Diffusion reaction in the interface of titanium/mild steel composite

The microstructure together with the formation and growth of reaction phases in the interfacial diffusion zone of the explosive cladding TA2/A3 has been investigated by means of OM, SEM, AES and XRD techniques. When the specimen annealed at temperature under theβ-Ti→α-Ti transformation, i. e. below 1 173 K, only TiC forms along TA2 side of interface and hinders the interdiffusion of Fe and Ti atoms, thus Fe₂Ti or FeTi is unable to occur. While heated up to the transformation temperature ofβ-Ti, e. g, over 1 223 K, the parabolic growth of intermetallic compounds of Fe₂Ti and FeTi with layer structure may form intergranularly and the formation ofβ-Ti orβ-Ti+α-Ti structure at the Fe enriched side of TA2 and the martensitic transformation products at the Fe-depleted side are observed owing to the diffusion of Fe. Furthermore, the growth ofβ-Ti transformation layer is revealed to follow the parabolic rule. Project supported by the Non-ferrous Metal Industry Corperation of China Synopsis of the first author Yang Yang, professor, Dr.-Ing., born on Nov. 11, 1963, has published more than twenty papers. Study fields are on the metallurgical effects of shock deformation on metals, interface of composite, roll and explosive cladding technologies, etc.     

Effect of tin on the reaction synthesis of ternary carbide Ti3AlC2

The effect of tin on synthesis of Ti₃AlC₂ by spark plasma sintering (SPS) from TiC/Ti/Al powders was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for phase identification and microstructure evaluation. The experimental results show that addition of tin can considerably accelerate the synthesis reaction of Ti₃AlC₂ and fully dense, essentially single-phase polycrystalline Ti₃AlC₂ could be successfully obtained by sintering 2TiC/1Ti/1Al/0.2Sn powders at 1200–1250 °C under a pressure of 30 MPa. SEM images show that Ti₃AlC₂ samples in about 2–5 μm thick and 10–25 μm long platelets can be obtained. The fracture toughness and flexural strength of Ti₃AlC₂ were 6.5±0.2 MPa·m^1/2 and 560±10 MPa, respectively. Funded by the National Natural Science Foundation of China (No.20771088, No.50572080) and Doctoral Foundation of Wuhan University of Technology (No.471-38650142)     

THE PROCESS AND MECHANISM OF TiAl－BASED ALLOY SYNTHESIZED FROM Ti AND AlP OWDERS

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Determination of isothermal section of Ag-Ti-Zr ternary system at 1 023 K

The isothermal section of Ag-Ti-Zr ternary system at 1023 K was determined by diffusion triple and electron probe microanalysis. The results indicate that four binary intermetallic phases of AgTi, AgTi2, AgZr and AgZr2 are found in Ag-Ti-Zr ternary system at 1 023 K. AgZr2 and AgTi2 form a continuous solid solution, namely Ag(Ti,Zr)2. Four three-phase regions: AgTi AgZr Ag, AgTi AgZr Ag (Ti, Zr)2, α-Zr β(Ti, Zr) Ag (Ti, Zr) 2 and α-Ti β(Ti, Zr) Ag (Ti, Zr)2 exist in the isothermal section. No ternary compound is observed.     

Effect of temperature on microstructure ofmolybdenum diffusion coating on titanium substrate

The halide-activated pack cementation process was used to form molybdenum diffusion coating on titanium substrate. The morphology, structure, elements diffusion distribution and microhardness of the coatings formed at different diffusion temperatures were studied. The results indicate that the coating is made up of deposition layer and diffusion layer, and the surface roughness of specimens is increased after diffusion. In the diffusion layer, the major phases are Mo and β-Ti phase with addition of α′-Ti phase and α″-Ti phase. And the phase composition of Mo →β→α″→α′ is formed for different Mo contents in the diffusion layer from outside to inside. The diffusion of Ti element is very obvious as well as Mo element. With increasing the diffusion temperature, the thickness of diffusion layer is increased rapidly, and the microhardness is changed more smoothly with diffusion depth, which shows the same distribution rules as the Mo content.     

