Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.     

Multi-layer CVD-SiC/MoSi2–CrSi2–Si/B-modified SiC oxidation protective coating for carbon/carbon composites

To further improve the oxidation resistance of coating for carbon/carbon (C/C) composites, a multi-layer CVD-SiC/MoSi₂–CrSi₂–Si/B-modified SiC coating was prepared on the surface of C/C composites by pack cementation and chemical vapour deposition method, respectively. The microstructures, oxidation and thermal shock resistance of the coating were studied. The influence of B content in pack powder on the microstructure and oxidation resistance of B-modified SiC coating was also investigated. The results show that the B-modified SiC coating prepared with 10 wt.% B exhibited the best oxidation protection ability for C/C composites at 1173 K. The multi-layer coatings could protect the C/C composites at 1173 K for 30 h and 1873 K for 200 h, and endure 30 thermal cycles between 1873 K and room temperatures. The oxidation resistance and thermal shock resistance is mainly attributed to their dense structure and self-sealing property.     

Pyrolysis characteristics and mechanism of hydrocarbon compounds for RDF

Abstract

With the help of pyrolysis gas chromatography–mass spectrometry (Py-GC-MS), high resolution thermogravimetric instrument and CHONS elemental analyzer, the pyrolysis products of refuse-derived fuel (RDF) in different months in one year are studied in this article. And the three kinds of RDF are denoted as A, B and C, respectively. High resolution thermogravimetric tests showed that obvious chemical reactions took place for RDF samples in the temperature ranges of 433–623?K, 623–773?K and 773–1173?K. RDF samples were tested by Py-GC-MS under 623?K, 773?K or 1173?K while He was as the balance gas, and the qualitative and semi-quantitative analysis results were obtained by introducing pyrolysis products into gas chromatography–mass spectrometry. According to the Py-GC-MS tests, the kinds and contents of pyrolysis products increased with the rising of the temperature, and RDF pyrolysis finished prior to 1173?K. Primary discussion for pyrolysis mechanism is progressed according to main pyrolysis products and the change of their relative contents, which can be used to provide the significant fundamental research of homogeneous reduction for NO_x removal.     

Acoustic emission as a technique to study breakaway oxidation and spalling behaviour of 2.25Cr-1 Mo steel

The onset of breakaway oxidation and cracking of the oxide scale formed on 2.25Cr-1Mo steel in air at 1173 K have been studied by the acoustic emission (AE) technique. AE parameters, i.e. events, ring-down counts, rise time, event duration and root mean square voltage show negligible increase during isothermal heating at this temperature, until a point where a sudden increase in AE activity is found. This point corresponds to the onset of breakaway oxidation. An enormous increase in AE activity after the start of cooling from the oxidation temperature has been attributed to separation of the oxide scale from the matrix. The peak amplitude distribution is measured and a derived b parameter is calculated. This has helped in distinguishing the phenomena of isothermal oxidation at 1173 K and internal cracking of oxide scale during cooling from 1173 K.     

The effect of strong magnetic field treatment on microstructure and room temperature compressive properties of NiAl–Cr(Mo)–Hf eutectic alloy

The Ni–33Al–28Cr–6Mo–0.2Hf (at.%) eutectic alloy was treated in a 10 T strong magnetic field at 1073 K, 1173 K and 1273 K for 1 h, respectively. Microstructure examination reveals that after the strong magnetic field treatment, Heusler particles (Ni₂AlHf) along eutectic cell boundaries distribute more uniformly, and moreover most Heusler particles have changed into Hf solid solution. In addition, after strong magnetic field treatment at 1173 K and 1273 K, the original eutectic cellular morphology is changed greatly; Cr(Mo) plates become spheroidizing and trend to align to the direction of strong magnetic field. The compression tests show that room temperature compressive ductility of the alloys with strong magnetic field treatment improves significantly, compared with the heat-treated alloy. The fracture characteristics change from debonding into transgranular cleavage, which indicates that the cohesion of grain boundaries get significant improvement.     

Direct oxidation route from metal to ceramic: Study on cobalt oxide

The study deals with the direct-oxidation kinetics of micronic-cobalt metal particles and its simulation for the complete transition from metal to ceramic. The simulation was also experimentally verified. All the three possible interfaces, Co/CoO, CoO/Co₃O₄ and Co₃O₄/O₂ (air), have been taken into consideration for the simulation. The complete oxidation kinetics has been investigated from the thermogravimetric studies under isothermal conditions in the temperatures 973–1173 K. A quantitative interpretation based on the diffusion of Co or oxygen ions through the grown oxide layer has been proposed. The activation energy for the oxidation kinetics calculated from the Arrhenius law was 161 ± 20 kJ mol⁻¹.     

