Influence of Sputtered NanocrystaUine Coating on Oxidation and Hot Corrosion of a Nickel-based Superalloy M951

The isothermal and cyclic oxidation behaviors in air and hot corrosion behaviors in Na2SO4 ＋ 25 wt% K2SO4 salt of M951 cast superalloy and a sputtered nanocrystalline coating of the same material were studied. Scanning electron microscopy, energy dispersive X-ray spectroscope, X-ray diffraction, and transmission electron microscopy were employed to examine the morphologies and phase composition of the M951 alloy and nanocrystalline coating before and after oxidation and hot corrosion. The as-sputtered nanocrystalline layer has a homogeneous y phase structure of very fine grain size （30-200 nm） with the preferential growth texture of （111） parallel to the interface. Adherent AI203 rich oxide scale formed on the cast M951 alloy and its sputtered coating after isothermal oxidation at 900 and 1000 ℃. However, when being isothermal oxidized at 1100℃ and cyclic oxidized at 1000 ℃, the oxide scale formed on the cast alloy was a mixture of NiO, NiAl2O4, Al2O3 and Nb205 and spalled seriously, while that formed on the sputtered coating mainly consisted of Al2O3 and was very adherent. Nanocrystallization promoted rapid formation of Al2O3 scale during the early stage of oxidation and enhanced the adhesion of the oxide scale, thus improved the oxidation resistance of the substrate alloy. Serious corrosion occurred for the cast alloy. The sputtered nanocrystalline coating apparently improved the hot corrosion resistance of the cast alloy in the mixed sulfate by the formation of a continuous Al2O3 and Cr2O3 mixed oxide layer on the surface of the coating, and the pre- oxidation treatment of the coating led to an even better effect.     

High-Temperature Oxidation Behavior of a Ni-Cr-W-Al Alloy

A study was conducted to examine the isothermal oxidation behavior of a new Ni-Cr-W-Al alloy in air at 1250℃ with different time. Oxidation kinetics was determined from weight-change measurements. The microstructure and composition of the oxide scale were investigated by means of scanning electron microcopy and X-ray diffraction. The results showed that the oxide scales of the alloy were a compact and continuous outer Cr2O3 and NiCr2O4 layer and an inner Al2O3 layer that was in dendrite shape. Oxides scales with good adherence were formed on the surface of the alloy, which made the alloy perform excellent high-temperature oxidation resistance.     

Effect of Y2O3 and Total Oxide Addition on Mechanical Properties of Pressureless Sintered β-SiC

The effect of Y2O3 and the total oxide volume fraction (Y2O3 Al2O3) on density and mechanical properties of low temperature (1770～1940℃) pressureless sintered β-SiC ceramics were presented. The optimum temperature for pressureless sintering of β-SiC was found to be～1850℃ and the optimum content of Y2O3 in the oxides was found to be between 40 and 57 wt pct. The highest sintered density was achieved by adding oxides at 14 vol. pct. Both of the highest strength and fracture toughness were achieved at～14 vol. pct oxide addition and yttria concentrations between 40 and 57 wt pct in the oxides. Hardness, on the other hand, was found to be the highest for samples with 14 vol. pct oxide addition and ～64 wt pct Y2O3 in oxides.     

Combined Effect of Aluminum Content and Layer Structure on the Oxidation Performance of Ti1-xAlxN Based Coatings

TiN,Ti1-xAIxN single layer coatings and TiN/Ti1-xAIxN multilayer coatings were deposited on SKH51 tool steel substrate by arc ion plating.The coatings were annealed in air to study the effect of aluminum and film structure on the oxidation performance.The surface morphology and structure were characterized by scanning electron microscopy and X-ray diffraction.The element distribution on the cross section was analyzed by electron probe microscopy.It is found that the oxidation resistance of these Ti1-xAlxN based coatings is mainly attributed to aluminum content in them.In comparison with the Ti1-xAlxN single layer coating,the TiN layer inserting into the Ti1-xAlxN in a multilayer coating increases the tendency of Ti diffusion toward the surface and forms a Ti-enriched top surface oxide layer,thus degrades the oxidation resistance.As far as the oxidation resistance is concerned in this study,Ti0.33Al0.67N single layer coating performs the best among all coatings.The kinetic of oxidation behavior of all coatings presents two definite stages.One is a slow oxidation growth which conforms to parabolic law,and the other presents severe mass gain with oxidation duration.The annealing time for severe oxidation initiation is responsible to Fe2O3formation in the oxide scale.     

