Oxidation of Metals最新文献

The nitridation of three austenitic high-temperature alloys in 95% N₂ + 5% H₂ environment at 1100 °C was evaluated in terms of gravimetry and investigated by SEM–EDS, EPMA and STEM. Samples made from Alloy 600, 253 MA and 353 MA were exposed for 1 day, 1 week and 3 weeks. Alloy 600 underwent very little nitridation, while 253 MA and especially 353 MA, were heavily affected by nitride precipitation. The nitridation of all three alloys had reached equilibrium after three weeks; the extent of nitridation depending on the chromium activity in the alloy. The kinetics of nitrogen ingress into the alloy depends on nickel concentration, while the rate-determining step in the nitridation process is the nucleation and growth of the nitride precipitates.

A novel high-entropy rare earth (Yb_0.25Sc_0.25Er_0.25Tm_0.25)₂Si₂O₇ or 4(YSET)_0.25 disilicate was fabricated through a solid-solution method to protect the underlying SiC substrate from harsh environment at elevated temperature. XRD analysis showed that the newly fabricated 4(YSET)_0.25 exactly matched with the constituent base Yb₂Si₂O₇ having a single stable β phase. The microstructure analysis showed that the powder was uniformly mixed. A CMAS exposure test was done to check the corrosion properties of 4(YSET)_0.25 at 1300 °C for 4 h and 48 h. The 4(YSET)_0.25 showed better resistance against CMAS after 48 h at 1300 °C, and a negligible amount of Ca was able to penetrate toward the 4(YSET)_0.25 substrate. The overall performance of 4(YSET)_0.25 against CMAS was far better than their single constituent elements.

Nowadays the production of steel from scrap in electric arc furnaces is the most common bridging technology to reduce CO₂ emissions. Depending on scrap quality, a non-negligible content of tramp elements such as Cu, Sn, or Ni is introduced into the steel. As their affinity to oxygen is lower than that of iron, they typically enrich at the steel/scale interface area and along grain boundaries during oxidation, which may result in quality problems. Oxidation processes are unavoidable in solid steel processing, and therefore, a deeper understanding of the occurring phenomena, such as intergranular oxidation and liquid metal infiltration of grain boundaries, is essential to continuously improve the product quality. In this study, oxidation experiments for slab reheating were performed by simultaneous thermal analysis under near-process conditions. For a clear statement on the role of tramp elements during oxidation, steel grades with and without tramp elements were investigated. The addition of the expected future contents of Cu and Sn does not affect external oxidation, but at the interface the presence of Cu and Sn leads to the formation of liquid Cu phases and infiltration of grain boundaries. The additional presence of Ni counteracts this formation, but due to its huge impact on iron activity it favors the formation of a rough steel/scale interface. In contrast with Ni, Cu and Sn hardly have any influence on iron activity. Numerical calculations based on a diffusion model and results of the well-known thermochemical software FactSage confirm these effects.

The influence of trace levels of O₂ and H₂O, contamination in an inert gas heat treatment atmosphere on the oxidation behvaiour of IN718 was investigated. Heat treatments consisted of holding IN718 at 1050 °C for 2 h in a combined thermogravimetric balance and gas chromatography-mass spectrometer (GCMS). Furnace atmospheres explored included 22–703 ppm O₂ and H₂O concentrations of 23–387 ppm. The GCMS measurements were able to quantify the O₂ and H₂O concentrations during heat treatment and revealed that oxidation became measurable at approximately 800 °C. The oxidation rate was parabolic during the 1050 °C isotherm, increasing linearly with an increase in either O₂ or H₂O concentration up to a value of 480 ppm. Beyond 480 ppm the oxidation remained constant and equivalent to that reported in air. A two layer surface oxide structure consisting of Cr₂O₃ and TiNbO₄ formed when the O₂, and H₂O content increased beyond 33 and 23 ppm respectively. Dry O₂ conditions (i.e. H₂O of approximately 25 ppm), caused spalling of the Cr₂O₃ oxide surface during cooling when the O₂ ppm was 124 ppm or above. In higher H₂O concentrations the Cr₂O₃ layer showed good adherence to the base metal and no cracking during cooling. The use of a He–5% H₂ carrier gas did not alter the oxidation rate significantly, but did increase the H₂O concentration, thus preventing oxide spalling during cooling.

In most cases, platinum-based alloys are mainly used in high-temperature oxidation environments, and mastering their oxidation behavior and the impact of oxidation on performance is crucial. Two new platinum-based high-temperature alloys, Pt-10Rh-0.5Zr and Pt-10Rh-0.5Zr-0.2Y, were designed and prepared in this study. The research focuses on the high-temperature oxidation behavior of the alloys in air and the influence of oxidation on the room temperature mechanical properties of the alloys. The results show that the relationship between oxidation weight loss and temperature of these two platinum-based alloys conforms to the Arrhenius equation within the temperature range of 1400–1600 ℃, and the oxidation resistance of Pt-10Rh-0.5Zr-0.2Y alloy is better than that of Pt-10Rh.0.5Zr alloy. Examination of the surface and fracture morphology of these oxidized platinum-based alloys revealed that zirconium and yttrium oxide particles, such as ZrO₂ and Y₂O₃, with different morphologies and structures were formed. The study also found that adding a small amount of zirconium and yttrium can significantly improve the room temperature ultimate tensile strength of Pt-10Rh alloy. However, after 20 h of high-temperature oxidation treatment at 1400 and 1500 °C, the tensile strength and plasticity at room temperature of both alloys showed a significant downward trend. Especially, the room temperature plasticity of Pt-10Rh-0.5Zr-0.2Y alloy decreased by more than 80% and exhibited a brittle fracture mode. Our research will contribute to the design and development of new high-temperature platinum-based alloys.

