MICROSTRUCTURES AND MECHANICAL PROPERTIES OF IN-SITU Al_2O_3/TiAl COMPOSITES BY EXOTHERMIC DISPERSION METHOD

In-situ Al2O3/TiAl composites were fabricated by pressure-assisted exothermic dispersion （PAXD） method from elemental powder mixtures of Ti, Al, TiO2, and Nb2O5. The microstructures and mechanical properties of the as-sintered composites are investigated. The results show that the as-sintered products consist of γ-TiAl, α2-Ti3Al, Al2O3, and NbAl3 phases. Microstructure analysis indicates that Al2O3 particles tend to disperse on the grain boundaries. Application of a moderate pressure of 35 MPa at 1200℃ yields Al2O3/TiAl composites with fine Al2O3 reinforcement and a discontinuous network linking by Al2O3 particles. The aluminide component has a fine submicron γ ＋α2 lamellar microstructure. With increasing Nb2O5 content, Al2O3 particles are dispersed uniformly in the matrix. The hardness of the composites increases gradually, and the bending strength and fracture toughness of the composites reach to the maximum value, respectively.

Microstructure and mechanical properties of WC-20wt% Co/ZrO_2(3Y)cermet composites

The cermet composites WC-20wt%Co/ZrO2（E6）with four different comtents of ZrO2（3Y）were prepared by normal vacuum sinter processing;the optical microscope and SEMwere used to characterize their microstructures.The hardness.bending strength and impact toughness of the specimens were determined.The experimental results show that ZrO2（3Y） particles in WC-20wt%Co matrix are sphcrical particles in different sizes which are distributed uniformly in Co phases and WC phases,the bending strength and impact toughness of the WC-20wt%Co cermet composites added ZrO2（3Y）improve remarkably.but the hardness values have little change.

Microstructure and mechanical properties of extruded Al/Al_2O_3 composites fabricated by stir-casting process

A novel two step mixing method including injection of particles into the melt by inert gas and stirring was used to prepare aluminum matrix composites （AMCs） reinforced with Al2O3 particles. Different mass fractions of micro alumina particles were injected into the melt under stirring speed of 300 r/min. Then the samples were extruded with ratios of 1.77 or 1.56. The microstructure observation showed that application of the injection and extrusion processes led to a uniform distribution of particles in the matrix. The density measurements showed that the porosity in the composites increased with increasing the mass fraction of Al2O3 and stirring speed and decreased by extrusion process. Hardness, yield and ultimate tensile strengths of the extruded composites increased with increasing the particle mass fraction to 7%, while for the composites without extrusion they increased with particle mass fraction to 5%.

Effect of nucleating agents on microstructure and mechanical properties of SiO_2-Al_2O_3-ZrO_2 glass-ceramics

SiO2-Al2O3-ZrO2 glasses with different nucleating agents were crystallized under special processing schedule. The microstructure and mechanical properties of the glass-ceramics in SiO2-Al2O3-ZrO2 system were investigated by differential scanning calorimetry, scanning electron microscopy, X-ray diffraction and three-point bending method. The results show that ZrO2 is not an effective nucleating agent in SiO2-Al2O3-ZrO2 system, while TiO2 is effective for the separation of spinel, and P2O5 facilitates solubility of ZrO2 in glass and crystallization. The main crystalline phases of the glass-ceramics are spinel, anorthite and tetragonal zirconia. With the increase of ZrO2 content in the glass, glass-ceramics show higher bending strength （120MPa） than others.