Role of Y4Al2O9 in High Temperature Oxidation Resistance of NiCoCrAlY-ZrO2.Y2O3 Coatings

NiCoCrAlY-ZrO2·Y2O3 coatings were deposited on the substrates by using a technology of combining electron,atom and ion beams （three beams）. Isothermal oxidation for these samples was performed at 1100℃ for 100-300 h. The results show that a thermally grown oxide （TGO） layer was formed between NiCoCrAlY layer and oxidation. The TGO contains α-Al2O3 and Y4Al2O9 etc. oxides. The intensity ratio of α-Al2O3/Y4Al2O9 was monotonously decreased with increasing oxidation time based on XRD （X-ray diffraction） analysis. The Y4Al2O9 phase plays the most important role in high temperature oxidation resistance at 1100℃. The related mechanism was also discussed.     

Improvement on the Oxidation Resistance of a Ti3Al Based Alloy by Cr1-xAlxN （0≤x≤0.47） Coatings

Cr1-xAlxN coatings have been deposited on a Ti3Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800-900℃ showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al2O3＋Cr2O3 mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800℃. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the AI content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.     

Oxidation of Hastelloy-XR Alloy for Corrosion-Resistant Glass-Coating

The oxidation behavior of Hastelloy-XR alloy was investigated to obtain the optimum surface condition for corrosion-resistant glass-coatings. The surface morphology of oxide scales changed significantly with variation of temperatureand oxygen partial pressure (po2 ). The oxidation kinetics was mainly parabolic independent of oxidation conditions.The oxide scales were consisted of inner Cr2O3 and outer spinel layers. The phase component of spinel layers wereMn1.5Cr1.5O4 and (Mn,Ni)(Cr,Fe)2O4 for the oxygen partial pressures po2<10 kPa and po2>10 kPa, respectively.The optimum oxidation condition to obtain an oxide scale for well-adhered glass-coating to the substrate was 1248 Kand po2 =0.01 kPa for the oxidation time of 43 ks.     

High-temperature oxidation behavior of modified 4Al alumina-forming austenitic steel: Effect of cold rolling

The oxidation behavior and mechanism of as-received and 30 % cold-rolled alumina-forming austenitic(AFA) steel were investigated in dry air at 700℃.The results show that the mass gain per unit area curves of as-received and 30 % cold-rolled steels subject to near-parabolic law before 100 h oxidation time.Two samples both show higher high-temperature oxidation resistance due to the formation of dense Al₂O₃ oxide scale.Gradual spallation of outer scale results in the formation of continuous and dense alumina scale.Dislocations can act as short-circuit diffusion channel for the diffusion of Al from alloy matrix to surface,and also provide nucleation sites for B2-NiAl phase,which ensure the continuous formation of Al₂O₃ scale.     

Comparison of Few-layer Graphene Prepared from Natural Graphite through Fast Synthesis Approach

We report the synthesis of high quality few-layer graphene on a large scale using high purity natural graphite from Sri Lanka. A novel thermal method was adapted to prepare graphene from intermediate graphite oxide, which was obtained by heating the intermediate at low temperature(above 150 °C) in air for 5 min and subsequent heating at 500 °C in Argon for 15 min. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, Raman spectroscopy etc. The properties and the performance of graphene were observed to depend on the graphite source. The reduced graphite oxide from Kahatagaha graphite source exhibits higher Brunauer–Emmett–Teller specific surface area ~500 m2g-1and stable specific capacity as an anode in Li-ion batteries, whereas Bogala graphite showed higher initial irreversibility and higher capacity as anode, exceeding the theoretical specific capacity of graphite. Both graphenes showed high electrical conductivity. The graphene,which exists in stacks of only a few layers, supposed to be 2–6 layers, would be promising for a vast variety of applications.     

Kinetic Parameters Estimation of MgO-C Refractory by Shrinking Core Model

Kinetics of oxidation of MgO-C refractories was investigated by shrinking core modeling of the gas-solid reactions taking place during heating the porous materials to the high temperatures. Samples containing 4.5~17 wt pct graphite were isothermally oxidized at 1000~1350℃. Weight loss data was compared with predictions of the model. A mixed 2-stage mechanism comprised of pore diffusion plus boundary layer gas transfer was shown to generally control the oxidation rate. Pore diffusion was however more effective, especially at graphite contents lower than 10 wt pet under forced convection blowing of the air. Model calculations showed that effective gas diffusion coefficients were in the range of 0.08 to 0.55 cm2/s. These values can be utilized to determine the corresponding tortuosity factors of 6.85 to 2.22. Activation energies related to the pore diffusion mechanism appeared to be around (46.4±2) kJ/mol. The estimated intermolecular diffusion coefficients were shown to be independent of the graphite content, when the percentage of the graphite exceeded a marginal value of 10.     

