• Journal of Magnetics
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• 제13권4호
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• pp.170-172
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• 2008

Fe-6.5% Si alloys have good magnetic properties due to their high electrical resistivity, very low magneto-striction, and low crystalline anisotropy. Despite their strong potential, these alloys have seldom been used in magnetic applications because of the very poor ductility of Si-steel above 3.0 wt% Si [1-4]. It is difficult to achieve compressed Fe-6.5% Si powder cores with excellent properties because of the low density due to poor ductility. In compressed powder cores, high density is essential in order to obtain high magnetization and permeability. In this study, an attempt was made to produce Fe-3%Si powder cores because the Fe-3.0 wt% Si alloys have relatively good magnetic properties and room temperature ductility. Gas atomized Fe-3.0 wt% Si powder was compressed into toroid shape cores. By reducing the Si content to 3.0 wt%, the hysteresis loss could be greatly reduced and thus the total core loss could be minimized. The total core loss is 600 mW/$cm^3$ at 0.1 T and 50 kHz.

• 한국재료학회지
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• 제14권2호
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• pp.126-132
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• 2004

The high strength steel sheets has been widely used as the automobile parts to reduce the weight of a vehicle. The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The 0.15C-4Mn and 0.15C-6.5Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and air cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $645^{\circ}C$ for 12 hrs. was about 46vol.% in hot rolled 0.lC-6.5Mn steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The tendency of tensile strength to increase with increasing the holding temperature was due to the decrease of retained austenite after cooling from the higher temperature of $670 ^{\circ}C$. The maximum strength-ductility combination was about 4,250 kg/$\textrm{mm}^2$ㆍ% when the hot rolled 0.lC-6.5Mn steel was reversely transformed at $645^{\circ}C$ for 12 hrs.

• 한국재료학회지
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• 제15권2호
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• pp.115-120
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• 2005

The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The $4\~8\%$ Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and furnace cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $625^{\circ}C$ for 6 hrs was about $50\;vol.\%$ in the $8\%Mn$ steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The maximum strength-ductility combination of 40,000 $MPa{\cdot}\%$ was obtained when the $8\%Mn$ steel reversely transformed at $625^{\circ}C$ for 12 hrs. However, it's property was significantly decreased at higher holding temperature of $675^{\circ}C$ resulting from the decrease of ductility.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2001년도 춘계학술대회 논문집
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• pp.146-151
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• 2001

Low temperature fatigue crack propagation ratio and characteristics of the pressure structural steel which is used for the low temperature pressure vessels. Fatigue crack properties was studied at room temperature of $25^{\circ}C$ and low temperature ranges $-60^{\circ}C,\; -80^{\circ}C \;and\; -100^{\circ}C$ with stress ratio of R=0.05, 0.1, 0.3 in the logarithmic relationship between the fatigue crack propagation rate (da/dN) and stress intensity factor $\DeltaK$, in low temperature case the relationship was extend to the range of low crack propagation rate. The fractured specimens were examined by SEM tested. That results showed specimen failed at low temperature exhibit the quasi-cleavage fracture formation, however, considerable ductility proceed final fracture.

• 한국재료학회지
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• 제26권12호
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• pp.709-713
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• 2016

Grain morphology, phase stability and mechanical properties in binary Ti-Al alloys containing 43-52 mo1% Al have been investigated. Isothermal forging was used to control the grain sizes of these alloys in the range of 5 to $350{\mu}m$. Grain morphology and volume fraction of ${\alpha}_2$ phase were observed by optical metallography and scanning electron microscopy. Compressive properties were evaluated at room temperature, 1070 K, and 1270 K in an argon atmosphere. Work hardening is significant at room temperature, but it hardly took place at 1070 K and 1270 K because of dynamical recrystallization. The grain morphologies were determined as functions of aluminum content and processing conditions. The transus curve of ${\alpha}$ and ${\alpha}+{\gamma}$ shifted more to the aluminum-rich side than was the case in McCullough's phase diagram. Flow stress at room temperature depends strongly on the volume fraction of the ${\alpha}_2$ phase and the grain size, whereas flow stress at 1070 K is insensitive to the alloy composition or the grain size, and flow stress at 1270 K depends mainly on the grain size. The ${\alpha}_2$ phase in the alloys does not increase the proof stress at high temperatures. These observations indicate that improvement of both the proof stress at high temperature and the room temperature ductility should be achieved to obtain slightly Ti-rich TiAl base alloys.

