Journal of Powder Materials

The Effect of Ni Addition on the Sintering Kinetics of Ball-milled Mosi_2 Powder Compacts: J Korean Powder Metall Inst. 1998;5(4):241-249.

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Abstract PDF: Sintering kinetics of ball-milled MoSi_2 was studied with the addition of Ni. MoSi_2 powder with the average particle size of 1 mutextrmm was obtained from ball-milling of 10 mutextrmm powder. Small amount of Ni was added to the ball-milled MoSi_2 powder by salt solution and reduction method. The powder was compacted into cylindrical shape at 200 MPa and isothermally sintered in a H_2 atmosphere at the temperature range of 1100~1400°C for 3~600 minutes. The changes of linear shrinkage and sintered density were monitored as a function of sintering time. The microstructure was observed by using optical microscopy and scanning electron microscopy. Phases were identified by X-ray diffratometer and electro-probe micro analysis. Sintering kinetics of Ni-added powder was compared to as-milled powder and the apparent activation energy was calculated from Arrhenius plot.

Synthesis of Titanium Silicides by Mechanical Alloying: J Korean Powder Metall Inst. 1998;5(4):250-257.

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Abstract PDF: The synthesis of titanium silicides (Ti_3Si, TiSi_2, Ti_5Si_4, Ti_5Si_3 and TiSi) by mechanical alloying has been investigated. Rapid, self-propagating high-temperature synthesis (SHS) reactions were observed to produce the last three phases during room-temperature high-energy ball milling of elemental powders. Such reactions appeared to be ignited by mechanical impact in an intimate, fine powder mixture formed after a critical milling period. During the high-energy ball milling, the repeated impact at contact points leads to a local concentration of energy which may ignite a self-propagating reaction. From in-situ thermal analysis, each critical milling period for the formation of Ti_5Si_4, Ti_5Si_3 and TiSi was observed to be 22, 35.5 and 53.5 min, respectively. Ti_3Si and TiSi_2, however, have not been produced even till the milling period of 360 min due to lack of the homogeneity of the powder mixtures. The formation of titanium silicides by mechanical alloying and the relevant reaction rates appeared to depend upon the critical milling period, the homogeneity of the powder mixtures, and the heat of formation of the products involved.

Initial Sintering Behaviour of the Powder Injection Molded W-15wt%Cu Nanocomposite Powder: J Korean Powder Metall Inst. 1998;5(4):258-264.

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Abstract PDF: The initial sintering behaviour of the powder injection molded (PIMed) W-l5wt%Cu nanocomposite powder was investigated. The W-Cu nanocomposite powder was produced by the mechanochemical process consisting of high energy ball-milling and hydrogen reduction of W blue powder-CuO mixture. Solid state sintering of the powder compacts was conducted at 1050°C for 2~10 hours in hydrogen at mosphere. The sintering behaviour was examined and discussed in terms of microstructural developments such as W-Cu aggregate formation, pore size distribution and W grain growth. The volume shrinkage of PIM specimen was slightly larger than that of PM(conventional PM specimen), being due to fast local densification in the PIM. Remarkable decrease of carbon and oxygen in the PIM enhanced local densification in the early stage of solid state sintering process with eliminating very fine pores less than 10 nm. In addition, such local densiflcation in the PIM is presumably responsible for mitigating of W-grain growth in the initial stage.

Effect of Sintering Atmosphere and Carbon Addition on Sintered Density of M3/2 Grade High Speed Steel Powder: Jin-Hwan Ahn, Jong-Seo Heo, Dong-Won Joo, Eun Jung, Jang-Hyun Sung; J Korean Powder Metall Inst. 1998;5(4):265-272.

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Abstract PDF: For the purpose of investigating the effect of sintering atmosphere and carbon addition on sintered density and microstructural characteristics, the M3/2 grade high speed steel powders with the addition of carbon are sintered in vacuum and 20%H_2/79%N_2/l%CH_4 gas atmosphere. With the addition of 0 wt%C, 0.45wt%C and 1.15 wt%C the optimum sintering temperatures decrease down to 1260°C, 1210°C and 1150°C respectively for the vacuum sintered specimen, and also decrease down to 1130°C, 1120°C and 1115°C for the gas sintered specimen. The threshold temperatures for full densification decrease steeply with increasing carbon content of the sintered specimen, while this temperatures are slowly decreased at high carbon content. The vacuum sintered specimen shows the primary carbides of MC and M_6C type at the optimum sintering temperature, and eutectic carbides of M_2C and Fe-Cr type are produced in the oversintered specimen. The gas sintered specimen exhibits M6C and Fe-Cr type primary carbides at the optimum sintering temperature. The eutectic carbides of M_6C and Fe-Cr type and MX type carbonitride are shown for the oversintered specimen in the gas atmosphere. The hardness of gas sintered specimen shows high value of 830-860 Hv due to the increment of carbide precipitation.

Microstructures and Repeated Usage-Properties of de-NO_x Transition Metals/ZSM-5 Catalyst Made by Mechanical Alloying Method: J Korean Powder Metall Inst. 1998;5(4):273-278.

