Kinetic characteristics of liquid phase sintering of mechanically activated W-15wt%Cu powder

The kinetic characteristics of W grain growth operated by diffusion controlled Oswald ripening （DOR） during liquid phase sintering were studied. A liquid phase sintering of W-15wt%Cu was carried out by pushing compacts into a furnace at the moment when the temperature increased to 1340℃ for different sintering times. The results show that liquid phase sintering produces the compacts with considerably low relative density and inversely, rather high homogeneity. On the basis of the data extracted from the SEM images, the kinetic equation of W grain growth, G^n = G0^n ＋ kt, is determined in which the grain growth exponent n is 3 and the grain growth rate constant k is 0.15 μm^3/s. The cumulative normalized grain size distributions produced by different sintering times show self-similar. The cumulative distribution function is extracted from the curves by non-linear fitting. In addition, the sintering kinetic characteristics of W-15wt%Cu compacts were also investigated.

Acting mechanism of F,K,and Na in the solid phase sintering reaction of the Baiyunebo iron ore

The effect of F,K,and Na on the solid phase reaction of the Baiyunebo iron ore was investigated by differential thermal analysis （DTA） and X-ray diffraction（XRD）.It has been identified that alkaline elements K and Na in the Baiyunebo ore instigate the formation of low melting point compounds Na2SiO3 and Na2O·Fe2O3 and the generation of molten state in the solid phase sintering.Element F in the Baiyunebo ore facilitates the formation of cuspidine compound 3CaO·2SiO2·CaF2 in the solid phase reaction.The cuspidine compound is kept in solid as one of the final products through the entire sintering process due to its high melting point.In the sintering process,CaF2and SiO2 react with CaO first and form 3CaO·2SiO2·CaF2 and 3CaO·2SiO2,so the formation of ferrites,Na2O·Fe2O3,and 2CaO·Fe2O3 is inhibited.

Liquid Phase Sintering (LPS) and Dielectric Constant of α-Silicon Nitride Ceramic

The spark plasma sintering （SPS） was applied to prepare α-Si3 N4 ceramics of different densities with magnesia, silicon dioxide, alumina as the sintering aids. The mechanism of liquid phase sintering （LPS） wus discussed and the factors influencing the density of the prepared samples were analyzed. The dielectric constant of sintered samples was tested. The experimental results show that the density can be controlled from 2.48 g/ cm^3 to 3.09 g/ cm^3 while the content of the sintering aids and the sintering temperature alter and the dielectric constant is closely dependent on the density of obtained samples.

Influence of SiO2 on bonding phase fluidity in iron ore sintering

The influence of SiO2 on bonding phase fluidity was studied by measuring the projected area of tablets prepared using mixtures of fine iron ore and chemical reagents after sintering. The microstructure of samples was observed by optical microscope and SEM. Moreover, the relationship between SiO2 and bonding phase fluidity and microstructure was analyzed by CaO-Fe2O3-SiO2 system 1 300℃ isothermal section diagram. The main experimental conditions are as follows： in the composition of the bonding phase,CaO is 10%, 15% ,and 20%; SiO2 is 2% and 6% ; sintering temperature is 1 250, 1 300 and 1 350 ℃ ; the sample cooling rate is quick. The results of the study show that.① The fluidity of the binding phase decreases with the increase of SiO2 content. Moreover,fluidity increases as the sintering temperature increases; however, when the CaO content is 10% （relatively low proportion）, the lifting temperature has limited effect on fluidity improvement. ② The hematite proportion of the microstructure increases as the SiO2 content increases. ③ When SiO2 content increases, the solid phase ratio in the bonding phase grows,while the liquid phase ratio diminishes. Moreover the extension of the SiO2 content in the liquid phase is larger than that in the solid phase.

β→α Transformation of γ-Phase in Sintered WC-Co Cemented Carbides

Varying the morphology and the structure of γ-phase (Co-base Co-W-C solid solution) by means of altering the cooling rate and the preparing method of liquid sintered WC-Co cemented carbides samples, the mechanism of fcc→hcp transformation of γ-phase in WC-Co alloy has been explored. The results show that, the cooling rate is an important affecting factor on fcc→hcp transformation of γ-phase and the fcc→hcp transformation is mainly a diffusive type when cooling WC-Co samples above room temperature

Directional migration behavior of cerium during sintering process of mischmetal doped cemented carbide

Three observation methods were used to investigate the existing form and the behavior of rare earth during the sintering process of high activity mischmetal （RE, with lanthanum and cerium） doped WC-8%Co-0.048%RE（mass fraction） alloy with low carbon-containing level by scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy （EDXS）, considering the fact that the addition amount of rare earth in the alloy is very minute. The directional migration process and mechanism of cerium were discussed. First, the sinter skin （surface） is observed. oxide on the sinter skin, and lanthanum in these cerium observed, and lanthanum containing phase/micro-zone in It is shown that there exists a dispersedly distributed cerium containing enrichment positions is very minute. Secondly, the polished section is the alloy is identified. Finally, based on the fact that the fracture of cemented carbide is resulted from the heterogeneous phase or other defects within the microstructure, the fracture surface is observed and cerium containing phase/micro-zone in the fracture source approximately 260 μm from the surface is identified. These combined observations reveal adequately the fact that lanthanum and cerium get separated and cerium predominantly migrates towards the surface during the sintering process.

