• Transactions of Materials Processing
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• v.24 no.3
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• pp.205-211
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• 2015

The current investigation was performed to study sliding-wear-rate deviation (wear-rate data scatter) in carbon steels with various microstructures. Pure iron, 0.2 wt. % C steel, 0.45 wt. % C steel, and bearing steel (AISI52100) were used for the investigation. These steels possess different microstructures. Microstructures of the pure iron, two carbon steel and the bearing steel were full ferrite, ferrite + pearlite and full pearlite, respectively. Depending on the carbon content, the carbon steel had different pearlite-volume fractions. Dry sliding wear tests of the steel were conducted using a ball-on-disk wear tester at a sliding speed of 0.1 m/s using a bearing ball (AISI52100) as a counterpart. Applied load and sliding distance were 100 N and 300 m, respectively. More than three (up to twelve) tests were conducted for each steel under the same conditions, and the mean deviations in the wear rate of the steel (microstructure) were compared. The wear-rate deviation in the steel with ferrite + pearlite microstructure was higher than that with ferrite microstructure, and the deviation decreased with the increase of pearlite volume fraction. The pure iron and the bearing steel specimens showed much less deviation. The high deviation observed from the ferrite + pearlite steel was attributed to irregular subsurface-crack nucleation and growth at the interface between the two micro constituents (ferrite and pearlite) during the wear test.

• Korean Journal of Metals and Materials
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• v.50 no.10
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• pp.735-743
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• 2012

The aim of this study was to analyze the effect of the cooling rate after heat treatment on the microstructure and mechanical properties of 2507 duplex stainless steels. Heat treatment was carried out at $1050^{\circ}C$ for 1 hr, followed by controlled cooling. The cooling rates were $175.6{\times}10^{-3}^{\circ}C/s$, $47.8{\times}10^{-3}^{\circ}C/s$, $33.3{\times}10^{-3}^{\circ}C/s$, $16.7{\times}10^{-3}^{\circ}C/s$, $11.7{\times}10^{-3}^{\circ}C/s$, $5.8{\times}10^{-3}^{\circ}C/s$ and $2.8{\times}10^{-3}^{\circ}C/s$, which resulted in variations of the microstructure, such as the fractional change of the ferrite phase and sigma phase formation. Fatigue, hardness, impact and tensile tests were performed on the specimens with different cooling rates. The precipitation of the ${\sigma}$ phase caused a hardness increase and a sharp decrease of toughness and tensile elongation. The fatigue limit of the sample with a cooling rate of $5.8{\times}10^{-3}^{\circ}C/s$ was 26 MPa higher than that of the sample with a cooling rate of $175.6{\times}10^{-3}^{\circ}C/s$. Our observations of the fracture surface confirmed that the higher fatigue resistance of the specimen with a cooling rate of $5.8{\times}10^{-3}^{\circ}C/s$ was caused by the delay of the fatigue crack growth, in addition to higher yield strength.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.2
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• pp.47-54
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• 1990

In this study, the effects of gas ratio($N_2:H_2$) and treatment time on the fatigue fracture behavior, fatigue crack growth behavior and corrosion fatigue fracture behavior for the ion-nitrided SM45C steel were investigated. The results show that the fatigue limit and corrosion fatigue strength increased in porportion to $N_2$ gas and treatment time for all kinds of specimen, used in the experiment. Compare to the non-nitrided specimen, the fatigue limit and the corrosion fatigue strength increased about $24{\sim}29%$ at $10^7$ cycles in air and $32{\sim}48%$ at $10^6$ cycles in 3% NaCl aqueous solution, respectively. Similar results were derived with SM45C steel under Compression-Tension $24{\sim}29%$ at $10^6$ cycles in air and $32{\sim}48%$ either in 3% NaCl aqueous solution or in tap water, respectively. Ion-nitrided SM45C steel showed a slow fatigue crack growth rate at relatively low range of ${\Delta}K$ compared to the non-nitrided specimen. To the contrary, its rate increase at higher range of ${\Delta}K$.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.258-292
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• 1996

