Soon H. Hong

Microstructures of binderless tungsten carbides sintered by spark plasma sintering process

Pure WC was sintered by spark plasma sintering (SPS) process for a binderless cemented carbide application. The relative density of spark plasma sintered WC was over 98% when the SPS temperature was 1700 °C under 50 MPa pressure. Grain growth of WC could be suppressed with full densification by shortening sintering time. When the initial WC powder size was varied from 0.57 to 4.06 μm, the sintered density decreased with decreasing WC powder size. However, WC with an initial powder size of 0.57 μm could be sintered to densification by addition of free carbon. The higher amount of surface oxide in the finer WC powder is considered to have caused decarbonization during the sintering process. For WC powders of 4.06 μm, abnormal grain growth occurred when the sintering time was over 1 min if the sintering temperature was over 1700 °C. Also, with the sintering temperature and sintering time where no abnormal grain growth occurred, abnormal grain growth occurred by addition of carbon. This result shows that the abnormal grain growth in WC sintered by SPS process can be controlled by carbon addition. The abnormal grain growth in sintered WC somewhat increased fracture toughness by crack deflection.

Generalized shear-lag model for load transfer in SiC/Al metal-matrix composites

The load-transfer efficiency of reinforcement, in cylindrical forms in metal-matrix composite (MMC), was analyzed based on the shear-lag model. Both the geometric shape and alignment of reinforcement were considered. The stress transferred to a misaligned whisker was calculated from differential equations based on the force equilibrium in longitudinal and transverse directions. A new parameter, defined as effective aspect ratio, was used to indicate the load-transfer efficiency of misaligned reinforcement. The effective aspect ratio was formulated as a function of aspect ratio and misorientation angle of reinforcement in MMC. A probability density function of misorientation distribution was used to estimate the strengthening effect of all misaligned whiskers distributed in the matrix. Considering the contributions of both effective aspect ratio and misorientation distribution on load-transfer efficiency, a generalized shear-lag model was proposed to explain the mechanical anisotropy of discontinuous reinforced MMC.

Effect of thermomechanical treatments on microstructure and properties of Cu-base leadframe alloy

The effect of thermomechanical treatments (TMT) on the microstructuresand properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy wasinvestigated. The Cu-base leadframe alloy was received as hot rolledplates with 8 mm thickness. The hot rolled plates were solutiontreated at 700°C or 800°C for 1 hour, and coldrolled with 40–85% reduction, then followed by aging treatment at450°C. The leadframe alloy solution treated at 800°Cshowed larger grain size of 15 μm comparing with the grain size of10 μm in leadframe alloy solution treated at 700°C. Theleadframe alloy with smaller grain size of 10 μm showed highertensile strength and lower electrical resistivity than that withlarger grain size of 15 μm. The dislocation density increased withincreasing reduction ratio of cold rolling from 40% to 85% andresulted in finer Ni2Si precipitates. Tensile strength increasedand electrical resistivity decreased with increasing reduction ratioof cold rolling due to the formation of finer Ni2Si precipitates.Two types of thermomechanical treatments were performed to enhance theproperties of leadframe alloy. One type of thermomechanical treatmentis to refine the grain size through the overaging, cold rollingfollowed by recrystallization. The recrystallization process improvedthe tensile strength to 540 MPa and elongation to 15% by reducing thegrain size to 5 μm. The other type of thermomechanical treatmentis to refine the precipitate size by two-step aging process. Thetwo-step aging process increased the tensile strength to 640 MPa andreduced the electrical resistivity to1.475 × 10−8 Ωm by reducing the size of Ni2Si precipitates to 4 nm.

Microstructure and mechanical properties of mechanically alloyed and solid-state sintered tungsten heavy alloys

The mechanical properties of solid-state sintered 93W‐5.6Ni‐1.4Fe tungsten heavy alloys fabricated by mechanical alloying were investigated. Blended W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm employing a ball-to-powder ratio of 20:1 and a ball filling ratio of 15%. A nanocrystalline size of 16 nm and fine lamellar spacings of 0.2 mm were obtained in mechanically alloyed powders at a steady state milling stage. Mechanically alloyed powders were consolidated into green compacts and solid-state sintered at 1300°C fo r1hi n ahydrogen atmosphere. The alloys sintered from mechanically alloyed powders showed fine tungsten particles (about 3 mm in diameter) and a relative density above 99%. The volume fraction of the matrix phase was 11% and the tungsten:tungsten contiguity was determined to be 0.74. The alloys exhibited high yield strengths (about 1100 MPa) due to their fine microstructures, but exhibited reduced elongation and impact energy due to a large area fraction of tungsten:tungsten boundaries and the low volume fraction of matrix phase.

Mechanical properties of WC–10Co cemented carbides sintered from nanocrystalline spray conversion processed powders

Abstract Mechanical properties and microstructures of nanocrystalline WC–10Co cemented carbides were investigated. The nanocrystalline WC–10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375 °C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC–10Co, commercial WC powders in a diameter range of 0.57–4 μm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr 3 C 2 and VC of varying amount were added into nanocrystalline WC–10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC–10Co cemented carbides, Co–W–C alloy was fabricated at the temperature of sintering process for the WC–10Co cemented carbides. The hardness of WC–10Co cemented carbides increased with decreasing WC grain size following a Hall–Petch-type relationship. The fracture toughness of WC–10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.

