Effects of post-annealing on the microstructure and mechanical properties of friction stir processed Al–Mg–TiO2 nanocomposites

Aluminum matrix nanocomposites were fabricated via friction stir processing of an Al–Mg alloy with pre-inserted TiO₂ nanoparticles at different volume fractions of 3%, 5% and 6%. The nanocomposites were annealed at 300–500 °C for 1–5 h in air to study the effect of annealing on the microstructural changes and mechanical properties. Microstructural studies by scanning and transmission electron microscopy showed that new phases were formed during friction stir processing due to chemical reactions at the interface of TiO₂ with the aluminum matrix alloy. Reactive annealing completed the solid-state reactions, which led to a significant improvement in the ductility of the nanocomposites (more than three times) without deteriorating their tensile strength and hardness. Evaluation of the grain structure revealed that the presence of TiO₂ nanoparticles refined the grains during friction stir processing while the in situ formed nanoparticles hindered the grain growth upon the post-annealing treatment. Abnormal grain growth was observed after a prolonged annealing at 500 °C. The highest strength and ductility were obtained for the nanocomposites annealed at 400 °C for 3 h.     

Effect of ageing on precipitation and impact energy of 2101 economical duplex stainless steel

The impact energy and microstructure of a thermally aged 2101 duplex stainless steel with composition of Fe–21.4Cr–1.2Ni–5.7Mn–0.23 N–0.31Mo were studied. The results showed that the room temperature impact energy of specimens decreased gradually with ageing temperature up to 700 °C and then increased with aging over 700 °C. The minimum value of impact energy was 37 J after 700 °C aging, which was only 34% of that for as-annealed specimens. For specimens aged at 700 °C, the room temperature impact energy decreased significantly after 3 min and was halved after 10 min. Fractographs showed that, with increasing aging time, the fracture morphology changed from fibrous fracture to transgranular and intragranular fracture. Scanning electron micrographs revealed that many precipitates were distributed along α/γ and α/α interfaces. The precipitates were extracted and confirmed by X-ray diffraction to be Cr₂N. Therefore, it can be concluded that precipitation of Cr₂N is the main reason for the decrease of impact energy in aged 2101 duplex stainless steel.     

Microstructure and mechanical properties of spark plasma sintered austenitic ODS steel

In this work, austenitic oxide dispersion strengthened (AODS) steel of composition Fe–16Cr–16Ni–1.5 W–0.21Ti–0.3Y₂O₃ (wt. %) was fabricated using two–stage ball milling followed by consolidation through spark plasma sintering (SPS). In the first–stage, mechanical alloying (MA) of ferritic powder and nano sized Y₂O₃ was carried out. This was followed by the addition of Ni in second–stage milling. SPS of the milled powder was carried out at 900, 950, 1000 and 1050 °C to explore the role of SPS temperature on density, microstructure as well as mechanical properties of the consolidated samples. A relative density of ～ 99% was obtained for samples sintered at 950 and 1000 °C. The as–sintered samples were subsequently solution annealed at 1075 °C for 2 h and water quenched. X–ray diffraction studies confirmed the presence of austenite in the consolidated and solution annealed samples. Electron back scatter diffraction analysis of solution annealed samples sintered at all the temperatures revealed a bimodal microstructure. The average grain size of 1.07 ± 0.72 µm was obtained for solution annealed samples sintered at 1000 °C. Yield strength and elongation of the same was measured as 851 MPa and 18%, respectively at room temperature. These values are the best combination of strength to elongation achieved on AODS alloys processed using MA and SPS, which makes this AODS steel much promising for high temperature applications.     

Effect of reaction temperature on crystallite size and sensing response of chromium oxide nanoparticles

Nanoparticles of chromium oxide have been synthesized following a precipitation technique at reaction temperatures 5, 27 and 65 °C. Synthesized powders were characterized using X-ray diffraction, transmission electron microscope and Brunauer–Emmett–Teller techniques to carry out structural and morphological analysis. The reaction temperature has been found to be playing a crucial role in controlling particle size. It has been observed that Cr₂O₃ nanoparticles synthesized at 27 °C were smaller as compared to those synthesized at 5 and 65 °C. Chromium oxide samples thus prepared were deposited as thick films on alumina substrates to act as gas sensors and their sensing response to ethanol vapour was investigated at different operable temperatures. It has been observed that all the sensors exhibited optimum response at 250 °C. The investigations revealed that sensing response of Cr₂O₃ nanoparticles synthesized at 27 °C was exceptionally higher than that of Cr₂O₃ nanoparticles synthesized at 5 and 65 °C.     

