Effect of binder compositions on microstructure, hardness and magnetic properties of (Ta,Nb)C-Co and (Ta,Nb)C-Ni cemented carbides

Daoush, W.M.,Park, H.S.,Lee, K.H.,Moustafa, S.F.,Hong, S.H. MPR Pub. Services 2009 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.27 No.4

Fabrication by powder technology routes, as well as microstructure characterization and measurement of hardness and magnetic properties of two types of (Ta,Nb)C sintered carbides having different composition of Ni and Co binders were carried out in the present study. Each carbide binder composition mixer was cold pressed in a uniaxial direction die at 400MPa followed by vacuum sintering at 1450<SUP>o</SUP>C. Nearly full dense (Ta,Nb)C sintered carbides with a relative density between 99.5% and 99.8% were obtained. The highest macrohardness values were observed for composition of (Ta,Nb)C-5wt%Co and (Ta,Nb)C-5wt%Ni of 1400 H<SUB>V</SUB> and 1300 H<SUB>V</SUB> respectively. The magnetic B-H hysteresis loops were measured at 0.8T by the VSM method. In the case of (Ta,Nb)C-Co, higher magnetic saturation values of 47emu/g were obtained for (Ta,Nb)C-30wt%Co and 13.5emu/g for (Ta,Nb)C-5wt%Co. However, the magnetic saturation values for the prepared carbide having a composition of (Ta,Nb)C-Ni displayed smaller magnetic saturation than the corresponding (Ta,Nb)C-Co carbides. On the other hand, the coercivities of the (Ta,Nb)C-Co has higher value than those of (Ta,Nb)C-Ni. As the metal binder phase was increased, the coercivity decreased. The influence of the distribution of Ta, Nb, Co and Ni in the carbide phase and the metal binder phase were investigated by EDAX compositional analysis and its relation with the hardness and the magnetic properties were studied.

High temperature ablation resistance of ZrN_p reinforced W matrix composites

Umer, M.A.,Lee, D.,Ryu, H.J.,Hong, S.H. MPR Pub. Services 2014 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.42 No.-

For the purpose of improving the high temperature ablation resistance of tungsten-based materials, a new class of composites was fabricated using particulate ZrN as reinforcement. The high temperature ablation behavior of the composites was investigated using oxyacetylene torch. Three different volume contents (up to 30vol.%) of ZrN were added to tungsten. It was observed that the addition of ZrN particles significantly enhanced the ablation resistance of the tungsten composites. Higher volume fraction of ZrN particles resulted in lower ablation rates. It was observed that ZrN particles, when exposed to the flame, formed a stable oxidation layer of ZrO<SUB>2</SUB> on the surface of the sample. This layer prevents further damage to the tungsten matrix, thus resulting in lower ablation rates. The effect of ZrN reinforcement content on the thermal behavior of the tungsten-based composites was also investigated. Thermal conductivity of the composites was seen to decrease as a function of temperature and reinforcement content. Lower values of thermal conductivity are helpful in enhancing the elevated temperature performance of the composites.

Analysis of the effect of solubility on the densification behavior of tungsten heavy alloys using the master sintering curve approach

Park, S.J.,Johnson, J.L.,Wu, Y.,Kwon, Y.S.,Lee, S.,German, R.M. MPR Pub. Services 2013 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.37 No.-

The densification behavior of 88W-8.4Ni-3.6Fe, 88W-8.4Ni-3.6Cu, and W-15Cu during heating is compared using the master sintering curve (MSC) approach. The MSC parameters, such as the work of sintering, activation energy, and densification ratio, are calculated from dilatometry tests, based on which the densification is classified into three regions with different sintering mechanisms. This approach enables contrast and comparison of the densification sensitivity to solubility, temperature, and heating rate. The master sintering curves are combined into a master sintering surface that includes integral work and the solubility of tungsten in the liquid phase.