In the last year great interest appears to YBCO thin films preparation on different substrate materials. Preparation of epitaxial film is a very difficult problem. There are many requirements to substrate materials that must be fullfilled. Main problems are lattice mismatch (misfit) and similarity of structure. From paper [1] or follows that difference in interatomic distances and angles of substrate and film is mire important problem than similarity of structure. In this work we present interatomic distances and angle relations between substrate materials belonging to ABCO4 group (where A-Sr or Ca, B-rare earth element, C-Al or Ga) of different orientations and YBCO thin films. There are many materials used as substrates for HTsC thin films. ABCO4 group of compounds is characterized by small dielectric constants (it is necessary for microwave applications of HTsC films), absence of twins and small misfit [2]. There most interesting compounds CaNdAlO4, SrLaAlO4 and SrLaGaO4 were investigated. All these compounds are of pseudo-perovskite structure with space group 14/mmm. This structure is very similar to structure of YBCO. SLG substrate has the lowest misfit (0.3%) and dielectric constant. For preparation of then films of substrates of this group of compound plane of <100> orientation are mainly used. Good quality films of <001> orientations are obtained [3]. In this case not only a-a misfit play role, but c-3b misfit is very important too. Sometimes, for preparation of thin films substrates of <001> and <110> orientations were manufactured [3]. Different misfits for different YBCO faces have been analyzed. It has been found that the mismatching factor for (100) face is very similar to that for (001) face so there is possibility of preparation of thin films on both orientations. SrLaAlO4(SLA) and SrLaGaO4(SLG) crystals of general formula ABCO4 have been grown by the Czochralski method. The quality of SLA and SLG crystals strongly depends on axial gradient of temperature and growth and rotation rates. High quality crystals were obtained at axial gradient of temperature near crystal-melt interface lower than 50℃/cm, growth rate 1-3 mm/h and the rotation rate changing from 10-20pm[4]. Strong anisotropy in morphology of SLA and SLG single crystals grown by the Czochralski method is clearly visible. On the basics of our considerations for ABCO4 type of the tetragonal crystals there can appear {001}, {101}, and {110} faces for ionic type model [5]. Morphology of these crystals depend on ionic-covalent character of bonding and crystal growth parameters. Point defects are observed in crystals and they are reflected in color changes (colorless, yellow, green). Point defects are detected in directions perpendicular to oxide planes and are connected with instability of oxygen position in lattice. To investigate facets formations crystals were doped with Cr3+, Er3+, Pr3+, Ba2+. Chromium greater size ion which is substituted for Al3+ clearly induces faceting. There appear easy {110} faces and SLA crystals crack even then the amount of Cr is below 0.3at.% SLG single crystals are not so sensitive to the content of chromium ions. It was also found that if {110} face appears at the beginning of growth process the crystal changes its color on the plane {110} but it happens only on the shoulder part. The projection of {110} face has a great amount of oxygen positions which can be easy defected. Pure and doped SLA and SLG crystals measured by EPR in the<110> direction show more intensive lines than in other directions which allows to suggest that the amount of oxygen defects on the {110} plane is higher. In order to find the origin of colors and their relation with the crystal stability, a set of SLA and SLG crystals were investigated using optical spectroscopy. The colored samples exhibit an absorption band stretching from the UV absorption edge of the crystal, from about 240 nm to about 550 m. In the case of colorless sample, the absorption spectrum consists of a relatively weak band in the UV region. The spectral position and intensities of absorption bands of SLA are typical for imperfection similar to color centers which may be created in most of oxide crystals by UV and X-radiation. It is pointed out that crystal growth process of polycomponent oxide crystals by Czochralski method depends on the preparation of melt and its stoichiometry, orientation of seed, gradient of temperature at crystal-melt interface, parameters of growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth. Growth parameters have an influence on the morphology of crystal-melt interface, type and concentration of defects.

• Corrosion Science and Technology
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• v.3 no.5
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• pp.198-208
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• 2004

Although there has been no general agreement on the mechanism of primary water stress corrosion cracking (PWSCC) as one of major degradation modes of Ni-base alloys in pressurized water reactors (PWR's), common postulation derived from previous studies is that the damage to the alloy substrate can be related to mass transport characteristics and/or repair properties of overlaid oxide film. Recently, it was shown that the oxide film structure and PWSCC initiation time as well as crack growth rate were systematically varied as a function of dissolved hydrogen concentration in high temperature water, supporting the postulation. In order to understand how the oxide film composition can vary with water chemistry, this study was conducted to characterize oxide films on Alloy 600 by an in-situ Raman spectroscopy. Based on both experimental and thermodynamic prediction results, Ni/NiO thermodynamic equilibrium condition was defined as a function of electrochemical potential and temperature. The results agree well with Attanasio et al.'s data by contact electrical resistance measurements. The anomalously high PWSCC growth rate consistently observed in the vicinity of Ni/NiO equilibrium is then attributed to weak thermodynamic stability of NiO. Redox-induced phase transition between Ni metal and NiO may undermine the integrity of NiO and enhance presumably the percolation of oxidizing environment through the oxide film, especially along grain boundaries. The redox-induced grain boundary oxide degradation mechanism has been postulated and will be tested by using the in-situ Raman facility.

• Journal of the Microelectronics and Packaging Society
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• v.24 no.1
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• pp.103-111
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• 2017

In this study, the thermomechanical stress and fatigue analysis of a high voltage and high current (3,300 V/1200 A) insulated gate bipolar transistor (IGBT) module used for electric locomotive applications were performed under thermal cycling condition. Especially, the reliability of the copper wire and the ribbon wire were compared with that of the conventional aluminum wire. The copper wire showed three times higher stress than the aluminum wire. The ribbon type wire showed a higher stress than the circular type wire, and the copper ribbon wire showed the highest stress. The fatigue analysis results of the chip solder connecting the chip and the direct bond copper (DBC) indicated that the crack of the solder mainly occurred at the outer edge of the solder. In case of the circular wire, cracking of the solder occurred at 35,000 thermal cycles, and the crack area in the copper wire was larger than that of the aluminum wire. On the other hand, when the ribbon wire was used, the crack area was smaller than that of the circular wire. In case of the solder existing between DBC and base plate, the crack growth rate was similar regardless of the material and shape of the wire. However, cracking occurred earlier than chip solder, and more than half of the solder was failed at 40,000 cycles. Therefore, it is expected that the reliability of the solder between DBC and base plate would be worse than the chip solder.

