• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.1254-1255
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• 2006

For precise property control of sintered products, it is important to understand accurately the packing density of the powder. We developed a packing simulation program that could make a packed bed of spherical particles having particle size distribution. In addition, the influence of the particle shape of the actual powder on the packing density was quantitatively analyzed. The predicted packing densities corresponded well to the actual data.

• 대한금속재료학회지
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• 제47권12호
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• pp.866-872
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• 2009

The effects of magnetic powder thickness on electromagnetic wave absorption characteristics in Fe-6.5Si-0.9Cr (wt%) alloy flakes/polymer composite sheets available for quasi-microwave band have been investigated. The atomized FeSiCr powders were milled by using attritor for 12, 24, and 36 h, powder thickness changed from $40{\mu}m$ to $3{\mu}m$ upon 36 h milling. The composite sheet, including thinned magnetic flakes, exhibited higher power loss in the GHz frequency range as compared with the sheets having thick flakes. Moreover, both the complex permeability and the loss factor increased with the decrease in thickness of the alloy flakes. Therefore, the enhanced power loss property of the sheets containing thin alloy flakes was attributed to the flakes of high complex permeability, especially their imaginary part. Additionally, the complex permittivity was also increased with the reduction of flake thickness, and this behavior was considered to be helpful for improvement of the electromagnetic wave absorption characteristics in the composite sheets, including thin alloy flakes.

• 한국분말재료학회지
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• 제7권4호
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• pp.197-204
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• 2000

Gas atomization mechanical alloying and hot pressing have successfully made high temperature Al-9.45Fe-4.45Cr alloy. The microstructure and mechanical properties of this alloy has been studied by using optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffractometer and compressive tester. It contains high concentration of transition elements of Fe and Cr, which form thermally stable dispersoids in the aluminum matrix. Proper oxidation of powders during ball milling strengthens the bulk extrudates by providing the obstacle particles. The oxide particles are very chemically and thermally stable and prevent the coarsening of the intermediate compounds.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.631-632
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• 2006

Cr-C-Ni composites were synthesized in situ from elemental powders of Cr, Ni and C by high energy milling followed by reactive sintering. The milled powders with the grain size in nano-scale were pressed to compacts and sintered. During the following thermal treatment at first the chromium carbide was formed and then the $Cr_3C_2-Ni$ cermets were sintered in one cycle. The interface between the binder phase and the carbide grains of the in situ composite has a good bonding strength as it is not contaminated with oxidation films or other detrimental surface reactions.

• 열처리공학회지
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• 제31권4호
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• pp.171-179
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• 2018

Copper powder dispersed with 4 vol.% of $Cr_2O_3$ was successfully produced by a simple milling at 210 K with a mixture of $Cu_2O$, Cu and Cr elemental powders, followed by Hot Pressing (HP) at 1123 K and 50 MPa for 2h to consolidate the milled powder. The microstructure of the HPed material was characterized by standard metallographic techniques such as XRD (X-ray Diffraction), TEM and STEM-EDS. The results of STEMEDS analysis showed that the HPed materials comprised a mixture of nanocrystalline Cu matrix and $Cr_2O_3$ dispersoid with a homogeneous bimodal size distribution. The mechanical properties of the HPed materials were characterized by micro Vickers hardness test at room temperature. The thermodynamic considerations on the heat of formation, the incubation time to ignite MSR (Mechanically induced Self-sustaining Reaction), and the adiabatic temperature for the heat of displacement reaction between the oxide-metal are made for the delayed formation of $Cr_2O_3$ dispersoid in terms of MSR suppression. The results of TEM observation and hardness test indicated that the relatively large dispersoids in the HPed materials are attributed to the significant coarsening for the high temperature consolidation; this leads to the low Vickers hardness value. Based on the thermodynamic calculation for the operating processes with a limited number of parameters, the formation kinetics and coarsening of the $Cr_2O_3$ dispersoid are discussed.

• 한국재료학회지
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• 제23권10호
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• pp.574-579
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• 2013

The milling and particulate characteristics of Al alloy-$Al_2O_3$ powder mixtures for a reaction-bonded $Al_2O_3$ (RBAO) process were studied. A commercially available prealloyed Al powder with Zn, Mg, Cu and Cr alloying elements (7475 series) was mixed with a calcined sinter-active $Al_2O_3$ powder and then milled in centrifugal milling equipment for ~48 hrs. The Al alloy-$Al_2O_3$ powder mixtures after milling were characterized and evaluated in various ways to reveal their particulate characteristics during milling. The milling efficiency of the Al alloy increased with a longer milling time. Comminution of the Al alloy particles started with its elongation, showing a high aspect ratio. With a longer milling time, the elongated Al alloy particle changed in terms of its shape and size, becoming equiaxially fine particles. Regardless of the milling efficiency of the Al alloy particles, all of the Al alloy particles repeatedly experienced strong plastic deformation during milling, giving rise to higher density of surface defects, such as microcracks, and leading to higher residual microstress within the Al alloy particles. The chemical reactions, oxidation behavior and hydration behavior of the Al alloy particles and the hydrolysis characteristics of their reaction with the environment were also observed during the milling process and during the subsequent powder handling steps.

