► A fusion (GTAW) and a solid state method (FSW) are used to weld Ti–6Al–4V alloy. ► Optimal parameters yielding defects-free weldments are identified. ► A very careful microstructural quantification ...of the FSW and GTAW weldments are performed. ► α′ Martensite formed only in FSWed samples and avoided in GTAWed samples. ► FSW process produced joints with considerably smaller HAZ and higher hardness values.
The obtained microstructures of a Ti–6Al–4V alloy welded by Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) were investigated and evaluated quantitatively. In the GTAW method, the effect of current was examined so that the samples were subjected to various currents between 90 and 120A. In the FSW process, samples were welded by different rotational speeds (450–850rpm). Non-destructive tests including Visual and Radiography Tests (VT and RT) were used to identify defect-free samples. The microstructural studies by electron microscopes revealed formation of different phases in the weld area of the samples welded via mentioned methods. The recorded peak temperatures in the weld regions compared favorably with the expectations about the evolved microstructures. A bi-modal microstructure was just obtained in the FSWed sample with a peak temperature below β transus temperature (T<995°C). α′ martensite phase, which is an acicular and strengthening phase in this alloy, was only observed in FSWed specimens.
Discontinuous dynamic recrystallization can occur during dynamic tensile extrusion of copper, which is subjected to uniaxial tensile strains of ~5 and strain rates up to 10
6
s
−1
in the extruded ...section. Through high-resolution transmission Kikuchi diffraction, we show that nucleation occurs through subgrain rotation and grain boundary bulging at boundaries between 〈001〉 and 〈111〉 oriented grains. The observed nuclei consist of subgrains with a size of approximately 200 to 400 nm.
The capability of the newly developed rheocasting (RC) technique in combination with the RheoMetal process for producing SiC particulate-reinforced AM50 and AZ91D matrix composites (Mg-based MMCs) ...was investigated. The quality of the MMCs was studied by analyzing the fraction of casting pores, number density of SiC clusters and the uniformity of SiC particles. Solid fraction, particle size and oxidation of SiC particles had strong impacts on the overall quality of the MMCs. The MMCs produced by 40% solid fraction and oxidized micron-sized SiC particles exhibited an excellent casting quality. A low-quality MMC was obtained when non-oxidized sub-micron sized SiC particles were employed. The results showed the formation of various types intermetallic particles and carbides such as MgO, Mg2Si, Al2MgC2, Mg2C3, Al4C3 as the interfacial reaction products of SiC/Mg alloy's melts. Mg hydride (α-MgH2) was also identified in inter-dendritic regions of the MMCs for the first time.
The modification of clinoptilolite as a natural zeolite via a simple route was carried out for CO2 adsorption. Cation exchange with Li+, Mg2+, and Ca2+, amine modification using monoethanolamine ...(MEA), triethanolamine (TEA) and hexyl amine, and ionic liquid modification using bmimX (X = PF6⁻, NO3⁻, Br⁻, Cl⁻, and bmim = 1-Butyl-3-methylimidazolium) were performed in a different amount of the modifiers. The samples were characterized by several methods (XRD, FT-IR, BET, and SEM), and adsorbed CO2 was evaluated utilizing adsorption isotherm at a wide range of pressure. The results showed enhancement in CO2 adsorption capacity for all the samples and 4.18, 3.58, and 4.35 times increase in CO2 adsorption were obtained where clinoptilolite/Li+, clinoptilolite/2% MEA and clinoptilolite/5% bmimPF6 were used as adsorbent at 4 bar pressure of CO2, respectively.
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•Modification of the natural zeolite using the simple and economical route.•Utilizing of cations, amines and ionic liquids for increasing of CO2 adsorption.•Enhancement of CO2 adsorption in the modified natural zeolite (clinoptilolite).•Increasing the CO2 adsorption capacity for some of the samples more than 4 times.
The FeCrAl alloy Kanthal APMT™ was exposed to N
2
–5%H
2
at 900 °C. Trace oxygen in the gases supplied an oxygen activity which was sufficient to render alumina and chromia thermodynamically stable. ...The exposures revealed that the oxide scale was penetrated by nitrogen exclusively at chromia domains. Microscopic analyses of the oxide scale did not reveal micro-cracks that could serve as points-of-entry for nitrogen. Instead it is suggested that nitrogen is transported through a dense chromia layer. Density functional theory was employed to investigate decisive nitrogen surface chemistry and transport properties in chromia and alumina. The study was used to validate that the complex redox chemistry of Cr
3+
as opposed to Al
3+
is a sufficient discriminating factor between alumina and chromia, facilitating N
2
dissociation and mobility of N in chromia.
