Solid state and liquid phase sintering of mechanically activated W–20 wt.% Cu powder mixture

W–20 wt.% Cu powder mixture was mechanically alloyed by high-energy ball milling for various times and the effect of mechanical alloying (MA) on the sintering response of the composite compacts was investigated. The densification, microstructure, hardness and electrical conductivity after solid phase sintering (SPS) and liquid phase sintering (LPS) were examined. It was shown that the microstructure of mechanically alloyed powder profoundly influence the sintering response, i.e. a meaningful relationship between the sintering kinetics and the milling time was observed. It is suggested that MA disintegrates the W–W particle networks and increases the contribution of solid phase sintering (SPS) of nanostructured Cu and W particles on the densification. Higher hardness and conductivity were achieved by prolonged MA and SPS, indicating a lower W–W contiguity of the milled powders compared with the conventionally prepared W–Cu composite. On the other hand, depression of the melting temperature of copper up to 145 °C was noticed by affording a prolonged MA. The lower melting temperature and finer distribution of the Cu particles in the W matrix enhanced the densification during LPS and improved the homogeneity and properties of the final product.     

Investigation on synthesis, pressureless sintering and hot pressing of chromium diboride

Experimental investigations were carried out on synthesis of chromium diboride through boron carbide reduction of Cr₂O₃. The products obtained were characterized by X-ray diffraction and the process optimized to prepare single-phase CrB₂ powder. Densification of CrB₂ was investigated by pressureless sintering and hot pressing. A maximum of 93% ρ_th was obtained by pressureless sintering at 1850 °C after a prolonged duration of 360 min. However, near theoretical density was achieved by hot pressing at 1600 °C and 35 MPa pressure for 2 h. The hardness and fracture toughness of fully dense CrB₂ was measured as 22 GPa and 3.5 MPa m^1/2, respectively. The mode of fracture in pressureless sintered samples is intergranular whereas that in hot pressed is transgranular.     

A method for long-term sulfidation of metal at low sulfur pressures and its application to sulfidation of an Fe-20 at.% Al alloy at 1023 K

A method of long-term Sulfidation that involves generating sulfur vapor from dissociation of Ag₂S or from transformation of NiS to Ni₃S_2+x is described. Sulfidation tests of iron samples showed the approach to be essentially valid. At 1023 K, Ag₂S dissociation of Ag₂S gives rise to atm and, in the presence of an inert carrier gas, efficient transport of sulfur to the metal was effected. Liberated silver grew in the form of discrete whiskers, hence did not interfere with continued dissociation of Ag₂S. Equilibration of NiS with Ni₃S_2+x occurs at atm at 1023 K but, with continued loss of sulfur to the metal, the nonstoichiometric Ni₃S_2±x predominates and the ambient is no longer constant. Thus for long-term studies, Sulfidation with NiS/Ni₃S_2+x mixtures is suitable only for metals that sulfidize slowly or at rates independent of the ambient . Using mainly the NiS-Ni₃S_2+x mixture as sulfur source, sulfidation tests up to 72 h at 1023 K were conducted on an Fe-20 at.% Al alloy. Sulfidation was apparently insensitive to the ambient and formation of a three-layer FeS/(Fe, Al)₃S₄/Al₂S₃ scale with internal precipitation zone proceeded at similar rates, and a relationship involving coupled diffusion is proposed. At longer exposure times, transformation of FeS to (Fe, Al)₃S₄ became extensive. Mechanical failure of the scale, followed by rapid healing beneath the original precipitation, also occured after extended reaction periods (>50 hr).     

Mechanical behavior and microstructure of Al-10wt.%Nb alloy fabricated by Mechanical Alloying

Al-10%Nb alloy powders were fabricated by mechanical alloying and their mechanical behavior and microstructure were investigated by means of tensile testing, differential scanning calorimetry, X-ray diffraction and electron microscopy. An intermetallic compound of Al₃Nb was partially formed in the mechanically alloyed powders. The grain size was 50 run after mechanical alloying for 20 hours, and increased to 500 nm after hot extrusion at 400°C. However the 20 size of the intermetallic compounds of precipitated Al₃Nb in an Al matrix, did not vary with hot extrusion. The density of the consolidated Al-Nb alloy was over 97% relatively with hot extrusion. Both the tensile strength and elongation decreased at the elevated temperature. As the temperature increased, the dimples in the fracture surface were large and of coalescent shape, and the fracture was caused by the precipitated phases of Al₃Nb.     

