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The components of gas-turbine engines operating in marine environments are highly susceptible to hot corrosion, which is typically classified as Type II (650-750 °C) and Type I (900-950 °C) hot-corrosion attack. Even though hot-corrosion has been widely investigated in the last 50 years, several critical questions remain unanswered and new ones have emerged based on recent observations that, in part, are associated with the increasing complexity of the alloy systems and the sulfate-deposit chemistries. The present work is focused on the Type II hot-corrosion mechanism for Co-base alloys. Observations for a CoCrAlY model alloy (isothermally exposed at 700 and 800 °C under different atmospheres, including: air and O2 with 100 and 1000 ppm SO2) suggest the rapid dissolution of Co (as Co-oxide) is not the controlling factor in the degradation mechanism, as was proposed by Luthra, since the γ-phase which is richer in Co, is not attacked as significantly as the Al-rich β-phase. To the contrary, it is suggested that Al (and Cr) is (are) the element(s) which is (are) removed first. A modified interpretation of the Type II hot-corrosion mechanism is proposed, which is based on the synergistic fluxing model developed by Hwang and Rapp. [ABSTRACT FROM AUTHOR]

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without NaSO deposit was examined at 700 °C in air and in SO-containing atmospheres. When coated with NaSO those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides. [ABSTRACT FROM AUTHOR]

Ni-Al coating alloys, which are commonly used in gas turbine engines operating in marine environments, are highly susceptible to hot corrosion attack. The effect of alloy composition and exposure conditions on the development of a protective alumina scale, which is important for the hot corrosion resistance of the alloy, and how they affect the transition of alumina from the θ to the α polymorph have been evaluated. A series of Ni-Al model alloys with a base composition of Ni-36 at.% Al, and 5 at.% additions of Cr, Pt and Si were exposed in dry air and in air-10%HO at 900 °C. The presence of water vapor in the gas led to higher oxidation rates and retarded the θ- to the α-AlO transformation. The oxidation behavior of the alloys and the alumina polymorph which formed differed depending on the alloying element considered. Additions of Cr accelerated the θ to α transformation, while Pt and Si retarded it. [ABSTRACT FROM AUTHOR]

A numerical study on 2D natural convection in cylindrical cavities during the sterilization of liquid foods was performed. The mathematical model was established on momentum and energy balances and predicts both the heating dynamics of the slowest heating zone (SHZ) and the lethal rate achieved in homogeneous liquid canned foods. Two sophistication levels were proposed in viscosity modelling: 1) considering average viscosity and 2) using an Arrhenius-type model to include the effect of temperature on viscosity. The remaining thermodynamic properties were kept constant. The governing equations were spatially discretized via orthogonal collocation (OC) with mesh size of 25 × 25. Computational simulations were performed using proximate and thermodynamic data for carrot-orange soup, broccoli-cheddar soup, tomato puree, and cream-style corn. Flow patterns, isothermals, heating dynamics of the SHZ, and the sterilization rate achieved for the cases studied were compared for both viscosity models. The dynamics of coldest point and the lethal rate F0 in all food fluids studied were approximately equal in both cases, although the second sophistication level is closer to physical behavior. The model accuracy was compared favorably with reported sterilization time for cream-style corn packed at 303 × 406 can size, predicting 66 min versus an experimental time of 68 min at retort temperature of 121.1 ℃. [ABSTRACT FROM AUTHOR]

In many laboratory hot corrosion (HC) experiments and other experiments involving sulfur commercial catalytic converters are used to catalyze the $${\text{SO}}_{ 2} + { 1}/ 2 {\text{O}}_{ 2} \to {\text{SO}}_{ 3}$$ reaction. The conversion rate of SO to SO highly depends on the catalyst used and thus the HC results, especially in low-temperature HC conditions, may vary significantly depending on the catalyst selected. Different elements present in the catalyst affect the SO to SO conversion and with it the degree of HC induced attack. Modern automotive catalysts have low activity for the oxidation of SO to SO. The conversion of SO to SO can be enhanced by modifications of the catalyst, e.g. by using a Pt-gauze or dip-coating the catalyst in a FeO solution. [ABSTRACT FROM AUTHOR]

Structural evolution during the synthesis of B2-NiAl intermetallic compound by mechanical alloying of equiatomic elemental mixtures was studied by Rietveld analysis, DSC and HTXRD. The lattice parameter, crystallite size, microstrain, amount of phase and ordering of the B2 phase were monitored as a function of milling time. Formation of the B2-NiAl phase shows a sigmoidal behavior, which suggests that Johnson-Mehl-Avrami nucleation and interface-controlled growth are the responsible mechanisms in the transformation. Almost complete transformation (~ 97 mol%) was obtained after 25 h of milling. A specific phase transformation sequence during milling was not absolutely determined, however, the sequence Ni + Al → NiAl → NiAl → B2-NiAl was identified by HTXRD. This sequence was confirmed by DSC. The transformation temperature of the B2-NiAl phase and the presence of additional intermetallic compounds show a direct dependence on the Ni-Al layer spacing. Using a production-scale Simoloyer horizontal Attritor Mill, the presence of NiAl phase was observed prior to the full synthesis of B2-NiAl. [ABSTRACT FROM AUTHOR]

