Dependence of the surface charge and the fluorine adsorption by γ-aluminum oxide on the solution temperature

The dependence of the surface charge of the γ-aluminum oxide and the fluorine adsorption on the solution temperature (20, 30, and 40°C), the pH (3.5–10), and the equilibrium concentration of fluorine in the solution (from 1.0 × 10⁻³ to 1.5 × 10⁻¹ M/l) is studied by the method of potentiometric titration and adsorption variations with the view to elucidate the nature of the processes that take place upon the removal of fluorine with the use of ECDM sludge of an aluminum alloy that was calcined at a temperature of 800 °C. The adsorption isotherms were processed using the Freundlich, Langmuir, and BET equations. The best coincidence with the experimental data is obtained with the use of the Langmuir equation. It is shown that both the solution temperature increase and the amount of fluorine adsorbed by the sample surface shift the pH_PZC to a more acid range. The fluoride adsorption occurs due to the exchange of the OH⁻-groups of the hydrated oxide surface for fluorine ions due to the interaction of the charged AlOH₂⁺ centers of the surface with F⁻ ions and due to the formation of hydrogen bonds of F⁻ and uncharged AlOH centers.     

Effect of electrolyte temperature on the nano-carbonitride layer fabricated by surface nanocrystallization and plasma treatment on a γ-TiAl alloy

The distribution of nano-carbonitrides produced by the treatments of surface nanocrystallization and plasma electrolytic carbonitriding on a γ-TiAl was investigated by means of figure analysis. The skewness and kurtosis of Gaussian shape distribution curves were studied and the effect of electrolyte temperature was determined. The usage of lower temperatures of the electrolyte is more suitable for achieving lower sizes of complex nano-carbonitrides. The surface roughness of treated samples was measured and it was observed that there is an optimum level of electrolyte temperature for surface roughness increase (difference between two measured data).     

Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

The microstructure and mechanical properties of eight γ-TiAl based alloys with compositions in the range Ti–44Al–8(Nb,Ta,Zr,Hf)–(0–0.2)Si–(0–1)B have been investigated to assess the possibility of improving the properties of γ-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48–2–2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti–44Al binary alloy. In some of the alloys the ω phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900°C. In alloys which contain both beta- and gamma- stabilisers, there is no single α phase field in the transformation sequence and instead there is a (α+β+γ) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of γ-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure.     

Effect of external loading on the martensitic transformation – A phase field study

In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-linear strain tensor and the effect of grain boundaries, is used to study the impact of applied stresses on an Fe–0.3%C polycrystalline alloy, both in two and three dimensions. The evolution of plasticity is computed using a time-dependent equation that solves for the minimization of the shear strain energy. Crystallographic orientation of the grains in the polycrystal is chosen randomly and it is verified that the said assumption does not have a significant effect on the final volume fraction of martensite. Two-dimensional (2-D) and three-dimensional (3-D) simulations are performed at a temperature significantly higher than the martensitic start temperature of the alloy with uniaxial tensile, compressive and shear loading, along with hydrostatic stresses. It is found that the 3-D simulations are necessary to investigate the effect of external loading on the martensitic transformation using the phase field method since the 2-D numerical simulations produce results that are physically incorrect, while the results obtained from the 3-D simulations are in good agreement with the empirical observations found in the literature. Finally, it is concluded that the given model can be used to predict the volume fraction of martensite in a material with any kind of external loading.     

A study on the two-step transient liquid phase diffusion bonding of K640 superalloy

A new technology, the two-step transient liquid phase diffusion bonding （TLP-DB） technology for cobalt-based K640 superalloy, was investigated. The method consists of a short-time high temperature heating to melt interlayer followed by isothermal solidification of liquid phase at a lower temperature than that of the conventional TLP-DB. The result indicates that the two-step TLP-DB can reliably produce an ideal joint with uniform chemical composition, which is superior to the joint welded by conventional TLP-DB in microstructure and mechanical properties. Bonding parameters of new process are 1 250℃ for 0. 5 h and 1 180℃ for 3 h. The high-temperature tensile strength of the joint by two-step TLP-DB reaches 74% of that of the base material on an equal basis, but the high-temperature tensile strength of the joint by conventional TLP-DB is only 58% of that of the base material.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

The Effect of Coatings on the Oxidation Kinetics of Intermetallic Titanium Alloys of the Ti2AlNb and γ-TiAl Systems

Protection of Metals and Physical Chemistry of Surfaces - The results of a study of the effect of aluminide coatings of the NiCrAlY and Al(Si) systems on intermetallic heat-resistant titanium...     

