MANUFACTURING OF ALUMINUM/FLY ASH COMPOSITE WITH LIQUID REACTIVE SINTERING TECHNOLOGY

The Al/fly ash composites are fabricated by liquid reactive sintering P/M process with fly ash particles as intensifying phases. The reactivity and newly formed phases during liquid sintering process have been analyzed by combing Thermochemicdl data base calculation and XRD characterization. The results show that some of constituents in fly ash have reacted with liquid aluminum so that the elemental Si, Fe, Ti as well as some amount of intermetallic compounds occur. The properties of aluminum/fly ash composites have been improved. With the fraction of fly ash increase, the composite density decreases; the hardness and the modulus of the composite increases, and the composite wear resistance are significantly increased. The fly ash reinforced composites represent a sort of low cost product with possible widespread applications in the automotive, small engine, and electromechanical machinery sectors.

Manufacturing a TiAl alloy by high-energy ball milling and subsequent reactive sintering

A TiAl alloy was fabricated by high-energy ball milling and subsequent reactive sintering from the mixed powders of Ti and Al. High-energy ball milling produced a kind of particular composite powders with an extremely fine altemative Ti and Al lamella structure. The composite powders not only possessed good consolidation and densification characteristics, but also resulted in the augment of nucleation rate of α and γ titanium aluminides during solid-phase reactive sintering After a series of processing, pressing, degassing, extrusion, and sintering, the resultant TiAl alloy presented high relative density and refined grain sizes of （α2 ＋ γ） lamella and γ phases. The compressive yield strength of the sintered TiAl reached 600 MPa at 800℃.

Pressureless reactive sintering mechanism of nanocrystalline Bi_2O_3-Y_2O_3 solid electrolyte

The nanocrystalline Bi2O3-Y2O3 solid electrolyte material was synthesized by pressureless reactive sintering process with Bi2O3 and Y2O3 nano mixed powder as raw materials, which was prepared by a chemical coprecipitation process. The study on the behavior of nano δ-Bi2O3 formation and its grain growth showed that the solid solution reaction of Y2O3 and δ-Bi2O3 to form δ-Bi2O3 occurs mainly in the initial stage of sintering process, and nano δ-Bi2O3 crystal grains grow approximately following the rule of paracurve （（D-D0）^2=K.t） during sintering process. After sintered at 600℃ for 2 h, the samples could reach above 96% in relative density and have dense microstructure with few remaining pores, the δ-Bi2O3 grains are less than 100 nm in size.

Toughened (Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composites fabricated by one-step reactive sintering with a unique SiB_(6) additive

High-entropy diboride has been arousing considerable interest in recent years.However,the low toughness and damage tolerance limit its applications as ultra-high-temperature structural materials.Here we report that a unique SiB_(6) additive has been first incorporated as boron and silicon sources to fabricate a high-entropy boride(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composite though one-step boro/carbothermal reduction reactive sintering.A synergetic effect of high-entropy sluggish diffusion and SiC secondary phase retarded the grain growth of the(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–51SiC composites.The small grain size was beneficial to shorten the diffusion path for mass transport,thereby enhancing the relative density to~99.3%.These results in an increase of fracture toughness from~5.2 in HEBS-1900 to~7.7 MPa·m^(1/2) in HEBS-2000,which corresponded to a large improvement of 48%.The improvement was attributed to a mixed mode of intergranular and transgranular cracking for offering effective pinning in crack propagation,resulting from balanced grain boundary strength collectively affected by improved densification,solid solution strengthening,and incorporation of SiC secondary phase.