Compressive deformation behavior of ternary compound Cr<Subscript>2</Subscript>AlC

The compressive properties of ternary compound Cr₂AlC at different temperatures and strain rates were studied. When tested at a strain rate of 5.6 × 10⁻⁴ s⁻¹, the compressive strength decreases continuously from 997 ± 29 MPa at room temperature to 523 ± 7 MPa at 900 °C. The ductile-to-brittle transition temperature is measured to be in the range of 700 to 800 °C. When tested in the strain rate range of 5.6 × 10⁻⁵ to 5.6 × 10⁻³ s⁻¹, Cr₂AlC fails in a brittle mode at room temperature, whereas the deformation mode changes from a brittle to a ductile as the strain rate is lower than 5.6 × 10⁻⁴ s⁻¹ when compressed at 800 °C. The compressive strength increases slightly with increasing strain rate at room temperature and it is less dependent on strain rate when tested at 800 °C. The plastic deformation mechanism of Cr₂AlC was discussed in terms of dislocation-related activities, such as kink band formation, delamination, decohesion of grain boundary, and microcrack formation.     

Microstructural characterization of a carbonitrided heat resisting alloy using focused ion beam-based techniques

Abstract

An austenitic Fe–25Cr–20Ni (wt.%) alloy was first nitrided in N₂–H₂ then carburised at 1000°C. The nitridation produced lamellar internal precipitates of Cr₂N which grew by discontinuous precipitation at a recrystallised austenite boundary formed at the precipitation front. Subsequent carburisation converted the Cr₂N to a mixture of chromium-rich M₇C₃+CrN, and released nitrogen into the austenite matrix. The submicron CrN precipitates were stabilized by a matrix supersaturated with nitrogen and exhibited a strong orientation relationship with the surrounding austenite. The rejected nitrogen diffused deeper into the alloy to form new, more finely spaced Cr₂N lamellae. Carbon diffusion overtook the nitride structure and precipitated finely spaced, lamellar M₂₃C₆ by a discontinuous precipitation process. This process is available to the carbides only when a prior boundary is present. This phenomenon, and the ability of nitridation to form a boundary and the inability of carburisation to do so, are interpreted using the energetics of nucleation.     

Effect of oxygen on the surface morphology of CuGaS2 thin films

Since the effect of oxygen is very significant during the heat treatment of the thin films, we study the effect of this during the annealing of CuGaS₂ thin films by two different types. In this study, CuGaS₂ thin films were deposited by vacuum thermal evaporation of CuGaS₂ powder on heated glass substrates at 200 °C submitted to a thermal gradient. The films are annealed in air and under nitrogen atmosphere at 400 °C for 2 h. In order to improve our understanding of the influence of oxygen during two annealing types on device performance, we have investigated our CuGaS₂ material by X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDX) and spectrophotometry. A correlation was established between the surface roughness, growth morphology and optical properties, of the annealed CuGaS₂ thin films. It was found that annealing of CuGaS₂ film in nitrogen atmosphere leads to a decrease of the mean grain size and to an evolution of a (112) preferred film orientation. Annealing in air results in the growth of oxide phases such as CuO and modifies the films structure and their surface morphology.     

The effect of annealing on the deformation behaviour and microstructure of crystallized Mg–23.5Ni (wt.%) alloy

Rapidly solidified amorphous Mg–23.5Ni (wt.%) ribbons were crystallized at 300 and 400 °C for 90 min. After annealing at 300 °C the microstructure was heterogeneous, consisting of rounded eutectic–lamellar domains, which contained magnesium grains smaller than 500 nm. In the case of ribbons annealed at 400 °C the microstructure, however, was homogenous, and composed of well-formed magnesium grains and Mg₂Ni particles. At room temperature both crystallized materials were brittle due to the high volume fraction of Mg₂Ni particles, but they exhibited some ductility with increasing test temperature. Above 200 °C, the microstructure of the ribbons annealed at 300 °C was characterised by the formation of particle free zones during the tensile test. This structure was not observed in the material annealed at 400 °C. Deformation behaviour and changes in the microstructure during plastic flow of both crystallized materials were explained according to grain boundary sliding mechanisms.     

