In-Hyung Moon

Dilatometric analysis on the sintering behavior of nanocrystalline W-Cu prepared by mechanical alloying

Abstract Unlike conventional W–Cu powder, the sintering of nanocrystalline W–Cu powder was significantly enhanced at solid phase sintering temperature. In the present study, in order to clarify this enhanced sintering behavior of nanocrystalline W–Cu powder prepared by mechanical alloying, the sintering behavior during heating stage was analyzed by dilatometry with various heating rates. The sintering of the nanocrystalline W–Cu powder was characterized by the densification of two stages, having two peaks in shrinkage rate curves. The temperature at which the first peak appeared was well below the Cu melting point and significantly dependent on the heating rate. On the basis of dilatometric data and microstructural observation, the possible mechanism for the enhanced sintering of the nanocrystalline W–Cu powder in the solid phase was attributable to the coupling effect of the sintering occurring inside as-milled powder and the sintering between powder particles.

Thermal conductivity of W–Cu composites at various temperatures

Abstract The effect of temperature on the electrical resistivity and thermal conductivity of W–Cu composites had been studied. In case of pure W and Cu, the thermal conductivity decreased with increase in temperature. However, the W–Cu composites showed increased thermal conductivity with increase of temperature up to 500°C, and then the conductivity decreased with increasing temperatures. This increase in thermal conductivity is explained by the increased number and movement of electrons as well as increased conduction paths for the electrons. The scattering of phonons is believed to be responsible for the decrease of thermal conductivity at the temperature of above 500°C.

SINTERING OF NANOSTRUCTURED W-Cu ALLOYS PREPARED BY MECHANICAL ALLOYING

Abstract Nanostructured (NS) powders with compositions corresponding to W-20wt%Cu and W-30wt%Cu were prepared by mechanical alloying. The microstructure and grain size of as-milled and annealed powders were analyzed by transmission electron microscopy. The compacted specimens were sintered at temperatures in the range 1000 °–1300 °, and then the microstructures of sintered parts were analyzed by scanning electron microscopy. Sintering of mechanically alloyed W-Cu alloys appears to be independent of Cu content, and may be explained in terms of recovery and grain growth in the mechanically alloyed powders as well as impurity activated sintering of W. After sintering, Cu pools are formed outside the mechanically alloyed powders. A relative sintered density of more than 95% is obtained by particle rearrangement during liquid-phase sintering, and the greatest homogeneity of W and Cu phases is achieved by sintering at 1200 °.

Densification behavior of mechanically alloyed W-Cu composite powders by the double rearrangement process

Department of Materials Engineering, Hanyang University, Seoul 133-791, Korea(Received January 22, 1998)(Accepted in revised form May 25, 1998)IntroductionTungsten-copper(W-Cu) composites are promising materials for thermal managing applications such asmicroelectronic devices because of the low thermal expansion coefficient of tungsten and the highthermal conductivity of copper (1–3). These materials have been produced by conventional Cu-infiltration sintering or by liquid-phase sintering (4, 5). The full densification of W-Cu composites byliquid-phase sintering is attained mainly by the tungsten particle rearrangement due to the capillaryforce and surface tension of Cu-liquid phase (6, 7) as well as solid-state sintering, because W and Cuhave no solubility under equilibrium condition (8). If the constituent phases are extremely finehomogeneous state, the fully dense parts with the homogeneous microstructure can be attained at thestage of particle rearrangement. To promote homogeneity, much works such as the coreduction methodof the raw oxide powders (9, 10) or mechanical alloying of the element powders (11, 12) have beenstudied. The mechanical alloying method is the attractive one from the engineering viewpoint becausethe nanostructured(NS) materials can be easily synthesized in large quantities. Nevertheless, the studiesof nanostructured W-Cu alloys have been focused only on the formation and the fabrication ofcomposite powders by mechanical alloying (11–13). And there were few researches of liquid-phasesintering behavior of NS W-Cu composite powders compared to nanostructure characteristic study.The authors have recently reported on the characteristics of nanostructured W-Cu alloys producedby mechanical alloying method (14, 15). In this study, the new concept of nanosintering was suggestedto explain the drastic grain growth of mechanically alloyed NS W-Cu powders during the solid-statesintering. The high sinterabilty of these mechanically alloyed NS W-Cu alloys was also obtained byliquid-phase sintering. In this present work, the enhanced sinterabilty of mechanically alloyed NS W-Cucomposite powders with sintering temperature is investigated and evaluated by the combination ofnanosintering and conventional liquid-phase sintering.ExperimentalElemental W (99.9% purity, 4.28mm) and Cu(99.5% purity, 50.42mm) powders were used for rawpowders in this experiment. The mechanical alloying was carried out in an attrition mill using a stainlesssteel vial and balls with a speed of 400 rpm. The powder charge was 20g, with balls to powders weightratio of 60:1. The W-20wt%Cu and W-30wt%Cu composite powders were prepared by milling for 100

