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The effect of aging heat treatment on microstructure evolution of the Ti-6Al-4Fe-0.25Si alloy with an initial microstructure of an elongated alpha was investigated. Aging treatments of the samples were carried out at 550℃ for up to 100 hours. The microstructure of the 5 hours heat-treated sample consisted of alpha grains, beta matrix and some TiFe intermetallic compounds that were precipitated from the beta matrix. Increasing the aging time to 10 hours, most of the beta matrix was decomposed to very fine alpha grains (~ 0.5㎛) and TiFe, and thus the volume fraction of the beta matrix was significantly decreased. EBSD analysis revealed that newly formed tertiary-alpha-grains in the vicinity of TiFe had high angle boundaries with respect to the primary and secondary alpha grains. As a result of these phase transformations during aging, the fraction of the alpha/alpha grain boundary was increased while that of the alpha/beta phase boundary was decreased.

keyword : titanium, TiFe intermetallic compound, aging, precipitation, electron backscatter diffraction

To analyze the factors on stretch-flangeability for 980 MPa-grade hot-rolled steels, two types of steels (Fe-Cr and Fe-Mo) were manufactured by hot-rolling. Manufactured steels at the low coiling temperature, such as 400 and 500℃, had poor stretch-flangeability due to un-uniformly distributed carbides and a large deviation of interphase hardness. However, when the coiling temperature was set at 650℃ with Fe-Cr steel, 998 MPa of ultimate tensile strength, 19% of total elongation and 65% of the hole expanding ratio were achieved by microstructural constituents of polygonal ferrite (PF) and granular ferrite (GF) dispersed with fine carbides (<50 nm). Therefore, the material to attain 980 MPa with superior formability was the Fe- Cr steel that was precipitation-hardened in polygonal ferrite and granular ferrite at the coiling temperature 650℃.

keyword : metals, rolling, precipitation, hardness test, stretch-flangeability

The aim of this study was to analyze high temperature deformation stability and properties of duplex stainless steels(DSS) according to annealing temperature. In order to analyze high temperature deformation stability, a number of compression tests were carried out with a stain rate of 10 -2 s -1 ~10 s -1 up to a compression ratio of 50% in a temperature range of 950-1300. The analysis of high temperature deformation stability of DSS was performed based on the Ziegler model. In order to analyze the high temperature properties of DSS, annealing treatments were conducted by isothermal holding for 1 hr at 950 to 1300 with 50 intervals followed by water cooling. The hardness and tensile tests were performed on specimens with different volume fractions of constituent phases, such as austenite, ferrite and sigma. The hardness and tensile strength of 2507 according to the annealing temperature are better than those of 2205. The strain rate sensitivity and Ziegler parameter are higher in 2205 than in 2507 as a whole, which implies that 2205 is better than 2507 in terms of forgeability at high temperature.

keyword : duplex stainless steel, high deformation stability, ziegler parameter, annealing

We report on the feasibility of producing 6xxx series Al alloy sheets using a combination of twinroll strip casting and asymmetric rolling. The Al alloy sheets produced in this study exhibited an excellent formability (r=1.2, ㅿr=0.17) and mechanical properties (σTS~260 MPa, ε>30%), which cannot be feasibly obtained via the conventional technique based on ingot casting and conventional rolling. The enhanced formability as evaluated in terms of r and ㅿr was clarified by examining the evolution of textures associated with strip casting and subsequent thermo-mechanical treatments. The evaluation of the formability leads us to conclude that the combined technique based on strip casting and asymmetric rolling is a feasible process for enhancing the formability of Al alloy sheets to a level beyond which the conventional technique can reach.

keyword : Al alloy, asymmetric rolling, strip casting, mechanical properties, textures, Al-Mg-Si

Low temperature δ-precipitation phenomenon below 2-step aging (718℃, 8 hr + 621℃, 8 hr) temperature of cold drawn Inconel 718 alloy wire was investigated. The investigation was carried out on wires with a cold drawn ratio of 0, 20, 50 and 70% using OM, SEM, XRD, TEM, and DSC. In microstructures of 50 and 70% drawn wire, many precipitates were found along the grain boundaries and the twin boundaries in deformation band. From the results of the XRD and TEM analysis, the precipitates were identified as platelike δ-phase. From the results of the DSC analysis, it was also found that a temperature of δ-precipitation decreases with an increase of the cold drawn ratio. We concluded that cold drawing of inconel 718 wire promotes the δ-precipitation, and under the condition of a high drawing ratio, the δ-phase could be precipitated at a temperature below the aging temperature (718℃).