Effect of atomic structure on preferential oxidation of alloys:amorphous versus crystalline Cu-Zr

The effect of structural order in the parent alloy substrate on the oxidation kinetics and oxide phase evolution was investigated for the thermal oxidation of amorphous Cu_(33at.%)Zr_(67at.%) and crystalline CuZr_2 alloys of identical compositions in the temperature range of 200–250?C. It was found that, besides the strong preferential oxidation of Zr in both alloys, the lack of structural order in the amorphous Cu_(33at.%)Zr_(67at.%)alloy results in much slower oxidation kinetics, as well as in distinctly different microstructures of the oxide overgrowth and its Zr-depletion zone in the wake of the ZrO_2 overlayer growth front. The experimental findings can be rationalized on the basis of the strikingly different atomic mobilities of Cu, Zr and dissolved O in the amorphous and crystalline alloys, which also results in different nucleation barriers for crystalline oxide nucleation. The thus obtained knowledge on the underlying oxidation mechanisms provides new and profound insights into the surface engineering of metallic alloys.     

Formation and Oxidation Behavior of Al—Cu—Fe Quasicrystal

Al62.5Cu25Fe12.5 alloy was prepared by arc melting .It was found that the formation of quasicrystalline phase is related to the condition of annealing,such as temperature and duration.Weight gain of Al-Cu-Fe quasicrystal during the oxidation at 700 and 800℃ in dry air was measured by means of thermal balance.The oxidation kinetics showed that the quasicrystal has good oxidation resistance.Only α-Al2O3 was formed on Al62.5Cu25Fe12.5 quasicrystal.The surface morphologies of Al-Cu-Fe quasicrystal after isothermal oxidation for different times were observed.     

Oxygen Permeation and Stability of Ce0.8Gd0.2O2-δ-PrBaCo2-xFexO5＋δ Dual-phase Composite Membranes

Dual-phase membranes of 60 wt% Ce0.8Gd0.2O2-δ-40 wt% Pr Ba Co2exFexO3 d（0 x 2） were prepared by a combined citrate and ethylene diamine tetraacetic acid（EDTA） complexing method. X-ray diffraction（XRD）results revealed the good chemical compatibility between ion-conducting phase CGO and electron-conducting phases PBC2 xFxO after sintering in air. The Fe ionic dopant had a significant effect on the phase structure stability and oxygen permeability under CO2 atmosphere, which was confirmed by XRD, thermogravimetrye differential scanning calorimetry（TGeD SC）, scanning electron microscopy（SEM） and oxygen permeation experiments. CGOeP BC0.5F1.5O dual-phase membrane demonstrated a stable oxygen permeation flux of2.71x10-7mol cm 2s 1with 50 mol% He/CO2 as the sweep gas at 925 C, and this value was much higher than that of perovskite-type membranes. The rate-limiting step in the oxygen permeation process changed from the bulk diffusion to the surface oxygen exchange when the CGOeP BC0.5F1.5O membrane thickness decreased to 0.8 mm or less. Due to the high oxygen permeation fluxes and the excellent structural stability under CO2 atmosphere, the CGOeP BC0.5F1.5O membrane is a great potential candidate material for separating oxygen from air in the oxy-fuel combustion techniques.     

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al2O3 scale.     

Processing of Ceramic Based Nanocomposite Using α-Al2O3 Powder and FeCl2 Solution as Starting Materials

Alumina-iron nanocomposite powders were prepared by a two-step process. In the first step, α-Al2O3-FeCl2 powder mixture was formed by mixing α-Al2O3 powders with FeCl2 solution followed by drying. In the second step, the FeCl2 in the dry power mixture was selectively reduced to iron particles. A reduction temperature of 750℃ for 15 min in dry H2 was chosen based on the thermodynamic calculations. The concentration of iron in FeCl2 solution was calculated to be 20 vol. pct in the final composite. Two techniques were used to produce composite bulk materials. The Al2O3 nanocomposite powders were divided to two batches. The first batch of the produced mixture was hot pressed at 1400℃ and 27 MPa for 30 min in a graphite die. To study the effect of oxygen on the Al2O3/Fe interface bonding and mechanical properties of the composite, the second batch was heat treated in air at 700℃ for 20 min to partially oxidize the iron particles before hot pressing. Characterization of the composites was undertaken by conventional density measurements, X-ray diffractometry （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electron probe micro analysis （EPMA）. The suggested processing route （mixing, reduction and hot pressing） produces ceramic-metal nanocomposite much tougher than the pure Al2O3. The fracture strength of the produced Al2O3/Fe nanocomposite is nearly twice that of the pure Al2O3. The presence of spinel phase, FeAl204, as thick layer around the Fe particles in the Al2O3 matrix has a detrimental effect on interfacial bonding between Fe and AI203 and the fracture properties of the composite.     

Investigation of La9:33Si6O26 Oxygen Ionic Conductor

La9.335i6O26 oxygen ionic conductor was synthesized by solid state reaction method. Its structure was deter- mined by single-crystal X-ray diffraction analysis at room temperature. The results showed that La9.33Si6O26 oxide has the apatite structure with space group P63/m. AC impedance measurements indicated that the oxides sintered in nitrogen have much higher conductivity than those sintered in air. The effects of grain boundaries on the conductivity were discussed.     

