Continuous Al2O3 Layer Research Articles

Insight into the oxidation behavior and excellent internal oxidation and nitridation resistance of Fe71.5-x(Ni, Cr, Ti)28.5Alx complex concentrated alloys by advanced characterization

For precipitate-strengthened alloys, the decomposition of the precipitates at the subsurface during high-temperature oxidation is detrimental to mechanical properties. In this study, we adjusted Al concentrations in Fe71.5-x(Ni, Cr, Ti)28.5Alx (x = 8, 12, and 16 at. %) complex concentrated alloys (CCAs) to precipitate elongated cuboidal L21 with higher density and Al concentration, as well as reduced inter-precipitate spacing. This is more conducive to the rapid diffusion of Al to the matrix and grain boundaries, promoting the formation of a continuous and compact Al2O3 layer with random crystallographic orientation at 800 °C in air. This not only inhibits the further decomposition of the L21 phase but also avoids internal oxidation and nitridation.

Revealing the environmental-assisted degeneration mechanism of an Al-containing dual-phase high entropy alloy in supercritical water

A dual-phase high entropy alloy (DP-HEA) containing 7.82 wt% Al was synthesized to withstand corrosive supercritical water. The long-term corrosion and quasi-static tensile tests reveal superior resistance to such environmental-assisted degeneration. The outstanding corrosion resistance can be attributed to the high aluminum content in B2 phase, which forms a continuous Al2O3 layer for protection, while the FCC phase is selectively corroded. Under external stress, stress corrosion cracking (SCC) initiates from surface oxide and penetrates uncompact mixed Cr/Al-rich oxide in deteriorated FCC regions or corroded phase interface, though network-shaped B2 can inhibit SCC propagation, which supports the brittle film rupture model.

Long-term oxidation resistance of CrAl(Si)N coatings deposited on TiAl-based alloy at 950 °C

CrAl(Si)N coatings were deposited on Ti-48Al-2Cr-2Nb alloy by arc ion plating. Long-term cyclic oxidation behaviors of the CrAl(Si)N-coated alloy were investigated at 950 °C. The oxidation resistance of the Cr0.4Al0.5Si0.1N coating was significantly better than that of the Cr0.5Al0.5N coating. The addition of Si to the CrAlN coating promoted the formation of an almost continuous inner Al2O3 layer in the oxide scale. The continuous and dense Cr2O3/Al2O3 oxide layer acted as a diffusion barrier and retarded the inward diffusion of oxygen and outward diffusion of the coating elements. However, interdiffusion between the CrAl(Si)N coatings and the alloy was serious and a thick interdiffusion zone formed.

The influence of cyclic creep on the long-term oxidation behavior of a directionally-solidified nickel-based superalloy

This work investigates the effect of cyclic creep on the long-term oxidation of a directionally-solidified nickel-based superalloy DZ445 up to 1800 h. The stress application of cyclic creep refines the oxides, increases the oxidation weight gain, reduces the oxidation rate exponent, and decreases the activation energy. It enhances the content of Al2O3 and TiO2 in the outermost layer and in the sub-outer layer of CrTaO4 in the film. It delays the formation of the continuous innermost layer of Al2O3 and the sub-inner layer that is rich in Al2O3. The change of phase content and type in the oxidation layers could be ascribed to the diffusion kinetic effect of cyclic creep stress in the superalloy.

High-Temperature Hot Corrosion Resistance of CrAl/NiCoCrAlY/AlSiY Gradient Composite Coating on TiAl Alloy

TiAl alloys are used in high-temperature components such as the turbine blades of aeroengines because of their excellent properties. However, TiAl alloys are prone to thermal corrosion when in near-ocean service. In order to solve this problem, a hot-corrosion-resistant CrAl/NiCoCrAlY/AlSiY gradient composite coating was prepared on the surface of the TiAl alloy. The phase composition and morphology of the coating were analyzed. Hot corrosion tests of the traditional NiCoCrAlY coating and CrAl/NiCoCrAlY/AlSiY gradient composite coating on a TiAl substrate were performed. The samples were coated with 75%Na2SO4 + 25%NaCl salt film and treated at 950 °C for 100 h, and the corrosion products were analyzed. The results indicate that compared with the TiAl substrate and traditional NiCoCrAlY-coated samples, the composite coating showed better hot corrosion resistance, only slightly cracking, and no corrosion loss occurred. This is mainly because the continuous Al2O3 layer can effectively resist the damage caused by the melting reaction in salt, and the Cr-rich layer can not only slow the mutual diffusion of elements but also generate a good corrosion resistance chromium oxide protective layer under serious corrosion. Moreover, the corrosion mechanism of the TiAl substrate, traditional NiCoCrAlY coating, and experimental composite coating was analyzed in detail.

