Microstructure Evolution of in situ （Ti3AIC ＋ TisSi3）/Ti3Al Composite Sheet with a Novel Quasi-continuous Chain Reinforcement Distribution Architecture Prepared by Using Roll Bonding and Reaction Annealing

The Ti3AI matrix composite sheet with a novel quasi-continuous chain reinforcement distribution was prepared by using roll bonding and subsequent reaction annealing with pure Ti and SiCp/AI foils. The （Ti3AIC ＋ TisSi3） reinforcements were produced by in situ reaction of the as-rolled Ti-（SiCp/AI） laminated composite sheet after a two-stage annealing treatment including first annealing at 660 °C and second annealing at 1250 °C. The microstructure evolution during the reaction annealing was investigated. In the first reaction annealing, TiAI3 was formed, and the SiC particles were pushed together due to the Kirkendall effect, and after the second reaction annealing, Ti3AI, Ti3AIC and Ti5Si3 were synthesized. In addition, the reinforcement distribution presents a quasi-continuous chain microstructure in Ti3AI matrix composite sheet.     

Performance of GH4169 brazed joint using a new designed nickel-based filler metal via cluster-plus-glue-atom model

A novel Ni-Cr-Si-B filler metal with the cluster formula of [Cr-Ni_(12)]B_2Cr + [B-Ni_8Cr]BSi Cr based on the cluster-plus-glue-atom model was designed for vacuum brazing GH4169 alloy. The effect of brazing temperature and brazing time on microstructure and shear strength of GH4169 alloy joints was investigated.The brazed seam was mainly composed of γ-Ni solid solution.(Nb, Ti)-rich phase and(Cr, Nb, Mo)-rich borides distributed in diffusion zones. The diffusion and aggregation of B, Cr, Nb, and Mo resulted in the variation of phase contrast and morphology of borides. Coarse precipitations in the joint brazed at1240℃ consisted of borides, Laves phase and δ phase. The shear strength of joints was principally dominated by the brittle precipitations in diffusion zone, and the homogenization of microstructure improved the room-temperature shear strength to 820 MPa with the high-temperature shear strength of 627 MPa for the joint brazed at 1240℃/20 min. The joint fractured in diffusion zone and brazed seam, and the existence of borides and Laves phase in diffusion zone provide the potential origin for crack growth.     

Synthesis of Single-Crystal TiO2 Nanowire Using Titanium Monoxide Powder by Thermal Evaporation

TiO 2 nanowires were synthesized successfully in a large quantity by thermal evaporation using titanium monoxide powder as precursor. X-ray diffraction results showed that all the products were pure rutile phase of TiO 2 . According to microstructural observations, the nanowires have two typical morphologies, a long straight type and a short tortuous type. The straight nanowires were obtained at a wide temperature range of 900-1050 ℃, while the tortuous ones were formed below 900 ℃. Transmission electron microscopy characterization revealed that both the straight and the tortuous nanowires are single-crystal rutile TiO 2 . The preferential growth direction of the nanowires was determined as [110] orientation according to electron diffraction and high-resolution image analyses. The morphological change of TiO 2 nanowires was discussed by considering the different atomic diffusion rates of Ti atoms caused by the phase transformation in Ti substrate at around 900 ℃.     

Segregation of Phosphorus and Precipitation of MNP-Type Phosphide at the Grain Boundary of IN706 Superalloy

The effect of phosphorus content and heat treatment on the segregation of phosphorus and precipitation of phosphide at the grain boundary of IN706 alloy has been investigated. The phosphide had a stoichiometry of MNP(M = Nb, Ti and N = Ni, Fe, Cr) and an orthorhombic crystal structure. The solubility of phosphorus in the grain matrix of IN706 alloy was determined to be between 0.008% and 0.013%.When soaked at 980 °C for 5 min, the phosphide was noticeably precipitated at grain boundaries with the grain size unchanged. When soaked at 980 °C for 3 h, the grains grew significantly, but the phosphide stayed at original grain boundaries which outlined the grain shape before soaking. Soaking at 1060 °C for 2 h fully dissolved the pre-existing phosphide but phosphorus atoms were not distributed uniformly in the grain matrix. When the holding time at 1060 °C was extended to 10 h, the phosphorus atoms were distributed uniformly in the grain matrix by sufficient diffusion, and the phosphide could only be formed at grain boundaries during the 980 °C soaking. The precipitation of phosphide mainly relied on the phosphorus segregation which was built up by diffusion.     