High temperature performance of dispersion-strengthened AlTi alloys prepared by mechanical alloying

Abstract Dispersion-strengthened AlTi alloys were prepared by the mechanical alloying (MA) process and hot consolidation method, and their high temperature mechanical properties and oxidation properties were investigated. The tensile strengths of MA AlTi alloys were substantially higher than those of conventional Al alloys such as 7075 and 2024 grades at temperatures above 150°C. The compressive strength and the yield strength of MA Al8wt.%Ti at 350°C with a strain rate of 1 × 10 −4 s −1 were 233 MPa and 186 MPa respectively. The measured parabolic rate constant k p in oxidation of the MA Al8wt.%Ti alloy at 600°C was a much lower value, approximately 1.12 × 10 −13 g 2 cm −4 s −1 , which revealed that the oxidation resistance of the MA alloy was much better than that of the 2024-TB Al alloy.

Processing and fracture toughness of nano-sized Cu-dispersed Al2O3 composites

Abstract An optimum route for the fabrication of Al 2 O 3 /Cu nanocomposites with sound microstructure and enhanced fracture toughness was investigated. Cu-dispersed Al 2 O 3 nanocomposites were obtained via hydrogen reduction and the pulse electric current sintering (PECS) of ball-milled Al 2 O 3 and CuO powder mixtures. Microstructural observation of the reduced powder revealed that Cu particles of about 30 nm size homogeneously surrounded the Al 2 O 3 powder. The nanocomposites, consolidated by PECS at 1250 °C for 5 min, exhibited full density and enhanced fracture toughness compared with monolithic Al 2 O 3 .

Micro metal powder injection molding of W-Cu nanocomposite powder

Increasing attention is being paid to micro metal injection molding as a manufacturing technology for miniature part. W-Cu nanocomposites have been used as heat sink and packaging materials in microelectronic devices. A micro injection molding technique will provide and appropriate tool to fabricate W-Cu nanocomposite materials for microcomponents. In the present study, a fundamental investigation of micro metal injection molding using W-Cu nanocomposite powder is reported. The densification behavior of W-Cu nanocomposite was examined in order to confirm the shape stability of microcomponents.

Nickel-enhanced grain growth in tungsten wire

Abstract The effect of Ni additive on recrystallization and grain growth in W wires has been investigated in order to explain the role of nickel in the activated sintering of tungsten, because the two phenomena are comparable with each other, being principally diffusion controlled and greatly influenced by the presence of Ni. The experiment was carried out in a bundle model of W wires in which the initial state of nickel could be appropriately controlled by arranging Ni-coated W wires together with non-coated wires. The recrystallization and grain growth processes were observed and analysed on the basis of the geometrical relation between the Ni source and the W wires. The development of the recrystallization front depends on the Ni diffusion path and rate. Nickel diffuses fast into the W wires through high diffusivity paths such as the surface and grain boundaries, inducing recrystallization at relatively low temperature. The apparent diffusivity of Ni in W wire, estimated by measuring the migration rate of the recrystallization front, is about 1.7 × 10 −9 cm 2 s −1 , while the surface diffusivity of Ni on W is estimated to be about 10 −5 cm 2 s −1 . The role of Ni in W is further discussed.

Formation of halo in Sb-InSb and Sn-Bi eutectic alloy systems

Abstract A revised model for the halo formation was proposed to modify the previous ones, mainly with regard to the unreasonable estimation of the interdendritic liquid composition. In the present work, the halo formation was explained on the basis of the competitive growth relation between halo and eutectic at the composition of coupled region boundary. Halo formation, therefore, is not confined to nf-f eutectic systems, but can be extended to nf-nf or f-f eutectic systems. The present model was qualitatively examined in Sb-InSb and Sn-Bi eutectic systems. Halo formation was observed to be dependent on growth rate. This growth rate-dependence can be expected in view of its close relation with the composition of coupled region boundary. However, its alloy composition dependence as predicted by the present model was not observed. This unexpected result was also briefly discussed.

On some physical properties of nanostructured Cu-Pb alloy prepared by mechanical alloying

Cu-Pb alloys have no solubility in the whole solid state and their physical properties are very different from each other. In the present study, nanostructured Cu-Pb alloy powders were synthesized by the mechanical alloying process, and their nanostructural characteristics were evaluated in order to elucidate the relationship between structure and properties. By appropriate control of mechanical alloying process variables, the Pb solid solubility in Cu matrix was increased up to 10 wt.%. The monotectic temperature of Cu-Pb alloy was also decreased by decreasing the crystalline size. The relation between the structure and properties of this nanostructured Cu-Pb alloy is discussed on the basis of the experimental results.