Spark Plasma Sintering of Boron Carbide Using Ti_(3)SiC_(2) as a Sintering Additive

Boron carbide has unique properties for wide application possibilities;however,poor sinterability limits its applications.One approach to overcome this limitation is the addition of secondary phases into boron carbide.Boron carbide based composite ceramics are produced by the direct addition of secondary phases into the structure or via reactive sintering using a sintering additive.The present study investigated the effect of Ti_(3)SiC_(2) addition to boron carbide by reactive spark plasma sintering in the range of 1700-1900℃.Ti_(3)SiC_(2) phase decomposed at high temperatures and reacted with B4C to form secondary phases of TiB2 and SiC.The results demonstrated that the increase of Ti_(3)SiC_(2) addition(up to 15 vol%)effectively promoted the densification of B4C and yielded higher hardness.However,as the amount of Ti_(3)SiC_(2) increased further,the formation of microstructural inhomogeneity and agglomeration of secondary phases caused a decrease in hardness.

Preparation and Microstructure of Porous ZrB_2 Ceramics Using Reactive Spark Plasma Sintering Method

Zirconium oxide （ZrO：） and boron carbide （B4C） were added to ZrB2 raw powders to prepare ZrB2 porous ceramics by reactive spark plasma sintering （RSPS）. The reactions between ZrO2 and B,C which produce ZrB2 and gas （such as CO and B2O3） result in pore formation. X-Ray Diffraction results indicated that the products phase was ZrB2 and the reaction was completed after the RSPS process. The porosity could be controlled by changing the ratio of synthesized ZrB2 to raw ZrB2 powders. The porosity of porous ceramics with 20 wt% and 40 wt% synthsized ZrB2 are 0.185 and 0.222, respectivly. And dense ZrB：SiC ceramic with a porosity of 0.057 was prepared under the same conditions for comparison. The pores were homogeneously distributed within the microstructure of the porous ceramics. The results indicate a promising method for preparing porous ZrB：based ceramics.

Reactive Hot-press Sintering of Al-B_4C Composites at Relative Low Temperature

A new process of reactive hot-press sintering with boron carbide(B4C) and aluminum powders was proposed to overcome difficulties in the sintering of dense B4C ceramic materials.The B4C powder with different content of pure metallic aluminum particle were milled,hot-pressed and sintered at 1600 ℃ for 1 hour.The mechanism of sintering at relative low temperature was analyzed.The phase constitution of the composites was determined.Effects of Al content on the hardness and fracture toughness of the composites were discussed.The results show that thermite reaction procedure in B2O3+Al was the mechanism of sintering at relative low temperature,B4C,Al2O3 and metallic aluminum are the major constituents of the composites.The microhardness of the composites decreases with the increasing of Al content,but the fracture toughness increase obviously.The composite with 5wt% Al content has the best microhardness and fracture toughness in all the composites.

Fe-Cr-C Cermet Made by Reaction Sintering

Fe-Cr-C cermet was prepared by reaction sintering of ferrochromium and graphite.The exothermic reaction was determined by DTA.The effect of carbon content on the porosity,hardness and oxidation resistance was investigated.The result showed that the cermet with 6%-7%of carbon has high hardness and oxidation resistance.

Relation Between Sintering Reactivity of Matrix and Thermal Shock Resistance of Ultra-low Cement Bonded Corundum-spinel Castables for Fired Purging Plugs

Purging plugs installed in the bottom of steel ladles are widely used for the secondary refining of high quality steel grades.The dynamic service conditions and temperature gradients caused by the cold inert gas blown through the plug during stirring create a strong thermal shock impact on the materials.This can affect its service life and restrict the safety and efficiency of steel making if the plug fails during use.In this work,the influence of the particle size distribution (PSD) and amount of reactive alumina on the sintering behavior of ultra-low cement bonded corundum-spinel based castables was investigated on lab scale.The relationship between sintering reactivity of matrix and thermal shock resistance of castables was evaluated in detail.Results show that the sintering of castables can be intensified by using finer reactive alumina.However,excessive sintering of the castable through finer reactive alumina is negative for thermal shock resistance.The microstructure characterization reveals that castables with more intense sintering show denser matrix structure,which is less effective in hampering crack propagation and therefore results in decline of their thermal shock resistance.