keyword : Inconel 718, alloys, cold working, precipitation, thermal analysis

In this study, a super carburizing treatment was applied to improve roller pitting fatigue life. It produced excellent properties of surface hardness and temper softening resistance by forming precipitation of fine and spherodized carbides on a tempered marstensite matrix through the repeated process of carburization and diffusion after high temperature carburizing step 1. The cycle II performed two times carburizing/diffusion cycle (process) after super carburization at 1,000℃ had fine and spherodized carbides to subsurface 200㎛. In this case, the carbide was (Fe,Cr)3C and there was not any massive carbides. In the case of Cycle II, the roller pitting fatigue life had a 6.15 million cycles. It was improved 48% compared to normal gas carburizing treatment.

keyword : super carburizing, fine and spherodized carbides, roller pitting fatigue Life, temper softening

We investigated the experimental method that uses the homogeneous precipitation method to prepare mica flakes-coated rutile-type titania pearlescent pigment with urea as a precipitant. TiO2 particles exhibit a high reflection of lights and optical properties with chemical stabilities, so they are appropriate for coating on luminescent pigments (mica). The coating principle of mica coated titania with various thicknesses was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tested by spectrophotometer. Mica with a particle size in the range of 40-60 ㎛was suspended in water, and metal sulphates and urea were added to the mixture, which was heated to boiling. The change in pH was continuously followed. The metal oxide and crystal structure were affected by the conditions of TiOSO4 concentration and reaction time with a sintering temperature the range of 800-1100℃.

keyword : oxides, mica, transmission electron microscopy, chemical synthesis, precipitation

In this study, we evaluated the fluidity of the Al-Zn based alloys which exhibit excellent mechanical properties. We conducted computer simulations of fluid flow using the results of DSC, DTA analysis and Javabased Materials Properties software (J. Mat. Pro). Such computer simulations were then compared with the results obtained from experimental observations. The computer simulation results and the experimental results were very similar in fluidity length. It was found that the fluidity length of Al-Zn alloys is improved by increasing the Zn content while decreasing the solidus temperature of an alloy. In addition, we elucidate the effect of Zn addition on variations in different mechanical properties and the microstructure characteristics of (Al-xZn3Cu0.4Si0.3Fe) x=20, 30, 40, and 45 wt% alloys fabricated by gravity casting.

keyword : Al-Zn alloys, fluidity, gravity casting, high strength, FDM

Thermal shock behavior of TiN-coated SUS 304 substrate was investigated using a laser ablation method. By short surface ablation with a pulse Nd-YAG laser, considerable surface crack and spalling were observed, whereas there were few oxidation phenomena, such as grain growth of TiN crystallites, nucleation and growth of TiO2 crystallites, which were observed from the coatings quenched from 700 in a chamber. The oxygen concentration of the ablated coating surface with the pulse laser also had a lower value than that of the quenched coating surface by Auger electron spectroscopy and electron probe micro analysis. These results were attributed to the fact that the properties of the pulse laser method have a very short heating time and so the diffusion time for oxidation was insufficient. Consequently, it was verified that the laser thermal shock test provides a way to evaluate the influence of the thermal shock load reduced oxidation effect.

keyword : thin films, thermo-mechanical processing, surface oxidation, auger electron spectroscopy, pulse Laser ablation method

The optical and electronic properties of Indium Tin Oxide (ITO) thin films deposited on a RFplasma treated glass substrate were investigated by X-Ray Photoelectron Spectroscopy (XPS), Ultra-violet Photoelectron Spectroscopy (UPS), Reflected Electron Energy Loss Spectroscopy (REELS). The modification of glass substrates was carried out by varying the time of the plasma surface treatment in an oxygen atmosphere. The focus of this research was to examine how the optical and electronic properties of ITO thin films change with the plasma treatment time. The surface energy increased since the carbon bonds were removed from the surface after the glass substrate received the surface treatment. The ITO thin films produced on the glass substrate with surface treatment showed that the high optical transmittance was approximately 85%. The measured band gap energy was as high as 3.23 eV when the plasma treatment time was 60 s and the work function after the treatment was increased by 0.5 eV in comparison to that before the treatment of 60 s. The ITO thin film exhibited an excellent sheet resistance of 2.79 Ω/□. We found that the optical and electronic properties of ITO thin films can be improved by RF-plasma surface treatment.

keyword : surface modification, ITO, REELS, UPS, XPS