Effects of vacuum hot pressing parameters on the tensile properties and microstructures of SiC-2124 Al composites

Abstract The effects of vacuum hot pressing temperature and pressure on tensile properties and microstructures of 20 vol.% SiC whisker (SiC w )-2124 Al composites were investigated. The tensile strength of SiC w -2124 Al composite increased rapidly with increasing vacuum hot pressing temperature up to 570 °C and became saturated above 570 °C. The increase in tensile strength with increasing vacuum hot pressing temperature is due to the enhanced densification of composite and reduced damage of whiskers from the softening of 2124 Al matrix with increasing volume fraction of liquid phase. The vacuum hot pressing pressure needed to be higher than 70 MPa to achieve densification of SiC w -2124 Al composite above 99%. A vacuum hot pressing pressure above 70 MPa was not helpful to enhance the tensile strength, since the aspect ratio of whiskers decreased owing to the damage during vacuum hot pressing. Combining the effects of the aspect ratio of whiskers and the density of composite, which are dependent on the vacuum hot pressing parameters, an equation is proposed to estimate the tensile strength of SiC w -2124 Al composite. The effect of porosity on tensile strength of SiC w -2124 Al composite was found to be more sensitive compared with powder metallurgy alloys or ceramics, since the most pores were located at whisker-matrix interfaces and reduced the load transfer efficiency between SiC whiskers and 2124 Al matrix.

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiCp/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO2 as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiCp/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10−6/K were obtained in 50–71 vol% SiCp/Al MMCs. The thermal conductivity of SiCp/Al composite decreased with increasing volume fraction of SiCp and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiCp/Al composite decreased with increasing volume fraction of SiCp, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turners model and the calculated thermal conductivity based on Maxwells model.

Mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy

Abstract The mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy from the elemental powders of W, Ni and Fe by a high energy ball mill in argon atmosphere was investigated. The mechanical alloying process parameters such as milling speed, milling time, ball-to-powder ratio and ball filling ratio were varied in order to investigate their influence on the microstructural evolution of mechanically alloyed powders. The mechanical alloying process proceeded following five distinct stages such as flattening stage, welding dominant stage, equiaxed particle fprming stage, random lamellar forming stage and steady state stage with increasing the milling time. The steady state stage of mechanical alloying was reached after milling for 48 hours with milling speed of 75 rpm, ball-to-powder ratio of 20:1 and ball filling ratio of 15%. Nanocrystalline grain size of 16 nm was obtained at the steady state stage of mechanical alloying. Mechanically alloyed powders were consolidated by cold isostatic pressing and followed by sintering at temperature ranged 1300–1485°C for 1 hour in hydrogen atmosphere. When liquid phase sintered at 1485°C, tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particles about 27μm than that from conventionally blended powders. The density of liquid phase sintered tungsten heavy alloy decreased with increasing the milling time due to the swelling during sintering. When solid state sintered below 1430°C, tungsten heavy alloy from mechanically alloyed powders showed ultra-fine tungsten particles about 3μm and showed high relative density above 97% insensitive to the milling time.

Effects of hot extrusion parameters on the tensile properties and microstructures of SiCw-2124Al composites

The effects of extrusion ratio and extrusion temperature on tensile strengths and microstructures of 20 vol.% SiCw-2124Al composites were investigated. The alignment of SiC whiskers along the extrusion direction in SiCw-2124Al composites was improved and the aspect ratio of SiC whiskers was decreased with increasing extrusion ratio from 10:1 to 25:1. The SiCw-2124Al composite exhibited the highest tensile strength at the extrusion ratio of 15:1. The aspect ratio of whiskers and the relative density of composite increased with increasing extrusion temperature from 470 °C to 530 °C. The tensile strength of SiCw-2124Al composite increased with increasing extrusion temperature up to 530 °C. Based on the concept of the load transfer efficiency of misoriented SiC whiskers in the 2124Al matrix, the two microstructural parameters of the alignment and aspect ratio of whiskers, which influenced on the tensile strength in an opposite way, were combined into a single parameter proposed as an effective aspect ratio. The highest tensile strength of SiCw-2124Al composite at the extrusion ratio of 15:1 is explained by the largest effective aspect ratio due to the balance of the alignment and aspect ratio of SiC whiskers. A modified phenomenogical equation describing the tensile strength of SiCw-2124Al composites as a function of microstructural parameters was proposed by introducing the effective aspect ratio as a substitute for the average aspect ratio of SiC whiskers.

High temperature deformation of Ti-(46-48)Al-2W intermetallic compounds

The high temperature deformation behavior of Ti‐46Al‐2W and Ti‐48Al‐2W intermetallic compounds have been investigated in isothermal compressive tests, performed at temperatures between 1000°C and 1200°C for strain rates between 10 3 and 10 1 s 1 . The stress‐strain curve during high temperature deformation exhibits a peak stress which is followed by a gradual decrease into a steady state stress with increasing the strain. The flow softening behavior after the peak stress is attributed to the effects of dynamic recrystallization during deformation. The dependence of flow stress on temperature and strain rate followed a hyperbolic sine relationship using the Zener-Hollomon parameter. The activation energies, Q, were measured as 449 kJ mol 1 and 394 kJ mol 1 , and the stress exponents were measured as 3.6 and 3.7 for Ti‐46Al‐2W and Ti‐48Al‐2W, respectively. The activation energy increased with decreasing Al content in TiAl-base intermetallic compounds. The coefficient between peak stress and Zener-Hollomon parameter, A, was not a constant, but was dependent on the activation energy. The peak stresses can be predicted well by using a normalized Zener-Hollomon parameter. The dynamic recrystallization rate and recrystallized grain size increased with increasing the temperature and with decreasing the strain rate.