughness decreases with increasing the temperature and time of the intercritical treatment. As the amount of martensite increases and the martensite coarsens with increasing the temperature and time, the fracture along the prior austenite grain boundaries becomes predominant and the fracture mode is changed from dimple type to low energy tear type. This fracture behavior is attributed to the increase in stress concentration susceptibility at the ferrite-coarser martensite interfaces along the prior austenite grain boundaries.

Effect of Mg content in Cu(Mg)/SiO2/Si multilayers on the resistivity after annealing in an oxygen ambient

The formation mechanism of self-aligned MgO layers obtained from Cu(Mg) alloys has been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing Cu(Mg)/SiO2/Si multilayer films in an oxygen ambient, resulting in a structure of MgO/Cu/MgO/SiO2/Si. Upon annealing at 300 °C or higher in an oxygen ambient, Mg segregates preferentially to the Cu surface until a dense, uniform MgO layer is formed. A growth limited thickness of the surface MgO was found to be about 150 A , providing substantial passivation of the exposed Cu surface. After a dense MgO layer forms, substantial Mg segregation to the SiO2 surface takes place. However, the formation of the interfacial MgO caused a sudden increase in resistivity after annealing for about 20 min, which can be due to the release of free Si being diffused into the Cu film by the reaction of Mg with Si in the SiO2. The optimum Mg contents in Cu(Mg) alloy films with various thickness were thus estimated to obtain the Cu(Mg) alloy multilayer film with substantially lower resistivity while retaining the beneficial properties of Cu passivation in an oxygen ambient.The formation mechanism of self-aligned MgO layers obtained from Cu(Mg) alloys has been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing Cu(Mg)/SiO2/Si multilayer films in an oxygen ambient, resulting in a structure of MgO/Cu/MgO/SiO2/Si. Upon annealing at 300 °C or higher in an oxygen ambient, Mg segregates preferentially to the Cu surface until a dense, uniform MgO layer is formed. A growth limited thickness of the surface MgO was found to be about 150 A , providing substantial passivation of the exposed Cu surface. After a dense MgO layer forms, substantial Mg segregation to the SiO2 surface takes place. However, the formation of the interfacial MgO caused a sudden increase in resistivity after annealing for about 20 min, which can be due to the release of free Si being diffused into the Cu film by the reaction of Mg with Si in the SiO2. The optimum Mg contents in Cu(Mg) alloy films with various thickness were thus estimated to obtain the Cu(Mg) alloy multilayer ...

Fracture behavior in medium-carbon martensitic Si- and Ni-steels

A study of fracture behavior in medium-carbon martensitic Ni- and Si-steels has been made. Charpy impact testing was conducted in order to investigate fracture mode as a function of test temperature. Whereas the transgranular cleavage fracture becomes the primary brittle fracture mode with decreasing test temperature in the Si-steel, intergranular fracture is the only brittle fracture mode observed at low temperatures in the Ni-steel. The different fracture behavior between these steels appears to be due to variation in intrinsic matrix toughness. Since Si may impair the intrinsic matrix toughness, the occurrence of transgranular cleavage fracture becomes relatively easy with decreasing test temperature. On the other hand, since Ni may improve considerably the intrinsic matrix toughness, the transgranular cleavage fracture is not able to occur although the test temperature decreases. Thus, the intrinsic matrix toughness can play a significant role in controlling the fracture behavior.

The effect of grain size on fracture behaviour in tempered martensite embrittlement for AISI 4340 steel

Abstract Tempered martensite embrittlement (TME) in AISI 4340 steel was studied for how variations in the test temperature and grain size affect the plastic flow. The grain size was changed by varying the austenitizing temperature in the range of 870–1200°C. For the evaluation of TME with test temperature, Charpy impact testing was performed in the range of −196-23°C. TME occurs because of an effective activation of intergranular brittle fracture in the 300°C tempered condition where grain boundary carbides are present, the ductile-brittle transition temperature (DBTT) increases with increasing grain size and the transition to brittle fracture is attributed to the occurrence of intergranular brittle fracture. This effect of grain size on the fracture behaviour indicates that the intergranular brittle fracture is controlled by the stress concentration susceptibility, i.e. the extent of dislocation pile-up at the grain boundaries, which increases with increasing grain size. In the 300°C tempered condition (in the presence of grain boundary carbides), the DBTT is higher by 70–150°C, compared with the 200°C tempered condition (nearly devoid of grain boundary carbides) where the transition to brittle fracture results from transgranular brittle fracture. A critical test temperature below which intergranular TME can occur is reduced with decreasing grain size. Therefore, intergranular TME can be produced by the occurrence of intergranular brittle fracture in the presence of grain boundary carbides, which can be more effectively activated as the stress concentration susceptibility increases with increasing grain size or with decreasing test temperature.