• Journal of Welding and Joining
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• v.31 no.1
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• pp.58-64
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• 2013

The following results were obtained, microstructures and tensile properties in arc brazed joints of DP(dual phase) steel using Cu-5.3wt%Sn insert metal was investigated as function of brazing current. 1) The Fusion Zone was composed of ${\alpha}Fe+{\gamma}Cu$ and Cu23Sn2. The reason for the formation of these solid solutions. Despite, Fe & Cu were impossible to solid solution at room temperature. It's melting & reaction to something of insert metal & Base Metal (DP Steel) by Arc. Brazing Process has faster cooling rate then Cast Process, Supersaturated solid solution at room temperature. 2) The increase Hardness of Fusion Zone was directly proportional to the rise of welding current. Because, ${\alpha}Fe+{\gamma}Cu$ phase (higher hardness than the Cu23Sn2.(104.1Hv < 271.9Hv)) Volume fraction was Growth, due to increasing the amount of base metal melting by High current. 3) The results of tensile shear test by Brazing, All specimens happen to fracture in Fusion Zone. On the other hand, when Brazing Current increasing tend to rise tensile load. but it was very small, about 26-30% of the base metal. 4) The result of fracture analysis, The crack initiate at Triple Point for meet to Upper B.M/Under B.M/Fusion Zone. This Crack propagated to Fusion zone. So ruptured by tensile strength. The Reason to in the fusion zone fracture, Fusion zone by Brazing of hardness (strength) was very lower then the base metal (DP steel). In addition the Fusion Zone's thickness in triple point was thin than the base metal's thickness in triple point.

• Journal of the Korean institute of surface engineering
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• v.51 no.6
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• pp.393-399
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• 2018

Formation behavior of aluminum anodic oxide (AAO) films on Al6061 alloy was studied in view of thickness, morphology and defects in the anodic films in 20 vol.% sulfuric acid solution at a constant current density of $40mA/cm^2$, using voltage-time curve, observation of anodized specimen colors and surface and cross-sectional morphologies of anodic films with anodization time. With increasing anodizing time, voltage for film formation increased exponentially after about 12 min and its increasing rate decreased after 25 min, followed by a rapid decrease of the voltage after about 28 min. Surface color of anodized specimen became darker with increasing anodizing time up to about 20 min, while it appeared to be brighter with increasing anodizing time after 20 min. The darkened and brightened surfaces with anodizing time are attributed to an increase in thickness of porous anodic oxide film and a chemical damage of the films due to heat generated by increased resistance of the film, respectively. Cross-sectional observation of AAO films revealed the formation of defects of crack shape at the metal/oxide interface after 15 min which prevents the growth of AAO films. Width and length of the crack-like defect increased with anodizing time up to 25 min of anodizing, and finally the outer part of AAO films was partly dissolved or detached after 30 min of anodizing, resulting in non-uniform surface structures of the AAO films.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.6
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• pp.389-393
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• 2000

High-quality $Er^{3+}$ doped Mg : $LiNbO_3$single crystal fibers were grown by a micro-pulling down ($\mu$-PD) method. Single crystal fibers were pulled down through the nozzle, at a pulling down rate of 0.5 mm/min and using a Pt crucible with a nozzle 1 mm in diameter in air atmosphere. Defects such as bubbles, cracks and inclusions were not detected in any of the grown crystals. The optical transmission of Er : Mg : $LiNbO_3$crystal was measured and the energy levels of $Er_2O_3$ ion could be calculated. The photoluminescence spectrum of crystal fibers showed an energy band emission with the strongest line corresponding to the $^4I_{3/2}{\to}^4I_{15/2}$transition. The concentration dependence of the entire wavelength region emission intensity upon excitation intensity measured emission intensity for the 3 mol% MgO doped fibers was larger than that for the 1, 5 mol% MgO doped fibers.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.16 no.3
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• pp.116-120
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• 2006

We have investigated plasma spray coated $Al_2O_3$ layers on Al-60 series substrates for development of wafer electrostatic chuck in semiconductor dry etching system. Samples were prepared without/with cooling bar on backside of samples, at various distances, and with different powder feed rates. There were many cracks and pores in the $Al_2O_3$ layers coated on Al-60 series substrates without cooling bar on the backside of samples. But the cracks and pores were almost disappeared in the $Al_2O_3$ layers on Al-60 series substrates coated with cooling bar on the back side of samples, 15 g/min. powder feed rate and various 60, 70, 80 mm working distances. Then the surface morphology was not changed with various working distances of 60, 70, 80 mm. When the powder feed rate was changed from 15 g/min to 20 g/min, the crack did not appear, but few pores appeared. Also the $Al_2O_3$ layer was coated with many small splats compared with $Al_2O_3$ layer coated with 15 g/min powder feed rate. The deposited rate of $Al_2O_3$ layer was higher when the process was done without cooling bar on the back side of sample than that with cooling bar on the back side of sample.