Yong Bum Choi

Tensile, Compressive and In-Plane/Inter-Laminar Shear Failure Behavior of CVI- and NITE-SiC/SiC Composites

A SiC/SiC composite is an attractive candidate material but it is a challenge to apply it to the practical components because of the inherent brittle-like failure and structural anisotropy. This study aims to evaluate the failure behavior of SiC/SiC composites by various test modes. Comparison between tensile and compressive strengths revealed the clear axial anisotropy of failure strength. The in-plane shear strength by the off-axial tensile method is invalid unless considering the mixed failure modes. Alternatively, it was demonstrated that the in-plane shear strength can successfully be obtained by the Iosipescu method. The true inter-laminar shear strength can be identified by the diametral compression method.

Low Pressure Casting Process of FeCrSi/A366.0 Alloy Composites and Their Characterization

Low pressure casting process was considered for fabrication of FeCrSi metal fiber reinforced A366.0 aluminum composites. FeCrSi/A366.0 alloy composite was fabricated by applied pressure 0.8MPa. The microstructure features, tensile strength and fatigue life of composites were investigated from room temperature to high temperature. It was confirmed that the FeCrSi metal fiber did indeed have a strengthening effect on the composite, lending it good mechanical properties and a good fatigue life at high temperatures.

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO2) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr2O3 film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.

Fabrication and characterization of unidirectional CF/Al composites

Abstract The unidirectional carbon fiber (CF) reinforced aluminum (Al) composites have been fabricated by the low pressure infiltration (LPI) of molten Al into porous CF preform. Prior to the fabrication of the unidirectional CF/Al composites, the unidirectional CF preform was prepared by sintering of CFs and copper (Cu) particles under the spark plasma sintering (SPS). The compression strength of CF preform was examined to determine the infiltration pressure of molten Al into CF preform. The effects of the different infiltration pressures and sizes of Cu particles on the densification of unidirectional CF/Al composites have also been investigated. The compression strength of CF preform increased with increasing of the contact area between CFs and Cu particles. The density of CF/Al composites improved with the increase of the infiltration pressure. The CF/Al composites, into which the bimodal Cu particles are added, have average particle sizes of 2.55 μm and 11.79 μm, with a high relative density of about 95% with the nearly homogeneous fiber distribution.

Influence of the specific surface area of a porous nickel to the intermetallic compound generated by reaction of a porous nickel and aluminum

A new process is proposed to fabricate an intermetallic compound reinforced aluminum alloy matrix composite by using the reaction between porous nickel and molten aluminum alloy. The intermetallic compound reinforced aluminum alloy composite was manufactured with the low-pressure infiltration process method. The amounts of the intermetallic compounds under 3 different specific surface area of porous nickel and the porosity inside composites were investigated. Porous nickel reacted with molten aluminum alloy at 973 K, and the intermetallic compound of Al3Ni was generated on the surface of the porous nickel. The generated intermetallic compound Al3Ni, was delaminated according to the difference of thermal expiation coefficient with nickel. And the intermetallic compounds moves in the direction of aluminum matrix. The area fraction of the intermetallic compounds increased with the increasing specific surface area of porous nickel. In addition, the defect inside the composite decreased by the increasing specific surface area of porous nickel.

Manufacturing process of dispersed intermetallic compounds Al alloy composites by using porous nickel

A new process is proposed to fabricate an intermetallic compound-reinforced aluminum alloy matrix composite using the reaction between porous nickel and molten aluminum alloy. The intermetallic compound-reinforced aluminum alloy composite was manufactured with the infiltration process method. Porous nickel reacted with the molten aluminum alloy at 973 K, and the intermetallic compound of Al3Ni was generated on the surface of the porous nickel. The generated intermetallic compound Al3Ni is delaminated due to different thermal expansion coefficient with nickel and moves in the direction of aluminum matrix. The effects of processing variables such as specific surface area of porous nickel, holding time after infiltrated molten aluminum alloy, and cooling rate on the formation and dispersion behavior of Al3Ni were investigated.