Effects of nitrogen content on the phase composition and high-temperature steam oxidation resistance of Cr0.5Al0.5Nx (x = 0.33, 0.81, 1.23) coatings on Zry-4 alloy

Cr0.5Al0.5Nx (x = 0.33, 0.81, 1.23) coatings were deposited on Zry-4 alloy using magnetron reaction sputtering technique to investigate their application as accident-tolerant fuel materials. The effects of nitrogen (N) content on the phase composition and high-temperature steam oxidation resistance of the coatings were systematically investigated. With the increasing N content, the phase composition of the coatings is mainly transformed from Cr(Al) to the cubic-CrAlN phase, which may be due to the formation of a greater number of CrN bonds and AlN bonds. The high-N content of the Cr0.5Al0.5Nx coating can significantly enhance the high-temperature steam oxidation resistance of zirconium alloy. Compared to the bare Zr alloy, the weight gain oxidation of the Cr0.5Al0.5N1.23 coating is reduced by 45 % after oxidation at 1200 °C for 1 h. The oxide layer thickness of the bare Zry-4 alloy is as high as 350 μm, while the oxide layer thickness of the Cr0.5Al0.5N1.23 coating is only about 12 μm. The complex oxide layer structure and an internal continuous Al2O3 layer are formed, effectively preventing the inward diffusion of O2−. In addition, with the increase of N content in the coatings, the degree of interdiffusion between the coating and the substrate is weakened. The formation of α-Zr(N) layer in the Cr0.5Al0.5N1.23 coating significantly inhibits the interdiffusion between the coating and the substrate.

Synergism of Al and Si occurs on corrosion of Ti3Al1−xSixC2 MAX in the static Pb-Bi eutectic at 550 ºC

MAX phase ceramics attract increased attention for lead-cooled fast reactors due to their promising compatibility with lead-bismuth eutectic (LBE). This work systematically investigates the corrosion behaviors of five Ti3Al1−xSixC2 MAXs with Al/Si complex A component in static LBE at 550 ºC up to 3000 h. Based on the lattice structure, mechanical properties, surface morphologies, and elemental composition during corrosion, a synergism of Al and Si during oxidation and diffusion is found, which contributes to a continuous and well-attached Al2O3 layer with improved LBE corrosion resistance. Therefore, the multi-element design will bring a broad space for the applications of MAX ceramics.

Development of Self-Passivating, High Strength Ferritic Alloys for CSP and TES Application

The addition of aluminum to ferritic stainless steels can result in self-passivation by the formation of a compact Al2O3 top layer, which exhibits significantly higher corrosion resistance to solar salt compared to a Cr2O3 surface layer. The development and qualification of realistic experimental methods for fatigue testing under superimposed salt corrosion attack will enable safe component design. Salt corrosion experiments were carried out at 600 °C with and without mechanical fatigue loading at a novel, self-passivating trial steel, using “solar salt” (60 wt.% NaNO3 and 40 wt.% KNO3). Cyclic salt corrosion tests at 600 °C under flowing synthetic air (without mechanical loading) showed that self-passivation to molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFerSCR (Salt Corrosion Resistant) steel. A compact, continuous Al2O3 layer was formed on the surface of the model alloys with Al contents of 5 wt.% and higher. A distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix.

Effect of Solution Heat-Treatment on the Oxidation Resistance of Ni-Base Single-Crystal Superalloy

To clarify the effect of solution heat-treatment on the oxidation resistance of Ni-base single-crystal superalloy TMS-238, the evaluation of dendrite/inter-dendrite segregation of alloying elements in the as-cast and heat-treated samples, and its effect on cyclic oxidation resistance were investigated. Cyclic oxidation test results at 1100 °C for up to 150 cycles clearly showed that the as-cast samples with element segregations had lower oxidation resistance compared to heat-treated samples with homogeneous structure. Further, for the as-cast sample, rapid growth and spallation of oxide consisting of NiO, Cr2O3, and Al2O3 were observed around the dendrite core for 10 cycles of oxidation. Analysis of sub-surface on sample isothermally oxidized at 1100 °C for 10 minutes showed that rapid oxide growth is due to the formation of discontinuous Al2O3 layer at dendrite core with lower Al concentration. Furthermore, in this study, the threshold value of Al concentration and Gibbs energy for the formation of continuous Al2O3 layer were estimated and determined to be around 5.2 wt pct and − 556.6 ± 0.5 kJ/mol, respectively. This indicated that the solution heat-treatment for TMS-238 should be conducted above 1305 °C for exhibiting oxidation resistance at 1100 °C, to meet the threshold value within the whole region between dendrite and inter-dendrite.