• 한국분말재료학회지
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• 제20권4호
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• pp.275-279
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• 2013

Fe-based oxide dispersion strengthened (ODS) powders were produced by high energy ball milling, followed by spark plasma sintering (SPS) for consolidation. The mixed powders of 84Fe-14Cr-$2Y_2O_3$ (wt%) were mechanically milled for 10 and 90 mins, and then consolidated at different temperatures ($900{\sim}1100^{\circ}C$). Mechanically-Alloyed (MAed) particles were examined by means of cross-sectional images using scanning electron microscopy (SEM). Both mechanical alloying and sintering behavior was investigated by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). To confirm the thermal behavior of $Y_2O_3$, a replica method was applied after the SPS process. From the SEM observation, MAed powders milled for 10 min showed a lamella structure consisting of rich regions of Fe and Cr, while both regions were fully alloyed after 90 min. The results of sintering behavior clearly indicate that as the SPS temperature increased, micro-sized defects decreased and the density of consolidated ODS alloys increased. TEM images revealed that precipitates smaller than 50 nm consisted of $YCrO_3$.

• 분석과학
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• 제6권2호
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• pp.197-205
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• 1993

닉켈과 크롬 20% 혼합분말을 실험실용 아트리터에서 시간을 달리하여 밀링하였고 입도분포, 미세조직 및 X-레이 회절 특성을 조사하였다. 합금화 상태를 확인하고자 포화자화값과 보자력값을 측정하였고 플라즈마 용해 잉곳 경우와 비교하였다. 기계적 합금화는 분말의 미세화 단계 후 크롬이 닉켈 속으로 확산이 일어나는 계면적의 증가에 의해서 진행하였다. 그러나 15시간 이상 밀링 후 서브 미크론 크기의 결정립으로 정상상태가 이루어졌음에도 불구하고 용해 잉곳과 같이 조성적으로 균일한 고용합금의 자성특성이 관찰되지 않았다. 밀링 시간이 길어질수록 결정립의 크기는 미세화되었으며 합금층이 증가하였다. 따라서 조성의 균질화는 분말의 소성변형에 의해서 일어나는 성분분말 사이의 계면적 증가와 계면내에서 입계 또는 전위와 같은 격자결함을 통해 크롬의 닉켈 속으로 확산에 의해서 율속되는 것으로 생각된다.

• 한국수소및신에너지학회논문집
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• 제17권4호
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• pp.418-424
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• 2006

In order to improve the activation properties of the $AB_2$ type hydrogen storage alloys for Ni-MH battery, the alloy surface was modified by employing high energy ball milling. The $Zr_{0.54}Ti_{0.45}V_{0.54}Ni_{0.87}Cr_{0.15}Co_{0.21}Mn_{0.24}$ alloy powder was ball milled for various period by using the high energy ball mill. As the ball milling time increased, activation of the $AB_2$ type composite powder electrodes were enhanced regardless of additives. When the ball milling time was small discharge capacities of the $AB_2$ type composite powder electrodes increased with the milling time. On the other hand for large milling time it decreased with increasing milling time. The maximum discharge capacity was obtained by ball milling for 3-4 min.

• 한국재료학회지
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• 제25권8호
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• pp.386-390
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• 2015

Fe-base superalloy powders with $Y_2O_3$ dispersion were prepared by high energy ball milling, followed by spark plasma sintering for consolidation. High-purity elemental powders with different Fe powder sizes of 24 and 50 mm were used for the preparation of $Fe-20Cr-4.5Al-0.5Ti-O.5Y_2O_3$ powder mixtures (wt%). The milling process of the powders was carried out in a horizontal rotary ball mill using a stainless steel vial and balls. The milling times of 1 to 5 h by constant operation (350 rpm, ball-to-powder ratio of 30:1 in weight) or cycle operation (1300 rpm for 4 min and 900 rpm for 1 min, 15:1) were applied. Microstructural observation revealed that the crystalline size of Fe decreased with an increase in milling time by cyclic operation and was about 15 nm after 3 h, forming a FeCr alloy phase. The cyclic operation had an advantage over constant milling in that a smaller-agglomerated structure was obtained. The milled powders were sintered at $1100^{\circ}C$ for 30 min in vacuum. With an increase in milling time, the sintered specimen showed a more homogeneous microstructure. In addition, a homogenous distribution of Y-compound particles in the grain boundary was confirmed by EDX analysis.