Processing of Ceramic Based Nanocomposite Using α-Al2O3 Powder and FeCl2 Solution as Starting Materials

Alumina-iron nanocomposite powders were prepared by a two-step process. In the first step, α-Al2O3-FeCl2 powder mixture was formed by mixing α-Al2O3 powders with FeCl2 solution followed by drying. In the second step, the FeCl2 in the dry power mixture was selectively reduced to iron particles. A reduction temperature of 750℃ for 15 min in dry H2 was chosen based on the thermodynamic calculations. The concentration of iron in FeCl2 solution was calculated to be 20 vol. pct in the final composite. Two techniques were used to produce composite bulk materials. The Al2O3 nanocomposite powders were divided to two batches. The first batch of the produced mixture was hot pressed at 1400℃ and 27 MPa for 30 min in a graphite die. To study the effect of oxygen on the Al2O3/Fe interface bonding and mechanical properties of the composite, the second batch was heat treated in air at 700℃ for 20 min to partially oxidize the iron particles before hot pressing. Characterization of the composites was undertaken by conventional density measurements, X-ray diffractometry （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electron probe micro analysis （EPMA）. The suggested processing route （mixing, reduction and hot pressing） produces ceramic-metal nanocomposite much tougher than the pure Al2O3. The fracture strength of the produced Al2O3/Fe nanocomposite is nearly twice that of the pure Al2O3. The presence of spinel phase, FeAl204, as thick layer around the Fe particles in the Al2O3 matrix has a detrimental effect on interfacial bonding between Fe and AI203 and the fracture properties of the composite.     

Processing of Ceramic Based Nanocomposite Using α-Al2O3 Powder and FeCl2 Solution as Starting Materials

Alumina-iron nanocomposite powders were prepared by a two-step process. In the first step, α-Al2O3-FeCl2 powder mixture was formed by mixing α-Al2O3 powders with FeCl2 solution followed by drying. In the second step, the FeCl2 in the dry power mixture was selectively reduced to iron particles. A reduction temperature of 750℃ for 15 min in dry H2 was chosen based on the thermodynamic calculations. The concentration of iron in FeCl2 solution was calculated to be 20 vol. pct in the final composite. Two techniques were used to produce composite bulk materials. The Al2O3 nanocomposite powders were divided to two batches. The first batch of the produced mixture was hot pressed at 1400℃ and 27 MPa for 30 min in a graphite die. To study the effect of oxygen on the Al2O3/Fe interface bonding and mechanical properties of the composite,the second batch was heat treated in air at 700℃ for 20 min to partially oxidize the iron particles before hot pressing. Characterization of the composites was undertaken by conventional density measurements, X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe micro analysis (EPMA). The suggested processing route (mixing, reduction and hot pressing)produces ceramic-metal nanocomposite much tougher than the pure Al2O3. The fracture strength of the produced Al2O3/Fe nanocomposite is nearly twice that of the pure Al2O3. The presence of spinel phase,FeAl2O4, as thick layer around the Fe particles in the Al2O3 matrix has a detrimental effect on interfacial bonding between Fe and Al2O3 and the fracture properties of the composite.     

铝-钢复合板结合界面研究

用显微硬度计、透射电镜、扫描电镜及能谱仪对铝-钢复合板结合界面进行了研究.结果表明,铝-钢复合板结合界面呈准正弦波形;结合界面两侧的铝元素和铁元素相互扩散,扩散的深度在微米数量级,爆炸后结合界面显微硬度增加,漩涡区的硬度增加为基体金属的3-6倍;结合界面由铝和铁的金属间化合物(FeAl、FeAl2、FeAl3、Fe24...     

氩弧熔覆Fe-Al金属间化合物复合涂层的组织与性能

为改善Fe-Al金属间化合物的力学性能及抗高温氧化性能,利用氩弧熔覆方法在Q235钢上制备了Fe-Al熔覆层和Fe-Al/Al_2O_3熔覆层。采用金相显微镜、X射线衍射仪、硬度计、磨粒磨损试验机对氩弧熔覆涂层进行显微组织结构观察和磨损性能测试。采用高温氧化试验对涂层的耐高温氧化性进行了研究。结果表明:Fe-Al熔覆层形成FeAl和Fe_3Al相,而Fe-Al/Al_2O_3熔覆涂层含有FeAl、Fe_3Al和Al_2O_3相;熔覆层的耐磨粒磨损性能优于基体且Fe-Al/Al_2O_3熔覆层优于Fe-Al熔覆层;熔覆层的耐高温氧化性能明显提高,在700℃下,Fe-Al熔覆层和Fe-Al/Al_2O_3熔覆层相比于基体分别提高了4.46和5.68倍。     

