Poor Oxidation Resistance Research Articles

FORMATION OF Ge-Y MODIFIED SILICIDE COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY

Nb-Si base ultrahigh temperature alloy is a promising candidate for high tempera- ture applications due to its high melting point, high strength and relatively low density at elevated temperatures. However, the poor oxidation resistance of this alloy has still be a major barrier to its high temperature applications. Ge and Y modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy have been prepared successfully by co-depositing Si, Ge and Y in order to improve its high temperature oxidation resistance. The structure, constituent phases and formation process of the coatings have been revealed using XRD, SEM and EDS. The results showed that the coatings had obvious layered structure which was composed of a (Nb, X)(Si, Ge)2 (X represents Ti, Cr and Hf elements) outer layer and a (Ti, Nb)5(Si, Ge)4 inner layer. The concentration of Ge in the coating increased with increase in the distance away from the coating surface. Compared with the single Y modified silicide coating, the addition of Ge did not change the coating structrue, but reduced the absorption and deposition of Si and retarded the coating growth. The coatings prepared at 1250 for different time (0—8 h) had similar structures and the coating growth kinetics followed a parabolic law.

PREPARATION AND OXIDATION BEHAVIOR OF ALU-MINIZED COATING ON TC4 TITANIUM ALLOYVIA FRICTION STIR LAP WELDING METHOD

The poor oxidation resistance of titanium and its alloys limits their use at elevated temperature.To solve this problem,a large amount of surface engineering techniques to produce antioxidation coatings on titanium alloys were utilized.In the present research,a solid-state processing method of friction stir lap welding(FSLW) was used to fabricate Al cladding or coating on the surface of TC4 titanium alloy,with lower cost and simpler operation which are still desirable for the coating preparation on titanium alloys.The lap joint structure was smartly transformed into an interface structure of coating.In this work,the Al coating with a thickness of 500μm was fabricated via multi-pass FSLW process using a slight plunge depth of tool-pin.The mechanical milling was used as a posttreatment for a suitable coating thickness.The oxidation testing was conducted at 700℃under air atmosphere.The microstructure,chemical composition analysis and phase determinations were performed using SEM,EDS and XRD methods.The evolutions of interlayer under the high-temperature oxidation procedure were detailed.It was found that the Ti-rich interlayer,with a thickness of 60μm, had a typical structure of mixed layers.The sufficient Al coating thickness played an important role in preventing the inter diffusion of oxygen,while the oxidation and melting phenomenon of Al coating occurred.The abundant Al content in the Al coating upper the interlayer,with a significant thickness, also benefited to the anti-oxidation performance and forming of the beneath Ti/Al interlayer at a rare oxygen environment due to the obstacle effect of the Al layer to oxygen diffusion,which exerted a main role in oxidation prevention for titanium alloy.As a result,the phases of outer surface were mainly Al_2O_3,Al_2Ti and Al_3Ti.The gradient distribution characteristic of Ti/Al interface structure occurred after the oxidation testing.

High temperature oxidation behavior of W–Cr–Nb Alloys in the Temperature Range of 800–1200 °C

A comparative study of high temperature oxidation behavior of the (W1−xCrx)90Nb10 (x=0.3, 0.5 and 0.6) alloys has been carried out at 800°C, 1000°C and 1200°C in static air. Microstructural study of the alloy samples conventionally sintered from the nanocrystalline elemental powders of W–Cr–Nb alloys shows the presence of a mixture of W-solid solution (Wss) and Cr2Nb phases. Comparison of the results of isothermal and cyclic oxidation tests shows the damage to be greater during the latter tests. The oxidation resistance during exposure at 800°C, 1000°C and 1200°C is found to be the highest for the (W0.4Cr0.6)90Nb10 alloy. Formation of WO3 appears to be responsible for poor oxidation resistance of (W0.7Cr0.3)90Nb10 alloy at 1200°C. On the other hand, growth of continuous Cr2WO6 scale appears to have significant role in protection against oxidation for the other two alloys at 1200°C. The formation of oxide scales containing Cr2WO6+NbWO5.5 and Cr2WO6+Nb2O5.3WO3 is found to be responsible for protection against oxidation at 800°C and1000°C, respectively.

