High-temperature Oxidation Experiments Research Articles

High-temperature oxidation resistance and wear properties of functionally graded CrHfNbTaTiN high-entropy nitride coating on Ti alloy

High-entropy nitrides have emerged as promising alternatives to traditional protective coatings due to their excellent comprehensive properties. However, there is still a long-standing technical challenge in the design and manufacture of hard and high-temperature stable high-entropy nitride coatings. This study utilized first-principles calculations to evaluate the thermodynamic stability between high-entropy nitrides and corresponding high-entropy metals, as well as titanium alloy substrates, confirming the rationality of the structural design of the functional gradient high-entropy ceramic coating. Subsequently, a double-glow plasma surface alloying technique were conducted to prepare a CrHfNbTaTiN functional gradient high-entropy ceramic coating on titanium alloy, followed by high-temperature cyclic oxidation experiments and high-temperature ball-on-disk wear tests. The research demonstrates that the gradient composition coating exhibits strong interfacial bonding, with an adhesion strength of around 40 N. It maintains excellent oxidation resistance in environments below 700 °C and displays good high-temperature wear resistance. This study contributes to providing insights and guidance for the further development of high-temperature, high-hardness, and wear-resistant materials.

Temperature dependence mechanism of high-temperature oxidation of transition metal silicide MoSi2

Transition metal silicides represented by MoSi2 have excellent oxidation resistance and are widely used as high-temperature anti-oxidation coatings in hot end components of power equipment. However, the mechanism of temperature-dependent growth of MoSi2 oxidation products has not been revealed. Therefore, this study investigated the formation characteristics of oxide film and silicide-poor compound on MoSi2 at temperatures of 1000 °C–1550 °C through high-temperature oxidation experiments, combined with microscopic Raman spectroscopy, scanning electron microscope, and x-ray diffraction (XRD) characterizations. The result showed that MoSi2 underwent high-temperature selective oxidation reactions at 1000 °C–1200 °C, forming MoO2 and SiO2 oxide film on the substrate. As the oxidation temperature increased to 1550 °C, after 100 h of oxidation, along with the disappearance of MoO2 and the phase transformation of SiO2, a continuous Mo5Si3 layer with a thickness of approximately 47 μm was formed at the SiO2–MoSi2 interface. Thermodynamics and kinetic calculations further revealed the mechanism of temperature-dependent growth of oxidation products (MoO2 and Mo5Si3) during high-temperature oxidation process of MoSi2. As the temperature increased, the diffusion flux ratio of O and Si decreased, leading to a decrease in oxygen concentration at the interface and promoting the growth of the Mo5Si3 layer. Its thickness is an important indicator for evaluating the oxidation resistance of MoSi2 coatings during service. This study provides experimental and mechanistic insights into the temperature-dependent growth behavior of Mo5Si3 during the high-temperature oxidation of MoSi2 coating, and provides guidance for predicting the service life and improving the oxidation resistance of silicide coatings.

High-Temperature Cyclic Oxidation Behavior and Microstructure Evolution of W- and Ce-Containing 18Cr-Mo Type Ferritic Stainless Steel.

Due to the recurrent starting and stopping operations of automobiles during service, their engines' hot ends are continually subjected to high-temperature cyclic oxidation. Therefore, it is crucial to develop ferritic stainless steels with better high-temperature oxidation resistance. This study focuses on improving the high-temperature cyclic oxidation performance of 18Cr-Mo (444-type) ferritic stainless steel by alloying with high-melting-point metal W and the rare earth element Ce. For this purpose, a high-temperature cyclic oxidation experiment was designed to simulate the actual service environment and investigate the high-temperature cyclic oxidation behavior and microstructure evolution of 444-type ferritic stainless steel alloyed with W and Ce. The oxide structure and composition formed during this process were analyzed and characterized using scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS) and electron probe X-ray micro-analyzer (EPMA), in order to reveal the mechanism of action of W and Ce in the cyclic oxidation process. The results show that 18Cr-Mo ferritic stainless steel alloyed with W and Ce exhibits an excellent resistance to high-temperature cyclic oxidation. The element W can promote the precipitation of the Laves phase between the matrix and the oxide film, and the small-sized Laves phase can inhibit the interfacial diffusion of oxidation reaction elements and prevent the inward growth of the oxide film. The element Ce can refine oxide particles and reduce the thickness of the oxide film. CeO2 particles within the oxide film can serve as nucleation sites for the formation of oxide particles from reactive elements, and they also contribute to pinning the oxide film, thereby enhancing its adhesion.