Structure and mechanical properties of ZrO_2-mullite nano-ceramics in SiO_2-Al_2O_3-ZrO_2 system

ZrO2-mullite nano-ceramics were fabricated by in-situ controlled crystallizing from SiO2-Al2O3-ZrO2 amorphous bulk. The thermal transformation sequences of the SiO2-Al2O3-ZrO2 amorphous bulk were investigated by X-ray diffraction, infrared spectrum, scanning electron microscope and differential scanning calorimetric. And the mechanical properties of the nano-ceramics were studied. The results show that the bulks are still in amorphous state at 900 ℃ and the t-ZrO2 forms at about 950 ℃ with a faint spinel-like phase which changes into mullite on further heating. ZrO2 and mullite become major phases at 1 100 ℃ and an amount of m-ZrO2 occur at the same time. The sample heated at 950 ℃ for 2 h and then at 1 100 ℃ for 1 h shows very dense and homogenous microstructure with ball-like grains in size of 20-50 nm. With the increase of crystallization temperature up to 1 350 ℃, the grains grow quickly and some grow into lath-shaped grains with major diameter of 5 μm. After two-step treatment the highest micro-hardness, flexural strength and fracture toughness of the samples are 13.72 GPa, 520 MPa and 5.13 MPa·m1/2, respectively.

Microstructure and Mechanical Properties of TiB_2-Al_2O_3 Composites

Effects of Al2O3 and Ni as the additives on the sinterability, microstructure and mechanical properties were systematic studied. The experimental results show that only a relative density about 96.2% of hot-pressing TiB2-30%Al2O3 can be attained due to the plate-like TiB2 particle and its random orientation and excessive Al2O3 grain growth. When sintering temperature is higher than 1 700 ℃, TiB2 grain growth can be found, which obvious improves flexural strength of TiB2 matrix but decreases toughness. It seems that mechanical properties of TiB2-Al2O3 composites are mainly depended on relative density besides grain growth. otherwise, they will be determined by relative density and TiB2 matrix strength together. Anyway, Al2O3 addition can weaken the grain boundary and thus improve the toughness of the materials. A flexural strength of 529 MPa, Vickers hardness of 24.8 GPa and indentation toughness of 4.56 MPa·m1/2 can be achieved inTiB2-30vol% Al2O3.

Effects of CeO_2 additives on the microstructure and mechanical properties of WC-Al_2O_3 composites

To improve the mechanical properties of WC-Al2O3 composites, the effects of trace amount of CeO2 additives on the microstructure and mechanical properties of the WC-Al2O3 composites prepared by hot pressing were investigated. The results revealed that the WC-Al2O3 composites doped with 0.1% CeOz possessed refined microstructure and enhanced mechanical properties compared with that of the undoped WC-Al2O3composites. Trace CeO2 suppressed the decarburization of WC, promoted the microstructural refinement, and improved the interface coherence of the WC matrix and Al2O3. When 0.1% CeO2 was added to the WC-Al2O3 composites, the effect of CeO2 resulted in the achievement of a relative density of 98.82% with an excellent Vickers hardness of 16.89 GPa, combining a fracture toughness of 9.85 MPa. m1/2 with an acceptable flexural strength of 1 024.05 MPa.

Fabrication and Mechanical Properties of Al_2O_3/TiAl Composites

Al2O3/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al TiO2 and Nb2O5, and the microstructure and mechanical properties were investigated. The results indicate the fine Al2O3 particles tend to disperse on the grain boundaries. The grain size of TiAl matrix decreases and the hardness increases with increasing Nb2O5 content. The bending strength and fracture toughness reach to a maximum when Nb2O5 content is 6 wt%, under 642 MPa and 6.69 MPa·m^1/2, respectively. Based on the fractography and the observation of crack propagation path, it is concluded that the strengthening and toughening of such composites at room temperature can be attributed to the refinement of the TiAl matrix, the deflection behavior in the crack propagation and the dispersion of Al2O3 particles.

Mechanical properties and plasma erosion resistance of BN_p/Al_2O_3-SiO_2 composite ceramics

BNp/Al2O3-SiO2 system ceramic matrix composites with different volume fractions （10%-60%） of hexagonal BN particulates （BNp） were prepared by hot-press sintering technique. Phase components, microstructure, mechanical properties and plasma erosion resistance were also investigated. With the increase of h-BNp content, relative density and Vickers＇ hardness of the composite ceramics decrease, while the flexural strength, elastic modulus and fracture toughness increase and then decrease. The plasma erosion resistance linearly deteriorated with the increase of BNp content which is mainly determined by the density, crystal structure and atomic number of the elements.