Formation of Intermetallic Compound in Iron-Aluminum Alloys

The mechanism of iron and aluminum intermetallics formation in the reactive sintering of iron and aluminum mixing powders has been studied by investigating iron aluminum diffusion couples.The couples were treated at 600℃,700℃,800℃,900℃ and 1000℃ respectively.It was found that an Al rich intermetallics FeAl 3 has formed in iron adjacent to the interface of iron and aluminum by aluminum diffusion into iron at 600℃ (below the eutectic temperature),and that in the case above 700℃ (above the eutectic temperature) there was a liquid,an intermetallics Fe 2Al 5 has formed in both side of the interface.The diffusion of iron and aluminum atoms is companied with the Fe Al reaction during the treatment under the both conditions.The diffusion coefficients of iron and aluminum and the activation energy were determined.The mechanism of the intermetallics formation in the couples is also discussed.

Reactive-sintering B_4C matrix composite for armor applications

The B4C matrix composite intended for armor applications still presents restrictions,such as low sintering density,production of large parts and inherited brittleness Herein,research on this topic is discussed in detail.First the material outlook of armor applications is organized from the development of composite armor and ceramic materials for armor to the B4C matrix composite and reactive-sintering method.In the second section,the technologies are reviewed for reactive pressureless sintering reactive hot-pressing sintering,reactive discharge plasma sintering and self-propagating high-temperature sintering Thereafter,our previous works on the TiB2/SiC/B4C composite and laminated Ti/B4C composite are employed to illustrate their microstructural evolution,phase transformation and fracture model.These studies provide a potential method for producing tough and high-strength ceramic composites for armor application.In the fina section,the mechanism,evaluation method and influencing factors of anti-penetration for ceramic armor and B4C matrix composite are reviewed.

Preparation of Composites by Nitro Aluminothermic Processes, over β–SiAlON Matrix in the SiAlON-SiC-Al2O3 System

Goal: The goal of the research is preparation of SiAlON-containing composite through nitro aluminothermic processes, by the methods of reactive sintering and hot compaction. Method: The composite CH-6 was obtained by the method of reactive sintering, with further grinding and hot compression in vacuum furnace at 16000°C, under 30 MPa pressure and 10-12 min standing at the final temperature. Precursor was prepared in a thermostat at 150°C temperature by double compression. Pressure equaled to 20-25 MPa. Results: Physical-technical properties of specimens prepared via hot compaction were investigated. Mechanical strength at compression is 1940 MPa;mechanical strength at bending is 490 MPa;elastic module is 199.5 GPa, HV-11.6 GPa. X-Ray diffraction analysis, electron microscopic and X-ray diffraction Microspectral analysis were used to investigate composite microstructure and phase composition. Composite formulation was defined, the main phases of which were: β-SiAlON, corundum and silicium carbide. Conclusion: Composite CH-6 has been selected from the obtained composites, which is characterized by relatively high physical-technical properties: strength, density and hardness. Materials can be used for making high refractory articles, such as jackets to secure thermocouples, furnace bedding, cutting tools for metal and wood treatment, in rocket spatial technology and others.

Effects of boron content on the microstructure and properties of B_(x)C-TiB_(2)(x=4.5,6.5 or 8.5)ceramic composites by the reactive spark plasma sintering

B_(x)C-TiB_(2)ceramic composites were fabricated via reactive spark plasma sintering using TiC and B as the raw materials.The impact of B/TiC mole ratios on the phase compositions,densification behaviors,microstructure,and mechanical properties of the ceramic composites were investigated.The results showed that the stoichiometry of‘B_(x)C’could be tailored by changing initial boron content and the obtained B_(4.5)C,B_(6.5)C and B_(8.5)C phases have the same crystal structure(R-3 m).The excess of B enhanced the reaction between TiC and B,which released a large amount of hot energy and promoted the densification of the composites.The TB8.5 composite sintered at 1900°C had the best comprehensive mechanical properties,with hardness and flexural strength of 40.36 GPa and 551 MPa,respectively.The formation of nano-sized TiB_(2)grains induced by reaction were beneficial for improving the mechanical properties of these composites.

