Sang-Pill Lee

Reaction sintering process of tyranno SA/SiC composites and their characterization

Abstract This paper deals with the efficiency of fiber preform preparation route for the fabrication of reaction sintering (RS) SiC f /SiC composites and their characterization, including density, microstructure and mechanical property. The applicability of carbon interfacial layer has been investigated in the RS process. The fiber preform was prepared by the consecutive slurry infiltration process, which associated with the combination of constant gas impregnation pressure and different magnitudes of cold pressure. The consecutive slurry infiltration process used for the preparation of fiber preform can be regarded as a promising technique for high density RS-SiC f /SiC composites, even if their mechanical properties depend on the magnitudes of cold pressure used. RS-SiC f /SiC composites entirely showed the morphology of near stoichiometric SiC phase in the intra-fiber bundle matrix, compared to that in the inter-fiber bundle matrix. The carbon interfacial layer was insufficient for the pseudo-ductile failure of RS-SiC f /SiC composites, even if some amount of fiber pull-out and interfacial delamination was observed in the tensile surface of bending test sample.

Effect of bonding interface on delamination behavior of drawn Cu/Al bar clad material

Cu/Al bar clad material was fabricated by a drawing process and a subsequent heat treatment. During these processes, intermetallic compounds have been formed at the interface of Cu/Al and have affected its bonding property. Microstructures of Cu/Al interfaces were observed by OM, SEM and EDX Analyser in order to investigate the bonding properties of the material. According to the microstructure a series of diffusion layers were observed at the interface and the thicknesses of diffusion layers have increased with aging time as a result of the diffusion bonding. The interfaces were composed of 3-ply diffusion layers and their compositions were changed with aging time at 400 °C. These compositional compounds were revealed to be η2, (θ+η2), (α+θ) intermetallic phases. It is evident from V-notch impact tests that the growth of the brittle diffusion layers with the increasing aging time directly influenced delamination distance between the Cu sleeve and the Al core. It is suggested that the proper holding time at 400 °C for aging as post heat treatment of a drawn Cu/Al bar clad material would be within 1 h.

Process, microstructure and flexural properties of reaction sintered Tyranno SA/SiC composites

Abstract The preparation routes of fiber preform for the fabrication of RS-SiC/SiC composites have been investigated, based on the mechanical property–microstructure correlations. Tyranno SA SiC fiber reinforced SiC composites have been fabricated by a reaction sintering process, which is associated with the consecutive slurry infiltration process of low pressure slurry impregnation with various filler particles and with the magnitudes of the cold pressures. The characterization of RS-Tyranno SA/SiC composites was evaluated by means of SEM, EDS and three point bending test. The consecutive slurry infiltration process of low impregnation pressure and cold pressure for the preparation of fiber preform provided the required density for RS-Tyranno SA/SiC composites (>2.9 mg/m3), even if there was a large amount of Si rich SiC phases in the matrix of the intra-fiber bundle. The flexural properties of RS-Tyranno SA/SiC composites depended on the magnitudes of cold pressure used for the preparation of the fiber preform.

Evaluation of intermediate phases formed on the bonding interface of hot pressed Cu/Al clad materials

The aim of the present study is to identify the properties of intermediate phases formed on the bonding interface of hot pressed Cu/Al clad materials by transmission electron microscopy and nano-indentation analyses. Cu/Al clad materials were fabricated by hot pressing under 200 MPa at 250 °C for 1 h and then heat treated at 400 °C for 1 h. Nano-indentation measurement was conducted to evaluate the nanohardness and modulus of the intermediate phases formed between the Cu/Al interfaces. A 3-tier diffusion layer was observed at the Cu/Al interfaces. Knoop microhardness values at the bonding interface were 7 to 11 times that of the Cu and Al matrix metals. The intermediate phases formed at the bonding interface were Al4Cu9, AlCu, and Al2Cu. A mapping analysis confirmed that the Al and Cu particles moved via mutual diffusion toward the intermediate phases formed at the bonding interface. The nanohardness values of η2-AlCu and γ1-Al4Cu9 were 4 to 7 times that of the Cu and Al matrix metals. Nanohardness and Knoop microhardness measurement curves exhibited similar tendencies. The rigidity values of the respective intermediate phases can be arranged in descending order as follows: γ1-Al4Cu9 > η2-AlCu > θ-Al2Cu.