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Abstract PDF: De-NO_x transition metals(Cu, Co)/ZSM-5 catalyst was made by mechanical alloying method, and their microstructures and repeated usage-properties were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The conversions ability of NO in the catalyst was measured. A part of ZSM-5 in CO/ZSM-5 composite powders was amorphous and the amorphous phase became less stable with increasing Co content. Conversion ability of NO in 10Cu/ZSM-5 powders decreased from 89% to 12% and that in 10Co/ZSM-5 decreased from 22% to 17% by 7 times conversion tests.

Microstructures and Mechanical Properties of HIPed Submicron WC-(5~20) mass% Co Cemented Carbides: J Korean Powder Metall Inst. 1998;5(4):279-285.

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Abstract PDF: The microstructures and mechanical properties of submicron WC-Co cemented carbides were investigated in relation to cobalt content. To inhibit the WC grain growth during sintering, VC was added as a inhibitor in each alloy with 3 mass% to the cobalt content. The WC-(5, 8, 10, 15, 20) mass% Co compacts were sintered at 1400°C for 30 min in vacuum. Some of WC-(5, 8, 10) mass% Co sintered compacts were HIPed with 120 atm at 1300°C for 1 hr. The shrinkages of all HIPed alloys were increased without depending on the cobalt contents and the sintered densities of them. The relative densities of the alloys were increased with the cobalt content and HIPing. The less the cobalt content, the larger the WC grain. Many contiguities of WC grains were found in WC-5 mass% Co alloy. The sizes and numbers of pores in the alloys were decreased by HIPing. And also the strength and the hardness of each alloy were increased. The maximum hardness was about 18.95 GPa in the WC-5 mass% Co alloy HIPed and the maximum transverse-rupture strength (T.R.S.) 3.2 GPa in the WC-20 mass% Co alloy sintered.

Microstructures and Mechanical Properties of TiC-Ni_3Al Cermet: J Korean Powder Metall Inst. 1998;5(4):286-292.

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Abstract PDF: Ni_3Al intermetallic compound has been tested as a binder phase, in order to improve the oxidation resistance and the mechanical properties of TiC-Ni cermet at a high temperature. The wettability of Ni_3Al on TiC and the optimum sintering condition were investigated in TiC-(30, 40) vol% Ni_3Al cermets with the sintering temperature (1380~1430°C) and time (30~99 min). The results are summarized as follows: 1) Ni_3Al showed good wettability on TiC above 1400°C ; 2) The shrinkages of the specimens increased with the sintering temperature, the sintering time and the binder content, whereas the relative densities were decreased; 3) Any other phase did not appeared in the microstructures of all sintered cermets. The grain sizes of TiC became larger as the sintering temperature and the sintering time as well as the binder content increased; 4) The hardness of the cermets decreased with increase in the sintering temperature and the sintering time as well as the binder content; 5) The transverse-rupture strength of the cermets increased with the sintering temperature and the sintering time, whereas it decreased with the binder content.

Improvments in Cost Reduction for Vacuum Sintering and Vacuum and Overpressure Sintering for Tungsten Carbides: Dieter Ermel; J Korean Powder Metall Inst. 1998;5(4):293-298.

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Abstract PDF: In all larger hardmetal workshops furnaces for dewaxing, vacuum sintering or vacuum and overpressure sintering are today's standard. The furnace technology is well established. Equipment specifications such as operating overpressure, determine sintering cost, product quality, safety and reliability of the furnace and ultimately influence the competitiveness of the hard metal procucer in the global market. Essential furnace requirements are an efficient utilization of the furnace, an environmental friendly dewaxing system, high temperature uniformity, metallurgical treatment with process gases, as well as reduced cooling time by means of rapid cooling. Examples of reduced sintering costs are described achieved using a new design of vacuum sintering furnace with an improved rapid cooling device, cooling times are reduced by up to 45%. Additionally, a cost comparison of two different designs of vacuum overpressure sintering furnaces are included.

The factors of dimensional change of Fe-Cu-C sintered objects: Masashi Fujinaga, Yoshitomo Suzuki; J Korean Powder Metall Inst. 1998;5(4):299-302.

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Abstract PDF: Dimensional accuracy is one of the most important issues in the production of sintered parts. The iron-copper-carbon system is commonly used alloys in sintered structural parts production. The dimensional control of these alloys, however, is not easy because of their complex sintering behavior. This study is an effort to clarify the influence of common factors on dimensional change of Fe-Cu-C sintered structural parts. We determined the effect of such various parameters as chemical composition, particle diameter, compact density, sintering temperature and sintering time on dimensional changes. Consequently, we obtained a useful formula to predict the final dimension in function of these parameters. The effect of typical impurities in copper powder on the dimensional change of sintered parts has also been described.

Recent Development of Science and Technology of Hard Materials in Japan: Koji Hayashi; J Korean Powder Metall Inst. 1998;5(4):303-311.