CrO3对Fe2O3+Al铝热反应系统反应过程的影响

用铝热-重力分离法制备了不含铁铝尖晶石（FeAl2O4）的陶瓷内衬复合钢管，并通过热力学计算分析了有关反应的优先顺序，结果表明，在Fe2O3-Al系统中强氧化剂CrO3与Al的反应并不是一步完成而是分步反应，FeAl2O4优先于Cr2O3与Al反应，因而加入CrO3添加剂可有效地去除陶瓷层中的尖晶石相，从而提高复合钢管的耐蚀性能。     

In-situ high temperature X-ray diffraction study of Co/SiC interface reactions

In situ experiments on the Co/SiC interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1–4s using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments with the apparatus. The time-temperature phase diagram of Co/SiC in N₂was determined. Co₂Si and CoSi were formed at the Co/SiC interface between 921 and 1573 K in N₂. The formation of CoSi obeyed the parabolic rate law. The value of the activation energy was 95 kJ/mol. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compressive strain caused by CoSi occurs on SiC.     

硬质相对冷喷涂FeAl金属间化合物涂层性能的影响

FeAl金属间化合物具有优良的物理性能和力学性能,但其室温塑性和断裂韧性低,限制了其工程应用.利用机械合金化制备了Fe(Al)固溶体合金粉末及Al2O3,WC硬质相增强的复合合金粉末,通过冷喷涂沉积涂层并结合后热处理原位反应制备了FeAl金属间化合物涂层及其复合涂层.利用扫描电镜(SEM)、X射线衍射仪(XRD)及显微硬度仪等研究了硬质相对球磨粉末组织结构、冷喷涂FeAl金属间化合物涂层组织结构及性能的影响.结果表明.硬质相可显著加速球磨粉末内部层状结构的细化程度,喷涂态涂层具有不同于传统热喷涂涂层的层状组织结构,热处理可实现喷涂态涂层中Fe(Al)固溶体向FeAl金属间化合物的原位转变,致使层状结构消失,获得无粒子界面的FeAl金属间化合物涂层,弥散分布的硬质相可显著提高冷喷涂FeAl金属间化合物涂层的强化稳定性.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

Application of reaction synthesis principles to thermal spray coatings

Reaction synthesis principles have been extended to plasma spraying to obtain coatings consisting of mixed oxide phases and iron aluminides. Elemental powders of iron and aluminium were fed through a d.c. plasma torch to deposit intermetallic coatings on carbon steel substrates. Carbon steel substrates were also pre-heated with a plasma flame to create an iron oxide surface on the substrate such that an exothermic thermite reaction takes place when molten splats of aluminium impinge the pre-heated substrate at sub- or supersonic velocities. A thermite reaction between iron oxide and aluminium allowed the formation of alumina, FeAl2O4, iron, and iron aluminide phases. The presence of FeAl2O4 and Al2O3 increased the surface hardnesses of the coating, and the hardnesses of the coatings are significantly higher than the hardnesses of steel substrate, and aluminium particles. X-ray analysis of the coatings, microstructural observations, and microhardness measurements suggest that plasma spraying conditions can be tailored to obtain coatings with high hardness values with in situ synthesized reinforcements (spinel and alumina) or iron aluminide phases. Aluminium-rich phases were observed in the as-deposited coatings when a mixture of aluminium and iron or aluminium and nickel were fed through the plasma gun in ratios equivalent to Fe3Al, FeAl, Ni3Al, and NiAl. In some cases, annealing allowed the formation of iron-rich or nickel-rich aluminide phases. High solidification rates of molten splats allowed very limited diffusional reactions between the splats of aluminium and iron, or aluminium and nickel because the available diffusional time for exothermic interfacial reactions is limited to a fraction of a second at best. Oxidation of part of the aluminium led to the formation of alumina in the as-deposited coatings, and therefore, a vacuum plasma spraying technique is desirable to obtain intermetallic phases. The results suggest that reactive spraying will allow deposition of coatings by utilizing the heats of reaction between the constituents, and reactive spraying will broaden the engineering applications of reaction synthesis techniques.     