Reactive SPS of Al_(2)O_(3)–RE:YAG(RE=Ce;Ce+Gd)composite ceramic phosphors

Ultrafine-grained Al_(2)O_(3)–rare earth:yttrium aluminium garnet(Al_(2)O_(3)–RE:YAG)(RE=Ce;Ce+Gd)composite ceramics were obtained for the first time by reactive spark plasma sintering(SPS)using commercially available initial oxide powders.The effect of key sintering parameters(temperature,dwell time,and external pressure(P_(load)))on densification peculiarities,structural-phase states,and luminescent properties of composites was studied comprehensively.Differences in phase formation and densification between Ce-doped and Ce,Gd-codoped systems were shown.Parameters of reactive SPS,at which there is partial melting with the formation of near-eutectic zones of the Al_(2)O_(3)–YAG system/coexistence of several variations of the YAG-type phase,were established.Pure corundum–garnet biphasic ceramics with an optimal balance between microstructural and luminescence performance were synthesized at 1425℃/30 min/30–60 MPa.The external quantum efficiency(EQE)of the phosphor converters reached 80.7%and 72%with close lifetime of~63.8 ns,similar to those of commercial Ce:YAG materials,which is promising for further applications in the field of high-power white light-emitting diodes(WLEDs)and laser diodes(LDs).

ZrB_(2)-SiC based composites for thermal protection by reaction sintering of ZrO_(2)+B_(4)C+Si

Synthesis and sintering of ZrB_(2)-SiC based composites have been carried out in a single-step pressureless reaction sintering(PLRS)of ZrO_(2),B_(4)C,and Si.Y_(2)O_(3) and Al_(2)O_(3) were used as sintering additives.The effect of ratios of ZrO_(2)/B_(4)C,ZrO_(2)/Si,and sintering additives(Y_(2)O_(3) and Al_(2)O_(3)),was studied by sintering at different temperatures between 1500 and 1680℃in argon atmosphere.ZrB_(2),SiC,and YAG phases were identified in the sintered compacts.Density as high as 4.2 g/cm^(3),micro hardness of 12.7 GPa,and flexural strength of 117.6 MPa were obtained for PLRS composites.Filler material was also prepared by PLRS for tungsten inert gas(TIG)welding of the ZrB_(2)-SiC based composites.The shear strength of the weld was 63.5 MPa.The PLRS ZrB_(2)-SiC composites exhibited:(i)resistance to oxidation and thermal shock upon exposure to plasma flame at 2700℃for 600 s,(ii)thermal protection for Cf-SiC composites upon exposure to oxy-propane flame at 2300℃for 600 s.

Effect of starting materials and sintering temperature on microstructure and optical properties of Y203:Yb^(3+)5 at%transparent ceramics

Y2O3:Yb^(3+)5 at%ceramics have been synthesized by the reactive sintering method using different commercial yttria powders(Alfa-Micro,Alfa-Nano,and ITO-V)as raw materials.It has been shown that all Y2O3 starting powders consist from agglomerates up to 5-7 μm in size which are fonned from 25-60 nm primary particles.High-energy ball milling allows to significantly decreasing the median particle size D50 below 500 nm regardless of the commercial powders used.Sintering experiments indicate that powder mixtures fabricated from Alfa-Nano yttria powders have the highest sintering activity,while(Y0.86La0.09Yb0.05)2O3 ceramics sintered at 1750℃for 10 h are characterized by the highest transmittance of about 45%.Y2O3:Yb^(3+)ceramics have been obtained by the reactive sintering at 1750-1825℃using Alfa-Nano Y2O3 powders and La2O3+Zr02 as a complex sintering aid.The effects of the sintering temperature on densification processes,microstructure,and optical properties of Y2O3:Yb^(3+)5 at%ceramics have been studied.It has been shown that Zr^(4+)ions decrease the grain growth of Y2O3:Yb^(3+)ceramics for sintering temperatures 1750-1775℃.Further increasing the sintering temperature was accompanied by a sharp increase of the average grain size of ceramics referred to changes of structure and chemical composition of grain boundaries,as well as their mobility.It has been determined that the optimal sintering temperature to produce high-dense yttria ceramics with transmittance of 79%-83%and average grain size of 8μm is 1800℃.Finally,laser emission at〜1030.7 nm with a slope efficiency of 10%was obtained with the most transparent Y203:Yb^(3+)5 at%ceramics sintered.

