To obtain an optimal powder bed condition for laser powder bed fusion (L-PBF), it is essential to clarify powder flow behaviors during the recoating process. In this work, the flow behaviors of Ti-6Al-4V powders were experimentally analyzed using a simple slope model by Particle image velocimetry (PIV) and were subsequently compared with the behaviors simulated using the similar model by discrete element method (DEM). The velocity of particles and the distribution were able to be obtained experimentally using PIV. In the DEM simulation, when the coefficient of restitution was increased, the angle between restituted particles and the slope and the average particle velocity were increased. Comparing those obtained by the DEM simulation, it was found that the velocity and distribution obtained by PIV analyses were close to those obtained by the DEM simulation at the coefficient of restitution of 0.3. These results suggest that the powder flow velocity analysis on the slope using PIV analyses is an effective method to validate the parameter of powder properties in DEM simulations.
The La-Co cosubstituted M-type strontium ferrite attracts attention as a base material for high-performance ferrite magnets. It is known that the uniaxial magnetic anisotropy of the material is enhanced by increasing the amount of Co by heat treatment under high oxygen pressure, but there is a problem in obtaining a pure sample. The present study investigated the conditions to obtain a single phase with increased La-Co substitution by heat treatments under several oxygen pressures. A single phase of M-type ferrite was obtained up to x = 0.35 at pO2 = 1 atm, and up to x = 0.65 at pO2 = 10 atm with the composition formula of Sr1-xLaxFe12-xCoxO19. The magnetic anisotropy is enhanced according to the Co concentration in these samples.
The fracture of WC-Co cemented carbides with different WC particle sizes was investigated experimentally, and then a discrete element method (DEM) simulation was performed considering such experiments. The bending strength of ultra-fine grained cemented carbide with a WC particle size of 0.4 μm was extremely high. The ultra-fine grained cemented carbide sample after bending test had numerous fracture pieces and the short length of the specimens without fracture. Initial structures consisting of matrix and defect (d) particles were used in the DEM simulations. The DEM simulation results for a sample of 2% d particle, which was considered ultra-fine grained cemented carbide by comparison with experimental results, were as follows: The sample with the highest load at fracture, the most d particle bond-breaking, and the shortest length of remaining specimen without particle bond breaking was the 2% d sample. The DEM simulation was considered to be very useful in understanding the ultra-fine grained cemented carbide’s strength and fracture behavior.
Joined plates of WC-Co cemented carbides with different Co content, WC grain size and carbon content were fabricated by sintering, grinding into rectangular plates, and then reheating (joining) the two plates together. Distribution of Co, microstructure and shape of such joined plates were investigated in detail. It was confirmed that Co migrated from higher Co content to lower one, from coarser WC grains to finer ones and from higher carbon content to lower one as shown in previous studies by Lisovsky and Fang. Shapes of the joined plates were distorted caused by volume change of each plate resulting from migration of liquid Co and solute WC through the interface of the joined plates. In the joined plates with different Co content, Co distributed discontinuously at the interface. This result can be explained by the fact that Co content at the both sides of the interface had been almost fixed just after two faces were joined. This study was done under the special condition using joined plates, but the results obtained suggest that the shape of industrial-made WC-Co cemented carbides should be distorted when distribution of Co becomes non uniform.
The uniformity of shrinkages and the distribution of grain sizes were investigated for flash-sintered 3 mol%Y2O3-ZrO2 compacts in direct current and alternating current (AC) electric fields using rectangular-shaped and circular truncated cone-shaped green compacts. The use of a high-frequency AC electric field enough to suppress electrode overvoltage was confirmed to be advantageous for uniform shrinkages, regardless of the shapes of the green compacts. However, the grain size distribution became larger in cone-shaped green compacts, the cross-sectional areas of which varied along the compact. The grain sizes in flashed cone-shaped green compacts were found to exhibit a near linear relation to current density, which provides a means for determining the grain size distribution upon the flash sintering of green compacts with different cross-sectional areas along an electric field.
The magnetic properties and microstructures of the HDDR powders fabricated from the NdFeB sintered magnets were investigated in order to improve their orientation. In case of fabricating from the strip cast alloys, the hydrogen-pulverized powders showed the dependence of the orientation on their powder sizes. On the other hand, the pulverized powders from the sintered magnets showed high orientation regardless of their powder sizes. This is due to inheritance of the high crystal orientations of the sintered magnets as starting materials. And the formation of the fine Nd2Fe14B crystals just at the surface area of the pulverized powders could be processed by the additional annealing under Ar atmosphere in advance of the HDDR treatment. As a result, the residual magnetization of the “capsule-structured” HDDR powders was improved from 100 to 125 emu/g due to remaining the unreacted Nd2Fe14B phases with high orientation at the inner core of the HDDR powders. This technique would provide the anisotropic NdFeB bonded magnets with high (BH)max, and the new environment-friendly recycling process of the NdFeB magnets with low cost.
Uniaxial pressing is a utilized technique to prepare powder compacts in regions of ceramics, metal, drug, and functional food. On the other hand, it is known that the inhomogeneous structures are induced by stress distribution resulting from applying the anisotropic stress field to the powder bed and they become the origin which forms defect degrading the performance of products. However, the generation process of inhomogeneous structures has not been clarified experimentally. Directly understanding the formation mechanism of the internal structure during uniaxial pressing is very important to control the particle-assembled structure in the powder bed and the quality of products. In this work, the change process in void morphology under uniaxial pressing was observed dynamic-two-dimensionally and intermittent-three-dimensionally by the novel observation system composed of swept-source optical coherence tomography (SS-OCT) and material testing machine. Moreover, the detected voids were evaluated quantitively to directly clarify particle-packing behavior on the initial stage of pressing.
