Sintering microstructure and properties of copper powder prepared by electrolyzation and atomization

The almost completely dense copper was prepared by ultrafine copper powder prepared with both methods of electrolysis and novel water-gas atomization through cold isostatic pressing(CIP)and sintering under atmospheric hydrogen.Fine copper powder possesses the higher sintering driving force,thereby promoting shrinkage and densification during the sintering process.The grain size of sintered samples by electrolytic copper powder is smaller than that prepared by the atomized copper powder,and the twin crystals are particularly prone to forming in the former sintered microstructure due to the raw powder with low oxygen content and high residual stress originating from the CIP process.The relative density of samples by electrolytic and atomized powder at 1000℃ sintering temperature achieves 99.3%and 97.4%,respectively,significantly higher than that of the powder metallurgy copper parts reported in the literature.Correspondingly,the ultimate tensile strength and yield strength of samples by both kinds of copper powder are approximately similar,while the elongation of the sintered sample by the electrolytic powder(60%)is apparently higher than the atomized powder(44%).The superior performance of samples fabricated by electrolytic powder is inferred from the full density and low oxygen level for there is no cuprous oxide in the grain boundaries.

Microstructure and compressive deformation of hypereutectic Al-Si-Fe based P/M alloys fabricated by spark plasma sintering

Al-Si-Fe based alloys are attractive light-weight structural materials for automotive engine components because of their high wear resistance, low density and low thermal expansion. Al-17Si 5Fe-2Cu-lMg-lNi-lZr alloys were produced in compact form by a spark plasma sintering （SPS） technique using gas atomized powders. The mean grain size of the compact was 530 nm, and fine equiaxed grains and uniformly distributed precipitates were observed in the compact. The compressive deformation behavior of the SPSed materials was examined at various temperatures and strain rates. All the true stress-true strain curves showed steady state flow after reaching peak stress. The peak stress decreased with increasing test temperature and decreasing strain rate. In the deformed specimens, the equiaxed grain morphology and the dislocation microstructure within the equiaxed grains were observed. These facts strongly indicated the occurrence of dynamic recrystallization during high temperature deformation of the present alloy.

The Effect of Sintering Temperature on Microstructure of High Chrome Bricks

The microstructure of high chrome bricks made at different sintering temperature are analyzed by SEM . The results indicate that the sintering temperature of high chromebricks has an optimum range , it is not the higher, the better, The high chrome bricks made at this -sintering temperature have the moderate crystal six in the matrix and of dense structure. The closed bonding structure could be obtained between grains and matrix and no crackle occurred.The high chrome bricks with this microstructure have the best dynamic properties.

Effect of glass fibre(GF) addition on microstructure and tensile property of GF/Pb composites fabricated by powder metallurgy

GF/Pb compositeswerefabricated by the method of powder metallurgy, and the density, microstructure and tensile propertywerecharacterized considering the size and content ofglass fibre （GF）. The results show that relative densities decrease with increasing GF fraction, and the 50μm-GF reinforced specimens exhibit a better densification than the 300μm-GF reinforced ones. The GF particles distribute quite uniformly inPb matrix, and the composites fabricated at low sintering temperature （〈200℃） possess fine-grain microstructure. The addition of GF significantly improves the strength of the Pb composites, and the ultimate tensile strength of the Pb composite reinforcedwith the addition of 50μm-0.5% GF（mass fraction）is about 30MPa higher than that of GF-free sample. For all composites groups, increasing the reinforcement content from 0.5%to 2%（mass fraction）results in a decrease in both tensile strength and ductility.

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.

Microstructures and properties of CuZrAl and CuZrAlTi medium entropy alloys prepared by mechanical alloying and spark plasma sintering

Equiatomic CuZrAl and CuZrAlTi medium entropy alloys were designed and synthesized by mechanical alloying and spark plasma sintering technique.The alloying behavior,phase evolutions,microstructures and properties of samples were investigated by X-ray diffraction,differential scanning calorimetry,field emission scanning electron microscopy,microscopy/Vickers hardness testing and electrochemical polarization measurement.The results indicate that the final products of as-milled alloys consist of amorphous phases.Ti addition improves the glass forming ability of as-milled alloys.The as-sintered CuZrAl alloy contains face-centered cubic（fcc）solid solution,Al_（1.05）Cu_（0.95） Zr and AlZr_2 phases at different sintering temperatures.With Ti addition,the as-sintered sample is only composed of intermetallics at 690°C,while fcc1,fcc2 and CuTi3phases are formed at 1100°C.CuZrAlTi-1100°C alloy exhibits the highest hardness value of 1173HV0.2owing to the high sintering density,solid solution strengthening and homogeneous precipitation of nano-size crystalline phase.CuZrAlTi-690°C alloy presents a similar corrosion resistance with304 Lstainless steel in seawater solution and further possesses the lower corrosion rate.