Selective Laser Melting of an Al–Fe–V–Si Alloy: Microstructural Evolution and Thermal Stability

Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si(wt%). The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al12(Fe,V)3Si and AlmF e metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 °C resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al12(Fe, V)3Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 k J/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min,which was resistant to coarsening exposed at 425 °C up to 500 h. The Al12(Fe,V)3Si and AlmF e phases were coarsened at 475 and 525 °C with increasing the exposure time. Coarsening of Al12(Fe,V)3Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600 °C resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which signi?cantly degraded the room temperature microhardness.     

Improvement on the Oxidation Resistance of a Ti3Al Based Alloy by Cr1-xAlxN （0≤x≤0.47） Coatings

Cr1-xAlxN coatings have been deposited on a Ti3Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800-900℃ showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al2O3＋Cr2O3 mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800℃. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the AI content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.     

Computer Simulation of Fe-Al-Si System Diffusion Couples

Diffusion couples, Fe-6.8 wt% Al-1.0 wt% Si/Fe and Fe-6.3 wt% Al-0.9 wt% Si/Fe were constructed and separately annealed at 1050°C for 3 h and at 1000°C for 64 h. The concentration profiles of Fe, Al and Si atoms in these couples were measured by electron probe micro-analysis (EPMA), while the diffusion behavior was also simulated by coupling thermodynamic and kinetic properties of Fe-Al-Si system. The simulation results were in good agreement with the measured concentration profiles showing the validity of dynamic parameters of Fe-Al-Si system. Calculation was made for Fe-7 wt% Al-1 wt% Si/Fe diffusion couples at 1000?C with different diffusion time. Silicon uphill was found under the influence of aluminum.     

Dissolution Mechanism of a Zr Rich Structure in a Ni_3Al Base Alloy

In the present research,the dissolution mechanism of a Zr rich structure during annealing of a Ni 3 Al base alloy containing Cr,Mo,Zr and B,was investigated.The annealing treatments were performed up to 50 h at 900,1000 and 1100 C.The alloy used in this investigation was produced by vacuum-arc remelting technique.The results show that at the beginning of the process,a mixed interface reaction and local equilibrium(long range diffusion) mechanism controls the dissolution process.After a short time,this mechanism changes and the dissolution mechanism of the Zr rich structure changes to only long range diffusion of Zr element.According to this mechanism,the activation energy of this process is estimated to be about 143.3 kJ·mol 1.Also the phases that contribute to this structure and the transformations that occur at the final steps of solidification of this alloy were introduced.According to the results,at the final step of solidification,a peritectic type reaction occurs in the form of L+ γ’→γ+Ni 7 Zr 2 and γ-Ni 7 Zr 2 segregates from the melt.Following this transformation,γ-Ni 7 Zr 2 eutectic separates from the remaining Zr rich liquid.The solidification process will be terminated by a ternary eutectic reaction in the form of L→γ+Ni 5 Zr+Ni 7 Zr 2.     

Lamellar Spacing Selection in Regular Eutectic Solidification at Low Velocity

Based on the Jackson and Hunt＇s analysis, the selection of lamellar spacing in regular eutectics was reexamined at low velocity. The isothermal assumption was released and the effective interface undercooling was determined by the weighted average of the eutectic phases. It is found that the lamellar spacing minimized the effective interface undercooling depends only on the intrinsic characteristics of a given system at a fixed velocity. In addition, the selection of lamellar spacing is related to the relationship between the average interracial undercoolings of the eutectic phases and the lamellar spacing. The selected lamellar spacing obviously deviated from that predicted by the Jackson and Hunt＇s analysis if the variation of the average interracial undercoolings of the solid phases with the lamellar spacing was markedly different at a constant growth rate.     

Effect of Heat Treatment on the Microstructure and Residual Stresses in (Ti,Al)N Films

(Ti,Al)N films were fabricated by arc ion plating (AIP) and then annealed within a range of temperatures from 200 to 500~0C for 30 min in vacuum. The results indicate that the average residual stresses decrease slightly from -5.84 to -4.98 GPa with increasing annealing temperature. The stress depth distribution evolves from a sharp "bell" shape to a mild "bell" shape, suggesting a more uniform stress state in the annealed films. The microstructure of the films was also investigated in detail. The as-deposited film consists of fine columnar crystals with an amorphous phase at the interface. During heat treatment, the columnar subgrain growth was observed; meanwhile, the phenomenon of crystallization has been identified at the interface. Further more, the relationship between the residual stresses and the microstructure of the films was explored and highlighted. In addition, there is no hardness degradation of the films during heat treatment.     