Effect of Additives on the Sintering of Amorphous Nano-sized Silicon Nitride Powders

Amorphous nano-sized silicon nitride powders were sintered by liquid phase sintering. The influences of the additives of Y2O3 and Al2O3 prepared by two different ways, the polyacrylamide gel method and the precipitation method, were investigated. The grain sizes of the additives prepared by the first method were finer than those of prepared by the latter method. When sintered at the same temperature, 1700 ℃, the average grain size of the silicon nitride is 0.3 um for the sample with the former additives, which is much finer than the one with the latter additives. The density of additives prepared by precipitation method is clearly lower than those of prepared by polyacrylamide gel method.

Effect of Mg composition on sintering behaviors and mechanical properties of Al-Cu-Mg alloy

Al-3Cu-Mg alloy was fabricated by the powder metallurgy(P/M) processes. Air-atomized powders of each alloying element were blended with various Mg contents(0.5%, 1.5%, and 2.5%, mass fraction). The compaction pressure was selected to achieve the elastic deformation, local plastic deformation, and plastic deformation of powders, respectively, and the sintering temperatures for each composition were determined, where the liquid phase sintering of Cu is dominant. The microstructural analysis of sintered materials was performed using optical microscope(OM) and scanning electron microscope(SEM) to investigate the sintering behaviors and fracture characteristics. The transverse rupture strength(TRS) of sintered materials decreased with greater Mg content(Al-3Cu-2.5Mg). However, Al-3Cu-0.5Mg alloy exhibited moderate TRS but higher specific strength than Al-3Cu without Mg addition.

Isothermal reduction kinetics and mineral phase of chromium-bearing vanadium–titanium sinter reduced with CO gas at 873–1273 K

Reduction of chromium-bearing vanadium–titanium sinter（CVTS） was studied under simulated conditions of a blast furnace, and thermodynamics and kinetics were theoretically analyzed. Reduction kinetics of CVTS at different temperatures was evaluated using a shrinking unreacted core model. The microstructure, mineral phase, and variation of the sinter during reduction were observed by X-ray diffraction, scanning electron microscopy, and metallographic microscopy. Results indicate that porosity of CVTS increased with temperature. Meanwhile, the reduction degree of the sinter improved with the reduction rate. Reduction of the sinter was controlled by a chemical reaction at the initial stage and inner diffusion at the final stage. Activation energies measured 29.22–99.69 k J/mol. Phase transformations in CVTS reduction are as follows： Fe_2O_3→Fe_3O_4→FeO→Fe; Fe_2TiO_5→Fe_2TiO_4→FeTiO_3; FeO·V_2O_3→V_2O_3; FeO·Cr_2O_3→Cr_2O_3.

Effects of Na/K ratio on the phase structure and electrical properties of Na_xK_(1-x)NbO_3 lead-free piezoelectric ceramics

Lead-free piezoelectric NaxK1-xNbO3（x = 0.3-0.8）（NKN） ceramics were fabricated by normal sintering at 1060°C for 2 h.Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content.The grain size of the produced dense ceramic was decreased by increasing Na content.A discontinuous change in the space distance was found at the composition close to Na0.7K0.3NbO3 ceramic, which indicates the presence of a transitional composition between two different orthorhombic phases, which is similar to the behavior of morphotropic phase boundary（MPB） in NaxK1-xNbO3 ceramics.Such MPB-like behavior contributes to the enhanced piezoelectric coefficient d33 of 122 pC/N, planar-mode electromechanical coupling coefficient kP of 28.6%, and dielectric constant εr of 703, respectively for the Na0.7K0.3NbO3 ceramic.Cubic temperature TC and the transitional temperature TO-T from orthorhombic to tetragonal phase are observed at around 420°C and 200°C, respectively.