Microstructural study of high-temperature Cr–Ni–Al–Ti alloys supported by first-principles calculations

Refractory metals present a potential for development of future high-temperature structural materials due to their high melting temperature and good high-temperature strength. However, their poor high-temperature oxidation resistance, creep resistance, and low fracture toughness at low temperatures have to be addressed. The microstructure of two Cr–Ni–Ti–Al alloys is characterized using XRD, LOM, SEM, and TEM. The microstructure of both alloys is comprised of very dilute Cr solid solution grains, intergranular L21 Ni2AlTi phase, and fine NiTi-rich B2 precipitates dispersed in the Cr-rich grains. The small spherical precipitates have a well-defined orientation relationship with the Cr-matrix, and the lattice mismatch between the precipitates and the Cr-alloy matrix is accommodated by the interface dislocations. In order to explain measured composition of L21 and B2, the site preference in these phases is further studied using the first-principles density functional theory (DFT) method at the low temperature limit. The theoretical calculations predict that Al prefers substituting for the Ti site in B2 NiTi. Structure analysis reveals that Al substituting for Ti results in the formation of strong Al–Ni bonds that have much shorter bond lengths than the Al substitution for the Ni site. The predicted Al site or Cr site preference is consistent with established Al–Ni–Ti ternary experiments. As for the L21-Ni2AlTi phase, DFT calculations predict that Cr prefers substitution with the Ni site over Al or Ti sites. This is related to the smallest relaxation effect and formation of stronger Al–Cr and Ti–Cr bonds that have much shorter nearest neighbor bond length compared to other substitution scenarios.

Enhanced ductility and oxidation resistance of Zn through the addition of minor elements for use in wide-gap semiconductor die-bonding materials

Pure Zn is one of the best die-attachment candidates for use in next-generation wide-gap semiconductor power devices operating at temperatures up to 300°C. However, it has certain drawbacks when used at high operating temperatures: poor ductility and limited oxidation resistance. In this study, we investigated the effect of adding minor elements – Ca, Mn, Cr, and Ti – in improving Zn ductility and oxidation resistance. This addition significantly reduced the grain size of the microstructure, thus improving the tensile strength and elongation of pure Zn. In addition, the minor elements addition significantly improved oxidation resistance of pure Zn. Consequently, because of higher ductility and oxidation resistance, the interconnection ability of Zn alloys as die-attachment candidates was significantly enhanced.

Hydrothermal co-doping of boron and phosphorus into porous carbons prepared from petroleum coke to improve oxidation resistance

Porous carbons (PC) have been extensively used in a wide variety of industries. However, their poor oxidation resistance, especially at high temperature (e.g.>370°C), has severely limited their applications such as catalysts under high temperature. Hydrothermal doping technology, as a newly emerged efficient approach for chemical modification, was for the first time introduced towards PC modification. The PCs used in this work were, by KOH activation, prepared from petroleum coke. The modified porous carbons (MPC) were achieved using hydrothermally co-doping boron and phosphorus into the PC followed by a heating treatment. The results revealed that the onset oxidation temperature of the MPC was greatly improved from 370°C to 600°C. The residual weight percent for MPC, an immediate and most critical indication of the oxidation resistance, was observed as high as 49% at 850°C, while that of the blank sample was <10% at an even lower temperature of 600°C. The achieved remarkable improvements on onset oxidation temperature and residue weight, substantiated by TGA, FTIR and Raman spectroscopy, confirmed the success of our unprecedented combination of co-doping and hydrothermal technologies to improve the oxidation resistances of PC materials.

Evaluation of a new Cr-free alloy as interconnect material for hydrogen production by high temperature water vapour electrolysis: Study in cathode atmosphere

For economic and ecological reasons, hydrogen is considered as a major energetic vector for the future. Hydrogen production via high temperature water vapour electrolysis (HTE) is a promising technology. A major technical difficulty related to high temperature water vapour electrolysis is the development of interconnects working efficiently for a long period. Working temperature of 800 °C enables the use of metallic materials as interconnects. High temperature corrosion behaviour and electrical conductivity of a new Cr-free Fe–Ni–Co alloy were tested in cathode atmosphere (H2/H2O) at 800 °C. The alloy exhibits a poor oxidation resistance but an excellent ASR parameter, as a result of the formation of a highly-conductive Cr-free surface spinel layer. Moreover, the role of water vapour and hydrogen was discussed and a diffusion mechanism in cathode atmosphere could be suggested.

Oxidation Behaviors of Two and Three Phase Mo-Si-B Alloys via Si Pack Cementation

Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. In order to resolve the poor oxidation resistance of the alloy system, in this study, the oxidation behaviors of Si diffusion coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of the constituent of the phase combination of Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the produced MoSi2 phase, as a result of the coatings, give an excellent oxidation resistance at prolonged high temperature exposure in air. The oxidation behaviors of uncoated and Si coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Electron Beam Welding Studies on Nb-Hf-Ti Refractory Alloy

Niobium, a refractory metal is mainly used as alloying addition in steels, superalloys, titanium and copper alloys. Being lightest refractory metal with high melting temperature, niobium based alloys are developed for high temperature applications of aerospace systems. However, poor oxidation resistance at elevated temperature limits its fabrication options and also requires oxidation protection in service. Among the fabrication methods, electron beam welding has been found to be a realistic option and the same has been studied in the present work. The paper presents the details of the Electron Beam Welding study carried out in developing the welding procedure for this alloy. An attempt has been made to correlate the weldment microstructure with the mechanical properties.