Low-temperature synthesis of zirconium metal using ZrCl₄-2Mg reactive mixtures

Park, K.T.,Nersisyan, H.H.,Lee, H.H.,Hong, S.I.,Cho, N.C.,Lee, J.H. MPR Pub. Services 2012 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.33 No.-

In this paper, we describe an attractive and rapid process for preparing zirconium metal (sponge and powder) by reacting zirconium tetrachloride with magnesium powder. The process was carried out in the combustion synthesis (shortly CS) mode using a ZrCl<SUB>4</SUB>-2Mg exothermic mixture, which provided a temperature of 1000-1100<SUP>o</SUP>C and a final product consisting of MgCl<SUB>2</SUB> and Zr sponge. The separation of Zr sponge from the reaction product was accomplished by dissolving MgCl<SUB>2</SUB> into warm acidified water. The average size of the Zr sponge particles ranged from about 1mm to about 10mm. Micrometer-sized (from 10 to 300μm) Zr powders were also obtained after mechanical processing of sponge. The characteristics of the Zr metal were established by X-ray diffraction, scanning electron microscopy, and inductively coupled plasma spectroscopy techniques.

Development of non-eroding rocket nozzle throat for ultra-high temperature environment

Park, D.Y.,Oh, Y.J.,Kwon, Y.S.,Lim, S.T.,Park, S.J. MPR Pub. Services 2014 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.42 No.-

The powder processing methods including powder metallurgy (P/M) and powder injection molding (PIM) techniques for tungsten (W)-rhenium (Re) were employed to produce a W-Re rocket nozzle. The composition of W-Re was determined by 25wt.% of Re to avoid the formatting brittle sigma (σ) phase. The samples for analysis of the densification behavior on sintering were prepared by die pressing and cold isostatic pressing (CIP). The feedstock for the PIM process was produced by mixing the W-25wt.% Re powder and binder system based on a wax-polymer with an optimum solid loading through the twin-extruder mixer. The injection molded specimens were debound to extract and decompose the binders via the solvent and thermal debindings. The debound samples were sintered in a hydrogen atmosphere. After sintering, hot isostatic pressing (HIP) was carried out in an argon atmosphere to enhance the density. The dilatometry experiments were performed to analyze and predict a densification behavior during sintering. The master sintering curve (MSC) model was used to characterize the densification behavior with a minimal set of preliminary experiments. The mechanical properties were evaluated through microstructure and chemical composition measured by EDX-SEM and X-ray diffraction (XRD). Finally, the eroding test was conducted using the W-25wt.% Re rocket nozzle produced by PIM under the high temperature. After carrying out erosion tests, the erosion rate, hardness and microstructure were evaluated.

A novel approach to synthesis of scandia-doped tungsten nano-particles for high-current-density cathode applications

Barik, R.K.,Bera, A.,Tanwar, A.K.,Baek, I.K.,Min, S.H.,Kwon, O.J.,Lee, W.S.,Park, G.S. MPR Pub. Services 2013 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.38 No.-

A novel synthesis approach for scandia-doped tungsten nano-powder using a sol-gel method is developed. It involves dissolving tungsten oxide at 300<SUP>o</SUP>C in the presence of a mixture of nitric acid, citric acid and ammonia. The dissolved tungsten oxide reacts with an aqueous solution of scandium nitrate in the liquid-liquid phase, which results in the homogeneous mixing of tungsten and scandium particles. A spherical shape particle was obtained due to the dissolving of tungsten oxide in the solution. Citric acid enhances the mixing of ions at the atomic scale, which affects the hydrolysis reactions and leads to the formation of the phase pure nano-particle. The synthesized nano-powder was characterized by SEM (Scanning Electron Microscopy), EDS (Energy-dispersive X-ray spectroscopy), TEM (Transmission Electron Microscopy), and XRD (X-ray Diffraction) analyses. The spherical morphology was observed via a SEM analysis and a narrow particle size distribution was noted by means of a TEM analysis. The XRD analysis of the powder showed the complete formation of the phase pure nano-particle with an average diameter of 50nm without any contamination by other materials.