Effect of Mn content on the high-temperature oxidation behaviors of Mn-substituted-for-Ni alumina-forming austenitic stainless steel

We studied high Mn-substituted-for-Ni AFA steel (Fe-14Cr-(6–12) Mn-10Ni-2.5Al-3Cu-Nb-C AFA steel) oxidation behaviors at 800 °C in air. (6–12) Mn AFA steel formed a three-layered scale: the outer layer is (Mn, Fe)-rich oxide, such as (Mn0·983Fe0.017)2O3, the intermediate is Cr-rich oxide, and the inner layer is Al2O3. (6–8) Mn-AFA steels formed a protective Al2O3 layer with stronger oxidation resistance than (10–12) Mn-AFA steels. The oxidation rate constants of 6Mn- and 8Mn-AFA steels are 1.7 × 10−3 and 2.4 × 10−3 mg2/(cm4·h), respectively, approximately 10 times higher oxidation resistance than that of 10Mn- and 12Mn-AFA steels. (10–12) Mn-AFA steels cannot form a continuous Al2O3 layer. The discontinuous Al2O3 layer allows the elements to diffuse outward, eventually forming Mn-Fe-Nb-rich oxide nodules on the surface of the steel. Mn concentrations with the potential to form protective alumina at 800 °C were identified.

Study on the oxidation resistance mechanism of self-healable NiAl coating deposited by atmospheric plasma spraying

A dense NiAl coating with low oxide content was deposited on the GH4202 superalloy substrate by atmospheric plasma spraying (APS) using carbon-containing Ni/Al composite powders. Compared with traditional APS NiAl coatings with high oxide content, this coating exhibits better long-term oxidation resistance at 900 °C in air. For the NiAl coating with low oxide content, it was found that the improved inter-splat bonding, due to limited oxide inclusions, promotes the formation of a continuous protective Al2O3 layer on the coating surface during oxidation. During long-term oxidation, the dispersion of oxide scales within the coating and element diffusion across splats result in the in-situ healing of unbonded inter-splat interfaces.

Effect of surface treatment and synergistic effect of O and S elements on hot corrosion behavior of GH738 superalloy

The hot corrosion behavior of GH738 superalloy after solution heat treated (SHT), room temperature machining (RTM) and low temperature burnishing (LTB) coated mixed salt (25% NaCl + 75% Na2SO4) on its surface was investigated at 700 °C. The results showed that the surface grain refinement of the LTB specimens and produced numerous multiple nano-twins, which was conducive to the diffusion of Cr and Al elements from the inside to the outside of the matrix. Dense and continuous Cr2O3 and Al2O3 layers were formed on the surface of the corrosion product layer and between the external corrosion layer and the surface of the matrix, respectively, which improved the hot corrosion resistance. The mechanism of the synergistic effect between O and S elements during corrosion process was found. The sulfide (SO3) reacted with the alloy elements in the matrix to form metal oxides and S, and this process promoted the diffusion of O element into the matrix. The O2 entering the internal corrosion layer and the O2 generated by the decomposition of SO3 together cause the partial pressure of oxygen in the matrix to rise, promoting the reoxidation of the metal sulfide to release S element. The oxygen in the matrix is consumed, resulting in the decrease of oxygen partial pressure. This leads to the reaction between S element and metallic elements with strong affinity to generate metal sulfide (such as TiS). In this process, O element promotes the diffusion of S element to the matrix.