HIGH TEMPERATURE FATIGUE BEHAVIOUR OF A CAST COBALT-BASE SUPERALLOY

Abstract The initiation and the propagation of fatigue cracks and the low cycle fatigue life of a cast cobalt base superalloy was studied at 293K, 973K and 1173K by optical and scanning electron microscopy. A substantial decrease in fatigue life occurred at 973 and 1173K when compared to room temperature life. A time-dependent bulk damage was evidenced at 1173K which was determined by quantitative microscopy for two plastic strain levels. High strain fatigue crack propagation experiments were carried out at room temperature and at 1173K. From these experiments the decrease of the overall fatigue life at high temperature was shown to result from a considerable reduction of the initiation period due to oxidation and also from a significant acceleration of the crack propagation rate in the presence of oxidation and bulk damage.     

Microstructural and mechanical properties of jointed ZrO2/Ti–6Al–4V alloy using Ti33Zr17Cu50 amorphous brazing filler

Ceramic ZrO₂ and metallic Ti–6Al–4V alloy are jointed by using a Ti₃₃Zr₁₇Cu₅₀ (at.%) amorphous alloy as a solder at 1123–1273 K in a high vacuum. It is demonstrated that the microstructure and mechanical properties are significantly influenced by the brazing temperature, the heat time and the cooling rate. The brazing seam jointing ZrO₂ with Ti–6Al–4V is composed of ZrO₂/Cu₂Ti₄O, (Ti,Zr)₂Cu/TiO, Ti₂O/CuTi₂, (Ti,Zr)₂Cu/CuTi₂/Ti–6Al–4V alloys and compounds, of which the increasing thickness weakens the shear strength as the brazing temperature, the heat time the cooling rate increase. The maximum shear strength of the brazing joints reaches 162 MPa with the optimal technical parameters: the brazing temperature of 1173 K, the heat time of 10 min and the cooling rate of 5 K/min. The fracture of the joint occurs in the brittle seam layer nearby the side of ZrO₂.     

Effect of Nb,Mo, V contents on oxidation resistance of Ti3Al based alloys

In the present paper, the effect of the contents of Nb, Mo, V on the oxidation properties (700°C, in air) of Ti₃Al based alloys has been studied. It has been shown that the alloys were oxidized rapidly as exposed at 700°C in the air. After 100 h exposure, oxygen-affected alloy surface layer of about 10 thickness has been formed on account of the poor protection of the oxide film. An addition of (11–13%)¹ Nb enhanced the oxidation resistance. The addition of Mo and V in the Ti₃Al–Nb system alloy reduced the oxidation resistance significantly.     

Short communication Low temperature forgeability of Ti–48Al–2Cr–2Nb alloy

Abstract

A two phase (γ + α) titanium aluminide alloy Ti–48Al–2Cr–2Nb, at.-% was isothermally forged along all the three axes in succession at an initial forging temperature of 1273 K, followed by another two sets of forgings at lower temperatures of 1173 and 1123 K. The combined effects of heavy multiaxial strains (? = 2·07, per set of forgings), progressively lower temperatures, and a moderate strain rate (10^-3 s^-1 ) transformed the microstructure to a fine and equiaxed shape. The microstructure developed is suitable for working at a low temperature.     

Structural and oxidation behavior of atmospheric heat treated plasma sprayed WC–Co coatings

In this study, structural and oxidation behavior of WC–Co coatings was analyzed during atmospheric heat treatment process between 150 °C and 1100 °C. Two types of WC–12%Co coatings with different particle size and morphology were deposited on steel substrates using Air Plasma Spraying. The coated samples were heat treated in atmosphere in different temperatures between 500 and 1100 °C. Microstructural evaluation, X-ray diffraction analysis and microhardness testing were performed before and after heat treatment. In this case, the results showed that, regarding increase hardness of coating samples based on increasing applied temperature, coatings kept their properties up to 500 °C. In addition, by increasing heat treatment temperature up to 1100 °C, oxidation process in coated layer accelerated and caused coating detachment from the coating-substrate interface.     

Investigation of active slip-systems in some body-centered cubic metals

Tensile tests were performed on high-purity W and Mo polycrystals at room temperature for a range of axial strain-rates 2.1 × 10⁻⁴–2.1 × 10⁻² s⁻¹. The critical resolved shear stress (CRSS) data was analyzed by using the analytical formulation for the strain-rate dependence of the CRSS derived in the kink-pair nucleation (KPN) model of flow stress in crystals with high intrinsic lattice friction. On evaluation of various microscopic slip-parameters of the model, the active slip-system in both W and Mo polycrystals was identified as {110}〈111〉. This is in good agreement with that deduced from the published data on the temperature dependence of the CRSS of these crystals as well as from the observed slip-lines on the deformed crystals reported in the literature. Moreover, the available data on the temperature dependence of the CRSS of Mo, Nb, Fe, V, and K crystals were also analyzed within the framework of the KPN model of flow stress. Peierls mechanism was found to be responsible for the CRSS of these metals; the active slip-systems in refractory metals Mo, Nb, Fe, and V were {110}〈111〉 and {211}〈111〉 whereas that in alkali metal K was {321}〈111〉.     