A new insight into the mechanism of Ce enhancing high temperature oxidation resistance of hot-formed steel

In this paper, the effect of Ce addition on high temperature oxidation behavior of hot-formed steel was studied by high temperature oxidation experiment. The structure and phase composition of surface oxide layer of specimens with different Ce content were systematically analyzed by SEM, EDS, XPS. The results showed that the addition of Ce promoted the formation of a dense oxide layer during the initial oxidation stage, and effectively weaken the oxidation degree of the steel surface. Meanwhile, the Ce2O3 particles segregation in Si–Cr rich phase reduced the concentration of vacancy defects in oxide layer and enhanced the adhesion between oxide layer and substrate. After Ce addition, the surface oxidation resistance of hot-formed steel is significantly improved.

High-temperature steam oxidation study of irradiated FeCrAl defueled specimens

Post irradiation examinations (PIE) were performed on irradiated iron-chromium-aluminum (FeCrAl) specimens. These FeCrAl specimens were fabricated at the US Department of Energy's Oak Ridge National Laboratory (ORNL). The experimental setup involved subjecting FeCrAl cladding, along with UO2 pellets, to irradiation in the Idaho National Laboratory Advanced Test Reactor (ATR). In parallel, the FeCrAl alloy tubing without UO2 pellets was irradiated at ORNL's High Flux Isotope Reactor (HFIR). After irradiation, the ATR-irradiated rodlet was transported to an ORNL hot cell, where it was sectioned into multiple samples for the PIE and severe-accident testing. The sectioning process revealed that the fuel was not bonded to the cladding and could be easily detached from sectioned cladding slices. Microstructural analysis of the fuel cross sections demonstrated no significant interaction between the fuel and the cladding. Additionally, high-temperature steam oxidation tests on defueled cladding segments showed minimal oxygen uptake even at 1200 °C. The ATR-irradiated specimens began to exhibit signs of enhanced oxidation upon reaching a temperature of 1300 °C. Furthermore, enhanced oxidation was observed on the inner surface of the ATR-irradiated FeCrAl specimen, which had been subjected to 1300 °C for a duration of 1 min. By contrast, high-temperature steam oxidation experiments indicated that the HFIR-irradiated FeCrAl cladding provided good thermal stability when exposed to 1300 °C for up to 4 h. Comparative analysis encompassing the oxidation behavior of the ATR-irradiated fueled FeCrAl, HFIR-irradiated unfueled FeCrAl, and unirradiated FeCrAl suggests that the fuel–cladding interaction, although not visible via standard microscale electron microscopy measurements, may accelerate the deterioration of FeCrAl cladding in beyond-design-basis accident scenarios.

Effect of Sc2O3 doping on YSZ TBCs: Morphologies, phase composition, mechanical properties, and high-temperature oxidation resistance

Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology, and then a high-temperature oxidation experiment was carried out at 1100 °C. Whereupon the morphologies, phase composition, and mechanical properties of YSZ ceramic coating and ScYSZ ceramic coating after different high-temperature oxidation time, as well as the high-temperature oxidation resistance of YSZ TBCs and ScYSZ TBCs, were comparatively studied. The results showed that after high-temperature oxidation, ScYSZ ceramic coating presented slighter cracking and exhibited better sintering resistance. After high-temperature oxidation, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was still a single T' phase, and their interplanar spacing became larger; nevertheless, the interplanar spacing of ScYSZ ceramic coating was smaller than that of YSZ ceramic coating. After high-temperature oxidation, ScYSZ ceramic coating had higher hardness, smaller elastic modulus, and smaller fracture toughness. In addition, ScYSZ TBCs exhibited better high-temperature oxidation resistance according to the morphology, equivalent thickness, high-temperature oxidation kinetics, and high-temperature oxidation thermodynamics of TGO.