Influence of Scandium on Magnesium and its Structure-Property Correlation

Magnesium alloys are the most demanded lightweight structural materials for different engineering applications such as aerospace, automobile, electronics, etc. However, the high temperature properties of Mg alloys are not comparable with its competitor, the Al alloys. Mg alloys are not recommended beyond 120°C due to their poor creep and oxidation resistance. In order to improve the high temperature properties of the magnesium alloys, rare earth containing Mg alloys were developed. Among these alloys, Mg-Sc alloys were found to be very interesting which exhibits better high temperature properties. In the present work, magnesium-scandium alloy was fabricated through liquid metallurgy route under inert cover. The alloy was characterized by optical microscopy, X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and hardness testing. The microstructural analysis reveals the α-Mg phase and the distribution of fine Mg-Sc intermetallic. It is observed from the DTA that the melting point of the base alloy has got enhanced by the addition of Sc. There is also an appreciable improvement in the hardness by the addition of Sc.

Production of MgO-X Refractory Material with Cellular Matrix by Colloidal Processing

The production in the siderurgy and foundry industry has changed considerably in the past years. Despite the new technologies and process, the use of magnesia carbon refractory remains constant. Namely the magnesia carbon refractory is widely used due low-priced cost, high refractivity, excellent corrosion resistance, thermal shock resistance, low thermal expansion, high slag penetration resistance and low wettability. The main disadvantages of use magnesia carbon refractories are the high carbon oxidation susceptibility and the formation CO and CO2 gases. As a result, tonne of CO and CO2 are expelled to the atmosphere. The use of open cell carbon-foam magnesia composite for refractory application can offer a substitute for the traditional refractory material since the high carbon content can be minored and the low mechanical strength and poor oxidation resistance of these materials can be improved.

The high-temperature oxidation behavior of Co-Re-Cr-based alloys

Co-Re-Cr-based model alloys have been developed for high-temperature applications beyond 1,200°C. The purpose of the present investigation is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. The first generation of this class of alloys showed a rather poor oxidation resistance during exposure to laboratory air. As a consequence of the lack of protectiveness of the oxide layer, the vaporization of rhenium oxide takes place during oxidation. It has been found that Si stabilizes the Cr2O3 scale, enhancing the oxidation resistance significantly.

Influence of Zr Addition on Oxidation Behavior of TiAl-Based Alloys

The alloys based on γ-TiAl have attractive properties as light weight heat-resisting material. The poor oxidation resistance is one of the drawbacks to limit their application. This results from the high growth rate of TiO2 in the scale and the spallation of the oxide scale during cooling. In the present study, the influence of Zr addition on the oxidation behavior of TiAl has been investigated with emphasis on scale adherence. The oxidation behavior of Ti-45Al (in mol %), Ti-45Al-6Nb alloys containing 0 - 0.2% Zr, Ti-Nb and Ti-Zr binary alloys were investigated. The oxide scale was analyzed by conventional methods including X-ray diffractometry, SEM and EPMA. The addition of Nb to TiAl results in a remarkable decrease in mass gain, while the Zr addition to TiAl results in a slight decrease. The Ti-45Al-Nb alloy with Zr shows scale spallation after long term exposure. Concerning the binary alloys, the addition of Zr to Ti more than 20 mol% accelerates the oxide growth significantly. The XRD for Ti-45Al-6Nb-0.2Zr after oxidation shows the formation of Zr oxides and Ti-Zr complex oxides in the scale. The scale spallation observed in this alloy is due to the enrichment of Zr at scale / alloy interface and formation of Zr oxide in the scale.

High-Temperature Oxidation Behaviour of Co-Re-Cr-Based Alloys: Limitations and Ways to Improve

Ni-based alloys are the most widely used alloy system in high-temperature applications. However, the use of Ni-based alloys is limited to temperatures below 1100°C. The experimental Co-Re-Cr-based alloys are promising for high-temperature applications for service temperatures beyond 1200°C. A complete miscibility in the Co-Re system allows to steadily elevate the melting point of the system with the rhenium content. In addition, rhenium takes the role as solid solution strengthening element. In the case of Co-based alloys, the oxidation resistance at high temperature is mainly based on the formation of a protective Cr2O3 scale. The purpose of the present investigations is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. Earlier investigations of the authors showed a rather poor oxidation resistance during exposure to laboratory air. Oxidation at 1000°C in air yielded an oxide scale that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. As a consequence of the lacking protectiveness of the oxide layer the vaporization of rhenium oxide takes place and hence leads to a rapid loss of Re. The aim of recent investigations is to study the effect of Si on the high-temperature oxidation behaviour of Co-Re-Cr alloys by means of kinetic and microstructural examinations. It was found that Si stabilizes the Cr2O3 scale, enhancing the oxidation resistance significantly. Hence, the synergetic effect of chromium with silicon could be considered as an encouraging perspective to improve the oxidation resistance of Co-Re-Cr alloys. Apart from that, other concepts to enhance the oxidation resistance of this class of materials are discussed, such as the formation of a borosilicate layer or protective Al2O3 scale on the substrate surface.