Thermal stability of nanocrystalline grains is a crucial factor that determines the unique microstructure and properties of the gradient nanostructured (GNS) materials at elevated temperatures. Nevertheless, oxidation is unavoidable for GNS metal materials utilized at high temperatures, potentially impacting the microstructure stability. In this study, we reveal the correlation between the high-temperature selective oxidation and the thermal stability of GNS layer through experiments and phase-field simulations. The improved oxidation resistance of GNS samples was ascribed to the excellent thermal stability of (Cr, Mn)₃O₄ oxides and a large proportion of low-energy twin boundaries. After prolonged oxidation, the GNS layer exhibited a bimodal microstructure. To analyze the elemental diffusion mechanism and microstructure evolution in the GNS layer, the phase-field simulation technique was employed. Selective oxidation led to the concentration of chromium reduced in the grain-boundary region, thereby diminishing the thermal stability of the grains and causing abnormal grain growth in the surface layer. Particularly, grain growth had a cumulative effect, the topmost grains coarsening will cause grain growth in the underlying layers, and subsequently, the grains in the interior region will also be gradually affected.

To address the considerable interest in LiF-BeF₂ (FLiBe) compatibility for fission and fusion reactor applications, static and flowing compatibility experiments were conducted to assess the compatibility with type 316H stainless steel. In static testing at 550° and 650 °C, small mass changes were measured and posttest characterization of the FLiBe showed increased levels of Fe, Cr, Ni and Mn in the salt. Adding Be in the static salt test reduced the dissolution of Fe and Ni. An initial assessment of mass transfer in flowing FLiBe without a Be addition was conducted using a monometallic 316H thermal convection loop (TCL) operated for 1000 h with a peak temperature of 650 °C. Similar to prior results in flowing FLiNaK salt, the 316H specimens exhibited small mass losses in the hot leg. Posttest characterization of the 316H specimens suggested Cr surface depletion in the hot and cold legs and possibly Fe deposition in the cold leg. To further understand this behavior, Cr and Fe dissolution was measured in static FLiBe at 550–650 °C.

The transition from external to internal oxidation of a binary Fe-10Cr alloy has been investigated in Fe/FeO Rhines pack (RP) and H₂/H₂O between 850 and 900 °C. Internal oxidation is facilitated by increasing temperature and presence of water vapor. A classical Wagnerian diffusion analysis predicts external oxidation for ferritic (BCC) Fe-10Cr and internal oxidation for austenitic (FCC) Fe-10Cr. The α-to-γ transformation is demonstrated to be the primary factor promoting internal oxidation in Fe–Cr around 900 °C. Water vapor is believed to promote internal oxidation due to a higher reactivity of H₂O compared to O₂ and higher preferential adsorption of the H₂O molecule.

The goal of this study was to use ¹⁸O-enriched water to better understand the role of H₂O in high-temperature oxidation. Seven model and three commercial M-Cr and M-Cr-Al alloys were studied in air with 10% of H₂O at 800 °C for 5 h. Oxygen from water vapor was more reactive than oxygen from the air and ¹⁸O enriched at the outermost layers of the formed Cr- and Al-rich oxides. Alloys with Al and/or Ti additions showed signs of internal oxidation but ¹⁸O was not enriched inside the alloy in locations with internal oxidation. Depending on the alloy Al content, the oxide went from Al oxidation beneath a chromia scale to external alumina scale formation.

The transformation of wüstite (FeO) in the oxide layer formed during high temperature oxidation (600 °C and 700 °C) on hot-worked tool steel was investigated. Wüstite plays an important role in the oxide layer of these steels used for hot working. However, understanding its transformation behavior during cooling is crucial for controlling the final oxide layer structure. Slow cooling rates have a significant influence on the final wüstite content, resulting in inaccurate representations of the composition of the oxide layer at temperatures above 570 °C. The aim of this study was to determine the influence of cooling rate on the wüstite content in the oxide layer after high temperature oxidation. It was found that for hot-worked steel samples oxidized at 700 °C or higher, a cooling rate of more than 1000 °C min⁻¹ is required to suppress the eutectoid transformation and maintain the realistic wüstite content. At lower temperatures (570 °C–600 °C), a cooling rate of more than 100 °C min⁻¹ is required to achieve the wüstite content observed at oxidation temperatures in the oxide layer. Overall, the hematite and magnetite contents also vart with the cooling rate, which is associated with changes in the wüstite content.