XPS study of the initial oxidation of the bulk metallic glass Zr<Subscript>46.75</Subscript>Ti<Subscript>8.25</Subscript>Cu<Subscript>7.5</Subscript>Ni<Subscript>10</Subscript>Be<Subscript>27.5</Subscript>

The surface oxidation behaviour of the bulk metallic glass Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 was investigated in situ by using X-ray photoelectron spectroscopy (XPS). The initial stages of oxidation at room temperature were studied by exposing the clean alloy specimen surface to varying doses of pure oxygen (up to 1,000 L) in an UHV chamber. Progressive oxidation of Zr, Be and Ti was observed with increasing doses, the major species in the oxide layer being Zr(IV) and Be(II) possibly existing as ZrO₂, BeO, while Cu and Ni remained in their elemental forms. High temperature in situ oxidation in the temperature range 423–653 K for a fixed oxygen dose of 300 L was also investigated. Oxidation of Be was observed at all temperatures, while a sharp decrease in the oxidation of Zr and Ti was observed for temperatures at 573 K and above. The results show a preferential oxidation of Be and Zr at room temperature, while at higher temperatures oxidation is controlled by the reduction of oxides of Zr and Ti and the diffusion of oxygen into the alloy bulk. The role of the dissolved carbon impurity in the reduction of the oxides is discussed.     

Titanium oxide films produced on commercially pure titanium by anodic oxidation with different voltages

Titanium oxide films produced on commercially pure Ti by anodic oxidation with different voltages were analyzed. Anodic oxidation was carried out at room temperature using 1.4 M H₃PO₄ electrolyte and a platinum counter-electrode, in potentiostatic mode under the following conditions: 50 V, 100 V, 150 V, 200 V and 250 V. It was observed that porous titanium layers were formed at all voltage values but morphological differences were observed. Initially, the film was thin but with increasing voltage it broke down locally and porous regions became evident due to the dielectric breakdown. The porosity and the pore size increased with the increasing voltage. The surface morphology in samples formed with 200 V had substantially different porous structures than those formed with other voltage values. The anodic film surface displayed pores and craters formed on the relatively flat ground oxide surface. AFM images showed that higher voltages produced thicker titanium oxide films.     

Thermal oxidation of vanadium-free Ti alloys: An X-ray photoelectron spectroscopy study

In the present work, X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of three alloys for biomedical applications: Ti–7Nb–6Al, Ti–13Nb–13Zr and Ti–15Zr–4Nb. The surface of these alloys was modified by annealing in air at 750 °C for different times with the aim of developing an oxide thick layer on top. The evolution of surface composition with annealing time was studied by XPS, and compared with the composition of the native oxide layer present on the samples before annealing. Two different oxidation trends were observed depending on the alloying elements and their corresponding diffusion kinetics, which give rise to different chemical species at the topmost layers. These results were linked with an evaluation of the biological response of the alloys by bringing them in contact with human peripheral blood mononuclear cells (PBMC).     

Temperature dependent work hardening in Ti–6Al–4V alloy over large temperature and strain rate ranges: Experiments and constitutive modeling

The plastic deformation behaviors of Ti–6Al–4V alloy over wide ranges of strain rate (from 10⁻⁴ to 10⁴ s⁻¹) and temperature (from 20 to 900 °C) are investigated by the quasi-static and dynamic uniaxial compression tests. The microstructure evolution of Ti–6Al–4V alloy at different temperatures is discussed. Material generates higher ductility and formability when temperature is higher than 500 °C, which leads to the decrease of work hardening rate. The true stress–strain responses are modeled with the JC, modified JC, KHL and modified KHL models. In detail, a temperature dependent work hardening function is introduced into the original JC and KHL models. The parameters of the four models for Ti–6Al–4V alloy are calculated by GA optimization method. The average standard deviations between the experimental and calculated flow stresses range from 4% to 13%, which validates the accuracy of the models. In addition, comparison of flow stresses at dynamic (10,000 s⁻¹), the work hardening rates at dynamic (7500 s⁻¹), as well as the quasi-static jump experiments were proposed to further validate the models. The modified JC and modified KHL models could characterize the temperature dependent work hardening effect for Ti–6Al–4V alloy over large strain rate and temperature ranges.     