A Comparison of Oxidation Behavior of Al Coatings Prepared by Magnetron Sputtering and Arc Ion Plating on γ-TiAl

利用XRD、EDS和SEM分析研究了磁控溅射和电弧离子镀2种工艺制备的Al涂层的微观结构、形貌和抗氧化性能。磁控溅射技术制备的均匀、致密的Al层拥有更为细小的晶粒组织。在氧化实验后,磁控溅射制备的Al涂层形成了一个由表层氧化层、次表层富Al层和互扩散层的保护性结构。相比之下,电弧离子镀制备的Al涂层表现出了更差的抗氧化性。这是由于在离子镀制备的Al涂层中发现的针孔可以为氧气的侵入提供通道,从而引起涂层的内氧化并最终导致涂层的剥离。结果表明,磁控溅射制备Al涂层具备更好的抗高温氧化性能。     

The use of ternary phase diagrams in the study of high temperature corrosion products formed on Fe–5 wt% Al alloys in reducing and oxidizing environments

Ternary phase diagrams, in conjunction with microscopy techniques and reaction product chemistries, were used to describe the possible “diffusion paths” and resulting morphologies that may occur during formation of corrosion scales from high temperature gaseous exposure. Iron with 5 wt% of aluminum was exposed to reducing and oxidizing environments at 700°C. Characterization of the surface reaction products was conducted using microscopy techniques with energy dispersive spectroscopy and electron probe microanalysis. Under both conditions, iron diffused outward to form surface reaction products, either iron sulfide or iron oxide. The ingress of sulfur or oxygen through the previously formed reaction products was found to produce internal corrosion phases within the alloy. By plotting chemical information acquired from the corrosion scales on ternary phase diagrams, development of the phase layer sequence and morphologies of the multiphase corrosion scales was schematically explained. This analysis was conducted using conventions summarized by Clarke (Trans. Am. Inst. Min. Engrs, 1963, 227, 1250) for plotting diffusion paths in multiphase ternary diffusion studies from solid-state reactions. By presenting the experimental data in this manner, the morphological development of the scales was related to the composition path on the ternary phase diagrams.     

Preconditioning of AISI 304 stainless steel surfaces in the presence of flavins—Part I: Effect on surface chemistry and corrosion behavior

Stainless steel AISI 304 surfaces were studied after a mild anodic polarization for oxide growth in the presence and absence of two derivatives of vitamin B2 (riboflavin and flavin mononucleotide) that can be secreted by metal-reducing bacteria and act as a chelating agent for iron species. The alterations in oxide chemistry were studied by means of surface-sensitive techniques such as X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analysis. The complementary electrochemical characterization revealed a preferential growth of an oxide/hydroxide iron-rich film that is responsible for an altered pit initiation and nucleation behavior. These findings suggest that as the corrosion behavior is determined by the interplay of the chemical and electronic properties, only a mild anodic polarization in the presence of redox-active molecules is able to alter the chemical and electronic structure of the passive film formed on stainless steel AISI 304. This helps to achieve a profound understanding of the mechanisms of microbially influenced corrosion (MIC) and especially the possible effects of the redox-active biomolecules, as they may play an important role in the corrosion susceptibility of stainless steel surfaces.     

Influence of O and Co on the early stages of sintering of WC–Co: a surface study by AES and STM

The influence of oxygen on the sintering behavior of WC–Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650°C results in a 2×2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM tip. Annealing at 850°C removes excess Co and leaves only the 2×2 surface. Oxygen exposure of the 2×2 surface results in a “clustered” cobalt oxide overlayer which on annealing at 750°C breaks up and restores the 2×2 structure as the metallic Co wets the surface.     

A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid.This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.     