Influence of current density on nano-Al_2O_3/Ni+Co bionic gradient composite coatings by electrodeposition

Metal and nano-ceramic nanocomposite coatings were prepared on the gray cast iron surface by the electrodeposition method. The Ni-Co was used as the metal matrix,and nano-Al2O3 was chosen as the second-phase particulates. To avoid poor inter-face bonding and stress distribution,the gradient structure of biology materials was found as the model and therefore the gradient composite coating was prepared. The morphology of the composite coatings was flatter and the microstructure was denser than that of pure Ni-Co coatings. The composite coatings were prepared by different current densities,and the 2-D and 3-D morphologies of the surface coatings were observed. The result indicated that the 2-D structure became rougher and the 3-D surface density of apices became less when the current density was increased. The content of nanoparticulates reached a maximum value at the current density of 40mA·cm^-2,at the same time the properties including microhardness and wear-resistance were analyzed. The microhardness reached a maximum value and the wear volume was also less at the current density of 40mA·cm^-2. The reason was that nano-Al2O3 particles caused dispersive strengthening and grain refining.

2Y_2O_(3-x)CeO_2对ZTM复相陶瓷相组成、显微结构、力学性能的影响

采用原位反应烧结工艺制备了 Zr O2 增韧莫来石 (ZTM)复相陶瓷。研究和分析了在 Y2 O3 为 2 mol%的情况下 ,Ce O2 添加量 (用 2 Y2 O3 -x Ce O2 )对 ZTM复相陶瓷相组成 ,显微结构及力学性能的影响。结果表明 ,当 Ce O2 添加量大于 4 .5mol%时 ,Ce O2 同 Zr O2 固溶 ,形成相对的 t-Zr O2 ;当 Ce O2 添加量在 1 .5～ 4 .5mol%时 ,Zr O2 晶格畸变 ,晶胞长大 ,缺陷增加 ,Ce O2 偏析、聚集 ,力学性能下降。当 Ce O2 添加量为 6.5mol%时 ,ZTM复相陶瓷的抗弯强度达到 3 70 Mpa,断裂韧性为 4 .8Mpa· m1/2 。

Effect of nano-sized Al2O3 reinforcing particles on uniaxial and high cycle fatigue behaviors of hot-forged AZ31B magnesium alloy

The effect of hot-forging process was investigated on microstructural and mechanical properties of AZ31 B alloy and AZ31 B/1.5 vol.%Al2 O3 nanocomposite under static and cycling loading. The as-cast alloy and composite were firstly subjected to a homogenization heat treatment at 450 ℃ and then an open-die forging at 450 ℃. The results indicated that the presence of reinforcing particles led to grain refinement and improvement of dynamic recrystallization. The forging process was more effective to eliminate the porosity in the cast alloy workpiece. Microhardness of the forged composite was increased by up to 80% and 16%, in comparison with those of the cast and forged alloy samples, respectively. Ultimate tensile strength and maximum tensile strain of the composite were improved by up to 45% and 23%, compared with those of the forged alloy in similar regions. These enhancements were respectively 50% and 37% in the compression test. The composite exhibited a fatigue life improvement in the region with low applied strain;however, a degradation was observed in the high applied strain region. Unlike AZ31 B samples, tensile, compressive and high cycle fatigue behaviors of the composite showed less sensitivity to the applied strain, which can be attributed to the amount of porosity in the samples before and after the hot-forging.