Improving the high-temperature oxidation resistance of CrN coating by gradient deposition considering internal stress effects

In this study, two different CrN coatings were deposited on the surface of Zr-4 via a multi-arc ion-plating method using gradient and non-gradient deposition processes. The effects of two deposition processes on the high-temperature oxidation resistance and internal stress of the coatings were investigated. The microstructure and phase structure of the coatings were characterized by scanning electron microscopy, X-ray diffraction (XRD), and energy dispersive spectrometry. The high-temperature oxidation resistance of the specimens was investigated by performing thermal shock experiments and simulated loss-of-coolant accident high-temperature steam oxidation experiments. The residual stresses of both coatings during deposition were determined by XRD analysis. In addition, the thermal stresses of both coatings during the high-temperature oxidation tests were modeled and calculated. Under long-term high-temperature oxidation conditions, the gradient-deposited CrN coating exhibited improved high-temperature oxidation resistance performance than that exhibited by non-gradient-deposited CrN coating. In addition, the residual stresses of gradient-deposited CrN coating during deposition and thermal stresses during high-temperature oxidation were significantly lower than those of the non-gradient-deposited CrN coating. The results obtained herein confirm the promising potential of gradient-deposited CrN coating as an accident-tolerant fuel cladding material.

Effect of refractory element addition on high temperature oxidation process of complex concentrated alloy coatings

The sluggish diffusion effect of complex concentrated alloys (CCAs) affords them considerable application potential in the field of high temperatures. In this study, the role of refractory elements in the high temperature oxidation process of denary complex concentrated alloy coatings (CCACs, namely NiCoCrFeAl0.8Si0.1(MoNbTaW)x, where x is 0.025, 0.05, 0.1 and 0.15) prepared via laser cladding was investigated. Four groups of the CCACs with different refractory contents were subjected to 100 h of high-temperature oxidation experiments, and the oxidation products and microscopic characteristics of the samples were observed via XRD and SEM, respectively. Atomic-scale characterisation and analysis of the oxidised samples were performed using TEM and XPS technology to explore the diffusion behaviour of elements in the high-temperature process. Results indicate that the solid solution limit of the CCACs is obtained by regulating the topological instability of the lattice matrix via the addition of refractory elements. Consequently, when 0.1 > x > 0.025, the coating is a body-centred cubic (BCC) + face-centred cubic (FCC); when x = 0.1, the coating maintains a single-phase FCC structure; and when x > 0.1, the coating changes to an FCC + BCC duplex structure. The x0.1 group exhibits the smallest relative strain among all coatings, which is conducive to maintaining the structural stability of the face-centred cubic at high temperatures and delaying atomic migration within the lattice. Furthermore, the oxidation kinetics curve of the CCACs with a single- phase FCC exhibits low Kp values. In a single- FCC lattice, the atom diffusion rate through volume slows down and grain boundary diffusion decreases. Considering the Weibull modulus for the hardness statistical value after coating oxidation, the modulus of single-FCC phase CCACs is also higher than others. This study has the potential to contribute to the application of high-temperature thermal protection for CCACs.

Preparation of VZrHfNbTa High-Entropy Alloy-Based High-Temperature Oxidation-Resistant Coating and Its Bonding Mechanism.

