Creep-Induced Phase Instability and Microstructure Evolution of a Nearly Lamellar Ti-45Al-8.5Nb-(W,B,Y)Alloy

Microstructure degradation and stress-induced transformation of a high Nb-containing TiAl alloy with nearly lamellar microstructure during creep were investigated. Tensile creep experiments were performed at 800, 850 and 900 °C under 150 MPa in air. Microstructures before and after creep tests were examined using scanning and transmission electron microscopy (SEM and TEM). Dislocations within the lamellar structure and β_o(ω) region and twin intersection in massive γ grains were investigated. Dislocation sliding played a critical role in the deformation of ω_o phase, which preferentially occurred on the (0002)ω_o plane. Possible deformation mechanisms were revealed. A stress-induced γ→α₂ phase transformation took place during the creep test at 850 and 900 °C. α₂ lamella could directly decompose into the ω_o phase at 850 °C. The instability of high-temperature microstructure can weaken the creep resistance and promote the plastic deformation of the lamellar matrix, thus could be detrimental to the creep properties. The correlations between creep properties and microstructure instability were discussed.     

γ′-cutting as rate-controlling recovery process during high-temperature and low-stress creep of superalloy single crystals

In the present study the pairwise cutting of the γ′-phase after high-temperature and low-stress shear creep deformation of superalloy single crystals was investigated using weak-beam and high-resolution transmission electron microscopy. Recently, a cutting process in the single-crystal superalloy CMSX-6 was observed [Acta mater., 45 (1997) 4251] where two γ-channel dislocations with different Burgers' vectors (b) jointly shear the γ′-phase in forming a superdislocation with an overall Burgers' vector of a[010]. This type of high-temperature and low-stress γ′-phase cutting mechanism was also observed for CMSX-4 in the present work, indicating that this mechanism is relevant for superalloy single crystals in general. Two different configurations have been observed associated with the pure edge a010 and the 45° a001 dislocations. The cores of these superdislocations are not compact, but rather are composed of two different a/2110 dislocations. The distance between the leading and the trailing superpartial dislocation for the pure edge a010 configuration is of the order of 25 Å. In all cases observed in the present study, the common superpartial is associated with the crystallographic slip system that is directly loaded (Schmid factor 1). The striking feature of the movement of the superdislocations in the γ′-phase is that the two superpartials need to move by a combined process of glide and climb. This requires diffusional exchange of atoms/vacancies between the leading and the trailing superpartial, in which case the process is self-fed and the overall vacancy equilibrium is not disturbed. It is also possible that one dislocation pair produces or absorbs vacancies so that its movement must be coupled to events which maintain overall vacancy equilibrium. Minimum creep rates can be rationalized on the basis of the fluxes associated with the movement of superdislocations in the γ′-phase.     

Creep behavior and deformation mechanisms in a nanocluster strengthened ferritic steel

Mechanically alloyed, nanostructured ferritic steels represent a class of alloys that can display high resistance to radiation and creep deformation, which are derived from the presence of nanoclusters, precipitates and solute segregation to the grain boundaries. The creep responses for a 14YWT nanostructured ferritic steel were measured over a range of temperatures and stress levels. The stress exponent was observed to vary non-linearly with applied stress; stress exponents were found to decrease with decreasing stress approaching unity at low stress. Transmission electron microscopy studies clearly demonstrated that creep deformation proceeds by a dislocation glide within nanoscale grains and that glide dislocations are attracted to and pinned by nanoclusters. In light of these observations, a new model of the creep response, inspired by the Kocks-Argon-Ashby model, is developed to explain the low creep rates and small stress exponents that are exhibited by these alloys.     

