Microstructural characterization of a platinum-modified diffusion aluminide bond coat for thermal barrier coatings

Microstructural and chemical evolution induced by thermal cycling of a platinum-modified diffusion aluminide bond coat was investigated with transmission electron microscopy (TEM), X-ray diffraction, (XRD) and electron microprobe analysis. As-fabricated, the bond coat was confirmed to be an ordered B2 structure, but the underlying microstructure was found to be modulated. Thermal cycling resulted in a primarily outward diffusion of Ni and the formation of a Ni-rich bond coat containing secondary L1₂ precipitates. Closer inspection of the bond coat revealed a transformation from its original B2 structure to a L1₀ martensite. In-situ TEM observations indicated that the martensite is stable at lower temperatures and that the parent B2 structure reappears at higher temperatures. These observations can be used to explain the variations in strength that have recently been measured in thermally cycled bond coats. The resulting transformation strain is also argued to play an important role in determining the accumulation of stress and strain in thermally cycled thermal barrier coatings (TBCs).     

Microstructural evolution in nickel during rolling from intermediate to large strains

High-purity nickel (99. 99 pct) with a grain size of 80 to 100 μm was deformed by cold-rolling from 37 to 98 pct reductions (von Mises effective strains ofεvm = 0. 5 to 4. 5). The deformation microstructures and texture at five strain levels were observed and characterized using transmission electron microscopy (TEM) and neutron diffraction. The microstructures evolved within a framework common to medium and high stacking fault energy fee polycrystals. This framework consists of structural subdivision by higher angle boundaries (geometrically necessary boundaries) at one volume scale and at a smaller volume scale by lower angle cell boundaries (incidental boundaries) for all strain levels. We have characterized the dislocation boundaries, including dense dislocation walls (DDWs), microbands (MBs), and lamellar boundaries (LBs) in terms of crystallographic and macroscopic orientations, morphology, and frequency of occurrence. The microstructural evolution is discussed with special emphasis on factors that contribute to the transition from structures characteristic of small and medium strain microstructures to those characteristic of large strain microstructures.     

Self-propagating high-temperature synthesis for the deposition of thermal-sprayed coatings

The paper reviews the relevant literature and results of the author’s research into self-propagating high-temperature synthesis (SHS) used to deposit protective thermal-sprayed coatings. The compositions of different SHS-produced powders, which are used to deposit thermal-sprayed coatings, are indicated. The combination of spraying and synthesis of the material to form coatings by the interaction of composite powder particles is considered. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 105–125, 2008.     

Modification of the structure of powder coatings on nickel and chromium-nickel bases by introducing nanoparticles of titanium diboride during electron-beam welding

Composite materials and coatings that contain titanium diboride in a metal host are widely used with the purpose of fabricating high-strength heat-resistant materials. Due to high hardness as well as heat and corrosion resistance, titanium diboride is a promising compound for use as a wear-resistant component of the composition “metal host-TiB₂.” The structure and mechanical properties of compositions TiB₂-PG-10N-01 and TiB₂-PKh20N80, which were obtained by a three-stage method involving the preliminary mechanical treatment of mixtures of elemental powders, the self-propagating high-temperature synthesis (SHS) reaction initiated in the activated mixture, and the subsequent mechanical treatment of the product of the SHS reaction, are investigated. To form the coatings, the method of electron-beam welding is applied.     

The influence of aluminum on the nickel and chromium diffusion into copper during the deposition and thermal treatment of gas-thermal coatings

The method of electron probe microanalysis is applied to analyze the distribution of elements over the diffusion layer obtained by the deposition on copper of gas-thermal coatings involving Al, Ni, Cr, Fe and subsequent thermal treatment. It is established that the penetration depth and concentration of the coating elements reach their maximal values in the case of the presence of an aluminum sublayer.     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Microstructural evolution of 6063 aluminum during friction-stir welding

The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.     

Microstructural characterization of electron beam-physical vapor deposition thermal barrier coatings through high-resolution computed microtomography

Thermal barrier coatings (TBCs), deposited using the electron beam-physical vapor deposition (EB-PVD) process, comprise a unique architecture of porosity capable of bridging the technological gap between insulation/life extension and prime reliance. The TBC microstructures consist of columnar structure, nucleated via vapor condensation, along with a high degree of intercolumnar porosity, thus providing enhanced stress relief on thermomechanical loading and also accommodating misfit stresses resulting from CTE mismatch. In this article, we report the characterization of these coatings using high-resolution synchrotron-based X-ray computed microtomography (XMT) at 1.3-μm resolution. Experiments focused on quantitative characterization/visualization of imperfections in these coatings and on the relative changes in microstructural features upon isothermal annealing. The influence of time/temperature of exposure was investigated and the results were correlated with elastic modulus. This article is based on a presentation made at the symposium “Characterization and Representation of Material Microstructures in 3-D” held October 8–10, 2002, in Columbus, OH, under the auspices of ASM International’s Phase Transformations committee.     