Synthesis of Al_2O_3/TiCN-0.2%Y_2O_3 Composite by Hot Pressing

Al2O3/TiCN composites were synthesized by hot pressing.The influences of components and HP temperature on mechanical properties,such as bending strength,breaking tenacity and Vickers hardness were investigated.The results showed that the mechanical properties of Al2O3/TiCN composite increased with temperature when hot pressing temperature is below 1650 ℃.The mechanical properties reached their maximums when the composites were sintered at 1650 ℃ for 30 min under hot pressing pressure of 35 MPa,the value of bending strength,breaking tenacity and Vickers hardness was 1015 MPa,6.89 MPa·m1/2,and 20.82 MPa,respectively.When hot pressing temperature was above 1650 ℃,density decreased because of decomposition with increased temperature,and mechanical properties dropped because of rapid growth of grains in size at high temperature.Microstructure analysis showed that the addition of Y2O3 led to the formation of YAG phase so as to inhibit the growth of crystals.This helped to improve breaking tenacity of the composites.TiCN particles with diameters of 1 μm dispersed at Al2O3 grain boundaries,inhibited grain growth and enhanced mechanical properties of the composites.SEM study of the propagation of indentation cracks showed that the bridge linking behavior between matrix and strengthening phase might lead to the formation of the coexisted field of crack deflection,branching and bridge linking.The mechanism of this phenomenon was that the addition of Y2O3 improved the dispersion of TiCN particles so as to enhance the tenacity of the composites.The breaking tenacity was changed from 5.94 to 6.89 MPa·m1/2.

Study on Mg PSZ Ceramics Doped with Y_2O_3 and CeO_2

Mg PSZ ceramics doped with Y 2O 3 and CeO 2 was prepared using traditional processing method. The fine grain PSZ ceramics( d c10 μm) sintered at low temperature(1550 ℃) was obtained by means of composition design. The effects of co stabilization of Y 2O 3, CeO 2 and annealing at 1100 ℃ on material composition, microstructure and mechanical properties were studied. The results show that Y 2O 3 and CeO 2 during annealing at 1100 ℃ can inhibit subeutectoid decomposition reaction effectively, and optimize nucleation and growth of t ZrO 2 precipitates in c ZrO 2 matrix phase. The materials show transgranular and intergranular fracture characteristics, and exhibit better mechanical properties owing to the cooperative effect of stress induced transformation toughening and microcrack toughening.

Emission property of Y_2O_3-doped Mo secondary emitter

Y2O3-doped Mo secondary emitters were prepared by liquid-liquid doping and solid-solid doping,respectively.The back-scattered scanning observation result indicates that the emitter prepared by liquid-liquid doping has fine microstructure whereas that prepared by solid-solid doping has large grain size.Y2O3-doped Mo emitter with small grain size prepared by liquid-liquid doping exhibits high emission property,i.e.,the secondary electron yield can get to 5.24,about 1.7 times that prepared by solid-solid doping.Moreover,Y2O3-doped Mo emitter exhibits the best emission performance among La2O3-doped Mo,Y2O3-doped Mo, Gd2O3-doped Mo and Ce2O3-doped Mo emitters due to the largest penetration depth of primary electrons and escape depth of secondary electrons in this emitter.The secondary emission of the emitter with small grain size can be explained by reflection emission model and transmission emission model,whereas only transmission emission exists in the emitter with large grain size.

Microstructural evolution and mechanical properties of h-BN composite ceramics with Y2O3-AlN addition by liquid-phase sintering

Hexagonal boron nitride(h-BN)composite ceramics were prepared by hot pressing with the addition of Y2O3 and AlN.The effects of different Y2O3–AlN contents on microstructural evolution,mechanical properties and thermal diffusion coefficients of h-BN composite ceramics were investigated.The results indicate that Y2O3–AlN forms a liquid phase during the sintering process achieving a good wettability with h-BN grains.In pure h-BN ceramic and h-BN composite ceramic with 40 wt%Y2O3–AlN,the h-BN grains grow well when controlled through solid-phase and liquid-phase diffusion,respectively.With the increase in Y2O3–AlN content,mechanical properties and thermal diffusion coefficients of h-BN composite ceramics first decrease and then increase,and the properties of h-BN composite ceramic with 10 wt%Y2O3–AlN are the inflection points.Such properties are highly related to the phase compositions,porosity and microstructure.