A STUDY ON FATIGUE DAMAGE OF SHAPE MEMORY ALLOY COMPOSITE USING NDE TECHNIQUE

TiNi shape memory alloy was used to recover the shape of transformed objects using its shape memory effect. This shape memory effect plays an important role inside metal matrix composite. A composite using shape memory alloy has a large advantage that can control crack initiation and propagation, when compared with other composites due to the shape memory effect of shape memory alloy under high temperature. In this study, TiNi/Al6061 and TiNi/2024 shape memory composites were fabricated by the hot press method, and a fatigue test was performed to evaluate the fatigue damage for the shape memory composites under room temperature and high temperature. The relationship of the crack growth rate and the stress intensity factor for these shape memory composites were clarified at both temperature conditions. The delay effect of crack propagation due to shape memory alloy was also evaluated under high temperature. In addition, an acoustic emission technique was used to evaluate the crack initiation and the control of crack propagation by shape memory effect under fatigue test nondestructively. The relationship between AE parameter and the degree of fatigue damage of the shape memory composites was discussed.

Effect of highly pressurized hydrogen gas charging on the hydrogen embrittlement of API X70 steel

During the use of API X70 steel as a pipeline structural material for the transportation of natural gas, hydrogen embrittlement can occur due to the hydrogen contained in natural gas. The aim of this study is to investigate the effects of the hydrogen content under high-pressure hydrogen gas conditions on the hydrogen embrittlement of air-cooled API X70 steel. The air-cooled API X70 steel was manufactured by hot rolling and was then air-cooled to room temperature. Tensile test specimens were held for 0 h, 1000 h, and 2000 h within a pressure vessel filled with 100% hydrogen gas at a gas pressure of 10 MPa, with the tensile tests then performed at room temperature. The microstructure of the API X70 steel consists of coarse polygonal ferrite, coarse pearlite, and fine acicular ferrite. The yield and tensile strength increased and elongation decreased considerably after a holding time of 2000 h compared to those of 0 h and 1000 h within the pressure vessel. The morphology of the fracture surface changed from ductile to brittle upon hydrogen gas charging. Secondary cracks were observed in both of the hydrogen-gas-charged specimens. No external cracks were formed on the surface of the tensile-tested specimen with a 0 h holding time; however, many external cracks were observed on the specimen surface subjected to hydrogen gas charging.

HIGH TEMPERATURE EROSION BEHAVIOR OF C/SiC COMPOSITE MANUFACTURED BY MELT SILICON INFILTRATION PROCESS

The A raw C/C composite with a density of 1.40g/cm3 was used in order to investigate the high temperature erosion behavior and mechanical properties of liquid silicon infiltrated C/SiC composites. The microstructures of the as-received and infiltrated composites were examined with an optical (OM) and a scanning electron microscope (SEM). The volume fraction of residual silicon and porosity was measured by using an image analyzer. The flexural test for as-received as well as melt Si infiltrated C/C composites was performed to estimate the mechanical strength of the C/SiC composites. Severe fiber damage was occurred after infiltration. SiC was formed by reaction between carbon of fiber surface and molten Si. Flexural strength was decreased after melt Si infiltration and pull-out of fiber was observed at the fracture surface of as-received material.