The θ-AlO → α-AlO phase transformation was investigated in thermally grown oxide formed on β-(Ni,Pt)Al bond coats during isothermal exposures at 900-1,200 °C in an argon atmosphere stream with the O partial pressure of 1 × 10 atm. Local curve fitting was used to evaluate the evolution of the parabolic rate constant, k, using a general kinetic model ( t = A + BΔ m + CΔ m), during the first 5 h of oxidation. All net mass-gain curves exhibited deviations from the classic parabolic model, Δ m = k t; a steady state regime was established only after 4 h of exposures, except for the sample oxidized at 1,100 °C. [ABSTRACT FROM AUTHOR]

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of α-AlO scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to α-AlO in addition to formation and growth of dense α-AlO is required to ensure full protection of the coating and base material over long periods. [ABSTRACT FROM AUTHOR]

The effect of three principal, independent, high-velocity oxygen fuel (HVOF)-processing parameters on the properties of NiCoCrAlY coatings deposited using commercial powders is reported here. The design of experiments (DoE) technique at a two-level factorial and a central composite rotatable design was used to analyze and optimize the HVOF spraying process. The deposition parameters investigated were (1) fuel flow, (2) oxygen flow, and (3) stand-off distance. The effect of these processing variables was evaluated using selected responses, including porosity and oxide content, residual stresses, and deposition efficiency. Coatings with low porosity as well as with low residual stress were obtained using high fuel-rich conditions at a stand-off distance between 250 and 300 mm. At shorter and longer stand-off distances, respectively, either excessive flattening of splats or un-molten condition occurred, resulting in high levels of porosity and residual stress. The response surface, the empirical relationships among the variables, and the response parameters allowed the selection of optimum deposition parameters and the improvement of coating properties. [ABSTRACT FROM AUTHOR]

In the present study, a commercial Fe-based metallic glass (MG) powder was deposited by laser cladding on 304 stainless steel substrates. The main purpose of this work is to study a range of processing conditions that allow the fabrication of well-established deposits. Experiments were conducted to evaluate the effect of the main processing variables on the mechanical properties and phases obtained in the coatings. XRD and SEM characterization was also performed to evaluate the phases and morphology of the deposits obtained. The results suggest that dense Fe-based MG cladded coatings require high thermal energy for deposition, and high dilutions were obtained in final deposits; however, the cooling rate is not enough to obtain 100% amorphous deposits. Although the MG structure observed in the precursor powders was not kept in the final deposits, the precipitation of crystalline phases resulted in very high hardness (> 700 HV0.1), which improved wear resistance against the substrate material. The processing of this alloy by laser cladding is promising when coatings with high hardness and wear resistance are required and can be a good alternative to the thermal sprayed process in which these alloys are conventionally processed. [ABSTRACT FROM AUTHOR]

In the present work, the structural, magnetic, and theoretical analysis of the Fe–Si alloy prepared by melting and heat-treated was performed. The ordered FeSi simple cubic (sc) phase was obtained by melting and heat treatment processes as determined by X-ray diffraction. The presence of the superstructure peak in the (312) crystalline direction confirms the high structural order reached. Using Mössbauer spectrometry (MS), a paramagnetic behavior with quadrupole splitting of SQ = 0.53 ± 0.02 mm/s was obtained. Although MS indicates paramagnetic behavior, vibrating sample magnetometry (VSM) showed ferromagnetic behavior with a coercive field of 25 Oe, associated with a small amount of Fe3Si segregations detected by scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). Using density functional theory (DFT), the crystalline structures for the simple cubic (sc) Fe50Si50, face-centered cubic (fcc) Fe3Si, and body-centered cubic (bcc) Fe3Si crystalline structures were simulated. Electron total density values were calculated to perform energetic comparisons with magnetic behavior. The electronic structures and magnetic properties of the Fe–Si alloys in different stoichiometric configurations were calculated by CASTEP, which employed first principles DFT. The density of states (DOS) and band structures were calculated together with magnetic properties. The results showed that the high value of the polarization spin for the fcc and bcc structures is due to the contribution of the high amount of Fe atoms above the Si atoms, which is reflected in an increase in the magnetic moment and that their presence could explain the ferromagnetic behavior observed by VSM. [ABSTRACT FROM AUTHOR]

The present contribution aimed at exploring the HVOF deposition process of bioactive multilayered HAp/titania composite coatings on Ti-6Al-4V substrates. These coatings can be regarded as functionally graded as the weight fraction of the constituent phases gradually changes layer by layer, from pure titania at the substrate-coating interface to pure HAp at the outer surface of the coating. Microstructural investigations were carried out on the graded coatings using scanning electron microscopy coupled with EDS microanalysis to confirm that the compositional gradient met the initial specifications. On the other hand, the in vitro properties of the coatings were studied in simulated body fluid (SBF) for periods ranging from 1 to 14 days. Moreover, mechanical characterization of both as-sprayed and soaked coatings in SBF was carried out by performing Vickers microhardness measurements through their cross section. The apparent interfacial toughness (KCa) of HAp/titania coatings, which is representative of their interfacial crack initiation resistance, was determined by performing indentation tests at the coating-substrate interface. Fracture toughness of both pure hydroxyapatite and functionally graded coatings was also calculated. The results revealed that the graded coatings produced in this work exhibited good reactivity and mechanical stability after being immersed in SBF indicating their potential for biomedical applications. [ABSTRACT FROM AUTHOR]

This paper deals with the deposition of LaZrO (LZO) and LaAlO (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness ( K ) of the annealed coating was only 1.04 MPa m. [ABSTRACT FROM AUTHOR]