Microstructure of interfacial reaction zone in SCS-6 SiC fibre reinforced super α2 composites

Abstract

The microstructure of the interfacial reaction zone in SCS-6 SiC/super α2 composites heat treated at 700°C for 3000 h was investigated by means of analytical transmission electron microscopy. The very fine grained reaction layer adjacent to the carbon coating of the SiC fibre was found to consist of two sub layers, determined to be (Ti, V)C and (Ti, V,Nb)₅Si₃. The second layer is (Ti,Nb)C with large equiaxed grainsfollowed by the third layer consisting of the (Ti,Nb)₃(Al,Si)C phase. This layer is separated from the matrix by a fourth layer with the phase composition (Ti,Nb)₅(Si,Al)₃. At some interface positions, the two layers of(Ti,Nb)C and (Ti,Nb)₃(Al,Si)C are separated by an additional layer of the (Ti,Nb)₃(Si,Al) phase. The thickening of the interfacial reaction zone at 700°C is mainly due to the layers of (Ti, Nb)₃(Al,Si)C and (Ti,Nb)₃(Si,Al). The growth of these two layers is probably responsible for the degradation of the mechanical properties of the composites.     

Diffusion bonding of Ti coated Zircaloy-4 and 316-L stainless steel

Diffusion bonding of Zircaloy-4 and Type 316-L stainless steel was carried out by coating the joining surfaces with Ti to minimize the interlayer effect. Bonding heat treatments were carried out in vacuum at 1000 °C for 4 h and 1050 °C for 1 h. The microstructure of the diffusion zone was investigated by scanning electron microscopy and the phases in the diffusion zone were analyzed by energy dispersive spectroscopy. It is observed that Ti coating at the interface produced a dendritic structure in the diffusion zone formed in the Zircaloy-4. The concentration of the dendrites increases with an increase in bonding temperature.     

Comparison of epitaxial growth of CoSi2 among Co/Ti, Co/Hf, and Co/Nb bilayers on (100)Si

Formation of epitaxial cobalt silicide film on (100)Si using Co/Ti, Co/Hf, and Co/Nb bilayers has been investigated. The degree of easiness in the epitaxial growth of CoSi₂ by annealing the metal bilayers on (100)Si at 800°C was found to strongly depend upon what thin metal layer was used as an epitaxy promoter. Perfect epitaxy of CoSi₂ was obtained using Co/Ti/(100)Si. Local epitaxy of CoSi₂ was obtained using Co/Ti/(100)Si, while epitaxy of CoSi₂ was not obtained for the Co/Nb/(100)Si system. Epitaxial growth of CoSi₂ in these Co/metal/Si systems seems to be related to the formation and decomposition of stable reaction barriers like Co–Ti–O and Hf–Si–O compounds at high temperatures. These stable reaction barriers formed at high temperatures make uniform diffusion of Co atoms possible, resulting in the growth of epitaxial CoSi₂.     

Effects of reaction products on the bond strength of the transition joint formed between titanium and stainless steel

Abstract

Solid state diffusion bonding was carried out between commercially pure titanium and 304 stainless steel in the temperature range 800 - 950°C for 120 min in vacuum under uniaxial load. The transition joints were characterised using optical and scanning electron microscopy. The study shows the presence of different reaction layers in the diffusion zone and their chemical compositions were determined by energy dispersive spectroscopy. The occurrence of different intermetallic phases such as FeTi, λ, χ, and σ has been predicted from the ternary phase diagram of Fe - Cr - Ti and they were confirmed by the X-ray diffraction technique. A maximum bond strength 76% of that of titanium was obtained for the diffusion couple processed at 800 ° C owing to finer size intermetallic compounds and the increment in joining temperature, which results in growth of brittle intermetallics leading to a sharp fall in the strength of the transition joints.     

Ti/Si(100)体系界面扩散反应动力学研究

采用移动边界条件下扩散问题的处理方法，综合界面反应和扩散两个过程对界面硅化物形成的影响，建立起Ti／Si（100）界面扩散反应动力学理论模型，并拟合快速热退火处理后试样界面Auger深度分析谱，得到Ti，Si在相应界质中扩散系数和表现反应活化能。研究结果表明，Ti／Si体系界面TiSi2生成经历了一个由反应动力学控制到扩散控制的过渡。Si从其晶格中解离并扩散到Ti／TiSi2界面是制约扩散过程的关键因素。     

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu-coated Ti – 6Al – 4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti – 6Al – 4V substrate. The formed intermetallic compounds at the interface between the Ti – 6Al – 4V substrate and Cu coating are CuTi₂, CuTi, Cu₄Ti₃, and β-Cu₄Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu-coated Ti – 6Al – 4V is 126.0 kJ mol⁻¹, and the pre-exponential factor is 0.1 m² s⁻¹. The tribological properties of the Cu-coated Ti – 6Al – 4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti – 6Al – 4V alloy. The wear resistance of the Ti – 6Al – 4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu – Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.     