Reaction Mechanism and Microstructure Evolution of Reaction Sintered h-BN

Hexagonal boron nitride ceramic（h-BN） based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N2 occurred vigorously at temperatures ranging from 1 000 ℃ to 1 300 ℃, which resulted in the generation of t-BN. When the temperature exceeded 1 450 ℃, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.

Effect of nitrogen introduction methods on the microstructure and properties of gradient cemented carbides

Gradient cemented carbides with the surface depleted in cubic phases were prepared following normal powder metallurgical pro-cedures.Gradient zone formation and the influence of nitrogen introduction methods on the microstructure and performance of the alloys were investigated.The results show that the simple one-step vacuum sintering technique is doable for producing gradient cemented carbides.Gradient structure formation is attributed to the gradient in nitrogen activity during sintering,but is independent from nitrogen introduced methods.A uniform carbon distribution is found throughout the materials.Moreover,the transverse rupture strength of the cemented carbides can be increased by a gradient layer.Different nitrogen carriers give the alloys distinguishing microstructure and mechanical properties,and a gradient alloy with ultrafine-TiC0.5N0.5 is found optimal.

Effect of cerium/lanthanum addition on microstructure and mechanical properties of Al7075 alloy via mechanical alloying and sintering

The effect of the Al-6Ce-3La（ACL） on the microstructural behavior of the Al7075 was investigated. Materials were synthesized by mechanical alloying with variation in the ACL content and milling time. Products were characterized and studied in the as-milled condition and mechanically evaluated after sintering. The synergetic effect of milling time and ACL content in the modified materials led to a reduction in the particle size. Results from electron microscopy showed a homogeneous dispersion of Ce/La phases up to 20 wt.% of ACL content after 10 h of milling. Mechanical evaluation under compressive test showed an improved performance for those alloys reinforced with 0.2 wt.% and 0.5 wt.% of ACL.

Effect of Sintering on Grain Boundary Microstructure and Electrical Properties of CaCu_3Ti_4O_(12) Ceramics

CaCu3Ti4O12 ceramic with a giant dielectric constant was synthesized by sol-gel method and sintered in three different sintering conditions： 1 035 ℃ for 48 h, 1 080 ℃ for 3 h and 48 h. The phase of the ceramics, the element distribution, the valance state of Ti ions at grain boundaries, and the electrical properties were characterized via X-ray diffraction（XRD）, energy dispersive X-ray analysis（EDAX）, X-ray photoelectron spectroscopy（XPS）, electrical conduction and dielectric measurement. The results demonstrate that the grain-boundary microstructure and the electrical properties are influenced by sintering conditions： 1 By raising sintering temperature, the Cu-rich and Ti-poor grain boundary was formed and grain resistivity was decreased. 2 By prolonging sintering time, the content of Ti3＋ near the grain boundary increased, leading to the decrease of the grain-boundary resistivity and the increase of the activation energy at grain boundary. The ceramic, sintering at 1 080 ℃ for 48 h, exhibited a small grain resistivity（60.5 ＊cm）, a large grain-boundary activation energy（0.42 e V）, and a significantly enhanced dielectric constant（close to 1×105 at a low frequency of 1×103 Hz）. The results of electrical properties accord with the internal boundary layer capacitor model for explaining the giant dielectric constant observed in Ca Cu3Ti4O12 ceramics.

Effect of Spark Plasma Sintering Temperature on Electrochemical Properties of La_(0. 82)Mg_(0. 18)Ni_(3. 50)Co_(0. 15) Alloy

Electrochemical properties of La_（0. 82）Mg_（0. 18）Ni_（3. 50）Co_（0. 15） alloys synthesized by spark plasma sintering（ SPS） were investigated based on the electrochemical measurements,physical parameters and microstructure observation. The sintering behavior of La_（0. 82）Mg_（0. 18）Ni_（3. 50）Co_（0. 15） alloys at the temperatures of 900,950 and 1 000 ℃ is characterized by four stages,i. e.,initial slight shrinkage,expansion,abrupt shrinkage and slight expansion. The maximum shrinkage displacement increases with increasing sintering temperature. All of the alloys consist of（ La,Mg）_2（ Ni,Co）_7 phase; additionally,temperatures of 900 and 950 ℃ are beneficial to the formation of（ La,Mg）（ Ni,Co）_3 phase,whereas the LaNi_5 phase is easy to form in the alloy synthesized by SPS at 1 000 ℃. The electrochemical measurements indicate an evident change of the electrochemical performance of the alloys associated with increasing the sintering temperature. The discharge capacity of the alloys first increases and then decreases as sintering temperature rises,whereas their cycle stability clearly grows all the time.Furthermore,the charging-discharging potential difference and discharging efficiency both demonstrate that the electrochemical properties of the alloy electrodes first augment and then decline with increasing sintering temperature.