Development of Self-Passivating, High-Strength Ferritic Alloys for Concentrating Solar Power (CSP) and Thermal Energy Storage (TES) Applications

Concentrating solar power (CSP) and thermal energy storage (TES) based on molten salts still lacks economic feasibility, with the material investment costs being a major drawback. Ferritic stainless steels are a comparatively cheap class of materials that could significantly contribute to cost reductions. The addition of aluminum to ferritic steel can result in self-passivation by forming a compact Al2O3 top layer, which exhibits significantly higher corrosion resistance to solar salt compared to the Cr2O3 surface layers typically formed on expensive structural alloys for CSP and TES, such as austenitic stainless steels and Ni-base super alloys. However, to date, no ferritic stainless steel combining Al2O3 formation and sufficient structural strength is available. For this reason, cyclic salt corrosion tests under flowing synthetic air were carried out on seven Laves phase-forming, ferritic model alloys (17Cr2-14Al0.6-1Nb2.6-4W0.25Si), using “solar salt” (60 wt. % NaNO3 and 40 wt. % KNO3). The Al content was varied to investigate the influence on the precipitation of the mechanically strengthening Laves phase, as well as the impact on the formation of the Al-oxide top layer. The W and Nb contents of the alloys were increased to examine their influence on the precipitation of the Laves phase. The salt corrosion experiments demonstrated that simultaneous self-passivation against a molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFerSCR (salt corrosion-resistant) steel. Microstructural examination unveiled the formation of a compact, continuous Al2O3 layer on the surface of the model alloys with Al contents of 5 wt. % and higher. Furthermore, a stable distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix, resulting in a combination of molten salt corrosion resistance and potentially high mechanical strength by a combination of solid solution and precipitation strengthening. These results show that high-strength ferritic alloys are suitable for use in CSP applications.

Enhanced high temperature oxidation resistance of discontinuous distributed TiBw reinforced TA15 titanium alloy by simple pack cementation method

Discontinuous distributed TiB whiskers reinforced TA15 titanium alloy (TiBw/TA15) is a potential high-temperature structural material but has poor oxidation resistance. In this paper, an aluminide coating was prepared by pack cementation to enhance the oxidation resistance of TiBw/TA15 at 800 °C. The influences of TiBw on the formation mechanism of the aluminide coating were revealed by density functional theory. The oxidation resistance mechanism of the aluminide coating was investigated by experimental and theoretical methods at both macroscopic and atomic levels. The good oxidation resistance was mainly attributed to the dense and continuous Al2O3 layer formed by the TiAl3 phase, which effectively prevented the diffusion of oxygen to the substrate.

The Effect of Dopants on Structure Formation and Properties of Cast SHS Alloys Based on Nickel Monoaluminide.

Alloys based on NiAl-Cr-Co (base) with complex dopants (base+2.5Mo-0.5Re-0.5Ta, base+2.5Mo-1.5Re-1.5Ta, base+2.5Mo-1.5Ta-1.5La-0.5Ru, base+2.5Mo-1.5Re-1.5Ta-0.2Ti, base+2.5Mo-1.5Re-1.5Ta-0.2Zr) were fabricated by centrifugal SHS metallurgy. The phase and impurity compositions, structure, mechanical properties, and the mechanism of high-temperature oxidation at T = 1150 °C were studied; the kinetic oxidation curves, fitting equations and parabolic rate constant were plotted. Al2O3 and Co2CrO4 were the major phases of the oxidized layer. Three layers were formed: I-the continuous Al2O3 layer with Co2CrO4 inclusions; II-the transitional MeN-Me layer with AlN inclusions; and III-the metal layer with AlN inclusions. The positive effect of thermo-vacuum treatment (TVT) on high-temperature oxidation resistance of the alloy was observed. The total weight gain by the samples after oxidative annealing decreased threefold (from 120 ± 5 g/m2 to 40 ± 5 g/m2). The phases containing Ru and Ti microdopants, which reduced the content of dissolved nitrogen and oxygen in the intermetallic phase to the values ∑O, N = 0.0145 wt.% for the base+2.5Mo-1.5Ta-1.5La-0.5Ru alloy and ∑O,N = 0.0223 wt.% for the base+2.5Mo-1.5Re-1.5Ta-0.2Ti alloy, were identified by transmission electron microscopy (TEM). In addition, with the significant high-temperature oxidation resistance, the latter alloy with Ti had the optimal combination of mechanical properties (σucs = 1644 ± 30 MPa; σys = 1518 ± 25 MPa).

The influence of water vapour on high-temperature oxidation of Al,Co,Cr,Ni-containing high entropy alloys under thermal shock conditions

The goal of this work is to determine the influence of water vapour on the corrosion properties of Al25Co25Cr25Ni25, Al20Co25Cr25Ni25Si5, Al20Co5Cr20Fe20Ni35 and Al10Co15Cr20Fe20Ni35 high entropy alloys under thermal shock conditions in synthetic air, air-10 %H2O and air-50 %H2O at 1000 °C. The oxidation kinetics suggest that H2O addition results in more significant scale spallation. Only Al25Co25Cr25Ni25 was capable of relatively insignificant mass loss, even at 50 %H2O, due to its capability of forming a more continuous Al2O3 layer. It can be concluded that 25 at% Al is required to ensure the appropriate protection in cyclic conditions against water vapour-containing corrosive environments at high temperatures.