Fabrication of solid-phase-sintered Si C-based composites with short carbon fibers

Solid-phase-sintered Si C-based composites with short carbon fibers（Csf/SSi C） in concentrations ranging from 0 to 10wt% were prepared by pressureless sintering at 2100°C. The phase composition, microstructure, density, and flexural strength of the composites with different Csf contents were investigated. SEM micrographs showed that the Csf distributed in the SSi C matrix homogeneously with some gaps at the fiber/matrix interfaces. The densities of the composites decreased with increasing Csf content. However, the bending strength first increased and then decreased with increasing Csf content, reaching a maximum value of 390 MPa at a Csf content of 5wt%, which was 60 MPa higher than that of SSi C because of the pull-out strengthening mechanism. Notably, Csf was graphitized and damaged during the sintering process because of the high temperature and reaction with boron derived from the sintering additive B4C; this graphitization degraded the fiber strengthening effect.

SINTERING BEHAVIOR OF Cu-Al ALLOY POWDER

The dynamic behaviour of transient liquid phase during sintering 5wt% Al-Cu alloy compacts with green density of 7.56g/cn￣3 is observed by means of high temperature metallographic microscopy. The structures and precipitating order of the phases are identified by means of DTA, TEM and composion analysis at the definite point and phase diagram. The results show that little Al-rich liquid phase resulting from eutectic reaction flows into the capillaries in Cu powder, because the peritectic reactions exhausts the liquid in high density compact,the composition homogenization needs longer sintering time. The remainder γ2-phase is discovered at place of the neck of sintered Cu particle and has a crystallographic relationship of (111)_(Cu)∥(033)_γ_2 after alloy is sintered at 900 ℃for 3h.

One step sintering of homogenized bauxite raw material and kinetic study

A one-step sintering process of bauxite raw material from direct mining was completed, and the kinetics of this process was analyzed thoroughly. The results show that the sintering kinetics of bauxite raw material exhibits the liquid-phase sintering behavior. A small portion of impurities existed in the raw material act as a liquid phase. After X-ray diffraction analyses, scanning electron microscopy observations, and kinetics calculations, sintering temperature and heating duration were determined as the two major factors contributing to the sintering process and densification of bauxite ore. An elevated heating temperature and longer duration favor the densification process. The major obstacle for the densification of bauxite material is attributed to the formation of the enclosed blowhole during liquid-phase sintering.

WC-Ni_3Al-B composites prepared through Ni+Al elemental powder route

In order to develop the liquid phase sintering process of WC-Ni3Al-B composites,the preparation process of WC＋Ni3Al prealloyed powder by reaction synthesis of carbonyl Ni,analytical purity Al and coarse WC powders was investigated.DSC and XRD were adopted to study the procedure of phase transformation for the 3Ni＋Al and 70%WC＋（3Ni＋Al） mixed powders in temperature ranges of 550-1200 °C and 25-1400 °C,respectively.The results demonstrate that the formation mechanism of Ni3Al depends on the reaction temperature.Besides WC phase,there exist Ni2Al3,NiAl and Ni3Al intermetallics in the powder mixture after heat treatment at 200-660 °C,while only NiAl and Ni3Al exist at 660-1100 °C.Homogeneous WC＋Ni3Al powder mixture can be obtained in the temperature range of 1100-1200 °C.The WC-30%（Ni3Al-B） composites prepared from the mixed powders by conventional powder metallurgy technology show nearly full density and the shape of WC is round.WC-30%（Ni3Al-B） composites exhibit higher hardness of 9.7 GPa,inferior bending strength of 1800 MPa and similar fracture toughness of 18 MPa-m1/2 compared with commercial cemented carbides YGR45（WC-30%（Co-Ni-Cr））.

Mechanical and corrosion properties of full liquid phase sintered WE43 magnesium alloy specimens fabricated via binder jetting additive manufacturing

This study investigates full liquid phase sintering as a process of fabrication parts from WE43(Mg-4wt.%Y-3wt.%RE-0.7wt.%Zr)alloy using binder jetting additive manufacturing(BJAM).This fabrication process is being developed for use in producing structural or biomedical devices.Specifically,this study focused on achieving a near-dense microstructure with WE43 Mg alloy while substantially reducing the duration of sintering post-processing after BJAM part rendering.The optimal process resulted in microstructure with 2.5%porosity and significantly reduced sintering time.The improved sintering can be explained by the presence of Y_(2)O_(3)and Nd_(2)O_(3)oxide layers,which form spontaneously on the surface of WE43 powder used in BJAM.These layers appear to be crucial in preventing shape distortion of the resulting samples and in enabling the development of sintering necks,particularly under sintering conditions exceeding the liquidus temperature of WE43 alloy.Sintered WE43 specimens rendered by BJAM achieved significant improvement in both corrosion resistance and mechanical properties through reduced porosity levels related to the sintering time.

