Materials Science Forum Vols. 534-536

Abstract: Low temperature sintering (Ni/Mn)CuZn ferrite was employed at most cases due to its co-firability with Ag (below 960 °C).The purpose of this study is to fabricate (Ni/Mn)CuZn ferrite sintered body with high-strength and high-frequency magnetic properties. Following is the procedure: firstly, (Ni/Mn)CuZn ferrite powder was synthesized under CO2 atmosphere at 500 °C from the mixed doxalate synthesized by co-precipitation method; then a small amount of boric acid [H3BO3] was added to the powder, and the (Ni/Mn)CuZn ferrite powder compact was prepared with Newton press and CIP methods; finally, (Ni/Mn)CuZn ferrite sintered body was fabricated by sintering at 900 °C under CO2 atmosphere. By this method, NiCuZn and MnCuZn ferrite sintered bodies with 0.5 mass% boric acid were obtained, the strength are 430 and 530 MPa respectively. The effect of various Mn addition on NiCuZn electromagnetic properties were studied.

Abstract: Mechanical properties of tungsten heavy alloys are dependent on many factors including the purity of the raw materials, their tungsten content, manufacturing parameters and the microstructure of the final compact. The main object of this research was to examine the effect of sintering conditions (temperature and time) on the microstructure of tungsten heavy alloys and how the resulting modification of the microstructure can be used to optimize their mechanical properties. Alloys composed of 88%, 93% and 95% wt. of tungsten with the balance of Ni: Fe in the ratio of 7:3 were consolidated into green compacts. Samples of each of the three resulting alloys were sintered at different temperatures (1350°C,1450°C and 1500 0C) for different sintering holding times (3 and 30 minutes) in hydrogen atmosphere. Standard metallographic procedures were used to obtain SEM micrographs. The mechanical properties of tungsten heavy alloys were found to be dependent on the microstructural parameters such as W particle size, solid volume fraction, connectivity and w-w contiguity. It was shown that the mechanical properties of the alloys, and especially their ductility, are harmed when tungsten grains are contiguous.

Abstract: 1mm-thick BLT ceramics were sintered in accordance with a bulk ceramic fabrication process. All XRD peaks detected in the sintered ceramics were indexed as the Bi-layered perovskite structure without secondary phases. Density was increased with increasing the sintering temperature up to 1050°C and the maximum value was about 98% of the theoretical density. The remanent polarization (2Pr) value of BLT ceramic sintered at 1050°C was approximately 6.5 μC/cm2 at the applied voltage of 4.5kV. The calculated electromechanical couping factor (kt) of it was about 5% and the mechanical quality factor (Qm) was about 2200. From these results, a BLT ceramic target for plused laser deposition (PLD) system was successfully fabricated.

Abstract: Growth behavior and kinetics of grains in a liquid matrix has been studied by computer simulation for various physical and processing conditions. The kinetics of growing and dissolving grains is considered to follow that of single crystals in a matrix. Depending on the crystal shape, i.e. rounded or faceted, different kinetic equations were adopted for growing grains and a single equation was assumed for dissolving grains. Effects of critical parameters such as step free energy, temperature, and liquid volume fraction were evaluated.

Abstract: The directions of further developments in the modeling of sintering are pointed out, including multi-scale modeling of sintering, on-line sintering damage criteria, particle agglomeration, sintering with phase transformations. A true multi-scale approach is applied for the development of a new meso-macro methodology for modeling of sintering. The developed macroscopic level computational framework envelopes the mesoscopic simulators. No closed forms of constitutive relationships are assumed for the parameters of the material. When a time-step of the calculations is finished for one macroscopic element, the mesostructures of the next element are restored from the initial state according to the history of loading. The model framework is able to predict the final dimensions of the sintered specimen on a global scale and identify the granular structure in any localized area for prediction of the material properties.

Abstract: The removal of oxygen during sintering by carbothermic reduction and the resulting carbon loss were studied for steel compacts prepared with the compositions Fe-3%Cr-0.5%Mo- 0.5%C and Fe-1.5%Mo-0.5%C, respectively, prepared from prealloyed powders. The compacts were prepared by pressing at 600 and 1000 MPa, and sintering was done at 1100 and 1300°C in vacuum. It showed that for the Cr-Mo steel, deoxidation strongly depends on the sintering temperature, in contrast to the plain Mo steel; at 1300°C very low oxygen levels were measured with the standard density compact while after pressing at 1000 MPa, still significant oxygen is contained, which is in agreement with the lower carbon loss measured. This indicates inhibition of final deoxidation by pore closure; nevertheless, the impact energy was very high and exceeded that of the Mo alloyed steels, indicating that finely dispersed oxides within the matrix are irrelevant if only the surface oxides have been completely removed.

Abstract: The particulate dispersive strengthened Cu-MoSi2 composites were prepared by a powder metallurgy process with aim to develop novel copper based composites of reasonable strength, high thermal conductivity and low thermal expansion coefficients. Compacted samples were sintered to over 90% of theoretical density. Microstructure of the composites was investigated by SEM while mechanical properties such as tensile strength, elongation and thermal properties such as thermal conductivity and thermal expansion coefficient (CTE) of the composites were examined as a function of the MoSi2 content and the process of fabrication. A comparative analysis of the mechanical and thermal properties of various Cu-matrix composites currently in use was given and the strengthening mechanisms for the Cu-MoSi2 composites were discussed.

Abstract: The nano-sized Fe powders were prepared by plasma arc discharge (PAD) process using pure Fe rod. The microstructure of the prepared nanopowders was evaluated and the effect of hydrogen gas in the chamber atmosphere was investigated. In addition, the sintering behavior of nanosized Fe powders was analyzed and compared with those of conventional micron powders. The prepared Fe nanopowders had nearly spherical shapes and consisted of metallic core and oxide shell structures. In PAD nanopowder sintering, the higher volume shrinkage at low sintering temperature was observed due to the reduction of surface oxide. The PAD nanopowders showed 6 times higher densification rate and more significant isotropic shrinkage behavior than those of micron sized Fe powders.

Abstract: A new process of pulsed electric current sintering was developed. It combines compaction with activated sintering effectively and can manufacture bulky nano-crystalline materials very quickly. Pulsed electric current sintering of high-energy ball-milled nano-crystalline iron-based powders is investigated in this work. A nanostructured steel is obtained with high relative density and hardness by this process. The average grain size of iron matrix is 58nm and the carbide particulate size is less than 100nm. The densification temperature of ball-milled powders is approximately 200°C lower than that of blended powders. When the sintering temperature increases, the density of as-sintered specimen increases but the hardness of as-sintered specimen first increases and then decreases. Microstructure analysis results show that the decrease of hardness is caused by the dramatic grain growth of iron matrix.

Abstract: This paper concerned with SPS (spark plasma sintering), hot pressing of sinter nanometer WC-Co powder and discussed the density, hardness, microstructures and grain sizes of the alloys sintered by different styles. The results showed that SPS could lower the sintering temperature, increased the density and circumscribed the growth of grain size of WC. Hot pressing sintering could produce high density alloys and play well on the grain growth, but its sintering temperature and sintering time were larger than SPS. Besides, the hardness of the sintered cemented alloys that was dependent on the grain size and densification could also be improved by SPS and hot pressing.