Fabrication of the Aluminum Matrix Composite by Ultrasonic Infiltration Technique

In order to infiltrate the molten aluminum alloy to the reinforcement preform by low pressure in casting process and acquire the high performance composites with high density, the effect of the ultrasonic vibration on the infiltration was investigated by model experiments using clear solution and glass or aluminum borate preform, which is correspond to a molten matrix and reinforcement, respectively. Ultrasonic vibration improves the wettability of liquid polyester resin on glass plate, dramatically. The final infiltration height and infiltration speed of liquid polyester resin in glass capillary were improved by the ultrasonic vibration. Furthermore, the infiltration speed of water to aluminum borate preform accelerated by ultrasonic vibration. This effect was more remarkable, when the infiltration height is lower or infiltration time is shorter. In actual, the molten aluminum alloy infiltrate to SiC preform using ultrasonic vibration easily and acquire the high dense composites without pores.

Effect of SiO2 amount on microstructures and tensile properties of alumina short fiber-reinforced composites by low-pressure infiltration method:

The tensile properties of two types of alumina (Al2O3 and Al2O3–SiO2) and short fiber-reinforced A366 alloy composite created by low-pressure infiltration have been studied. The applied pressure and temperature of molten alloy are 0.4 MPa and 1073 K, respectively. The composite containing 10 vol.% of fiber preform was sectioned and the microstructure was observed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Tensile properties of Al2O3 fiber-reinforced A366 alloy composites were investigated, in conjunction with investigation of effects of amounts of SiO2 sol added as binder to fabricated preform and effects of changed chemical composition of Al2O3 fiber. The composite with SiO2 sol of 8 mass% has higher relative density in comparison with composite with SiO2 sol of 2 mass%. SiO2 sol raised the relative density of composite by the reaction with aluminum. In addition, the ultimate tensile strength of composite which has Al2O3 fiber-reinforced A366 alloy composite containing SiO2 sol of 8 mass% was approximately 20% higher than that of Al2O3–SiO2 fiber-reinforced A366 alloy composite containing SiO2 sol of 8 mass%.

Fabrication of Vapor Grown Carbon Fiber Reinforced Aluminum Composites by Spark Sintering

As vapor grown carbon fiber (VGCF) possesses the good mechanical properties, high thermal conductivity, high electrical conductivity and low thermal expansion, VGCF/ Al composites are expected to be suitable materials for a high performance radiator. In this study, VGCF were dipped and treated with ultrasonic vibration in some kinds of solution at first. Then, the mixed powders including three kind of average particle size of Al (1, 3 and 30µm) was milled by wet process with three kinds of solution, which are acetone, ethanol and butanol. Ethanol is most suitable for mixing solution because of homogeneous distribution of VGCF and Al powders. The mixed powders were spark-sintered in order to obtain dense VGCF/Al composites. The densification mechanism of VGCF/Al composites was divided into the plastic deformation (2nd stage) and creep deformation (3rd stage) after the 1st stage of rectangular wave pulse discharge. The densification rate of VGCF/Al composite powder depended on Al powder for matrix, but independent on VGCF. VGCF are dispersed uniformly in the VGCF/1 µm Al composite. But the aggregations of VGCF exhibited a preferential orientation in VGCF/30 µm Al composite, which results from the deformation of Al powders under the uniaxial pressure during the hot pressing in sintering process.

Fabrication of Carbon Nano-Fiber / Aluminum Composites by Low-Pressure Infiltration Method

In this study, the fabrication of carbon containing aluminum composites was attempted by using low-pressure infiltration method. At first, porous preform containing vapor grown nano-fiber (VGCF) and pure aluminum powder was fabricated by spark plasma sintering (SPS) method. Porosity in preform was controlled by changing the applied pressure during plasma sintering. Consequently, the porous preform with 40-50vol％ in porosity was obtained, which has enough compression strength for low-pressure infiltration (<1MPa). Then, the molten pure aluminum infiltrated to porous preform with 0.4MPa in applied pressure at 1023K, and consequently we can obtain the composite with 62-86% in density. The electrical and thermal conductivity of composites was affected by the porosity, strongly.