TiC-47 mass% Ti-20 mass% Mo (Mo added alloy) and TiC-52 mass% Ti-21 mass% Mo2C (Mo2C added alloy) were prepared through powder metallurgy. The TiC grain size of Mo2C added alloy sintered at 1673 K was about 3 μm, which was smaller than that of the Mo added alloy. The mechanism was considered as follows. First, when C diffuses from Mo2C to α-Ti, the Mo phase containing defects such as vacancies precipitates. Second, the solid solution of Mo and Ti is promoted by the mutual diffusion between the precipitated Mo phase and α-Ti, and the transformation from α-Ti to β-Ti occurs. Third, the solute diffusion coefficient of β-Ti is much larger than that of α-Ti, so C diffuses from TiC to β-Ti, producing TiC0.6. Meanwhile, Mo has a smaller free energy for carbide formation than Ti has and then TiC0.6 cannot contain Mo, so Mo is concentrated in β-Ti adjacent to precipitated TiC0.6. This suppresses the growth of TiC0.6, and then the grain size of TiC0.6 becomes smaller. The transverse-rupture strength of the Mo2C added alloy was 20% higher than that of the Mo added alloy, reaching 0.95 GPa. At that time, both alloys have almost the same hardness.
In the present study, we have examined the wear properties of the sintered pure iron subjected to two distinct heat treatments such as nitriding-quenching (NQ) and carburizing-quenching (CQ). Based on our current observations, the martensite layer was formed on the surface layer following each treatment, whereas the hardness of the NQ martensite was much higher than that of the CQ one. The wear of the CQ specimen was slightly smaller than that of the NQ martensite, despite the lower value of the initial hardness. Meanwhile, the hardness of the CQ surface after the sliding tests significantly elevated relative to the NQ surface resulting in the better wear resistance. EBSD analysis demonstrated that the plastic deformation on the CQ surface along the sliding direction. Furthermore, the micro area X-ray diffraction along the surface layer of the CQ surface showed that a small amount of the retained austeite which reduced locally during the test. Therefore, the CQ-treated surface showed the excellent wear resistivity due to the surface hardening by the stress-induced transformation of the retained austenite dispersed in the martensite, in addition to the strain hardening of the martensite itself. In contrast, the worn surface of the NQ specimen showed slight plastic deformations of the ferrite grains beneath the martensite layer, but not in the surface martensite layer. This deformation under the martensite layer was due to the hardness gap between inward and the heat-treated surface, and might contribute to form the concave profile on the sliding surface. Consequently, this study could demonstrate such the difference in the wear mechanisms between the CQ and the NQ specimens.
We report synthesis and characterization of structural and physical properties in YbInCu4 and Yb-rich Yb1+xIn1-xCu4. We have measured the powder X-ray diffraction, the temperature dependence of the magnetic susceptibility between 2 and 300 K in the magnetic field of 1 T, the high-field magnetization up to 72 T and the temperature dependence of the electrical resistivity in the magnetic field of 0 and 14 T to study the change of physical properties by substitution and to understand the origin of the valence transition.
The powder X-ray diffraction patterns suggest that Yb1+xIn1-xCu4 crystallized in the cubic C15b-type Laves structure. We newly discovered a two-step magnetic anomaly in the high-field magnetization process interpreted by crystal field splitting effects. We finally made a magnetic phase diagram in H-x, which is consisting of 4 phases. Three phase boundaries are defined by valence transition from Yb valence fluctuation to Yb trivalent localized magnetism, change of the ground state due to crystal field splitting effects and disturbance of Kondo lattice.
Well-defined SiO2-coated Fe nanoparticles with various SiO2 thickness from 1.2 to 27.8 nm have been successfully prepared by controlling the amount of tetraethyl orthosilicate and by the subsequent reduction of SiO2-coated Fe3O4 nanoparticles with CaH2. The saturated magnetization of the SiO2-coated Fe nanoparticles increased with decreasing SiO2 thickness. The saturated magnetization of the SiO2-coated Fe nanoparticles with SiO2 thickness of 2.7 nm or more have slightly decreased by 192 hr and did not change above 192 hr throughout atmospheric exposure, whereas that with SiO2 thickness of 1.2 nm steeply decreased in 24 hr and continued to decrease above 24 hr.
Molecular dynamics simulations were performed to study six grain boundaries of α-alumina (Al2O3) with a glassy phase of anorthite (CaAl2Si2O8) composition. We calculated excess energy, diffusion constant and ratio of excess volume with different thickness of the glassy film. It was found that excess energy for some grain boundaries exhibited a minimum. When the thickness of the glassy film was thick adequately, excess energy corresponded to the energy of alumina-glass interface and they were different for each interface. Diffusion constants depended on the thickness of the glassy film. The diffusion constant of thin film was smaller than that of thick film. Excess volume was the maximum when the thickness of the glassy film was 0.2~0.3 nm. When the atomic arrangement of the crystals didn’t fit each either, the excess volume of the grain boundary with the glassy film was smaller than that of the pure grain boundary. When the glassy film width was nm order, the atomic arrangement of the glassy phase was regular and the atomic diffusion behavior was approached that of a solid (crystalline) phase. We need to consider not only solid-liquid interface but also solid-solid (crystalline) interface for the structure of ceramics made by liquid phase sintering.