Titanium nitrides as novel reactants for in-situ synthesis of TiB_(2)-based composites with improved properties

A novel in-situ reactive approach based on the reactions among TiN,aluminum and boron has been developed to synthesize TiB_(2)-based composites including TiB_(2)-h BN(TB)and TiB_(2)-h BN-AlN(TBA).Fully dense ceramics with fine-grained microstructure were successfully obtained via spark plasma sintering at 1850℃/60 MPa/5 min.Microstructure analysis suggests h BN flakes were homogenously distributed in the TiB_(2)matrix.For AlN,however,they were elongated into plate-like grains during sintering,in which lots of defects in terms of stacking faults and twinning structures were observed.The mechanical and thermophysical properties of as-sintered ceramics were comprehensively investigated and compared.In-corporating AlN significantly improved the flexure strength,hardness,fracture toughness and thermal conductivity of TiB_(2)-h BN ceramics.The electrical conductivity of TB(3.06×10^(6)S/m)is larger than that of TBA(2.35×10^(6)S/m)at room temperature,but the value(6×10^(5)S/m)was lower than that of TBA(6.9×10^(5)S/m)at 1173 K.Based on the measurement of electrical and thermal conductivity,electron and phonon contributions to thermal conductivities of TB and TBA were calculated and their temperature dependences were illustrated.

Comparative Study on the Corrosion Resistance of Al–Cr–Fe Alloy Containing Quasicrystals and Pure Al

Al-Cr-Fe alloy containing quasicrystals has been consolidated using spark plasma sintering（SPS）. Its corrosion resistance properties were comparatively investigated with pure Al by electrochemical methods in 3.5 wt% NaCl solution. Their corrosion current density was also compared with that of three commercial steels-316 stainless steel, AISI 440C stainless steel and AISI H13 tool steel. Al-Cr-Fe alloy exhibits nobler corrosion potential and evident passivation with a potential range of around 150 mV while no passivation of pure Al sample is seen. The corrosion resistance of Al-Cr-Fe alloy is less than that of pure Al, but is close to that of 316 stainless steel and superior to that of AISI 440C stainless steel and AISI H13 tool steel.

Microstructure improvement related coercivity enhancement for sintered NdFeB magnets after optimized additional heat treatment

The micro structure, especially the Nd-rich phase and the grain boundary, in sintered NdFeB magnets plays an important role in magnetic reversal and coercivity mechanism. To better understand the effects of the microstructure on the coercivity, we investigated the microstructure and properties improvements of a commercial sintered NdFeB magnet after optimized additional heat treatment. The coercivity is enhanced from 1399 to 1560 kA/m. This enhancement has been explained in terms of the evolution of the grain boundary structure, and the formation of continuous thin layers of Nd-rich phase is important for high coercivity. The micromagnetic simulation together with the numerical analysis based on the nucleation model suggest that the reversed magnetic domains nucleate mainly at the interface of multijunctions of Nd_2 Fe_（14）B grains with high stray fields during the demagnetization process. Both improved anisotropy fields at grain boundaries and reduced stray fields at multi-junction Nd-rich phases contribute to the coercivity enhancement. This work has importance in understanding the crucial micro structure parameters and enhancing the obtainable properties for sintered NdFeB magnets.

Preparation and properties of W–Cu–Zn alloy with low W–W contiguity

The W–Cu–Zn alloy with a-brass matrix and low W–W contiguity was prepared by method of electroless copper plating combined with spark plasma sintering（SPS） method.The effects of process and parameters on the microstructure and mechanical properties of the alloy were investigated.The W–Cu–Zn alloy with a relative density of 96 % and a W–W contiguity of about 10 % was prepared by original fine tungsten particles combined with wet mixing method and SPS solid-state sintering method at 800℃ for 10 min.The microstructure analysis shows that Cu–Zn matrix consists of nano-sized a-brass grains,and the main composition is Cu3Zn electride.The nano-sized Cu was coated on the surface of tungsten particles by electroless copper plating method,and the fairly low consolidation temperature and short solid-state sintering time result in the nano-sized matrix phase.The dynamic compressive strength of the W–Cu–Zn alloy achieves to1000 MPa,but the alloy shows poor ductility due to the formation of the hard and brittle Cu3Zn electrides.The fine-grain strengthening and the solution strengthening of the Cu–Zn matrix phase are responsible for the high Vickers microhardness of about 300 MPa for W–Cu–Zn alloy.