Journal of Powder Materials

Activated Sintering of W and W-Cu System(II): J Korean Powder Metall Inst. 1999;6(2):119-131.

A binder system for low carbon residue and debinding behaviors in injection molding of NdFeB powder: J Korean Powder Metall Inst. 1999;6(2):132-138.

Abstract PDF: A new binder system and debinding process for low carbon residue in the injection molding of Nd(Fe, Co)B powder are investigated. In the injection molding of magnetic materials, it is demanded to reduce carbon residue which deteriorates their magnetic properties. The binder system developed is composed of polyethylene glycols (PEGs) and polypropylene (PP). PEG was selected as a major binder is component to be extracted in a molecular state by solvent extraction in ethanol, which step would leave no residue. PP was selected as a minor binder component to be subsequently removed by thermolysis which step would leave carbon residue. The behaviors of solvent extraction with the variations of PEG molecular weight, temperature, and time were examined. The dependency of residual carbon content on thermolysis atmosphere was also studied. Opened pore channels introduced in a green body by the solvent extraction and microstructures of the sintered magnets were observed using SEM.

Synthesis of TiC-Ni Based Cermet Powders and Microstructures of Sintered Compacts Prepared by Reaction Milling: J Korean Powder Metall Inst. 1999;6(2):139-144.

Abstract PDF: The pure Ti, Ni and carbon powders were reaction milled to synthesize the TiC-Ni based cermet powders with ultrafine microstructures. After milling, the ultrafine TiC or amorphous Ti-Ni phase was obtained, respectively, according to the milling condition. The effects of milling variables on the synthesizing behavior of the powders were investigated in detail. The sintered TiC-Ni based cermet of the reaction milled powders consisted of very fine TiC of 0.2~1.5mum, as compared with that of a commercial cermet of 3~5mum. This demonstrates the potenial of reaction milling as an effective processing route for the preparation of cermet materials.

Synthesis of Ni Silicides by Mechnical Alloying: J Korean Powder Metall Inst. 1999;6(2):145-151.

Abstract PDF: Nickel silicides (Ni_5Si_2, Ni_2Si and NiSi) have been synthesized by mechanical alloying (MA) of Ni-27.9at.9at%Si, Ni-33.3at% and Ni-50.0at% powder mixtures, respectively. From in situ thermal analysis, eash citical milling period for the formation of the three phases was observed to be 40.2, 34.9 and 57.5 min, at which there was a rapid increase in temperature. This indicates that rapid, self-propagating high-temperature synthesis (SHS) reactions were observed to produce the three phases during room-temperature high-energy ball milling of elemental powders. Each Ni silicide, Ni and Si, however, coexisted for an extended milling time even after the critical milling period. The powders mechanically alloyed after the critical period showed the rapid increase in microhardness. The Hv values were found to be higher than 1000kgf/mm2. The formation of nickel silicides by mechanical alloying and the relevant reaction rates appeared to be influenced by the critical milling period and the heat of formation of the products involved (Ni_5Si_2rightarrow-43.1kJ/mol.at., Ni_2Sirightarrow-47.6kJ/mol.at., NiSirightarrow-42.4kJ/mol.at).

HIP Consolidation and Effect of Process Variables on Micristructure for Ren\'e 95 Superalloy Powders: J Korean Powder Metall Inst. 1999;6(2):152-162.

Abstract PDF: The present study is concerned with the effect of PM process variables on the microstructure by using atomized superalloy powders. It is suggested that the inhomogeneity of composition is strongly dependent on the process variables. The contents of segregation elements of plasma rotating electrode process (PREP) powders are larger than those of Ar atomization (AA) powders. As HIP treatment temperature in-increases, the secondary phases on the prior particle boundaries (PPB) have continuous,uniform distribution and high density, but the amount of PPB decreases suddenly at 1150°C. Segregated phases on the PPB are identified to be MC type carbide. Brittle MC type carbides on the PPB provide fracture initiation sites and preferred fracture path, thereby leading to intergranular type brittle fracture.

Effects of Solid Lubricants on Microsteucture and Mechanical Proerties of Sintered Valve Seats for Automobile Engine: J Korean Powder Metall Inst. 1999;6(2):163-170.

Abstract PDF: Engine valve seat is a functionally important part for maintaining engine performance. The progress of automotive technology has increased the severity of conditions to which it is exposed, especially as regards LPG fuel engines and turbocharger. Therefore, it requires excellent heat and wear resistance to meet the severe condition inside the engine. In the present study, effects of solid lubricants such as CaF_2, MnS and MoS_2 on microstructure and mechanical properties of sintered Fe alloys for valve srats have been investigated for the development of valve seat material with high temerature wear resistance. As a results of engine simulation test, 0.5 wt% CaF_2 specimen showed the most excellent property, but in the overall aspect of view valve recession has increased with increasing the amount of solid lubricants.

Characteristics of Ti-Ni-(XCu) Shape Memory Alloy Powders made by Gas Atomization Process: J Korean Powder Metall Inst. 1999;6(2):171-177.

Abstract PDF: Ti-45.2at.%Ni-5at.%Cu and Ti-40.2at.%Ni-10atat.%Cu alloy powders were fabricated by gas atomization process. The microstructures, Shape, hardness and phase transformation behaviors of the powders were investigated by means of optical microscopy, scanning electron microscopy, micro-hardness measurement, x-ray diffraction analyses and differential scanning calorimetry. The hardness of the Ti-Ni-XCu alloy powders decreased as Cu-content increased. The x-ray diffraction analyses were carried out for powders without heat treatment, and those that treated at 850°C for an hour in a vaccum state(105 torr) and then quenched into ice water. The intensity of B19t phase increased with heat treating. The monoclinic B19t martensite was formed in the Ti-Ni-XCu alloy powders during cooling.

Fabrication of MoSi_2-TiC Composite Powders by Mechanical Alloying: J Korean Powder Metall Inst. 1999;6(2):178-185.

Abstract PDF: MoSi_2-TiC composite powders were fabricated by in-situ reaction through mechanical alloying. Also the monolithic MoSi_2 as well as TiC were synthesiced by mechanical alloying for comparison. An abrupt increase of vial surface temperature was detected due to a sudden reaction between elemental powders during milling. The reaction time for synthesis of composite powders decreased with increasing the content of (Ti+C) powder. It was found that a significant decrease of Ti grain size was observed with increasing the milling time. And the synthesis reaction of MoSi_2-TiC composite powders were largely dependent on the reaction between Ti and C powders.

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