Microstructural characterization of plasma sprayed Al2O3-40 wt.% TiO2 coatings on high density graphite with different post-treatments

Graphite is one of the candidate materials proposed for application in pyrochemical reprocessing plants involving aggressive molten chloride environment. Post treatments are promising techniques for the improvement of properties of thermal spray coatings for different industrial applications. In the present work, the effect of post treatments like vacuum annealing (VA) and laser melting (LM) on the microstructure and chemical modification of plasma sprayed Al₂O₃-40 wt.% TiO₂ coatings over high density (HD) graphite substrates has been investigated. When compared with sprayed coatings (SC), VA coatings showed cluster morphology and LM coatings exhibited homogenous microstructure. On laser melted surfaces networks of cracks were observed. XRD studies showed that the metastable γ-Al₂O₃ phase present in the SC is transformed to stable α-Al₂O₃ after post treatments. In LM coatings Al₂TiO₅ phase was more predominant in contrast to SC and VA coatings. The microhardness enhancement was observed in case of LM coating compared to the VA and SC. Due to elimination of coating defects in LM samples, there is a considerable reduction in the surface roughness.     

Improvement of the hydrogen-storage kinetics of Mg by reactive mechanical grinding with 10 wt.% Fe2O3 and 5 wt.% Ni

The addition of Fe₂O₃ to Mg is believed to be able to increase the hydriding rate of Mg, and the addition of Ni is thought to be able to increase the hydriding and dehydriding rates of Mg. A sample Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) was prepared by mechanical grinding under H₂ (reactive mechanical grinding). The as-milled sample absorbed 4.61 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.59 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min, and desorbed 3.83 wt.% hydrogen at 593 K under 1.0 bar H₂ for 60 min. The activated Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) had a slightly higher hydriding rate at the beginning of the hydriding reaction but a much higher dehydriding rate compared with the activated Mg-10 wt.%Fe₂O₃. prepared via spray conversion. After hydriding-dehydriding cycling, Fe₂O₃ was reduced, Mg₂Ni was formed by the reaction of Mg with Ni, and a small fraction of Mg was oxidized.     

Microstructure and properties of Cr3C2-modified nickel-based alloy coating deposited by plasma transferred arc process

The nickel-based alloy with 30 wt.% chromic carbide (Cr₃C₂) particles has been deposited on Q235-carbon steel (including 0.12 wt.% C) using plasma transferred arc (PTA) welding machine. The microstructure and properties of the deposited coatings were investigated using optical microscope, scanning electron microscope (SEM) equiped with X-ray energy spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), microhardness testes, and sliding wear test. It was found that the γ(Ni, Fe), M₇(C,B)₃, Ni₄B₃, and (Cr,Fe)₂B phases existed in the Cr₃C₂-free nickel-based alloy coating obtained by PTA process. The typical hypoeutectic structure and composition segregation in the solid solution could be found clearly. The addition of 30 wt.% Cr₃C₂ particles led to the existing of Cr₃C₂ phase and the microstructure changing from hypoeutectic structure into hypereutectic structure. The composition segregation in the solid solution could not be found clearly. The average microhardness of the Cr₃C₂-free nickel-based alloy coating increased by 450-500 HV after the addition of 30 wt.% Cr₃C₂ particles. The partial dissolution of Cr₃C₂ particles led to the enrichment of carbon and chromium in the melten pool, and hence caused the formation of more chromium-rich carbides after the solidification process. The undissolved Cr₃C₂ particles and the increasing of chromium-rich carbides was beneficial to enhance the hardness and wear resistance of the Cr₃C₂-modified nickel-based alloy coating deposited by PTA process.     