Ultra-high Temperature Oxidation-Resistant Alloys (UTORAs) have received a lot of attention due to the increased research demand for deep space exploration around the world. However, UTORAs have the disadvantages of easy oxidation and chalking. So, in this study, a UTORAs is prepared by hot-press sintering on VZrHfNbTa (HEA: High Entropy Alloys can generally be defined as more than five elements by the equal atomic ratio or close to the equal atomic ratio alloying, the mixing entropy is higher than the melting entropy of the alloy, generally forming a high entropy solid solution phase of a class of alloys.) a substrate coated with hafnium. The bonding mechanism, resistance to high-temperature oxidation, and hardness of the sample tests are carried out. The results show that zirconium in the matrix will diffuse into the hafnium coating during the high-temperature sintering process and form the HfZr alloy transition layer, the coating thickness of the composite is about 120 μm, and the diffusion distance of zirconium in the hafnium coating is about 60 μm, this transition layer chemically combines the hafnium coating and the HEA substrate into a monolithic alloy composite. The results of high-temperature oxidation experiments show that the oxidation degree of the hafnium-coated VZrHfNbTa composite material is significantly lower than that of the VZrHfNbTa HEA after oxidation in air at 1600 °C for 5 h. The weight gain of the coated sample after oxidation is 56.56 mg/cm2, which is only 57.7% compared to the weight gain of the uncoated sample (98.09 mg/cm2 for uncoated), and the surface of the uncoated HEA shows obvious dents, oxidation, and pulverization occurred on the surface and interior of the sample. In contrast, the coated composite alloy sample mainly undergoes surface oxidation sintering to form a dense HfO2 protective layer, and the internal oxidation of the hafnium-coated VZrHfNbTa composite alloy is significantly lower than that of the uncoated VZrHfNbTa HEA.

Impact of Si concentration on microstructure and high temperature oxidation behavior of IN718 coating by plasma cladding: An experimental and first principle study

To improve the properties of IN718 coatings, Si-modified IN718 coatings with an equiaxed structure and enhanced high-temperature oxidation resistance were fabricated by plasma cladding in this paper. The addition of Si aggravated the precipitation of the Laves phase and Cr/Nb segregation. First-principles calculations revealed that the increase in repulsive forces between NbFe atoms and CrNi atoms is the essential reason for the promotion of Cr/Nb segregation. The hardness of IN718 increases with Si addition, mainly attributed to grain refinement and dislocation strengthening. High-temperature oxidation experiments revealed that the 3Si-IN718 coating generated a double oxide layer at 1000 °C, with Cr2O3 in the external layer and SiO2 in the internal layer; therefore, the oxidation resistance of the coating was greatly enhanced. Si addition significantly reduced the internal oxidation of Al/Nb and cracking of the oxide layer. However, Si has a modest contribution to oxidation resistance, and the oxide layers were all Cr2O3 at 900 °C. According to the thermodynamic calculations, the elemental activity of Si only increased significantly above 900 °C, which may be the reason for the different results at different experimental temperatures. Calculations of surface adsorption illustrate that Si doping enhances the ability of the Ni-Fe-Cr surfaces to bind O atoms and raises the potential barrier for O atoms to diffuse into the sublayer, thereby enhancing the resistance of the IN718 coating to high-temperature oxidation.

Study on crack healing performance of Al2O3/SiCw/TiSi2 new ceramic tool material

In consideration of the crack sensitivity in ceramics, a new alumina-based ceramic material with excellent crack healing ability was successfully fabricated by adding SiC whiskers and TiSi2 particles. Different sizes of cracks were prefabricated on the surface of the material with the diamond indenter, and corresponding crack healing behavior was studied by high temperature oxidation experiments in a muffle furnace under air. The influences of healing temperature, healing time, and crack size on the surface morphology and flexural strength of the material were investigated in detail. When the crack width was 0.68 μm, the appropriate temperature and duration time for crack healing and full-strength recovery were 800 °C for 60 min or 1000 °C for 30 min, respectively. The maximum crack size capable for self-healing of the ceramic material was 2c = 250–300 μm and w = 0.68 μm, and the corresponding healing condition was of 800 °C/60 min. The crack healing of Al2O3/SiCw/TiSi2 ceramics was primarily driven by the high temperature oxidation reactions of Ti5Si3 and SiCw. Ti5Si3 was a new phase formed during sintering, and its formation could be described by thermodynamically feasible reactions. The new crack healing mechanism was revealed deeply, including both the local crack bridging generated by the in-situ development of TiO2 and the liquid phase filling of SiO2 intermediate oxides (Si2O3 and SiO). This research conclusion has great significance for the repairing and remanufacturing of ceramic materials with crack defects.