Oxidation Behaviors of Pure Ti Thermal Plasma Spray Coated Mo-Si-B Alloys

Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. The unstable MoO3 phase is naturally produced when the alloys were exposed at low and /or high temperature in an air atmosphere. In order to resolve the poor oxidation resistance of the alloy system, several attempts have been made via surface coatings and/or component additions. In this study, the oxidation behaviors of the Ti powder thermal spray coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of precursor Ti coatings on Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the Ti powder was tightly bonded and reacted with the surface of the substrate, and TiO2 layer was formed at the outer surface of the coated Ti layer as a result of oxidation exposure. The oxidation behaviors of pure elemental component coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

First-Principles Study of Nb Doping Effect on the Diffusion of Oxygen Atom in γ-TiAl

The effect of Nb doping on the diffusion of oxygen in γ-TiAl is studied by the use of first-principles. Our calculated results showed that the diffusion barriers of oxygen in γ-TiAl are increased by the Nb doping. And the effect of Nb doping dies down as the distance between the oxygen atom and doped Nb atom increases. Accordingly, the improvement of the poor oxidation resistance of γ-TiAl by Nb doping may be caused by suppressing the diffusion of oxygen atom in γ-TiAl.

Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene

This study evaluated the tradeoffs amongst fatigue crack propagation resistance, wear resistance, and oxidative stability in a wide variety of clinically-relevant cross-linked ultra-high molecular weight polyethylene. Highly cross-linked re-melted materials showed good oxidation and wear performance, but diminished fatigue crack propagation resistance. Highly cross-linked annealed materials showed good wear and fatigue performance, but poor oxidation resistance. Moderately cross-linked re-melted materials showed good oxidation resistance, but moderate wear and fatigue resistance. Increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance. This is the first study to simultaneously evaluate fatigue crack propagation, wear, oxidation, and microstructure in a wide variety of clinically-relevant ultra-high. The tradeoff we have shown in fatigue, wear, and oxidation performance is critical to the material’s long-term success in total joint replacements.

Effect of CaO composition on oxidation and burning behaviors of AM50 Mg alloy

Oxidation and burning behaviors were studied for CaO added AM50 Mg composites which were manufactured by conventional melting and casting processes without SF 6 protective gas. CaO added AM50 Mg composites show the stable oxidation resistance, while AM50 Mg alloys show the poor oxidation resistance. The effects of CaO addition on the burning resistance under ambient, nitrogen and dry air atmospheres were examined for CaO added AM50 Mg composites. With increasing CaO addition, the burning temperature increases under ambient, nitrogen and dry air atmospheres. The burning temperatures of small test specimen under all conditions greatly increase even by 0.3% CaO (mass fraction) addition into AM50 Mg alloys.

Microstructure evolution and wear properties of bulk MoSi 2 fabricated by field activated sintering

Field activated sintering (FAS) was employed to fabricate dense bulk MoSi 2 from Mo and Si powders. During the FAS process, Si was melted first, and then Mo was dissolved leading to a precipitation of MoSi 2. A eutectic reaction of MoSi 2–Mo 5Si 3 resulted from a combined effect of Mo–Si exothermic reactions and field induced Joule heating. With hyper-stoichiometric amount of Si (Mo/Si = 1:2.06), the FAS MoSi 2 is free of Mo 5Si 3 phase. Subsequently, an un-lubricated sliding wear test of MoSi 2 against carbon steel was conducted. Both normal load and temperature have crucial points related to a transition from mild to severe wear. Sliding speed and relative density have a negative effect on wear rate. In addition, at room temperature the wear is with a protective layer composed of oxide and transferred substance, whereas poor oxidation resistance leads to spalling of the layer and brittle fracture at an elevated temperature up to 500 °C.

Effect of dysprosium addition on the cyclic oxidation behaviour of CoNiCrAlY alloy

The cyclic oxidation behaviour of a Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without the addition of 0.2 wt.% dysprosium was investigated at 800 and 1100 °C in static laboratory air. The Dy-containing alloy showed a faster θ- to α-alumina transformation and significantly less weight gain than Dy-free alloy. Under cyclic oxidation at 1100 °C, Dy addition produced a continuous and protective Al 2O 3 scale. The Dy-free alloy exhibited poor oxidation resistance. Scale spallation led to the development of a complex oxide scale and internal precipitation: (Al,Cr) 2O 3 on the surface, followed by a Al 2O 3 layer, then (Al,N) precipitates alone beneath the external scale.