A microscopic study of the effect of γ-radiation on Pb1 − x Mn x Se epitaxial films

The surface morphology of n- and p-type epitaxial films of lead manganese selenide (Pb_1?x Mn _x Se (x = 0.01)) on barium fluoride substrates of the (111) orientation in the initial state and after exposure to gamma-radiation is studied by atomic force microscopy. It is shown that the surface relief undergoes modification in the absorbed dose region of 5 ≤ D _γ ≤ 35 kGy. The p?n conductivity inversion at 10 ≤ D _γ ≤ 25 kGy is found from the characteristics of the surface structures and confirmed by the electrophysical measurements. It is revealed that the radiation resistance of these films is violated at a dose above 35 kGy.     

IFHTSE Global 21: heat treatment and surface engineering in the twenty-first century: Part 9 – Influence of materials science on heat treatment and surface engineering

Abstract

Significant progresses have been made in the past decades in the discipline of materials science, which include, but not limit to, the synthesis of new materials, advancement in analytical and experimental techniques, sustainable and environmentally friendly processing technologies, computational material science and nanotechnology. These developments have major influences on the research and application of heat treatment and surface engineering, and provide new opportunities to engineer the surfaces of new and conventional materials using advanced technologies to meet the ever increasing demands in surface and subsurface related properties. This survey gives a brief review on some aspects of heat treatment and surface engineering which keep pace with new developments in materials science. The specific areas being examined include: (i) advanced analytical techniques; (ii) sustainable and environmentally friendly technologies; (iii) surface engineering of emerging new materials such as intermetallic compounds, shape memory alloys and biomaterials; (iv) the search for superhard coatings and surface nanostructured materials; (v) mathematical modelling of surface engineering systems.     

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.     

A comparative study on microstructural evolution and surface properties of graphene/CNT reinforced Al6061−SiC hybrid surface composite fabricated via friction stir processing

A comparative study on the surface properties of Al−SiC−multi walled carbon nanotubes (CNT) and Al−SiC−graphene nanoplatelets (GNP) hybrid composites fabricated via friction stir processing (FSP) was documented. Microstructural characterization reveals a more homogeneous dispersion of GNPs in the Al matrix as compared to CNTs. Dislocation blockade by SiC and GNP particles along with the defect-free interface between the matrix and reinforcements is also observed. Nanoindentation study reveals a remarkable ∼207% and ∼27% increment in surface nano-hardness of Al−SiC−GNP and Al−SiC−CNT hybrid composite compared to as-received Al6061 alloy, respectively. On the other hand, the microhardness values of Al−SiC−GNP and Al−SiC−CNT are increased by ∼36% and ∼17% relative to as-received Al6061 alloy, respectively. Tribological assessment reveals ∼56% decrease in the specific wear rate of Al−SiC−GNP hybrid composite, whereas it is increased by ∼122% in Al−SiC−CNT composite. The higher strength of Al−SiC−GNP composite is attributed to the mechanical exfoliation of GNPs to few layered graphene (FLG) in the presence of SiC. Also, various mechanisms such as thermal mismatch, grain refinement, and Orowan looping contribute significantly towards the strengthening of composites. Moreover, the formation of tribolayer by the squeezed-out GNP on the surface is responsible for the improved tribological performance of the composites. Raman spectroscopy and various other characterization methods corroborate the results.     

A comparative study on the electrochemical behaviour of amorphous and nanocrystalline cobalt–tungsten electrodeposited coatings

Amorphous and nanocrystalline cobalt–tungsten coatings were electrodeposited from a citrate-ammonia bath on copper substrates. Both coatings showed a nodular surface morphology, but a microcrack network was observed in the amorphous coating. The cyclic voltammograms of both deposits revealed anodic and cathodic low-current plateaus around the open circuit potential, exhibiting a passive behaviour. Mott–Schottky analysis showed that the passive films exhibit n-type semiconductivity behaviour and that formed on the amorphous coating showed higher crystal defects. Electrochemical impedance spectroscopy revealed that the amorphous coating has higher corrosion resistance than the nanocrystalline one at both open circuit and anodic potentials. This was attributed to the higher pore resistance of passive film formed at the open circuit potential and more chemical stability of the amorphous coating which reduces its dissolution at the anodic potential. The plugging of the microcrack network in the amorphous coating by corrosion products eliminated the negative effect of microcracks.