Variation of the mechanical properties of tungsten heavy alloys tested at different temperatures

The high-temperature mechanical properties of 95W-3.5Ni-1.5Fe and 95W-4.5Ni-0.5Co alloys were investigated in the temperature range of room temperature to1100℃. The yield strength and tensile strengths declined gradually, and the ductility of both alloys increased as the testing temperature was increased to 300℃. All the three properties reached a plateau at temperatures between 300 and 500℃ in the case of 95W-3.5Ni-l.5Fe and at temperatures between 350 and 700℃ in the case of 95W-4.5Ni-0.5Co. Thereafter, the ductility as well as yield and tensile strengths decreased considerably.

Ultrashort-time liquid phase sintering of high-performance fine-grain tungsten heavy alloys by laser additive manufacturing

Liquid phase sintering(LPS)is a proven technique for preparing large-size tungsten heavy alloys(WHAs).However,for densification,this processing requires that the matrix of WHAs keeps melting for a long time,which simultaneously causes W grain coarsening that degenerates the performance.This work develops a novel ultrashort-time LPS method to form bulk high-performance fine-grain WHAs based on the principle of laser additive manufacturing(LAM).During LAM,the high-entropy alloy matrix(Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2))and W powders were fed simultaneously but only the matrix was melted by laser and most W particles remained solid,and the melted matrix rapidly solidified with laser moving away,producing an ultrashort-time LPS processing in the melt pool,i.e.,laser ultrashort-time liquid phase sintering(LULPS).The extreme short dwell time in liquid(-1/10,000 of conventional LPS)can effectively suppress W grain growth,obtaining a small size of 1/3 of the size in LPS WHAs.Meanwhile,strong convection in the melt pool of LULPS enables a nearly full densification in such a short sintering time.Compared with LPS WHAs,the LULPS fine-grain WHAs present a 42%higher yield strength,as well as an enhanced susceptibility to adiabatic shear banding(ASB)that is important for strong armor-piercing capability,indicating that LULPS can be a promising pathway for forming high-performance WHAs that surpass those prepared by conventional LPS.

Influence of Gd_2O_3 and Yb_2O_3 Co-doping on Phase Stability,Thermo-physical Properties and Sintering of 8YSZ

The role of multicomponent rare earth oxides in phase stability, thermophysical properties and sintering for ZrO2-based thermal barrier coatings （TBCs） materials is investigated. 8YSZ codoped with 3 mol% Gd2O3 and 3 mol% Yb2O3 （GYb-YSZ） powders are synthesized by solid state reaction for 24 h at various temperatures. As temperature increases, stabilizers are dissolved into zirconia matrix gradually. Synthesized at 1 500 °C, GYb-YSZ is basically composed of cubic phase. GYb-YSZ exhibits excellent phase stability and sinters lower than 8YSZ by nearly three times. The thermal conductivity of GYb-YSZ is much lower than that of 8YSZ, and the thermal expansion coefficient of GYb-YSZ is comparable to that of 8YSZ. The influence of Gd2O3 and Yb2O3 co-doping on phase stability, thermal conductivity and sintering of 8YSZ is discussed.

Microstructure and mechanical properties of liquid phase sintered silicon carbide composites

Silicon carbide (SiC) composites were prepared by hot-press sintering from α-SiC starting powders with BaAl2Si2O8 (BAS). The effects of additives on densification, microstructure, flexural strength, and fracture behavior of the liquid phase sintered (LPS) SiC composites were investigated. The results show that the served BAS effectively promotes the densification of SiC composites. The flexural strength and fracture toughness of the SiC composites can reach a maximum value of 454 MPa and 5.1 MPa·m1/2, respectively, for 40% (w/w) BAS/SiC composites. SiC grain pullout, crack deflection, and crack bridging were main toughening mechanisms for the sintered composites.

Development of calcium stabilized nitrogen rich α-sialon ceramics along the Si_(3)N_(4):1/2Ca_(3)N_(2):3AlN line using spark plasma sintering

Calcium stabilized nitrogen rich sialon ceramics having a general formula of Ca_(x)Si_(12-2x)Al_(2x)N_(16) with x value(x is the solubility of cation Ca in α-sialon structure)in the range of 0.2-2.2 for compositions lying along the Si_(3)N_(4):1/2Ca_(3)N_(2):3AlN line were synthesized using nano/submicron size starting powder precursors and spark plasma sintering(SPS)technique.The development of calcium stabilized nitrogen rich sialon ceramics at a significantly low sintering temperature of 1500℃(typically reported a temperature of 1700℃ or greater)remains to be the highlight of the present study.The SPS processed sialons were characterized for their microstructure,phase and compositional analysis,and physical and mechanical properties.Furthermore,a correlation was developed between the lattice parameters and the content(x)of the alkaline metal cation in the α-sialon phase.Well-densified single-phase nitrogen rich α-sialon ceramics were achieved in the range of 0.53(3)≤x≤1.27(3).A nitrogen richα-sialon sample possessing a maximum hardness of 22.4 GPa and fracture toughness of 6.1 MPa·m^(1/2) was developed.