先进的电子断层扫描技术在材料科学中的发展——基于透射电子显微镜和扫描透射电子显微镜(英文)

介绍了基于透射电子显微镜和扫描透射电子显微镜的电子断层扫描技术在材料科学领域的最新进展。详细地描述各种电子断层扫描的类型:基于透射电子显微镜的断层扫描技术包括明场断层扫描、暗场断层扫描、弱束暗场断层扫描、环形暗场断层扫描和能量过滤断层扫描;基于扫描透射电子显微镜的断层扫描技术包括高角环形暗场断层扫描、环形暗场断层扫描,非共格明场断层扫描、电子能量损失谱断层扫描和X射线能谱断层扫描。报道了优化的倾转系列,比如双轴倾转、同轴倾转、锥形倾转以及等斜率倾转等。总结了先进的重构算法包括离散迭代重构技术、压缩传感算法以及等斜率算法。最后,提出了电子断层扫描技术在材料科学中的发展趋势。     

粒度调控对钨阴极表面发射微区的影响

为了在微米尺度探究未分级钨粉、分级钨粉制备钨基体在阴极热电子发射性能上的差异,深入了解阴极表面微区电子发射过程和阴极发射机理,本文采用新型深紫外激光发射电子显微镜/热发射电子显微镜系统（DUV-PEEM/TEEM）对两种钨粉制备的阴极表面进行了光电子、热电子成像分析。结果显示,与未分级钨粉相比,分级钨粉制备钨基体的显微组织更加均匀,且闭孔率由1.44%降至0.47%;光电子+热电子联合成像精确得出两种阴极的发射均主要集中于孔隙及孔隙中的活性物质上;通过热电子图像对比及分析,分级钨粉制备阴极的热电子发射微区面积更大、分布更均匀;且在1050℃工作条件下,分级钨粉制备阴极的脉冲电流密度为30.79A/cm2,发射斜率为1.38;综合分析显示其具备更理想的本征热电子发射能力和发射均匀性,这对于了解阴极发射性能及改善热阴极制备工艺有一定的指导作用。     

The effect of roll gap geometry on microstructure in cold-rolled aluminum

Microstructure and texture are analyzed through the thickness of two aluminum plates cold-rolled 40% with different roll gap geometries. It is found that both texture and microstructure are strongly affected by the rolling geometry. After rolling with intermediate-size draughts a rolling-type texture is developed throughout the plate thickness. In this case, grains are subdivided by extended planar dislocation boundaries preferentially aligned at an angle of 40 ± 15° to the rolling direction. In the plate rolled with small draughts, shear texture components appear in the intermediate layers. In these layers, extended planar dislocation boundaries are frequently found to be inclined closely to the rolling direction. The subsurface and central layers of this plate exhibit microstructures similar to those in the plate rolled with intermediate draughts. It is suggested that the development of different textures and microstructures at different depths is related to the activation of different slip systems due to through-thickness strain gradients.     

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.     

Glycolthermal synthesis and characterization of hexagonal CdS round microparticles in flower-like clusters

Hexagonal CdS round microparticles in flower-like clusters were synthesized by glycolthermal reactions of CdCl₂ and thiourea as cadmium and sulphur sources in 1,2-propylene glycol (PG) at 100-200 °C for 10-30 h. Phase and morphology were detected using X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM, TEM). The products were pure phase of hexagonal wurtzite CdS. The quantitative elemental analysis of Cd:S ratio was detected using energy dispersive X-ray (EDX) analyzer. Raman spectrometer revealed the presence of fundamental and overtone modes at 296 and 595 cm⁻¹, corresponding to the strong 1LO and weak 2LO modes, respectively. Photonic properties were investigated using UV-visible and photoluminescence (PL) spectroscopy. They showed the same absorption at 493-498 nm, and emission at 431 nm due to the excitonic recombination process. A possible formation mechanism was also proposed, according to experimental results.     

An Investigation of Carbon Content and Consolidation Temperature on Microstructure and Properties of Hafnium Boride Ceramic

This paper discusses the use of Plasma Pressure Compaction to consolidate hafnium diboride powders. The effect of carbon addition on densification was studied. The influence of consolidation temperature on microstructure, density, and hardness of the bulk sample is presented. The interrelationship between microstructure and properties of the bulk sample are rationalized in light of the intrinsic influence of carbon content and the extrinsic influence of consolidation temperature.     