Effect of Y2O3 dispersoids in 80Ni-20Cr alloy on the early stages of oxidation at low-oxygen potential

Specimens of a 80Ni-20Cr type alloy, with and without Y₂O₃ dispersoid particles, were oxidized at 1000°C in H₂/H₂O mixtures where the partial pressure of oxygen (P _{O 2}) was varied between 10₃ and 10₂₄ atm. Oxide particles nucleated homogeneously on both alloys, and preferential nucleation on dispersoid particles at the surface was not observed. Continuous Cr₂O₃ films formed slightly faster at aP _{O 2} of 10^–21 atm on the alloy containing the dispersoid, but the difference was negligible at higher pressures. Oxidation atP _{O 2}=10^-19 and 10^–21 atm involved both the formation of Cr₂O₃ and the evaporation of chromium. Thin films of -Al₂O₃ were observed on both alloys after oxidation atP _{O 2}.     

Oxidation behaviour of Ti-46.7Al-1.9W-0.5Si in air and Ar-20%O2 between 750 and 950 °C

The oxidation behaviour of an intermetallic alloy, Ti-46.7Al-1.9W-0.5Si, was studied in air and Ar-20%O₂ atmospheres at 750, 850 and 950 °C. Oxidation of the alloy followed a parabolic rate law at low temperature (750 °C) in both environments. The alloy oxidised parabolically in air and at a slower rate in Ar-20%O₂ at 850 °C. Following a parabolic oxidation for a relatively short exposure period (72 h) at 950 °C, the oxidation rate was reduced after prolonged exposure (up to 240 h) in air. The alloy oxidised in a slower manner in the Ar-20%O₂ atmosphere at 950 °C. Higher oxidation rates were observed in air than in Ar-20%O₂ at all three experimental temperatures. Multi-layered scales developed in both environments. The scale formed in air consisted of TiO₂/Al₂O₃/TiO₂/TiN/TiAl₂ layers, ranging from the surface to the substrate—whilst the scale developed in the Ar-20%O₂ atmosphere comprised of the sequence TiO₂/Al₂O₃/TiO₂/Al₂O₃/Ti₃Al/substrate. The two layers of Al₂O₃ in Ar-20%O₂ were more effective in providing protection of the substrate against high temperature corrosion than the single layer of Al₂O₃ formed in air.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Production of in situ aluminum-titanium diboride master alloy formed by slag-metal reaction

Al-TiB₂ master alloys have received much attention in recent years owing to their potential as efficient grain refiners for aluminum foundry alloys. In this study, the process of production of master alloys was investigated to develop a low cost method, namely, slag-metal reaction. This method can be used to fabricate Al-TiB₂ master alloy in situ from the TiO₂-H₃BO₃-Na₃AlF₆ and Al system. Since the price of the raw materials is low and the technology is simple, the processing technique appears to reduce the cost of the master alloy. Because of exothermic reactions, not much energy is needed to melt materials. In this process, Titanium diboride particles were formed in situ through the reactions of TiO₂, H₃BO₃ and Na₃AlF₆. Results showed that when the aluminum melted, the condensed TiB₂ particles that formed in situ were spherical with an average diameter of 1 μm. Furthermore, these TiB₂ particles were distributed uniformly through the master alloy.     

Effects of sintering additives on microstructure and mechanical properties of TiB2-WC ceramic-metal composite tool materials

TiB₂-WC ceramic-metal composite tool materials were fabricated using Co, Ni and (Ni, Mo) as sintering additives by vacuum hot-pressing technique. The microstructure and mechanical properties of the composite were investigated. The composite was analyzed by the observations of scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The microstructure of TiB₂-WC ceramic-metal composites consisted of the fine WC grains and uniform TiB₂ grains. The brittle phase of Ni₃B₄ and a few pores were found in TiB₂-WC-Ni ceramic-metal composite. A lot of pores and brittle phases such as W₂CoB₂ and Co₂B were formed in TiB₂-WC-Co ceramic-metal composite. The liquid phase of Co was consumed by the reaction which led to the formation of the pores and the coarse grains of TiB₂. The pores, brittle phases and coarse grains of TiB₂ were harmful to the improvement of the mechanical properties of the composite. The sintering additive of (Ni, Mo) had a significant effect on the density and the mechanical properties of TiB₂-WC ceramic-metal composite. The formation of intermetallic compound of MoNi₄ inhibited the consumption of liquid phase of (Ni, Mo). The liquid phase of (Ni, Mo) not only inhibited the formation of the pores and the coarse grains of TiB₂ but also strengthened the interface energy between WC and TiB₂ grains. The grain size was fine and the average relative density of TiB₂-WC-(Ni, Mo) ceramic-metal composite reached 99.1%. The flexural strength, fracture toughness and Vickers hardness of TiB₂-WC-(Ni, Mo) ceramic-metal composite were 1307.0 ± 121.4 MPa, 8.19 ± 0.29 MPa m^1/2 and 22.71 ± 0.82 GPa, respectively.     