Rotational coating of zinc on Nd-Fe-B powders with improved antioxidation performance

A newly developed rotational coating process has been used to coat Nd-Fe-B flake-shape powders with Zn. A uniform layer of Zn as thin as 1.5 μm can be coated on the Nd-Fe-B powders under a proper rotational coating condition. The coercivity decreases by about 4.23% and magnetization decrease by only 1.64% after Zn coating. The antioxidation, on the other hand, is greatly improved. The large mass increase during oxidation starts at 400 °C for the Zn-coated powders, 150 °C higher than that of the uncoated powders. In addition, during the high-temperature oxidation experiment from 30 °C to 450 °C, the mass of uncoated powders increases by 2.82%, while the mass of Zn-coated powders only increases by 0.40%. These results prove that the rotational coating is a very effective way to enhance the oxidation resistance of Nd-Fe-B powders, owing to the high-quality coating layer and its good isolation ability for the matrix. The process is also low-cost and easy to achieve mass production, making it a promising technique for producing high-performance Nd-Fe-B powders.

The microstructure and high-temperature oxidation resistance of Si-rich Mo-Si-B coatings prepared by ultrasonic vibration assisted laser cladding

Si-rich Mo-Si-B powders (Mo-62Si-5B at%) were deposited on Nb-Si based alloy by laser cladding assisted with ultrasonic vibration. The obtained coatings mainly consist of MoSi2 and (Mo, X)5Si3 (X = Nb and Ti). With the introduction of ultrasonic vibration during laser cladding, the lath-shaped MoSi2 phase transforms into cellular gradually. Meanwhile, the discrepancy on the microstructure of the top region and the bottom region is eliminated gradually, indicating that the cavitation and acoustic streaming of ultrasonic vibration not only refines the microstructure, but also homogenizes the element and phase distribution in the coatings. High-temperature oxidation experiment at 1250 ℃ were conducted to assess the oxidation resistance of the coatings. Compared with Nb-Si alloy, the coated specimens have much lower oxidation weight increase. Furthermore, with the assistance of ultrasonic vibration, the oxidation resistance of the coating is enhanced further. The oxide scales on the coatings consist of two layers: an upper amorphous aluminoborosilica with TiO2 particles dispersed and a lower crystalline SiO2 layer. With the introduction of ultrasonic vibration, the microstructure of the coatings is refined, resulting in more phase and grain boundaries in the coatings. Therefore, the diffusion of the elements to the surface is accelerated and the protective oxide scale can be formed quickly.

Effects of thermal and mechanical properties of Hf and Ta-doped NiCoCrAlY bonding layer alloys on the high-temperature oxidation resistance of thermal barrier coatings

The MCrAlY bonding layer alloy was prepared by vacuum melting, and the alloy composition was NiCoCrAlY and NiCoCrAlYHfTa. The high-temperature cyclic oxidation experiment was carried out in air at 1050∘C until it failed. The results show that Hf and Ta have a significant influence on the oxidation resistance of the alloy. The doped specimens have longer oxidation lifetimes compared to the undoped samples. The study also found that the Hf would diffuse out and combine with O to form HfO2 in the thermally grown oxide (TGO) layer, which would improve the adhesion and reduce the wrinkles of the oxide layer. Ta in the doped specimens hinders the diffusion of Al and reduces the oxidation rate, while improving the thermal mismatch. Therefore, the specimens doped with both Hf and Ta have the highest oxidation lifetimes, though the oxidation lifetime was 10 times longer than that of the undoped specimens. It is indicated that the doping of Hf and Ta is an important method to improve the oxidation lifetime of thermal barrier coatings (TBCs).