Microstructural evolution of titanium matrix composite coatings reinforced by in situ synthesized TiB and TiC by laser cladding

Titanium-based composite coatings reinforced by in situ synthesized TiB and TiC particles were successfully fabricated on Ti6Al4V by laser cladding using Ti-B_4C-Al or Ti-B_4C-C-Al powders as the precursor materials.The microstructural and metallographic analyses were made by X-ray diffraction(XRD),optical microscope(OM),scanning electron microscopy(SEM),and electron probe microanalysis (EPMA).The results show that the coatings are mainly composed ofα-Ti cellular dendrites and a eutectic transformation p...     

Study on the isothermal oxidation behavior in air of Ti3AlC2 sintered by hot pressing

The isothermal oxidation behavior at 900–1300°C for 20 h in air of bulk Ti₃AlC₂ with 2.8 wt% TiC sintered by means of hot pressing was investigated in the work. The isothermal oxidation behavior generally followed a parabolic rate law. The parabolic rate constants increased from 1.39×10⁻¹⁰ kg²·m⁻⁴·s⁻¹ at 900°C to 5.56×10⁻⁹ kg²·m⁻⁴·s⁻¹ at 1300°C. The calculated activation energy was 136.45 kJ/mol. It was demonstrated that Ti₃AlC₂ had excellent oxidation resistance due to the continuous, dense and adhesive protect scales consisted of a mass of α-Al₂O₃ and a little of TiO₂ and/or Al₂TiO₅. In principle, the oxide scale was grown by the inward diffusion of O²⁻ and the outward diffusion of Ti⁴⁺ and Al³⁺. The rapid outward diffusion of cations usually resulted in the formation of cracks, gaps, and holes.     

Effect of Y2O3 on cracking susceptibility of laser-clad Ti-based composites coatings

Ti-based composite coatings reinforced by in situ synthesized TiB and TiC were deposited on Ti6AlaV substrates by laser cladding. The effects of Y2O3 on the microstructure and cracking susceptibility of the coatings were investigated in details. It is shown that a small amount of Y2O3 addition can significantly refine the microstructure of the coatings by hastening spheroidization of the primary phase structure. The maximum refinement in microstructure was obtained with the optimum （2 wt%） addition of Y2O3. Moreover, it can increase the volume fraction of TiC and reduce the residual stress of the coatings due to the decrease in lattice distortion of the α（Ti） matrix. All of these factors lead to the reduction in cracking susceptibility of the coatings containing Y2O3 on the premise that the hardness of the coatings is improved. The fracture toughness of the coatings without and with Y2O3 （2 wt%） is 8.32 and 17.36 MPa.m1/2, respectively. Scanning electron microscope examination reveals a transition of the fractured surfaces from cleavage fracture to quasi-cleavage fracture resulting from the Y2O3 addition.     

Fabrication of Fine-Grained Si3N4-Si2N2O Composites by Sintering Amorphous Nano-sized Silicon Nitride Powders

1 IntroductionAs high-temperature structure materials ,nitride ce-ramics possess the excellent mechanical properties ,highmelting temperature ,low density, high elastic modulusand strength, and good resistance to creep, wear andoxidation[1-3]. The intrinsic brittleness and hardness ofsilicon nitride ceramics , however , make it difficult andcostly to machine into complex-shaped components[4 ,5].The fracture toughness of sintered silicon nitride ceramicsmust beimprovedif these ceramics areto be…     

Preparation and thermoelectric properties of SiO<Subscript>2</Subscript>/<Emphasis Type="Italic">β</Emphasis>-Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript> nanocomposite materials

A series of SiO₂/β-Zn₄Sb₃ core-shell composite particles with 3, 6, 9, and 12 nm of SiO₂ shell in thickness were prepared by coating β-Zn₄Sb₃ microparticles with SiO₂ nanoparticles formed by hydrolyzing the tetraethoxysilane in alcohol-alkali-water solution. SiO₂/β-Zn₄Sb₃ nanocomposite thermoelectric materials were fabricated with these core-shell composite particles by spark plasma sintering (SPS) method. Microstructure, phase composition, and thermoelectric properties of SiO₂/β-Zn₄Sb₃ nanocomposite thermoelectric materials were systemically investigated. The results show that β-Zn₄Sb₃ microparticles are uniformly coated by SiO₂ nanoparticles, and no any phase transformation reaction takes place during SPS process. The electrical and thermal conductivity gradually decreases, and the Seebeck coefficient increases compared to that of β-Zn₄Sb₃ bulk material, but the increment of Seebeck coefficient in high temperature range remarkably increases. The thermal conductivity of SiO₂/β-Zn₄Sb₃ nanocomposite material with 12 nm of SiO₂ shell is the lowest and only 0.56 W·m⁻¹·K⁻¹ at 460 K. As a result, the ZT value of the SiO₂/β-Zn₄Sb₃ nanocomposite material reaches 0.87 at 700 K and increases by 30%.     