The thermal stability in air of hot-pressed diboride matrix composites for uses at ultra-high temperatures

The resistance to oxidation in ambient air at a temperature up to 1600 °C of two hot-pressed diborides matrix composites, both containing 19.5% v/o SiC and 3 v/o HfN (as sintering aid), was investigated. The diboride matrix was based on HfB₂ or a ZrB₂/HfB₂ mixture (volume ratio ≈ 1). Both the materials were subjected to repeated heating-cooling cycles at 1600 °C, and a 20 h exposure at 1450 °C in flowing dry air. Modest weight gains and limited corrosion depths highlighted a rather good thermal stability. In accordance with the thermo-gravimetric test at 1450 °C, the oxidation kinetics for both the composites superbly fit a para-linear law. The introduction of the SiC particles provided tangible benefits for the resistance to oxidation. One of the oxidation products, a borosilicate glass, sealed pores and coated the exposed faces, greatly limiting the inward transport of oxygen towards the internal oxide/diboride interfaces.     

Fabrication and oxidation behavior of Cr2AlC coating on Ti6242 alloy

An ∼ 5 µm Cr₂AlC coating was synthesized on near-α titanium alloy Ti6242 using an industrially sized magnetron sputtering coater. Isothermal oxidation at 700 °C and 800 °C, and cyclic oxidation at 700 °C of the bare alloys and coated specimens were investigated in air. The results indicated that the Ti6242 alloy faced serious oxidation problems at 700 °C and 800 °C. Repeated formation and spallation of the multilayered oxide scale on the Ti6242 alloy occurred during oxidation testing. The coated specimens exhibited much better oxidation behaviour as compared to the bare alloy. A continuous Al-rich oxide scale formed on the coating surface during the initial oxidation stages. The oxide scale and coating itself acted as diffusion barriers blocking the further ingress of oxygen and protected the substrate alloy from oxidation. The oxidation mechanisms of the bare alloy and the coated specimens were investigated based on the experimental results.     

The kinetics and morphological development of oxide scales on cobalt-iron alloys (0–70 wt.% Fe) at 1200°C

The oxidation kinetics of Co-Fe alloys containing up to 70 wt.% Fe have been determined at 1200°C in oxygen at pressures ranging from 10^–4 to 1 atm. These kinetics, which were parabolic, resulted from metal diffusion through the scales as the rate-controlling process. The scales were either one-phase cobalto-wustite (CoFe)O or two-phase, containing growth of spinel cobalt ferrite (CoFe)₃O₄ in the cobalto-wustite matrix. The particular shape of the relationship, exhibiting a maximum for the parabolic oxidation constant vs. the iron alloy composition, is interpreted as being caused by the combined opposing effects of more rapid metal diffusion in cobalto-wustite and diffusional resistance of the spinel growths in cobalto-wustite with increasing iron alloy content.     

Hydrogen-sorption and thermodynamic characteristics of mechanically grinded TiH1.9 as studied using thermal desorption spectroscopy