Self-propagating high-temperature synthesis of promising ceramic materials for deposition technologies of functional nanostructured coatings

Taking into account the demand for composite targets or precursors for ion-plasma technologies for the deposition of functional nanostructured coatings, this work is a review of recently obtained and previously unpublished results of synthesis in the combustion mode of a series of chemical classes of systems differing in regards to the mechanisms of combustion and structure formation. The experimental results for self-propagating high-temperature synthesis are presented for the following systems: Ti-Al-C, Cr-Al-C, Ti-Cr-Al-C, Cr-B, Ti-Cr-B, Ti-Ta-C, and Ti-Si₃N₄-Al-C. The compositions of reaction mixtures and conditions of obtaining the most interesting and needed materials with high mechanical properties and resistance to high-temperature oxidation are found.     

化学气相沉积ZrO_2涂层的表面微观形貌演变

采用ZrCl4-CO2-H2-Ar体系,在不同温度或氢气流量条件下采用常压化学气相沉积法(APCVD)通过不同沉积流程在C/C样品表面制备ZrO2涂层。用X射线衍射分析仪(XRD)和扫描电镜(SEM)分析涂层的物相组成和形貌特征。结果表明:随沉积温度升高,ZrO2涂层表面形貌由小颗粒堆积态向大尺寸多晶转变;氢气流量为0时,只在局部区域出现了少量ZrO2,当氢气流量为400mL/min时,ZrO2涂层晶粒尺寸较大且晶体学平面特征最明显;在氢气流量为800mL/min时沉积出现了副产物ZrC与C;在氢气流量为1600mL/min时,出现沉积副产物ZrC;连续沉积时,得到的ZrO2涂层为山峰状形貌。     

Microstructural evolution during laser cladding of M2 high-speed steel

Laser cladding of gas-atomized M2 high-speed steel on the mild steel substrate was performed using scan rates of 1 to 10 mm/s, scan line spacings of 0.1 to 0.5 mm, and powder feed rates of 1 to 10 g/min, for a given laser power of 400 W. This article presents a detailed study of the microstructural evolution during laser cladding. The effect of scan rate, scan line spacing, and powder feed rate on cooling rate can be described in terms of the cladding-layer thickness, i.e., the thinner the layer, the higher the cooling rate. The degree of metastability in the laser-clad microstructure can be understood in terms of the lattice parameter of the bcc phase. The lattice parameter of the bcc phase increased with increasing layer thickness and reached a maximum value at a thickness of 0.3 mm. Correspondingly, the microstructure varied from a cellular or dendritic structure of δ ferrite and austenite to a mixture of martensite and retained austenite. However, further increasing the layer thickness led to a decrease of both the lattice parameters of the bcc phase and the proportion of retained austenite in the martensite. This was accompanied by an increase of the amount of carbide at the prior austenitic grain boundaries and a decrease of the carbon content in the martensite and retained austenite.     

2524铸态合金均匀化过程中微观组织的变化

在不同温度和不同保温时间条件下对2524铸态合金进行均匀化处理,利用扫描电镜(SEM)、透射电镜(TEM)、差示扫描量热法(DSC)和金相显微镜(OM)等,研究2524铸态合金在均匀化过程中微观组织的变化,并分析在不同的均匀化处理状态下T相(即Al20Cu2Mn3相)的析出情况。结果表明:2524铸态合金中存在严重的枝晶偏析,大量的非平衡共晶相连续地沿晶界分布,主要的第二相为Al2Cu和Al2Cu Mg;主要合金元素不同程度地富集在晶界,富集程度为CuMgMn;最佳的均匀化处理制度为500℃/48 h,均匀化处理后,Al2Cu和Al2Cu Mg基本回溶至基体,只余少量难溶的Al Cu Fe Mn相散布在晶界,该均匀化制度与均匀化动力学方程的计算结果基本一致;随均匀化处理温度升高,T相粒子的尺寸逐渐增大而数目逐渐减少。     

Microstructural evolution at the bonding interface during the early-stage infrared active brazing of alumina

Infrared brazing of Al₂O₃ and alloy 42 using a silver-base active braze alloy was investigated at 900 °C for 0 to 300 seconds, with a heating rate of 3000 °C/min. Experimental results show that Ti₃(Cu, Al)₃O intermetallic with various amounts of Al is observed in the reaction layer and plays an important role in the early stage of reactive wetting. A two-layer structure is observed at the reaction interface brazed at 900 °C for 5 seconds. The reaction layer close to the alumina contains large amounts of Al, so the mass balance of the system is maintained. The growth of the reaction layer is not rate controlled by diffusion within the first 120 seconds. After 120 seconds, the rate controlling mechanism of the reaction layer becomes the diffusion control, satisfying the parabolic law. Dynamic wetting angle measurements using a traditional vacuum furnace at the heating rate of 10 °C/min demonstrate that the wetting angle rapidly decreases within the first 150 seconds, especially 0 to 80 seconds, and eventually stabilizes after 600 seconds.