Microstructure and mechanical properties of Al-B4C composite at elevated temperature strengthened with in situ Al2O3 network

This study evaluated the mechanical properties and thermal properties of Al-12 vol%B4 C composite at elevated temperature strengthened with in situ Al2 O3 network.The composite was fabricated using powder metallurgy(PM)with raw materials of fine atomized aluminum powders,and the associated microstructures were observed.At 350℃,the composite had ultimate tensile strength of UTS=137 MPa,yield strength of YS0.2=118 MPa,and elongation ofε=4%.Besides,the mechanical properties of the composite remained unchanged at 350℃after the long holding periods up to 1000 h.The excellent mechanical properties and thermal stability at 350℃were secured by in situ am-Al2O3 network that strengthened the grain boundaries.The interfacial debonding and brittle cracking of B4 C particles were the main fracture mechanisms of the composite.In addition,the influence of sintering temperature and rolling deformation on the microstructures and mechanical properties was studied.

Phase evolution, microstructure and properties of Y_2O_3-doped TiCN-based cermets

Y2O3-doped TiCN-based cermets were prepared by pressureless sintering with powders TiC, TiN, Ni, etc. as main starting materials. The influence of sintering processes and Y2O3 on properties of TiCN-based cermets were investigated. The phase composition of TiCN-based cermets almost had no change with Y2O3 addition. The fullly densified TiCN-based cermets were achieved by P-2 sitering process. The fracture surface showed lots of small dimples caused by hard phase particles pulling-off, and the left hard phase particles were attached to the arborous dendritic matrix. The Vickers hardness, fracture toughness and bending strength of TiCN-based cermets increased firstly and then decreased with the increment of Y2O3 content. When Y2O3 contents were both 0.8 wt.%, compared with the P-1 sintered samples, the Vickers hardness, fracture toughness and bending strength of the P-2 sintered sampies reached 14.84 GPa, 8.66 MPa-m1/2 and 660.4 MPa, which were increased by 7.9%, 6.1% and 45.8%, respectively.

First-principles study of structure,mechanical and optical properties of La-and Sc-doped Y2O3

The electronic,mechanical and optical properties of La-and Sc-doped Y2O3 were investigated using firstprinciples calculations.Two doping sites of Sc and La in Y2O3 were modeled.The calculated values of the energy of formation show that the most energetically favorable site for a La atom in Y2O3 is a d-site Y atom,while for Sc a b-site Y atom is the more stable position.The calculated band gap shows a slight decrease with increasing La or Sc concentration.The calculated results for the mechanical and optical properties of Y(2-x)RxO3(R=Sc or La,0<x≤0.1875)show that La-or Sc-doped Y2O3 would have enhanced strength,and thus an ability of resisting external shocks,and increased hardness and mechanical toughness.These improved mechanical properties are achieved without sacrificing the optical properties of the doped compounds.So the doping of La or Sc in Y2O3 is permissible in the preparation of Y2O3 transparent ceramics,of course,doping of La or Sc will benefit the sintering of transparent ceramics.

Effects of Nano-Y_2O_3 and Sintering Parameters on the Fabrication of PM Duplex and Ferritic Stainless Steels

Here we report the effects of nano-Y203 addition, sintering atmosphere and time during on the fabrication of PM duplex and ferritic stainless steels composites by dual-drive planetary milling of elemental Fe, Cr and Ni powders followed by conventional pressureless sintering. Yttria-free and yttria-dispersed duplex and ferritic stainless steels are fabricated by conventional sintering at 1000, 1200 and 1400 ℃ temperatures under argon atmosphere. In another set of experiment, yttria-free and yttria-dispersed duplex and ferritic stainless steels are consolidated at 1000 ℃ for l h under nitrogen atmosphere to study the effect of sintering atmosphere. It has been found that densities of duplex and yttria- dispersed duplex stainless steel increase from 71% to 91% and 78% to 94%, respectively, with the increase in sintering temperature. Similarly, hardness value increases from 257 to 567 HV25 in case of duplex, and from 332 to 576 HV25 in yttria-dispersed duplex stainless steel. X-ray diffraction analysis shows the domination of more intense austenite phase than ferrite at higher sintering temperature and also in nitrogen atmosphere. It is also evident that addition of yttria enhances phase transformation from a-Fe to 7-Fe. Duplex and yttria-dispersed duplex stainless steels exhibit the maximum com- pressive yield strength of 360 and 312 MPa, respectively.