Effect of rhenium on corrosion behavior of electrodeposited Co-Cu alloy coating at room temperature and high temperature

Rhenium (Re) is a high-performance refractory metal. Adding rhenium to the alloy can significantly improve the overall performance of the alloy. This paper proposes a simple way to prepare rhenium alloy coatings. The microstructure and XRD results show that the addition of rhenium has a certain effect on the alloy coating. Co-Cu-Re alloy coating had a corrosion current density of 3.2 10−6 (A/cm2) in a solution of 3.5% NaCl, which was 3.4 times lower than that of the mild steel substrate and 2.4 times lower than that of the Co-Cu alloy coating. The charge transfer resistance (Rct) is 4512 (Ω·cm2), which is 6 times larger than that of the mild steel substrate and 1.1 times larger than that of the Co-Cu alloy coating. The electrochemical test results show that the addition of rhenium element can enhance the corrosion resistance of the alloy coating. In the high-temperature oxidation experiment, the oxidation rate constant of the rhenium-containing coating was calculated to be the smallest compared with the mild steel substrate and the Co-Cu alloy coating, and it was observed that the addition of rhenium can effectively inhibit the interdiffusion of elements, indicating that the rhenium-containing coating has stronger high-temperature oxidation resistance. Finally, through the XPS characterization of the alloy coating, the mechanism by which rhenium protects the alloy coating in a high-temperature environment is explained. The addition of rhenium has a broader significance in improving the overall performance of the alloy coating, and will promote its wide application in various fields.

Research on the oxidation resistance and ultra-high frequency thermal fatigue shock failure mechanisms of the bilayer and multilayer nano-coatings on cemented carbide tools

The multilayer nano-coating shows excellent thermal stability and oxidation resistance which could be attributed to the multielement composition and multilayer structure. It is widely used in the machining of hard-to-cut materials, such as titanium alloy. The formation of serrated chip during cutting process of Ti-6Al-4V would result in the ultra-high frequency thermal fatigue shock on the coating surface, and this could induce the thermal fatigue failure of coating. In this study, the ultra-high frequency (20 kHz) thermal fatigue shock testing was developed to investigate the thermal fatigue failure mechanisms of the bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating. It was achieved by using the pulsed laser system to irradiate the coating surface under different shock cycles, which could produce alternating thermal stress on coating surface. This creates a close simulation of the actual working conditions in high-speed machining of Ti-6Al-4V with serrated chip produced. Moreover, the high temperature oxidation experiments were performed to study the oxidation resistance of coating. The failure mechanisms of coating/substrate system under the constant thermal loads and dynamic thermal loads were compared and analyzed. The cutting performance of related coated tools was evaluated after turning Ti-6Al-4V. The correlation between the thermal fatigue shock resistance of coatings at ultra-high frequency and the wear behavior of coated tools was built. It can be concluded that the multilayer TiSiN/TiAlN nano-coating showed better properties of the resistance to thermal fatigue shock in machining Ti-6Al-4V.

A comparison of thermo-oxidation kinetic measurements of Zircaloy-4 in light and heavy water steam

High temperature oxidation experiments of Zircaloy-4 have been previously reported using various gaseous environments including light water steam and synthetic air, but no experiments involving heavy water steam oxidation have been reported in the open literature. Understanding the reaction kinetics in heavy water steam is important for CANDU nuclear generating stations, which are moderated and cooled by heavy water. The long standing assumption has been that the oxidation behaviour is the same for both environments; the objective of this work is to validate that assumption. The oxidation behaviour of Zircaloy-4 was investigated in this work under light water steam and heavy water steam to compare the reaction kinetics at temperatures ranging from 500 ∘C to 1250 ∘C to simulate an accident scenario. For this work, a NETZSCH 449 F1 Simultaneous Thermal Analyzer was used to perform thermogravimetric measurements of Zircaloy-4 specimens to measure the mass changes and oxidation reaction rates. A comparison was then drawn and benchmarked to available data in the open literature for similar experiments. Additionally, Scanning Electron Microscopy and Energy-dispersive X-Ray Spectroscopy analyses were performed for a few specimens to analyse and identify oxidation behaviour over the specified temperature range.

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO2-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO2 reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al2O3 layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO2 doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.