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Abstract PDF: Hard materials such as hardmetal, coated hardmetal, cermet, ceramics and diamond or c-BN sintered compact are a kind of grain-dispersed alloy with high volume of hard particles. These are used for cutting tools, wear-resistant tools, rock bits, high pressure apparatus, etc. The annual production in Japan is about 1.7 billion dollars (200 billion yen). This is greatly owed to the development in science and technology which has been accomplished by applying new concepts such as fine or uniform grain microstructure, orientation of crystal grains, functionally graded material, artificial lattice and coherent bonding in recent years. In this review, the development in recent years in Japan is briefly summarized.

Effect of Mechanical Alloying on Combustion Densification of MoSi_2: Hyung-Sang Park, Jin-Seong Park, Mi-da Ka, Kwang-Seon Shin, Yong-Seong Kim; J Korean Powder Metall Inst. 1998;5(4):312-318.

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Abstract PDF: The effect of the mechanical alloying of elemental Mo and Si powders on the combustion densification behavior of MoSi_2 was investigated. The ignition temperature of the combustion reaction of the mechanically alloyed powder was measured to be significantly lower than that of the powder mixture prepared by the low energy ball milling process. The densification of the products after the combustion reaction under compressive pressure from the mechanically alloyed powders, however, was found to be poorer than that of the products from the ball milled powder.

Powder Forging of Rapidly Solidified hi-Si Alloy with Back Pressure: T. Kohno, K. Kawase, M. Otsuki, K. Morimoto; J Korean Powder Metall Inst. 1998;5(4):319-323.

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Abstract PDF: Powder forging with a back pressure was investigated for production of automobile and compressor parts made of a rapidly solidified Al-Si alloy powder. Disk-shaped green compacts made of a rapidly solidified Al-Si alloy powder were hot forged, and hubs were formed by loading back pressure on their top. The influences of the back pressure and die temperatures on forgeabiliy and properties of parts made of a rapidly solidified Al-Si alloy powder were examined. This method was also applied to the production of a scroll part. The results of these studies are summarized as follows : 1. A back pressure on the hub top is very effective for consolidation and preventing crack formation in the hub. 2. When a back pressure tess than 98 MPa is applied, the forging pressure increases by the same amount of the applied back pressure. With more than 98 MPa, the forging pressure increases further due to an increased friction at the hub side. 3. Die temperatures higher than approximately 670k are needed in order to consolidate well the hub top without cracks.

Computer Simulation of Sintering and Grain Growth: Hideaki Matsubara; J Korean Powder Metall Inst. 1998;5(4):324-328.

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Abstract PDF: This paper is aimed to study the computer simulation of sintering process for ceramics by Monte Carlo and molecular dynamics methods. Plural mechanisms of mass transfer were designed in the MC simulation of sintering process for micron size particles; the transfer of pore lattices for shrinkage and the transfer of solid lattices for grain growth ran in the calculation arrays. The MD simulation was performed in the case of nano size particles of ionic ceramics and showed the characteristic features in sintering process at atomic levels. The MC and MD simulations for sintering process are useful for microstructural design for ceramics.

Development of Tungsten Dispersed Copper Based Alloy and its Physical Property: Akira Mishima, Shigeya Sakaguchi; J Korean Powder Metall Inst. 1998;5(4):329-333.

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Abstract PDF: Copper-10 wt. % tungsten alloyed powder was obtained by co-reduction of mixed tungsten-trioxide and copper oxide powders at 973 K for 7.2 Ks. In the alloy obtained by pressure-assisted sintering of this co-reduced powder, ultra fine tungsten particles (about 100nm) were dispersed uniformly in the copper matrix. At room temperature, the hardness of this alloy was Hv151 and the electrical conductivity was 85% IACS. After annealing at 1173 K for 3.6 Ks, the hardness and electrical conductivity were Hv147 and 84% IACS, respectively, and were same as before annealing. It was confirmed that the hardness and electrical conductivity of this alloy were hardly influenced by annealing condition since the microstructure of this alloy is highly stabilized.

Microstructure and Properties of Nano-Sized Ni-Co Particulate Dispersed Al_2O_3 Matrix Nanocomposites: Sung-Tag Oh, Sando Mutsuo, Niihara Koichi; J Korean Powder Metall Inst. 1998;5(4):334-339.

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Abstract PDF: In purpose of introducing the inverse magnetostrictive properties into the structural ceramics, Al_2O_3 based nanocomposites dispersed with nano-sized Ni-Co particles were studied. The composites were fabricated by the hydrogen reduction and hot-pressing of Al_2O_3 and NiO-CoO mixed powders. The mixtures were prepared by using Ni- and Co-nitrate (Ni(NO_3)_2;cdot;6H_2O;and;Co(NO_3)_2;cdot6H_2O) as source materials for the Ni-Co particles. Microstructural observations revealed that nano-sized Ni-Co particles were dispersed homogeneously at Al_2O_3 grain boundaries. High strength above 1 GPa was obtained for the Al_2O_3/10 wt% Ni-Co nanocomposite fabricated by a controlled powder preparation process. The inverse magnetostrictive response to applied stress was obtained due to the presence of dispersed Ni-Co particles, which indicates a possibility to incorporate new functions into the structural ceramics without loosing the mechanical properties.

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