Microstructure and properties of Al_2O_3 dispersion-strengthened copper fabricated by reactive synthesis process

Al2O3 dispersion-strengthened copper alloy was prepared by reactive synthesis and spark plasma sintering（SPS） process. Studies show that nano-sized c-Al2O3 particles with 27.4 nm mean size and 50-nm interval are homogeneously distributed in copper matrix. The density of SPS alloy is about 99 %, meanwhile, the electrical conductivity of sintered alloy is 72 % IACS and the Rockwell hardness can reach to HRB 91.

Highly electro-conductive B_(4)C-TiB_(2) composites with threedimensional interconnected intergranular TiB_(2) network

To achieve lightweight B_(4)C-based composite ceramics with high electrical conductivities and hardness,B_(4)C-TiB_(2) ceramics were fabricated by reactive spark plasma sintering(SPS)using B_(4)C,TiC,and amorphous B as raw materials.During the sintering process,fine B_(4)C-TiB_(2) composite particles are firstly in situ synthesized by the reaction between TiC and B.Then,large raw B_(4)C particles tend to grow at the cost of small B_(4)C particles.Finally,small TiB_(2) grains surround large B_(4)C grains to create a three-dimensional interconnected intergranular TiB_(2) network,which is beneficial for an electro-conductive network and greatly improves the conductivity of the ceramics.The effect of the B_(4)C particle size on the mechanical and electrical properties of the ceramics was investigated.When the particle size of initial B_(4)C powders is 10.29µm,the obtained B_(4)C-15 vol%TiB_(2) composite ceramics exhibit an electrical conductivity as high as 2.79×10^(4) S/m and a density as low as 2.782 g/cm^(3),together with excellent mechanical properties including flexural strength,Vickers hardness(HV),and fracture toughness(KIC)of 676 MPa,28.89 GPa,and 5.28 MPa·m^(1/2),respectively.

A new class of high-entropy M_(3)B_(4) borides

A new class of high-entropy M3B4 borides of the Ta_(3)B_(4)-prototyped orthorhombic structure has been synthesized in the bulk form for the first time.Specimens with compositions of(V0.2Cr0.2Nb0.2Mo0.2Ta0.2)3B4 and(V0.2Cr0.2Nb0.2Ta0.2W0.2)_(3)B_(4) were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors.The sintered specimens were〜98.7%in relative densities with virtually no oxide contamination,albeit the presence of minor(4-5 vol%)secondary high-entropy M5B6 phases.Despite that Mo_(3)B_(4) or W_(3)B_(4) are not stable phase,20%of M03B4 and W3B4 can be stabilized into the high-entropy M3B4 borides.Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N.This work has further expanded the family of different structures of high-entropy ceramics reported to date.

In-situ ZrB2-hBN ceramics with high strength and low elasticity

ZrB2 ceramics with various hexagonal BN(hBN)additions up to 37 vol%were reactively densified by spark plasma sintering using powder mixtures containing ZrB2,ZrN and boron.ZrN-boron based additives effectively promoted the densification process,ZrB2 ceramics reached>99%relative density at 2000℃and an applied pressure of 60 MPa with only 5 vol%in-situ formed hBN,whereas the relative density of pure ZrB2 was only 91.2%at the same conditions.Increasing thehBN contents,the morphology of hBN grains gradually changed from quasi-spherical to flake dominated,which has substantial influence on their mechanical properties.In-situ ZrB2-10 vol%hBN ceramics demonstrated high flexural strength of 597±22 MPa,relatively low Young’s modulus of 406 GPa and good machinability,especially for the impressively large strain to failure(1.47×10^-3)which is superior to most of their counterparts in the ZrB2 based particulate reinforced ceramics.