Effect of hydro co-extrusion on microstructure of duo-cast Al 3003/Al 4004 clad materials

The effects of hydro co-extrusion on the microstructure changes of aluminum hybrid duo-cast Al 3003/Al 4004 clad materials were studied. The specimen of duo-cast Al 3003/Al 4004 clad materials was in circle shape, and was composed of Al 3003(outside) and Al 4004(inside) materials. The specimen was extruded by the hydro co-extrusion equipment. The manufacturing conditions of the specimen were 423 K in temperature and 5 in extrusion ratio. The dimensions of the specimen were 80 mm in diameter of the Al 4004 material and 35 mm in thickness of the Al 3003 material before the hydro co-extrusion process, and 30 mm in diameter and about 5 mm in thickness after the extrusion process, respectively. The microstructure and the hardness for two specimens were investigated. The hardness value of cross section in the duo-cast Al 3003/Al 4004 clad materials before the extrusion process was increased in form of the parabola toward the center. However, after the extrusion process, it was almost constant in the portion of Al 4004 material. Lots of big voids above 1 mm in diameter in the specimen existed in the interfacing region of Al 3003 and Al 4004 materials before the extrusion process. These big voids disappeared after the process of hydro co-extrusion.

Stability Analysis of Axially Moving Simply Supported Pipe Conveying Fluid

ABSTRACT The dynamic instability and natural frequency of an axially moving pipe conveying fluid are investigated. Thus, the effects of fluid velocity and moving speed on the stability of the system are studied. The governing equation of motion of the moving pipe conveying fluid is derived from the extended Hamiltons principle. The eigenvalues are insystem via the Galerkin vestigated for the pipe method under the simple support boundary. Numerical examples show the effects of the fluid veloc-ity and moving speed on the stability of system. Moreover, the lowest critical moving speeds for the simply supported ends have been presented. * 1. 서 론 내부에 유체를 가지는 파이프 구조물은 배관계 및 일반적인 기계장치에서 흔히 볼 수 있다. 파이프 내부에 흐르는 유체는 파이프 시스템의 동적특성 및 안정성에 큰 영향을 미치기 때문에 유체의 질량비와 유속에 의한 시스템의 안정성 해석에 관한 연구는 오랜 기간 많은 연구자들에 의하여 연구되어져 오고 있다 (1~3) . 또한, 축방향으로 움직이는 구조물은 기계 구동 벨트, 자기 테이프, 종이 등 여러 공학적, 산업적 분야에서 쉽게 접할 수 있다. 특히, 이송되는 보 구조물의 형태는 평판 톱의 블레이드나 밸브 액추에이터 등에서 그 예를 찾아 볼 수 있다. 이처럼 시스템의 불안정을 유발하는 주요 파라교신저자; 정회원, 동의대학교 기계공학과E-mail : isson92@deu.ac.krTel : (051)890-2239, Fax : (051)890-2232* 동의대학교 기계공학과** 한국폴리텍 VI대학 달성캠퍼스 자동차과 미터인 이송속도와 유속이 연성되어 전체 시스템의 안정성에 미치는 영향을 해석하는 것은 공학적으로 매우 중요한 문제라 할 수 있다. Wickert와 Mote

Microstructure and Thermal Shock Properties of SiC Materials

The thermal shock properties of SiC materials were investigated for high temperature applications. In particular, the effect of thermal shock temperature on the flexural strength of SiC materials was evaluated, in conjunction with a detailed analysis of their microstructures. The efficiency of a nondestructive technique using ultrasonic waves was also examined for the characterization of SiC materials suffering from a cyclic thermal shock history. SiC materials were fabricated by a liquid phase sintering process (LPS) associated with hot pressing, using a commercial submicron SiC powder. In the materials, a complex mixture of and powders was used as a sintering additive for the densification of the microstructure. Both the microstructure and mechanical properties of the sintered SiC materials were investigated using SEM, XRD, and a three point bending test. The SiC materials had a high density of about 3.12 Mg/m3 and an excellent flexural strength of about 700 MPa, accompanying the creation of a secondary phase in the microstructure. The SiC materials exhibited a rapid propagation of cracks with an increase in the thermal shock temperature. The flexural strength of the SiC materials was greatly decreased at thermal shock temperatures higher than , due to the creation of microcracks and their propagation. In addition, the SiC materials had a clear tendency for a variation in the attenuation coefficient in ultrasonic waves with an increase in thermal shock cycles.