Formation of B2 intermetallic NiAl and FeAl by mechanical alloying

Nanostructud B2 intermetallic compounds NiAl and FeAl have been prepared by mechanical alloying (MA) the elemental powder mixtures and subsequent heating. The structural evolution during MA was monitored by in situ thermal analysis and X-ray diffraction (XRD). The final products were characterized by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the nanocrystalline intermetallic compound NiAl, which is difficult to disorder by milling, was synthesized directly after an exothermic explosive reaction; whereas FeAl compound was formed after a thermal process of asmilled Fe(Al) solid solution obtained through interdiffusion during MA. The large heat of formation of NiAl compound is the main driving force for the exothermic explosive reaction, and the difference in diffusivity between NiAl system and FeAl system is suggested to be the main cause of the different behaviors of formation between NiAl and FeAl compounds by MA.     

Solid-state reactions underlying mechanochemical synthesis in the Fe-Al system

The sequence of solid-state transformations in the mechanochemical synthesis of Fe-Al intermetallics according to the reactions Fe + Al = FeAl, FeAl + 2Fe = Fe₃Al, and 2FeAl + 3Al = Fe₂Al₅ in powder mixtures has been studied by X-ray diffraction, differential scanning calorimetry, and Mössbauer spectroscopy. The results indicate that the process involves the formation of atomic configurations that may become nuclei of stable or metastable intermediate phases. Prolonged milling leads to homogenization of the synthesis product and the formation of a solid solution or an intermetallic phase with a low degree of long-range order. Complete ordering of the intermetallic phase can as a rule be achieved by heating.     

The Creep and Thermal Stability Characteristics of a Unidirectionally Solidified Al2O3/YAG Eutectic Composite

Compressive creep characteristics at 1773, 1873, and 1973 K, oxidation resistance over 1000 h at a temperature of 1973 K in ambient air, and the thermal stability characteristics at 1973 K in ambient air of a unidirectionally solidified Al2O3/YAG eutectic composite were evaluated. At a test temperature of 1873 K and a strain rate of 10–4/s, the compressive creep strength of a eutectic composite manufactured by the unidirectional solidification method is approximately 13 times higher than that of a sintered composite with the same chemical composition. The insite eutectic composite also showed greater thermal stability, with no change in mass after an exposure of 1000 hours at 1973 K in ambient air. The superior high-temperature characteristics are closely related to such factors as (1) the in-situ eutectic composite having a microstructure, in which single crystal Al2O3 and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries and (2) no amorphous phase is formed at the interface between the Al2O3 and the YAG phases.     

Microstructural evolution of Al–Fe powder mixtures during high-energy ball milling

High-energy ball milling has been performed on FexAl1-x powder mixtures with x=0.75, 0.50, 0.25 and 0.20. X-ray diffraction, Mossbauer spectroscopy and electron microscopy have been used to characterize the samples milled for different times and annealed in a differential scanning calorimeter. It is found that, during milling, there is diffusion of both elements into each other, with a prevalence of iron diffusion into aluminium, at least in the early stages of the process. This behaviour is more pronounced in the aluminium-rich samples. The growth of the Fe(Al) and Al(Fe) solid solutions has been observed for x0.5, different from the lower iron concentrations where the Fe(Al) phase has not been detected. The annealing of pre-milled samples favours the formation, depending on the sample composition and on the annealing temperature, of intermetallic phases such as Fe3Al, FeAl, Fe2Al5 and FeAl3. © 1998 Chapman & Hall     

Reactive wetting of rutile by liquid aluminium

Sessile drop wetting experiments of liquid Al on polycrystalline rutile (TiO2) were conducted in the 973.1273 K temperature range under a low total pressure (9.3 10.3 Pa, Ar) and a low oxygen partial pressure ( 1.33 10.7 Pa), as a function of temperature and time. A non-wetting (150., 973 K, t >120 min.) to partial wetting (85., 1273 K, 50.60 min.) transition reflects reactive wetting characteristics. Microstructural investigations of the metal-ceramic interface shows that TiO2 is reduced by liquid Al, resulting in the formation of Al2O3. The steady-state contact angle at 1273 K of Al on -Al2O3 and Al on rutile are very similar, and the role of Ti segregation is minimal. It appears that spreading of the Al drop on TiO2 is governed by the reduction reaction at the solid-liquid interface. The measured activation energy corresponds well to the activation energy for volume diffusion of Al, Ti and O in rutile and the volume diffusion of Al in polycrystalline -Al2O3.     