Influence of post-deposition annealing in oxygen ambient on metal–organic decomposed CeO2 film spin coated on 4H-SiC

Post-deposition annealing (PDA) was performed in oxygen ambient at different temperatures (600, 800, 1,000 and 1,150 °C) onto metal–organic decomposition (MOD) prepared CeO₂ film spin coated on n-type 4H-SiC substrate. Effects of PDA onto the physical and electrical characteristics of MOD-derived CeO₂ were investigated. Four orientations [(111), (200), (220), and (311)] of CeO₂ peaks were revealed by X-ray diffraction analysis in all of the samples with a preferred orientation in (200) direction. However, α-Ce₂O₃ and cerium silicate (Ce₂Si₂O₇) interfacial layers emerged at 1,150 °C. As annealing temperature increased, grain size of films was increased but microstrains were decreased. Electrical results indicated that negative effective oxide charge and slow trap density were decreased as temperature increased. The lowest interface trap density was perceived by sample annealed at 1,000 °C. However, the highest electric field was obtained by sample annealed at 1,150 °C. Reasons that contributed to this observation were discussed.     

Origin of photoluminescence peaks in Ge–SiO2 thin films

Ge–SiO₂ thin films were prepared by the RF magnetron sputtering technique on p-Si substrates from a Ge–SiO₂ composite target. The as-deposited films were annealed in the temperature range of 300–1000 °C under nitrogen ambience. The structure of films was evaluated by X-ray diffraction, X-ray photoemission spectroscopy and Fourier transform infrared absorption spectroscopy. Results show that the content of Ge and its oxides in the films change with increasing annealing temperature (T_a), the photoluminescence (PL) characteristics are closely dependent on the contents of Ge and its oxides in SiO₂ matrix. The dependence observed strongly suggests that the PL peak at 394 nm is related to the existence of GeO and 580 nm to that of Ge nanocrystal (nc-Ge) in the films.     

Phase formation sequence of Cr2AlC ceramics starting from Cr–Al–C powders

The reaction process of Cr₂AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr₂AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr₅Al₈, Cr₂Al and Cr₇C₃ were the intermediate phases appearing in turn in the heating process. The amount of Cr₂AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.     

Effects of 200 keV Ar-ions irradiation on the structural and optical properties of reactively sputtered CrN films

Modification in structural and optical properties of chromium-nitride (CrN) films induced by argon ion irradiation and thermal annealings were investigated using various experimental techniques. CrN films deposited by d. c. reactive sputtering on Si substrate were implanted with 200 keV argon ions, at fluences of 5–20 × 10¹⁵ ions/cm². As-implanted samples were then annealed in vacuum, for 2 h at 700 °C. Rutherford backscattering spectrometry, X-ray diffraction, cross-sectional (high-resolution) transmission electron microscopy and spectroscopic ellipsometry (SE) measurements were carried out in order to study structural and optical properties of the layers. After irradiation with 200 keV Ar ions a damaged surface layer of nanocrystalline structure was generated, which extended beyond the implantation profile, but left an undamaged bottom zone. Partial loss of columnar structure observed in implanted samples was recovered after annealing at 700 °C and CrN started to decompose to Cr₂N. This layer geometry determined from transmission electron microscopy was inferred in the analysis of SE data using the combined Drude and Tauc-Lorentz model, and the variation of the optical bandgap was deduced. The results are discussed on the basis of the changes induced in the microstructure. It was found that the optical properties of the layers are strongly dependent on the defects' concentration of CrN.     