Microstructure and mechanical properties of ultrafine grained Mg15Al alloy processed by equal-channel angular pressing

An as-cast magnesium alloy with high Al content Mg15Al was subjected to equal-channel angular pressing (ECAP) through a die with an angle of ϕ=90° at 553 K following route B_c. It is found that the network β-Mg₁₇Al₁₂ phases in the as-cast Mg15Al alloy are broken into small blocks and dispersed uniformly with increasing numbers of pressing passes. Moreover, many nano-sized Mg₁₇Al₁₂ particles precipitate in the ultra-fine α-Mg matrix. The grains are obviously refined. However, the grain structure is inhomogeneous in different areas of the alloy. The average size of the primary phase α-Mg is reduced to about 1 μm while grains of around 0.1–0.2 μm are obtained in some two-phase areas. With additional ECAP passes (up to 8), coarsening of the grains occurs by dynamic recovery. Room temperature tensile tests show that the mechanical properties of Mg15Al alloys are markedly improved after 4 ECAP passes. The ultimate tensile strength and elongation to failure increase from 150 MPa to 269.3 MPa and from 0.05% to 7.4%, respectively. Compared with that after 4 passes, the elongation to failure of the alloy increases but the strength of the alloy slightly decreases after 8 ECAP passes. Fracture morphology of the ECAP-processed alloy exhibits dimple-like fracture characteristics while the as-cast alloy shows quasi-cleavage fractures.     

NMR research on cement clinker and its structures in early age hydration

Clinker has long been regarded as a critical factor for cement hydration and solidification.α-C2S and β-C2S in 2CaO·SiO2(C2S) phase and C3S Monoclinic 1(C3S M1) and C3S Monoclinic 3 (C3S M3) in 3CaO·SiO2 (C3S) phase were clearly recorded in the 29Si MAS NMR spectra.The content of C3S phase in the clinker deduced from the fine peak analysis coincides with the phase quantification analysis calculated by the Taylor-Bogue method based on XRF,which also accords to the statistical data in industrial production.NM...     

Microscopic phase-field simulation of Cr atom substitution character during formation of Ll2 and DO22 phases in Ni-Cr-Al alloy

The simulations of Cr atom substitution character during the formation of L12 and DO22 phases in Ni-Cr-Al alloy were performed at 873 K based on microscopic phase-field model. It is found that the substitution of Cr is affected by Cr and Al contents and limits of occupation probabilities of Cr atom in L12 phase are present. The precipitate is single L12 phase when the component is less than the limit, Cr atoms substitute the Al sublattices in Ll2 phase, and both of atoms Al and Cr occupy the β-sites and complex phases Ni3（Al1-xCrx） are formed; Cr atoms enter Ni sites when Al and Cr contents exceed the limit, and substitute β-sites or both of α- and β-sites. The DO22 phase is formed at the boundary of Ll2 phase.     

Effects of β-TCP Ceramics on Intracellular Ca~(2+) Concentration,Mineralization of Osteoblast and Protein Structure

β-TCP, as one of calcium phosphates ceramics, exerts perfect biocompatibility and osteoconductivity, and is clinically used as a bone graft substitute for decades. Consequently, the effects of β-TCP ceramics on intracellular Ca2+ concentration, mineralization of osteoblast and BSA protein structure were studied. Results showed that β-TCP could increase the intracelluar Ca2+ concentration and mineralization of osteoblast, indicating that β-TCP ceramics could take part in the organic metabolism and the degrad...     