This study concerns the interfacial microstructure and texture development in three textured CVD α-Al2O3 coatings using X-ray diffraction, transmission Kikuchi diffraction, scanning transmission ...electron microscopy and energy dispersive X-ray analysis. It is well known that the performance of these types of coatings relies on the degree and type of texture. The aim of this work is to study the microstructure of three different CVD α-Al2O3 layers when deposited onto a Ti(C,N,O) bonding layer. The coatings were deposited onto cemented carbide/cobalt substrates (WC/Co).
It was observed that grain boundary diffusion of W and Co occurred through the TiN and Ti(C,N) layers to the bonding layer/α-Al2O3 interface. This may disturb the alumina layer nucleation and early growth. Interfacial porosity was observed at the bonding layer/α-Al2O3 interface. The number of voids that were detected in the (0001) and (011¯0)-textured coatings was similar when H2S was not used during the initial deposition step. When H2S was present during the nucleation step deposition of the α-Al2O3 more voids were introduced at the interface for the (0001)-textured samples. The alumina grain morphology developed from small (~100 nm) equiaxed grains at the start of the alumina coating to larger (several microns) columnar grains at the top of the coating. The inner part of the alumina coatings had a more random orientation. The texture changed either: (i) gradually over several grains, or (ii) more abruptly from one grain to another; from more random orientations to the desired texture. The desired texture develops earlier when using H2S at the start for the (0001)-textured coatings, while the (011¯0)-texture development benefits from the absence of H2S. Thus, in this study, H2S promotes (0001) texture and interfacial void formation.
•Diffusion of W and Co in Ti-rich layers up to alumina•Higher fraction of voids when H2S introduced in the nucleation step of the alumina•More random orientation of grains at the inner part of the alumina•Gradual or abrupt texture development from random to desired texture•H2S promotes (0001) texture and interfacial void formation.
This letter focuses on the capability of Transmission Kikuchi Diffraction (TKD) in Scanning Electron Microscope (SEM) for obtaining microstructural and micro-textural information from nano-grained ...oxide scales formed on a FeCrAl alloy. Orientation maps, with an indexing rate of 85%, showed the formation of grains in the range 20–300nm. TKD revealed the existence of an orientation relationship at the alloy/oxide interface as well as the presence of a single grain (40nm) with spinel structure in the alumina scale. A pre-tilted sample holder was designed for TKD investigations at short working distances with minimized mechanical drift of the thin foils.
•The TKD technique was successfully used for analyzing nano-grained oxide scales.•TKD orientation maps with high indexing rates (>80%) were obtained.•High resolution phase identification was done on grains as small as 40nm.
The thesis deals with high temperature corrosion behavior of some selective stainless steels and Ni-base alloys with applications in power generation technologies, e.g. boilers fired by biomass and ...waste. The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using in-situ ESEM method. Besides, the effects of thermal cycling on the oxidation behaviour of a Ni-base alloy HR-214 was studied in air at 1200°C.The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. The in-situ results were validated by ex-situ exposures, i.e. reference tube furnace. The alloys were corroded in the matter of minutes in the studied environment. Quite evidently, the severest oxidation attack occurred locally in the vicinity of KCl particles, where oxide crusts and oxide shells/rims (consisting of Fe-, Cr- and Al- oxides) were formed. STEM studies showed that all the three alloys formed a thin base oxide scale. Chlorine-induced oxidation caused chlorination of the alloys as evidenced by detection of chlorine below the protective scales.In the case of the Ni-base alloy HR-214, both isothermal and cyclic exposures led to the formation of a duplex oxide morphology, composed of a columnar alumina layer overlaid by a complex Ni(Al,Cr)2O4 spinel. It was evident that thermal cycling resulted in the formation of vertical cracks in the multi-layered scale. Additionally, STEM/EDX revealed outwards transport of Cr through the cracks/ alumina grain boundaries, which caused thickening of the outer spinel layer.Moreover, an attempt was made to develop the newly introduced TKD method to study nano-sized oxide scales. This was conducted by (a) designing a dedicated sample holder, (b) specimen preparation and (c) acquisition parameters. These efforts made it possible to achieve data-rich TKD orientation maps (with indexing rates > 85%). Thus, the technique was effectively employed to obtain useful information from the microstructure and microtexture of the fine-grained oxide scales. Besides, the technique provided information concerning the crystallographic orientation relationship at oxide/oxide and oxide/alloy interfaces.
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