Influence of growth parameters on the dopant configuration of nitrogen-doped graphene synthesized from phthalocyanine molecules

Synthesis of nitrogen-doped graphene (NDG) via chemical vapor deposition (CVD) using phthalocyanine, a solid precursor containing carbon and nitrogen, is reported. The effect of the growth parameters (temperature, time, and carrier gas) on the surface morphology, dopant configuration, and conductivity of the films was studied. The NDG films were synthesized at different substrate temperatures of 1050 °C, 950 °C, and 850 °C for different growth times of 5–15 min in the presence of an Ar?+?H₂ gas mixture. Significantly, pyrrolic-N type defects are observed predominantly after 5 min of growth time. At 1050 °C, pyrrolic N content is around 45.4% after 5 min of growth which decreased to 24.1% after 15 min of growth, while the graphitic-N content increased from 41.2 to 76% at the same time. It is demonstrated that the conversion of pyrrolic type of nitrogen to graphitic nitrogen defects can be arrested by changing the carrier gas from Ar?+?H₂ to Ar. The pyrrolic-N content increased to 64% by changing the gas from Ar?+?H₂ to Ar at 15 min. The electrolyte gated field-effect transistors were fabricated using the obtained films, and dopant-dependent mobility was observed. The mobility for pyrrolic-N-dominated film is 13.6 cm² V^?1 s^?1 increasing to 62.8 cm² V^?1 s^?1 for graphitic-N-dominated film.

     

Microstructure and dielectric properties of CaCu3Ti4O12 ceramic

CaCu₃Ti₄O₁₂ (CCTO) was prepared by the solid state technique. The sample was calcined at 900 °C/12 h and sintered at 1050 °C/24 h, then subjected to XRD to ensure CCTO formation. The microstructure was observed by SEM. XRD results identified both samples as single phase CCTO, whereas the microstructure shows abnormal grain growth and large pores. Sintering was studied in the temperature range of 950–1050 °C for 3–12 h. Increasing sintering temperature enhances the density and secondary formation of Cu₂O. A clear grain boundary and dense microstructure were observed. The results show that the sample sintered at 1040 °C/10 h yields a clearly uniform grain size with the highest ε_r (33,210).     

A study on the influence of Mo, Al and Si additions on the microstructure of annealed dual phase Cr–Ta alloys

The effects of additions of 5 at.% Mo, Al and Si on the long-term annealed microstructures of a two phase Cr–Cr₂Ta alloy have been studied. Following 200 h at 1300 °C, the lamellar eutectic constituent of all the alloys disintegrated into discrete particles of the Laves phase embedded within a Cr-rich solid solution phase, along with the formation of fine Laves phase precipitates. One of the predominant differences between the three alloying additions was the extent of the C14 to C15 polytypic transformation of the Cr₂Ta-based Laves phase. With Mo and Al additions, the Cr₂Ta Laves phase transformed from C14 to either C15 or intermediate hexagonal polytypes following 200 h annealing at 1300 °C. In contrast, Si additions stabilised the C14 polytype, with no transformation to other polytypes observed after prolonged annealing at 1000, 1100 and 1300 °C.     

Influence of heat treatment on the fatigue life of a laser-welded NiTi alloy wire

The influence of heat treatment on the rotating–bending fatigue of a laser-welded superelastic NiTi alloy wire was investigated. In comparison with the as-welded and annealed at 500 °C for 1 h specimens, the specimen annealed at 400 °C for 1 h shows the best fatigue resistance. The difference of fatigue life in the heat-treated materials is attributed to the different Ti₃Ni₄ precipitate size. Heat treatment to produce smaller coherent precipitates (10 nm) improves the fatigue resistance of the laser-welded NiTi alloy wire. When the Ti₃Ni₄ precipitates become coarse, the fatigue resistance decreases.     

Molten salt synthesis of Cr3C2-coated flake graphite and its effect on the physical properties of low-carbon MgO–C refractories

To improve the performance of low-carbon MgO–C refractories, the Cr₃C₂-coated flake graphite was synthesized by molten salt method, then the Cr₃C₂-coated flake graphite was added to low-carbon MgO–C refractories. In this work, the effects of reaction temperature, Cr/C mole ratio, and holding time on the synthesis of Cr₃C₂-coated flake graphite were studied. Furthermore, the effect of Cr₃C₂-coated flake graphite on the physical properties of low-carbon MgO–C refractories was evaluated. The results indicated that when the reaction temperature was 950 °C, Cr/C molar ratio was 1/2 and holding time was 3 h, the synthesized Cr₃C₂-coated flake graphite had fewer surface micropores and the crystallinity of Cr₃C₂ grains was high. In addition, the Cr₃C₂-coated flake graphite exhibited excellent oxidation resistance. MgO–C refractories containing 2 wt% Cr₃C₂-coated flake graphite exhibited more excellent physical properties. The results of nano CT detection system showed that the addition of Cr₃C₂-coated flake graphite promoted the densification of MgO–C refractories.     