复杂铝热反应的平衡热力学分析

利用最小自由能原理对复杂铝热反应(Fe₂O₃+Cr₂O₃+CrO₃+NiO+O₂+N₂+TiO₂+C+S+CaO+Al)过程进行了平衡热力学分析,计算了铝热剂成分、预热温度、惰性添加剂对平衡产物成分、反应热和绝热温度的影响。结果表明,平衡产物中除了Al₂O₃,Fe,Cr,Ni等产物外,还有可能生成TiC, Cr₇C₃, CrN, AlNi,Ni₃Al,FeAl₂O₄,CaS,TiN等物质。随着铝热剂中Al含量的增加,平衡产物中生成铝镍金属间化合物的倾向增加。CaO有除S的作用,此外,CaO还使体系的绝热温度降低。提高预热温度,体系的绝热温度升高。     

反应等离子喷涂Fe-Al2O3-FeAl2O4复合涂层的反应机理研究

金属/陶瓷复合涂层具有金属的韧性和陶瓷的高强度、高硬度等优点,利用反应等离子喷涂技术将Fe2O3/Al复合粉制备成金属/陶瓷复合涂层,以X射线衍射(XRD)、扫描电镜(SEM)、能谱仪(EDS)等分析技术研究了该复合涂层的反应机理和涂层性能.结果表明:涂层具有以层状的FeAl2O4、Al2O3为骨架,球形的Fe及部分FeAl为弥散相的复合组织;复合粉体反应形成涂层的过程是分步进行的,而且在熔滴到达基体后部分反应仍继续进行;一定程度上,由于反应过程受到Al元素的扩散限制,同时等离子喷涂的冷却速度较高,使得涂层中主要含有FeAl2O4及少量的FeO.     

On the deformation behavior of single crystalline Fe---Al

Compression tests were performed on two single crystalline alloys with the B2 (CsCl) crystal structure, FeAl and Fe-45Al. Testing was done in air and test temperatures ranged from room temperature to 1273 K. Results show that limited {123} slip was produced in FeAl at low temperatures followed by the addition of {110} slip. {211} slip was produced in the Fe-45Al alloy up to 1073 K after which {110} slip was also observed. The flow stress vs. temperature curve of FeAl showed high compressive stresses up to 973 K followed by a large decrease in stress with increasing temperature. The flow stress versus temperature curve for Fe-45Al showed a very weak temperature dependence up to 973 K followed by a gradual decrease in stress with increasing temperature. Below 973 K, the flow stress values of Fe-45Al were calculated to be 50 to 60% of the values of FeAl.     

Atomistic modeling of plastic deformation in B2-FeAl/Al nanolayered composites

In this work, the deformation response of the B2-FeAl/Al intermetallic composites, as a model material system for nanolayered composites comprised of intermetallic interfaces, has been explored. We use atomistic simulations to study the deformation mechanisms and the interface misfit dislocation structure of B2-FeAl/Al nanolayered composites. It is shown that two sets of dislocations are contained in the interface misfit dislocation network and are correlated with the initial dislocation nucleation from the interfaces. The effects of layer thickness on the uniaxial deformation response of the B2-FeAl/Al multilayers are investigated. We observed that under compressive loading the smaller proportion of the FeAl layers leads to the lower overall flow stress. Under tensile loading, the void formation mechanism is investigated, suggesting the interface structure and the dislocation activities in the FeAl layers playing a significant role to trigger the strain localization which leads to void nucleation commencing at the interface. It is also found that the deformation behavior in the “weak” Fe/Cu interface behaves substantially different than that of the “strong” FeAl/Al interface. The atomistic modeling study of the nanolayered composites here underpinned the mechanical response of “strong” intermetallic interface material systems. There is no void nucleation during the entire plastic deformations in the Fe/Cu simulations, which is attributed to much higher dislocation density, more slip systems activated, and relative uniformly distributed dislocation traces in the Fe phase of the Fe/Cu multilayers.

     

HP40Nb钢热浸镀Al-Si高温氧化行为及组织研究

为提高高温抗氧化性能,对HP40Nb钢进行了热浸镀Al-10%(质量分数)Si,并进行不同温度扩散处理,研究了不同扩散处理试样在1000℃条件下的高温氧化行为,通过SEM,EDS和XRD分析了经不同扩散处理后的渗层在高温氧化过程中的组织结构变化.结果表明:经800℃/4h扩散处理,渗层由内层(NiAl+ Cr3 Si)...     

Effect of Al and C on structure and mechanical properties of Fe–Al–C alloys

Abstract

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe₃Al with ordered DO₃ structure and Fe₃AlC_0·5 precipitates with L′1₂ structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.