Synthesis of (Ti,W, Mo,V)(C,N) nanocomposite powder from novel precursors

(Ti, W, Mo, V)(C, N) nanocomposite powders with globular-like particle of ∼10–100 nm were synthesized by a novel method, namely carbothermal reduction–nitridation (CRN) of complex oxide–carbon mixture, which was made initially from salt solution containing titanium, tungsten, molybdenum, vanadium and carbon elements by air drying and subsequent calcining at 300 °C for 0.5 h. Phase composition of reaction products was discussed by X-ray diffraction (XRD), and microstructure of the calcined powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that the synthesizing temperature of (Ti, W, Mo, V)(C, N) powders was reduced greatly by the novel precursor method. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N) is at only 1200 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the complex oxide–carbon mixture and its unusual honeycombed structure.     

A combined composition and morphology study of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings

The composition and morphology of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings are studied by a variety of complementary analytical techniques. Morphology of the alloy deposits is shown to change significantly with Co content in the alloy coating. An increase in the Co content in the range of 0.7-9 wt.% Co in Zn-Co and Zn-Co-Fe alloys results in a change in grain shape from angular to nodular and a further increase up to 10 wt.% Co corresponds to a characteristic growth mode. In the range of 10-29 wt.% of Co, the deposit contains two types of grains, i.e. one with low Co content (5-7 wt.%) and another with higher Co content (i.e. 15-35 wt.%). Zn-Co and Zn-Co-Fe alloys with Co contents of or higher than 32 wt.% Co show a homogeneous structure, which can be considered to be nanocrystalline in nature. The presence of two or more phases is not desired in terms of enhanced local corrosion by (micro-)galvanic coupling of phases while the single phase or nanocrystalline coatings provide good corrosion protection properties.     

Reinforcement of Al–Fe–Ni alloys with the in situ formation of composite materials

One of the most effective methods for the improvement of the mechanical properties of metals is their reinforcement with non-metallic materials. In the present work powder of K₂TiF₆ and KBF₄ was added in an Al–Fe–Ni alloy while the alloy was in liquid form at 1060 °C with a 5 wt.% mixture of powders and with simultaneous stirring for 30 min. The liquid was squeeze-casted at 150 bar. The as-cast specimens were examined with electron microscopy and X-ray diffraction. SEM analysis revealed that the as-formed material is composed by needle-like crystallites along with a dentritic form and an interdendritic phase. The composition of the needle-like crystallites may presumably be expressed by the formula (Fe-Ni)Al₃. The rest of the matrix consists of almost pure Al grown dentritically, while the interdendritic phase contains Fe and Ni dissolved in Al. EDS analysis also proved the existence of spots with high Ti concentration, which probably refer to the Ti–B compounds. Finally TEM verified the presence of nanocrystals in the matrix.     

In situ composite formation in the Ni–(Cu)–Ti–Zr–Si system

In situ composites were synthesized by arc melting Ni–(Cu)–Ti–Zr–Si alloys. The X-ray diffraction patterns of rapidly cooled cast strips show a primary Ni(Ti, Zr) B2 structure superimposed on the diffuse scattering maxima from the amorphous phase. Compression test results show that the composite starts to yield at 1200 MPa and fractures at 1900 MPa.     