Isobaric thermal desorption spectroscopy and X-ray diffraction analysis were used to study the influence of mechanical dispersion during high-energy milling γ-TiH_1.9 hydride upon its hydrogen-sorption characteristics, temperature and enthalpy of the γ → β (TiH_1.9 → Ti[H]_β) phase transition at isobaric heating the sample under hydrogen atmosphere with speed 3°/min. Isobars of hydrogen thermal desorption in the regions of the γ and β phases of the Ti-H₂ system at pressures of 0.1, 0.25, 0.315 and 0.45 MPa of hydrogen in the reactor have been derived. Experimental data obtained for initial titanium hydride and mechanically grinded for 20 min in a planetary ball mill have been used for construction of Van’t Hoff plots and for determination of enthalpy of formation of γ-hydride from solid solution of hydrogen in bcc titanium. Our experimental data reveal that 20 min high-energy influence on titanium hydride powder leads to increasing the specific surface of the samples from 0.13 to 8.58 m²/g and to significant (more than 250°) decreasing the temperature of the beginning of hydrogen release when heating the sample (i.e., to a decrease of thermal stability of mechanically activated TiH_1.9). However, mechanical dispersion does not change the temperature of the γ → β phase transition. It has been established that high-energy milling TiH_1.9 powder causes the effect of a decrease of enthalpy of the formation of γ-hydride from 248 kJ/mole H₂ to 175 kJ/mole H₂.     

Preparation of Ti3AlC2 by mechanically activated sintering with Sn aids

The mechanically activated sintering process was adapted to synthesize Ti₃AlC₂ using 3Ti/Al/2C/0.05Sn powder mixtures. The result showed that the powders containing TiC, Ti₃AlC₂ and Ti₂AlC were obtained by mechanical alloying (MA) 3Ti/Al/2C powders. Addition of appropriate Sn reduced the content of Ti₂AlC and enhanced the synthesis of Ti₃AlC₂ significantly. The powders with highest content of Ti₃AlC₂ were obtained by MA 3Ti/Al/2C/0.05Sn powders. Through pressureless sintering the mechanical alloyed powders at 900–1100 °C for 2 h, the high purity Ti₃AlC₂ material with fine organization was produced.     

Interactive effect of cerium and aluminum on the ignition point and the oxidation resistance of magnesium alloy

This paper focused on the interactive effect of cerium (Ce) addition and aluminum (Al) content in magnesium alloy on ignition point and oxidation resistance. Ce content played an important role in improving the oxidation resistance of Mg alloy. Ignition point ascended with increasing Ce content. 0.25 wt% Ce content in Mg alloys could greatly improve tightness of the oxide film of Mg alloys. However, when Ce content in the alloy exceeded its solid solubility, ignition point descended. Furthermore, Al content in the alloy also influenced the ignition point. The higher the Al content was, the lower the ignition point.     

Rapid fabrication of the Ti3SiC2 bonded diamond composite by spark plasma sintering

A mixture of Ti/Si/TiC/diamond powders was employed to fabricate the Ti₃SiC₂ bonded diamond composite using the spark plasma sintering-reactive synthesis method. The addition of diamond does not inhibit the synthesis of Ti₃SiC₂ in the sintered product. In the matrix Ti₃SiC₂ grains developed lamellar morphology with an average length size of 5-10 μm. Ti₃SiC₂ matrix displays good pullout strength with diamond, and the Ti₃SiC₂ bonded diamond material exhibits good wear resistance.     

The effect of sliding speed and amount of loading on friction and wear behavior of Cu–0.65 wt.%Cr alloy

Friction and wear behavior of a peak aged Cu–0.65 wt.%Cr alloy was investigated. The friction and wear experiments were run under ambient conditions with a pin-on-disk tribometer. Experiments were performed using various applied normal loads and sliding velocities. The tribological behavior of the studied alloy was discussed in terms of friction coefficient, wear loss and wear mechanism.Friction coefficient and wear loss have shown large sensitivity to the applied normal load and the sliding velocity. At the sliding velocity of 0.3 m/s weight loss increased from 6.9 to 51 mg by increasing the normal load from 20 to 40 N. At higher sliding velocity minimum weight loss is achieved at 60 N normal load. So it can be seen that with increasing normal load wear rate decreases due to the formation of a continuous tribofilm which consists of Fe–Cu intermetallic. Varying of friction coefficients in different conditions of normal load and sliding velocity is correlated to the wear behavior.The analysis of worn surfaces by XRD and SEM showed that an increase in normal load and sliding velocity creates an intermetallic wear-induced layer, which modifies the wear behavior of the alloy. The XRD result indicates that new phase of Cu_9.9Fe_0.1 is generated on worn surfaces of the pin specimens during the wear tests. There is a significant correlation between the micrograph of worn surfaces and the wear rate of specimens.