In-situ synthesis of tungsten nanoparticle attached spherical tungsten micro-powder by inductively coupled thermal plasma process

Han, C.,Na, H.,Kim, Y.,Choi, H. MPR Pub. Services 2015 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.53 No.1

Tungsten spherical micro-powder, tungsten nanoparticle, and tungsten nanoparticle attached spherical micro-powder were synthesized by manipulating feedstock and plasma gas composition in the present study. Powder preparation strategy is simple and phase evolution pathway of each component was tailored by exploiting thermos-physical properties of feedstock and thermal plasma reactivity. Irregular W aggregates underwent melting-solidification pathway and accordingly, spherical micro-powders were obtained. On the other hand, low boiling point WO<SUB>3</SUB> micro-powders were vaporized and reduced in Ar-H<SUB>2</SUB> thermal plasma. After that, vapor species condensed to nanoparticles which were composed of α-W and β-W. These results demonstrated that phase evolution pathway and resulting powder properties were determined by feedstock powders. In this context, W and WO<SUB>3</SUB> blended powders were prepared by a simple blending and they were fed into Ar-H<SUB>2</SUB> thermal plasma. Owing to individual interactions of in-flight particles with thermal plasma and co-flight of spherical W micro-powders and W nanoparticles, nanoparticle dispersed spheroidized micro-powder was in-situ synthesized. 3 dimension architecture of the synthesized nanoparticle dispersed micro-powder was revealed by ultrasonic detachment.

Fabrication and mechanical properties of powder metallurgy tantalum prepared by hot isostatic pressing

Kim, Y.,Kim, E.P.,Noh, J.W.,Lee, S.H.,Kwon, Y.S.,Oh, I.S. MPR Pub. Services 2015 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.48 No.-

The fabrication process of a powder metallurgy (P/M) tantalum product with full density and fine microstructure was developed by using cold and hot isostatic pressing techniques. In order to increase the compact density and make the uniform density distribution, cold isostatic pressing (CIPing) of tantalum powders was conducted. Prior to hot isostatic pressing (HIPing), the CIPed billet was encapsulated and degassed to remove the contaminants in the container. After degassing, HIPing was performed twice and full densification of the tantalum powders was accomplished, regardless of powder size. The effect of processing conditions on the microstructure and mechanical properties of P/M tantalum billets was investigated. As the number of processing steps and temperature increased, the grain size of HIPed tantalum billets increased. Moreover, contrary to the Hall-Petch relation, the mechanical strength was increased in spite of increasing the grain size. This is because the oxygen content of the billets increased with rising in temperature and the number of processing steps. Therefore, in case of tantalum, it is found that the mechanical properties of tantalum may be highly influenced by the amount of interstitial elements, especially oxygen, rather than microstructural properties.

High-temperature compression behavior of W-10wt.%Cu composite

Lin, D.,Han, J.S.,Kwon, Y.S.,Ha, S.,Bollina, R.,Park, S.J. MPR Pub. Services 2015 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.53 No.2

Hot deformation behavior of tungsten-copper (W-Cu) composite at elevated temperature was investigated by hot-compression testing using a Gleeble 3500 thermo-mechanical simulator. W-10%Cu composite was fabricated by copper infiltration on porous tungsten skeleton (CIT) method. The porous tungsten skeleton was prepared by powder injection molding (PIM) process using tungsten powders and wax-polymer binder system. The hot deformation equations for W-10%Cu composite were determined through strain-stress relations at elevated temperatures, where the strain rate is related to static stress, deformation temperature and activation energy. The effects of deformation temperature and strain rate on the mechanical properties of W-10wt.%Cu composite were investigated through flow strain-stress curves at elevated temperatures and its microstructures before and after deformation.

Spark plasma sintering of WC-Co tool materials prepared with emphasis on WC core-Co shell structure development

Lee, S.,Hong, H.S.,Kim, H.S.,Hong, S.J.,Yoon, J.H. MPR Pub. Services 2015 INTERNATIONAL JOURNAL OF REFRACTORY METALS AND HAR Vol.53 No.1

Quasi-nano-sized WC core-Co shell structure composites were successfully fabricated by the wet chemical reduction method by introducing uniformly dispersed WC powders of 300-nm size into the Co(OH)<SUB>2</SUB>, which was subsequently reduced to Co shell structure by drop-wise titration of H<SUB>3</SUB>PO<SUB>2</SUB>. X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) identified WC core-Co shell phases which were subsequently ball-milled, compacted and finally densified at 1400-1450<SUP>o</SUP>C by spark plasma sintering. WC-Co sintered composites thus prepared were analyzed to examine densification, hardness and fracture behavior: reasonably adequate post-sintering properties are attributable to improved WC core-Co shell structure afforded by the optimized mechanical alloying and SPS sintering processes.