熔融6061和4043铝合金在纯钛表面的反应润湿

利用改良座滴法研究了高真空条件下熔融6061和4043铝合金在600,650,700℃分别与纯钛(TA2)的反应润湿行为。结果表明:Al/Ti体系属于典型的反应润湿,铝合金中微量的Si元素在界面上产生了明显富集且满足热力学条件;界面上形成了富Si的致密的层片状Ti_7Al_5Si_(12)相,致密层产生后阻碍熔体润湿母材;Ti_7Al_5Si_(12)相的分解及三相线附近疏松的粒状Al_3Ti相产生后能够破除钛表面的氧化膜,进而促进润湿;6061/TA2和4043/TA2两润湿体系铺展动力学均可由反应产物控制(Reaction Product Model)模型描述,整个润湿铺展过程分为两个阶段,即先呈指数铺展、后呈线性铺展;6061铝合金对应两个阶段的铺展活化能分别为56kJ/mol和112kJ/mol,4043铝合金以指数铺展为主,铺展活化能为47kJ/mol,Ti_7Al_5Si_(12)相的分解对应于指数铺展阶段。     

Dissolution behaviour of Ti/Cu contact reaction

Abstract

This paper investigates the base metal dissolution behaviour of Ti/Cu contact reaction couples at temperatures of 880–940°C and holding times of 1–180 s. The results show that once the temperature reached the eutectic point of Ti and Cu (875°C), the liquid formed quickly. For Ti/Cu contact reaction couples, the formation of eutectic liquid phase had a direction affected by the contact assembly, i.e. the eutectic liquid phase formed first at the Cu side, and then spread along the depth direction of Cu. The width of the eutectic liquid zone when Ti was placed on Cu was wider than when Ti was placed under Cu. Also, the shape of the upper liquid zone was wave-like while the shape of the lower liquid zone was planar, the formation mechanisms of the two liquid zone are related to the spreading behaviour of the eutectic liquid at the Ti/Cu interface. The formation process of Ti/Cu eutectic liquid zone was similar to that of the traditional solid-state diffusion zone, and the relationship between the width of the liquid zone and holding time obeyed a square root law.     

Growth kinetics of Pd2Si from evaporated and sputter-deposited films

The growth kinetics of Pd₂Si formed by palladium films either vacuum deposited or sputter deposited onto vacuum-evaporated, sputter-deposited or single-crystal silicon substrates were investigated by backscattering spectrometry (BS). The impurity contents of the films and substrates were characterized by both secondary ion mass spectrometry and BS. In all cases, Pd₂Si is the only silicide phase detected after vacuum annealing up to 300°C and the growth mechanism is always diffusion limited. The rate of the diffusion was found to differ and the activation energies for Pd₂Si formation varied between 0.9 and 1.5 eV. These changes are correlated with the presence of impurities in the deposited palladium and silicon films. Also, sputtered palladium films do not form epitaxial Pd₂Si layers on (111) Si substrates as vacuum-deposited palladium films do. The conclusion is that rates of reaction and morphology of Pd₂Si layers grown by thermal annealing depend on the deposition system.     

基于真空热压扩散法的金刚石/Ti界面生成机制

采用真空热压扩散法在聚晶金刚石表面制备Ti层,探究金刚石表面金属化过程中的界面生成机制。利用扫描电子显微镜和X射线衍射仪,分析了钛层的表面形貌、界面结构和界面间的物相组成,采用能谱仪对界面进行了元素分析,计算了聚晶金刚石与Ti层之间界面的扩散带宽度及生成TiC的化学反应吉布斯自由能变。研究结果表明:在聚晶金刚石表面形成了平整、致密的Ti层,在聚晶金刚石与Ti层界面之间存在C、Ti和Co元素的扩散,在结合界面处产生了一定宽度的元素扩散带,同时在金刚石表面生成了点状TiC。真空热压扩散法实现了金刚石与Ti层的化学结合,可以提高金刚石与Ti层的结合强度。