• 한국주조공학회지
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• 제2권2호
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• pp.2-9
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• 1982

This paper is concerned with the improvement of impact and tensile Properties of spheroidal graphite cast iron of the following duplex matricess which were heat treated in the eutectic transformation temperature range (that is, $({\alpha}+{\gamma})$ coexisting range) ; ferrite-martensite, ferrite-bainite and ferrite-pearlite. The absorbed energy and maximum load was measured by recording the load-deflection curve with instrumented Charpy impact testing machine in the temperature range from $+100^{\circ}C$ to $-196^{\circ}C$. It was found the ferrite-bainite duplex matrix showed the highest toughness among the above matrices in the room temperature and the low temperature range. Comparison of this matrix to ferrite-pearlite matrix(that is, as cast) showed a lowering of $27^{\circ}C$ in the nil-ductility transition temperature (NDT) and a lowering of $40^{\circ}C$ in the ductile-brittle transition temperature (TrE), Which seems to result from the finner dimple pattern observed using miorofractography.

• 연구논문집
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• 통권29호
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• pp.163-171
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• 1999

A new titanium alloy system. Ti-xFe-ySi (x,y=0-4 wt%). was designed and characterized with the point at low cost and high strength for casting applications. Fe improved room and elevated temperature mechanical properties owing to solid solution hardening and beta phase stabilization. Si yielded titanium silicides and Si addition over 1 wt% resulted in poor ductility due to coarse silicide chains at prior beta boundaries. The optimum composition was found to be Ti-4Fe-(0.5-1)Si in the viewpoint of tensile strength and ductility which are comparable to the Ti-6Al-4V. The metal-mould reaction was also examined for Ti-xFe and Ti-xSi binary alloy system. The thickness of surface reaction layer w as not affected significantly with Fe content, while it was decreased with Si content. In the Ti-4Si alloy, no reaction layer was found. The depth of surface hardening layer was about $200\mum$ regardless of the mould materials.

• 열처리공학회지
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• 제25권4호
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• pp.190-195
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• 2012

This study aims to investigate and compare the microstructures and room temperature tensile properties for AZ91 and ECO-AZ91 (AZ91+0.3%CaO) alloys in as-cast, T4 and T6 states, respectively. In as-cast state, the ECO-AZ91 alloy has finer microstructure than the AZ91 alloy. The AZ91 alloy exhibits greater ductility, while YS and UTS are inferior to those of the ECO-AZ91 alloy. After T4 treatment, most of ${\beta}$ compounds disappear in the AZ91 alloy, whereas ${\beta}$ phase is still observed in the ECO-AZ91 alloy due to its enhanced thermal stability, resulting in lower values of ductility and UTS. In T6 state, YS and UTS are better in the ECO-AZ91 alloy.

• 열처리공학회지
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• 제26권1호
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• pp.1-6
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• 2013

The B2-ordered NiAl has attracted much attention as one of the candidates as a next generation high temperature material, because it has a high melting temperature, a low specific gravity and an excellent high temperature oxidation resistance. However, the application of NiAl to structural materials needs the improvement of its brittleness at room temperature. The study was carried out on the relation between several properties of NiAl and some variation of Ni content within NiAl phase, which means deviations from the stoichiometric composition. The main results were as follows; (i) Good ductility was obtained at the testing temperature more than 1073 K irrespective of Ni content. (ii) Increasing Ni content offered preferable tensile properties. (iii) Every NiAl with varying Ni contents showed the superior oxidation resistance.

• 소성∙가공
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• 제17권6호
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• pp.412-419
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• 2008

Using lightweight materials in vehicle manufacturing in order to reduce energy consumption is one of the most effective approach to decrease pollutant emissions. As a lightweight material, magnesium is increasingly employed in automotive parts. However, because of its hexagonal closed-packed(HCP) crystal structure, in which only the basal plane can move, the magnesium alloy sheets show low ductility and formability at room temperature. Thus the press forming of magnesium alloy sheets has been performed at elevated temperature within range of $200^{\circ}C{\sim}250^{\circ}C$. Here we try the possibility of sheet metal forming at room temperature by adopting incremental forming technique with rotating tool, which is so called as rotational-incremental sheet forming(RISF). In this rotational-incremental sheet forming the spindle tool rotates on the surface of the sheet metal and moves incrementally with small pitch to fit the sheet metal on the desired shape. There are various variables defining the formability of sheet metals in the incremental forming such as speed of spindle, pitch size, lubricants, etc. In this study, we clarified the effects of spindle speed and pitch size upon formability of magnesium alloy sheets at room temperature. In case of 0.2, 0.3 and 0.4mm of pitch size with hemispherical rotating tool of 6.0mm radius, the maximum temperature at contact area between rotating tool and sheet metal were $119.2^{\circ}C,\;130.8^{\circ}C,\;and\;177.3^{\circ}C$. Also in case of 300, 500, and 700rpm of spindle speed, the maximum temperature at the contact area were $109.7^{\circ}C,\;130.8^{\circ}C\;and\;189.8^{\circ}C$.