Study of microstructure and corrosion kinetic of steel guitar strings in artificial sweat solution

Corrosion processes of strings on musical instruments occur frequently. Such processes significantly influence sound quality, but also human health because metals may provoke skin allergy. Therefore, in this study the corrosion process of six guitar strings and metal ions release associated with nickel allergy was monitored after immersion in sweat solution for 28 days. Dissolution of metal ions was measured as a function of time, and the changes of the samples after 28 days were studied by SEM/EDS. The results have shown that the amounts of dissolved metal ions in corrosive solution are decreasing in the following order: Fe³⁺ < Sn²⁺ < Mn²⁺ < Si⁴⁺ < Ni²⁺. The corrosion of electric guitar strings in artificial sweat solution was determined by the solubility of samples. The electric guitar strings E6, A5 and D4 corrosion kinetic in artificial sweat followed parabolic rate relation, while g3, h2 and e1 strings followed linear rate relation. After the 4 weeks of monitoring, the quantities of the nickel eluted ions did not exceed the limits prescribed by standard regulations. Therefore, such items do not present a threat to human health.     

Local structural fluctuation in Pd–Ni–P bulk metallic glasses examined using nanobeam electron diffraction

Local structural fluctuation in relation to the medium range order (MRO) in a Pd–Ni–P bulk metallic glass was examined using nanobeam electron diffraction (NBED) technique. We found diffraction spots with strong intensities in the NBED patterns taken from any observation sites in the specimen. This indicates a presence of MRO regions densely formed in the specimen. The diffraction spots in NBED were normally dispersed around positions corresponding to the first halo-diffraction ring in selected area diffraction. A low-temperature annealing led to form a nanocrystalline microstructure consisting of unidentified phosphides. In the course of annealing, the MRO structures deduced from the NBED patterns have no structural similarity to the phosphides found in the primary crystallization stage. The MRO structure changes into a similar structure with the primary crystals just before the crystallization. A discussion is made for the MRO structure and its relation to the glass stability and also to the phosphide nucleation in the primary crystallization.     

Improvement of the creep resistance of FeCo–2V alloy arising from a small addition of Cr

This work presents our recent findings that a small addition (0.5 at. %) of Cr to FeCo–2V alloy leads to a great improvement in creep resistance. High resolution electron microscopy was applied to study microstructural evolution of the Cr added alloy during the creep process performed at 600 °C under 200 MPa. At an initial step of the creep, there appear plate-like precipitates with bcc/fcc structure as well as some rod-like ones with hcp structure. A coherent relationship is identified between the precipitates and bcc FeCo matrix. With prolonging the creep, the rod-like hcp precipitates are revealed to remain in the bcc matrix, showing a good stability under the creep condition and in turn resulting in piling-up of dislocations to a great extent around the precipitates. In addition, the Cr added alloy is shown to have a large stress exponent of 8.4, indicating a strong interaction between dislocations and the hcp precipitates.     

Synthesis and characterization of neodymium oxide nanoparticles

The nanometric precursors of neodymium oxide of various morphology from fibrous to well-dispersed spheroidal were prepared via a solvothermal reaction routes. The precursors and their thermal evolution to neodymium oxide phase were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that the reaction parameters, kind of solvent as well as neodymium salt used played a key role for the product formation of desired morphology and structure. Similarly, kind of neodymium oxide precursor determined the morphology and the crystal structure (haxagonal or cubic) of the final oxide. The potential application of Nd₂O₃ precursors prepared by solvothermal method as convenient material for preparation of homogeneous thin coatings on planar substrates is shown.     

The effect of manufacturing process induced near-surface deformed layer on the corrosion behaviour of AA2198-T851 Al–Cu–Li alloy

ABSTRACT

The effect of the near-surface deformed layer on the corrosion behaviour of AA2198-T851 Al–Cu–Li alloy has been investigated using microscopy and electrochemical techniques. This shows that a deformed layer developed by rolling was more active but only exhibited superficial attack compared to a mechanically polished surface, which produced severe localised corrosion associated with the presence of hexagonal T1 particles. The rolled surface became increasingly corrosion resistant with time, but when coupled, galvanically enhanced severe localised corrosion sites developed on the surface over time.     

TEM STUDY OF C-B_4C-SiC COMPOSITES WITH SILICON ADDITIVE

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