Effect of quenching on microstructure and wear‐resistance of Fe–10Cr–1.5B–2Al Alloy

Cast Fe–10Cr–1.5B–2Al alloy was quenched at different temperatures. The effects of quenching temperature on microstructure and hardness and wear‐resistance of Fe–10Cr–1.5B–2.0Al alloy were investigated by means of the optical microscopy, the scanning electron microscope, X‐ray diffraction, energy dispersive spectrometer, Vickers hardness and Rockwell hardness tester, and the MM‐200 block‐on‐ring wear testing machine under dry friction condition. The results indicate that the as‐cast microstructure of Fe–10Cr–1.5B–2.0Al alloy consists of ferrite, pearlite and netlike eutectics which are distributed in the grain boundary. The eutectics mainly include herringbone M₂B and chrysanthemum M₇(C, B)₃. The matrix gradually turns into single martensite with the increase of the quenching temperature. The type of borocarbides has no obvious change after quenching. The netlike boride almost totally fractures and transforms from the fish‐bone structure to the graininess. There is some retained austenite in the quenched structures when the quenching temperature is more than 1100 °C. When the quenching temperature is in a range of 1000 °C to 1100 °C, the hardness and wear resistance show a sharp increase with an increase of temperature, and show a slight decrease after surpassing 1100 °C.     

Structure and luminescence properties of TiO2:Er3+ nanocrystals annealed at different temperatures

Erbium doped TiO₂ nanocrystals with the structures of anatase, pyrochlore Er₂Ti₂O₇, and rutile, characterized by X-ray diffraction, have been obtained at different annealing temperatures from 300 °C to 900 °C. The nanocrystalline size for anatase TiO₂ is reduced with increasing doped erbium concentration. Following ultraviolet 325 nm irradiation, the intensity of the green emission is the most intense for the TiO₂:Er³⁺ nanocrystals with a structure of pyrochlore Er₂Ti₂O₇, which evolves from the structure of anatase annealed at 800 °C. Moreover, following ultraviolet 325 nm and infrared 980 nm irradiation, the visible emission spectra for the nanocrystals annealed at 900 °C change drastically. Correspondingly, the structure of anatase disappears, while that of rutile becomes dominant, which indicates that phase transformation occurs.     

The effect of micro alloying elements (vanadium,titanium) additions on the austenite grain growth behavior in medium carbon steel containing nitrogen

The austenite grain growth behavior of microalloying elements free steel (nitrogen steel) and micro alloyed steel (V−N steel and V−Ti−N steel) was investigated. The equilibrium dissolving behavior of precipitates was calculated by thermodynamic software and the morphology was observed by tunneling electron microscope. Moreover, grain growth kinetics was analyzed through theoretical calculation. Results show that the austenite grain size of V−N steel and V−Ti−N steel are significantly refined by the undissolved precipitates compared to nitrogen steel. Due to higher dissolving temperature of (vanadium, titanium)(carbon, nitrogen), the austenite grain of V−Ti−N steel keeps fine and increases slowly from 900 °C to 1250 °C. The difference of activation energy between V−N steel and V−Ti−N steel was supposed to come from the effect of different kinds of precipitates on the austenite grain growth. Compared to the vanadium rich (vanadium, titanium)(carbon, nitrogen), the titanium rich (vanadium, titanium)(carbon, nitrogen) is larger and more stable in view of its existence at higher temperature. The decrease of pinning forces can be attributed to the decrease of volume fraction and increase of radius of (vanadium, titanium)(carbon, nitrogen). Compared with critical grain sizes, the larger experimental grain sizes lead to grain growth from 900 °C to 1250 °C.