Thermal Growth and Nanomagnetism of the Quasi-one Dimensional Iron Oxide

Quasi-one dimensional iron oxide nanowires with flat needle shape were synthesized on the iron powders by a rather simple catalyst-free thermal oxidation process in ambient atmosphere. The characterization by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman and high-resolution transmission electron microscopy (HRTEM) revealed that these nanostructures are single crystalline α-Fe2O3. The various dimensions with 40-170 nm in width and 1-8 μm in length were obtained by tuning the growth temperature from 280 to 480℃. A surface diffusion mechanism was proposed to account for the growth of quasi-one dimensional nanostructure. The typical α-Fe2O3 nanowires synthesized at 430℃ had a reduced Morin temperature TM of 131 K in comparison with their bulk counterpart. The coercivitis Hc of these nanowires are 321 and 65 Oe at 5 and 300 K, respectively. The temperature of synthesis also has important effects on the magnetic properties of these nanowires.     

Enhanced oxidation resistance of Mo-12Si-8.5B alloys with ZrB_2 addition at 1300℃

Mo-12Si-8.5B and Mo-12Si-8. B-1.0 wt%ZrB_2 alloys were fabricated using mechanical alloying, followed by hot-pressing. Both alloys exhibited uniform microstructure, with the Mo_3 Si and Mo_5SiB_2 phases distributing dispersedly in the α-Mo matrix. Mo-12Si-8.5B-1.0 wt%ZrB_2 showed a finer-grained microstructure than Mo-12 Si-8.5 B alloy owing to the addition of ZrB_2. The results of isothermal oxidation tests at 1300℃ in air revealed that Mo-12Si-8.5B and Mo-12Si-8.5B-1.0 wt%ZrB_2 alloys initially suffered a transient stage with high mass loss due to the volatilization of MoO_3, and then achieved a steady stage owing to the formation of a protective borosilicate scale on the alloy surface. Especially, the transient stage of Mo-12Si-8.5B-1.0 wt%ZrB_2 alloy was shortened to be less than 300s, and the mass loss of this stage was reduced by at least 88% compared with that of Mo-12Si-8.5B alloy, indicating a significant improvement in the oxidation resistance. The addition of ZrB_2 not only resulted in a continuous borosilicate scale quickly covering the entire base alloy during the transient stage, but also improved the protectiveness of the borosilicate scale of the steady stage by bringing out a large number of ZrO_2/ZrSiO_4 particles embedded discontinuously in the borosilicate scale, which effectively restricted the inward diffusion of oxygen by acting as diffusion barriers and decreased the thickness of inner oxide layers in particular.     

Characterization of the Protective Surface Films Formed on Molten AZ91D Magnesium AHoy in SO2/Air Atmospheres in a Sealed Furnace

Surface films that formed on molten AZ91D magnesium alloy in SO_2/air cover gases at 680 ℃ in a sealed furnace were characterized by scanning electron microscopy,energy dispersive spectroscopy,X-ray diffraction and Auger electron spectroscopy.It is revealed that the film formed on molten AZ91 D alloy surface in cover gas with high air content can prevent the molten AZ91 D alloy from oxidation and ignition.The surface film contained three elements,namely magnesium,oxygen and sulfur,and was mainly composed of MgO and MgS.The properties of the film depended on air content in the cover gas and holding time.Thermodynamic calculation showed that MgSO_4 was the stable phase,and it was concluded that the formation of MgSO_4was important for the formation of the protective surface film in SO_2/air atmospheres.     

Plasma Post Oxidation of Plasma Nitrocarburized SKD 61 Steel

Plasma nitrocarburizing and plasma oxidizing treatments were performed to improve the wear and corrosion resistance of SKD 61 steel.Plasma nitrocarburizing was conducted for 12 h at 540℃in the nitrogen, hydrogen and methane atmosphere to produce theε-Fe-（2-3）（N,C） phase.The compound layer produced by plasma nitrocarburising was predominantly composed ofε-phase,with a small proportion ofγ′-Fe-4 （N,C） phase. The thickness of the compound layer and the diffusion layer are about 10μm and about 200μm,respectively. Plasma post oxidation was performed on the nitrocarburized samples with various oxygen/hydrogen ratio at constant temperature of 500℃for 1 h.The very thin magnetite （Fe-3O-4） layer of 1-2μm in thickness on top of the compound layer was obtained.Anodic polarization test revealed that plasma nitrocarburizing process contributed a significant improvement of corrosion resistance of SKD 61 steel.However,the corrosion characteristics of the nitrocarburized compound layer was deteriorated by oxidation treatment.