A comparative study on modified Johnson Cook,modified Zerilli–Armstrong and Arrhenius-type constitutive models to predict the hot deformation behavior in 28CrMnMoV steel

The true stress-strain data from isothermal hot compression tests on Gleeble-3500 thermo mechanical simulator, in a wide range of temperatures (1173–1473 K) and strain rates (0.01–10 s⁻¹), were employed to establish the constitutive equations based on modified Johnson Cook, modified Zerilli–Armstrong, and strain-compensated Arrhenius-type models respectively to predict the high-temperature flow stress of 28CrMnMoV steel. Furthermore, a comparative study has been made on the capability of the three models to represent the elevated temperature flow behavior of this steel. Suitability of the three models were evaluated by comparing the accuracy of prediction of deformation behavior, correlation coefficient, average absolute relative error (AARE) and relative errors of prediction, the number of material constants, and the time needed to evaluate these constants. The results showed that the predicted values by the modified Johnson Cook and Zerilli–Armstrong models could agree well with the experimental values except under the strain rate of 0.01 s⁻¹. However, the strain-compensated Arrhenius-type model could track the deformation behavior more accurately throughout the entire temperature and strain rate range.     

Structure and mechanical properties of as-cast Ti–5Nb–xCr alloys

As-cast Ti–5Nb and a series of Ti–5Nb–xCr with Cr content ranging from 1 to 13 mass% prepared by using a commercial arc-melting vacuum-pressure casting system were investigated. Commercially pure titanium (c.p. Ti) was used as a control. X-ray diffraction (XRD) for phase analysis was conducted with a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens. The fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that these alloys obviously had different structures and mechanical properties with the addition of various amounts of Cr. When 1 mass% Cr was added, the structure was comprised mainly of the α′ phase, which was also found in Ti–5Nb. With the addition of 3 mass% Cr, α′ and α′′ phases were appeared. When the Cr content was increased to 5 mass% or greater, the β phase was completely retained. Moreover, the ω phase was detected in the Ti–5Nb–5Cr and Ti–5Nb–7Cr alloys. The largest quantity of ω phase and the highest bending modulus were found in the Ti–5Nb–5Cr alloy, while the Ti–5Nb–9Cr alloy had the lowest bending modulus. Moreover, the high strength/modulus ratios of the Ti–5Nb–3Cr (22.5) and Ti–5Nb–9Cr (21.3) alloys demonstrate its advantage for use as implant materials. Also, these two alloys exhibited the better elastic recovery angles of 28.3° in Ti–5Nb–3Cr and 22.2° in Ti–5Nb–9Cr. In the current search for better implant materials, α′ + α′′ phase Ti–5Nb–3Cr and β phase Ti–5Nb–9Cr alloys with low modulus, ductile property, excellent elastic recovery capability and reasonably high-strength seem to be the most feasible alloy for orthopedic and dental applications if some other necessary properties are obtained.     

Effect of sintering temperature on the properties of Cu-Co ferrites prepared by oxalate precipitation method

Polycrystalline Cu-Co ferrite powder was synthesized following oxalate precipitation method. The samples of the compound Cu_0.5Co_0.5Fe₂O₄ were heated at different temperatures in the range of 773-1173 K and were characterized by X-ray diffraction and SEM techniques. The results of XRD show the formation of single-phase cubic spinel structure. The lattice parameter showed a minimum value for the sample heated at 1073 K. It has been observed that grain size increases with the increase in temperature and is maximum (3.2 μm) for the powder sintered at 1173 K.     

Low energy implantation to inhibit wear in N+ ions implanted WC–Co composite

This work discusses the influence of nitrogen ion (N⁺) implantation on wear resistance of WC–Co composite. The WC–Co samples were bombarded at low N⁺ ions energies of 20 and 30 keV and doses of 10¹⁷ and 2 × 10¹⁷ ions cm⁻². Tribological tests were conducted against cylindrical 100Cr6 pin at 200 N load and 180 mm s⁻¹ speed. The tests use water lubrication and four sample types with Co binder content ranging in 6.5–25%. The X-ray spectra reveal that implantation is able to transform the original [CFC] Co structure of virgin surface to harder amorphous phase. However, it was found that excessive low binder content alters the wear behavior on non-implanted samples since it causes wear rate transition from 0.59 × 10⁻⁷ to 2.1 × 10⁻⁷ mm³/(mm² s) imposing hence instable wear regime. The SEM micrographs confirm the formation of transferred film within the implanted worn surface owing to (i) an enhancement in Co flow and (ii) a generation of oxides (Fe₂O₃, Fe₃O₄, Co₂O₃, WO₂). While the formed film acts to inhibit severe abrasion, the material removal process combining cobalt flow and carbide grains pull-out seems to be associated with oxidation mechanisms to be accentuated with energy increase. The most improvements in wear resistance were observed on samples with the highest Co content and the results were found more sensitive to N⁺ ions implantation energy than dose.