High-temperature oxidation resistance of ceramic coatings on titanium alloy by micro-arc oxidation in aluminate solution

Ceramic coatings with aluminum titanuate (Al2TiO5) were prepared on Ti–6Al–4V alloy using pulsed bi-polar Micro-arc Oxidation (MAO). The micromorphology and phase composition of the micro-arc-oxidition ceramic coatings on the titanium alloy were characterized by X-ray powder diffraction (XRD), and scanning electron microscopy (SEM) respectively. The results revealed that the distinct discharge channels and pores on the surface of the micro-arc-oxidition coatings appeared, and these channels were connected in the molten state. The electrolyte concentration was inversely proportional to the coating hardness; additionally, the coating prepared with sodium aluminate and sodium hypophosphite concentrations of 4 ​g/L and 0.5 ​g/L, respectively, was the most refined after high-temperature sintering, and it was demonstrated to better prevent oxidation. Increasing the electrolyte concentration coincided with fluctuating coating thermal shock resistance. The thermal shock resistance of the coating respectively prepared with sodium aluminate, and the sodium hypophosphite concentrations of 4 ​g/L and 0.5 ​g/L was the highest. Additionally, the high-concentration coatings performed significantly better than the low-concentration coatings. The oxidation resistance of the coating samples was also significantly higher than that of the TC4 titanium alloy substrate. The adhesion strength between the coatings and the substrate with and without the sealing treatment was measured by tensile tests. Then, the high-temperature oxidation performance of the coating samples with and without the sealing treatment was investigated by conducting a high-temperature oxidation experiment at a calcinating temperature of 500 ​°C. The results indicate that the adhesion strength between the coatings and substrate was high for the as-prepared and sealed micro-arc oxidation samples regardless of whether they were calcined. The high-temperature oxidation mass increase curves for the sealed and unsealed coating samples calcined at 500 ​°C for 500 ​h revealed that the high-temperature-oxidation-induced mass increase of the coating samples sealed with a sodium silicate solution was much lower than that of the titanium alloy substrate. Thus, the sealing treatment significantly improved the high-temperature oxidation resistance of the TC4 titanium alloy. Lastly, the high-temperature oxidation behavior at 500 ​°C was analyzed and discussed.

Microstructure and properties of Mg–Ca–Zn alloy for thermal energy storage

Mg-based alloys with a suitable phase change temperature, high phase change latent heat and excellent compatibility with Fe are currently the research hotspot in the field of latent heat storage. Herein, based on the Pandat software calculations, five different Mg–Ca–Zn alloys have been designed by changing Ca or Zn content. It's found that Mg–15Ca–30Zn alloy exhibits a suitable phase change temperature, a high heat storage value, a small solid-liquid coexistence temperature range, and leading to ease of processing according to the results of Pandat solidification simulation. Subsequently, the microstructure, thermophysical properties and high-temperature oxidation properties of Mg–Ca–Zn alloys were systematically investigated. The experimental results revealed that the Mg–Ca–Zn alloys are mainly composed of primary α-Mg phase, Mg2Ca phase, MgZn phase and Ca2Mg6Zn3 phase. The melting enthalpy of Mg–15Ca–30Zn alloy (eutectic alloy) is the highest among these five alloys due to high volume of α-Mg + Mg2Ca + MgZn + Ca2Mg6Zn3 eutectic structure and Ca2Mg6Zn3 ternary phases with high binding energy, whereas Mg–15Ca–40Zn alloy exhibits a large heat storage value per unit volume. The results of high-temperature oxidation experiments indicate that Mg–15Ca–30Zn and Mg–15Ca–40Zn alloys have good oxidation resistance at 350 °C due to the dense oxide layer of CaO + ZnO with good protection, and the oxidation resistance of Mg–15Ca–40Zn alloy is better than that of Mg–15Ca–30Zn alloy at 400 °C due to the increase of highly dense ZnO (1 < PBR <2) content. The obtained results confirm the suitability of the studied metal alloys for latent heat energy storage.