Simultaneously improving mechanical properties and oxidation resistance of Ti-bearing high-entropy superalloys at intermediate temperature via silicon addition

Ti-bearing high-entropy superalloys (HESAs) often suffer from severe intergranular embrittlement and terrible oxidation degradation at intermediate temperatures. Here we showcase that minor Si addition can effectively mitigate the intergranular embrittlement and improve the oxidation resistance of the a (Ni2Co2FeCr)92Ti4Al4 HESA at 700 °C simultaneously. Experimental analysis revealed that the intergranular G phase induced by 2 at% Si addition can effectively suppress the inward diffusion of oxygen along grain boundaries at 700 °C, thus enhancing the tensile ductility of the alloy from ∼8.3% to ∼13.4%. Besides, the 2 at% Si addition facilitated the formation of a continuous Al2O3 layer during oxidation, contributing to a remarkable reduction in the growth rate of the oxide scale to a quarter of the Si-free HESA. Our results demonstrate that Si can be a favorable alloying element to design advanced HESAs with synergistically improved thermal-mechanical performance.

Effect of Si on the Oxidation Behaviors of Ti3 Al1-x Six C2 at 1000 °C.

In this work, Ti3 Al1-x Six C2 (x=0, 0.2, 0.4, and 0.6) with Al/Si solid solution structure are synthesized, and the effects of Si on their oxidation behaviors at 1000 °C are evaluated. The addition of Si not only contributes to the formation of Ti5 Si3 impurity but also affects the composition of the oxide scale. Particularly, the incorporation of Si in the TiO2 lattice is demonstrated, which alters the formation energy of the (110) plane in TiO2 , thus leading to the preferential growth of Si-doped TiO2 to dendritic congeries. Moreover, the Si addition is believed to affect mass transportation during the oxidation process, which accelerates the formation of a continuous Al2 O3 layer in the oxide scale. With an optimized Si content, the oxidation of Ti3 Al1-x Six C2 is restrained. However, with excess Si content, the continuity of the resulting Al2 O3 layer is destroyed, thus the oxidation rate rises again.

Effect of elements distribution on oxidation behavior of a nickel-based single crystal superalloy

The oxidation behavior of a fourth-generation nickel-based single crystal superalloy with different heat treatments were studied at 1100 °C in air. The specimens were divided into three groups, one as-cast group, and two heat-treated groups: solution treated and fully heat treated. The elemental distribution is mainly affected by the solution temperature, the higher the solution temperature, the more uniform the distribution of elements. Heat treatment can effectively reduce the difference in oxidation behavior between dendritic region and interdendritic region. However, due to the inability to form a continuous Al2O3 layer, the oxidation resistance of heat-treated specimens degrades.

Evolution of coating layers during high-temperature annealing and their effects on magnetic behavior of Fe(Si) soft magnetic composites

Low-loss Fe(Si) soft magnetic composites (SMCs) with atomic-layer-deposition coated layer were successfully prepared in this work. The continuous, compact and uniform Al2O3 layer is found to form tight bonding with the powder base. Evolution of the coating layers has been investigated under different annealing temperatures and closely linked to the magnetic performance of the composites. Results indicate that 1100 °C was the optimal annealing temperature, at which the Fe(Si) SMCs showed lowest core loss of 1237 mW/cm3 and the highest permeability of 99.7 simultaneously (100 mT/100 kHz). Integrity of the coating layer ensures the maximum grain growth and stress removal of powder particles at the highest possible temperature, so as to reduce the hysteresis loss. Formation of high resistivity oxides and silicates (Al2O3, Fe2SiO4 and 3Al2O3·SiO2) after annealing result in a low inter-particle eddy current loss. Besides, high temperature vacuum annealing is helpful to eliminate impurity atoms and decrease the anomalous eddy current loss. The improvement in effective permeability caused by high-temperature annealing is mainly attributed to the relaxation of residual stresses and the reduction of defects. While excessive temperature leads to the decomposition and destruction of the insulation layer, resulting in a significant increase in hysteresis loss, inter-particle and anomalous eddy current loss.