Structure and properties of TiB<Subscript>2</Subscript> thin films deposited at low temperatures using RF magnetron sputtering

The TiB2 thin films were deposited on steel substrates using RF magnetron sputtering technique with the low normalized substrate temperature （0.1〈Ts/Tm〈0.2）. Microstructure of these films was obtained by field emission scanning electron microscope （FESEM） and the grazing incidence X-ray diffraction （GIXRD） characterization, while the composition of films was obtained using Auger emission spectroscopy （AES） analysis. It was found that the TiB2 thin films were overstoichiometric with the B/Ti ratio at 2.33 and the diffusion of Ti and B atoms on the substrate surface was greatly improved at 350 ℃. Moreover, a new dense structure, named ＂equiaxed＂ grain structure was observed by FESEM at this substrate temperature, Combined with FESEM and AES analysis, it was suggested that the ＂equiaxed＂ grain structure was located in Zone 2 at the normalized substrate temperature as low as 0.18.     

激光熔覆钛基复合涂层的显微组织和硬度分析

采用激光熔覆快速非平衡合成方法制备了原位反应合成TiB增强钛基复合材料.用Y2O3、Ti和B的混合粉末在Ti-6Al-4V基体表面激光熔覆制得TiB/Ti复合涂层.采用X线衍射仪(XRD)、扫描电子显微镜(SEM)、能量散射光谱仪(EDS)和硬度测试等方法,研究了原位合成TiB/Ti复合涂层的显微结构和显微硬度.结果显示:激光熔覆层的相结构主要为α-Ti和TiB两相,TiB增强相均匀地分布于复合涂层中,熔覆层的硬度值高于基体Ti合金的硬度值1倍以上,Y2O3含量(质量分数w,全文同)为1％的激光熔覆涂层内部的增强相组织最为均匀、细小,且硬度值也最高,平均硬度(HV)值约为830.     

Electrolysis expansion performance of semigraphitic cathode in [K3AlF6/Na3AlF6]-AlF3-Al2O3 bath system

The electrolysis expansion of semigraphitic cathode in [K₃AlF₆/Na₃AlF₆]-AlF₃-Al₂O₃ bath system was tested by self-made modified Rapoport apparatus. A mathematical model was introduced to discuss the effects of α _CR (cryolite ratio) and β _KR (elpasolite content divided by the total amount of elpasolite and sodium cryolite) on performance of cathode electrolysis expansion. The results show that K and Na (potassium and sodium) penetrate into the cathode together and have an obvious influence on the performance of cathode electrolysis expansion. The electrolysis expansion and K/Na penetration rate increase with the increase of α _CR. When α _CR=1.9 and β _KR=0.5, the electrolysis expansion is the highest, which is 3.95%; and when α _CR=1.4 and β _KR=0.1, the electrolysis expansion is the lowest, which is 1.28%. But the effect of β _KR is correlative with α _CR. When α _CR=1.6 and 1.9, with the increase of β _KR, the electrolysis expansion and K/Na penetration rate increase. However, when α _CR=1.4, the electrolysis expansion and K/Na penetration rate firstly increase and then decrease with the increase of β _KR. Foundation item: Project (2005CB623703) supported by the Major State Basic Research and Development Program of China; Project (2008AA030502) supported by the National High-Tech Research and Development Program of China     

Hot deformation behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy with acicular microstructure

The characteristics of hot deformation of an α+β titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si with acicular microstructure were studied using isothermal hot compressive tests in a strain rate range of 0.01-10 s-1 at 860-1 100 °C. The true stress-true strain curves of alloy hot-compressed in the α+β region exhibit a peak stress followed by continuous flow softening; whereas in the β region,the flow stress attains a steady-state regime. At a strain rate of 10 s-1 and in a wide temperature range,the alloy exhibit...     

Effects of Ti,Al and Hf on niobium silcides formation in the Nb-Si <Emphasis Type="Italic">in situ</Emphasis> composites

The effects of alloying elements Ti, Al and Hf on niobium silicides formation in the Nb-Si in situ composites have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The binary, ternary and multicomponent alloys have been fabricated by vacuum non-consumable arc melting method. The results show that Ti tends to stabilize Nb₃Si phase, while Al promotes the direct formation of β-Nb₅Si₃ phase with a tetrahedral D8 _m structure. Exceptionally, it seems that Hf is beneficial to the formation of γ-Nb₅Si₃ phase with a hexangular D8₈ structure. For the multicomponent Nb-Si in situ composites, the cooperative effects of different elements on niobium silicides formation basically maintain the character of ternary system.     

Fabrication and mechanical properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/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.