Inhibited aluminization of an ODS FeCr alloy

Aluminide coatings are of interest for fusion energy applications both for compatibility with liquid Pb-Li and to form an alumina layer that acts as a tritium permeation barrier. Oxide dispersion strengthened (ODS) ferritic steels are a structural material candidate for commercial reactor concepts expected to operate above 600 °C. Aluminizing was conducted in a laboratory scale chemical vapor deposition reactor using accepted conditions for coating Fe- and Ni base alloys. However, the measured mass gains on the current batch of ODS Fe-14Cr were extremely low compared to other conventional and ODS alloys. After aluminizing at two different Al activities at 900 °C and at 1100 °C, characterization showed that the ODS Fe-14Cr specimens formed a dense, primarily AlN layer that prevented Al uptake. This alloy batch contained a higher (> 5000 ppma) N content than the other alloys coated and this is the most likely reason for the inhibited aluminization. Other factors such as the high O content, small (~ 140 nm) grain size and Y-Ti oxide nano-clusters in ODS Fe-14Cr also could have contributed to the observed behavior. Examples of typical aluminide coatings formed on conventional and ODS Fe- and Ni-base alloys are shown for comparison

Mechanistic-Based Lifetime Predictions for High-Temperature Alloys and Coatings

Increasing efficiency is a continuing goal for all forms of power generation from conventional fossil fuels to new renewable sources. However, increasing the process temperature to increase efficiency leads to faster degradation rates and more components with corrosion-limited lifetimes. At the highest temperatures, oxidation-resistant alumina-forming alloys and coatings are needed for maximum lifetimes. However, lifetime models accurate over the extended application durations are not currently available for a wide range of candidates and conditions. Increased mechanistic understanding and relevant long-term data sets will assist in model development and validation. Current progress is outlined for applying a reservoir-type model to Fe-base alloys and coatings. However, more work is needed to understand environmental effects, such as the presence of H2O, and to extend the current model to NiCrAl and NiCr alloys. As the critical performance factors are better understood, it will be easier to evaluate new materials in laboratory screening experiments

Performance of chromia- and alumina-forming Fe- and Ni-base alloys exposed to metal dusting environments: The effect of water vapor and temperature

Fe- and Ni-base alloys including an alumina-forming austenitic alloy were exposed for 500h under metal dusting environments with varying temperature, gas composition and total pressure. For one H2–CO–CO2–H2O environment, the increase in temperature from 550 to 750°C generally decreased metal dusting. When H2O was added to a H2–CO–CO2 environment at 650°C, the metal dusting attack was reduced. Even after 5000h at a total pressure of 9.1atm with 20%H2O, the higher alloyed specimens retained a thin protective oxide. For gas mixtures containing little or no H2O, the Fe-base alloys were less resistant to metal dusting than Ni-base alloys

Effect of H2O and CO2 on the Oxidation Behavior and Durability at High Temperature of ODS-FeCrAl

International audienceCyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O2, H2O and CO2. Compared to 1 h cycles in dry O2, exposure in air + 10 vol.% H2O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO2 + 50 % H2O + 0.75 % O2 had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO2 + 50 % H2O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O2 was added to the CO2/H2O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000

Role of boron on the Spark Plasma Sintering of an α-SiC powder

International audienceThis study deals with the role of non-oxide sintering aids such as boron carbide (B4C) or – free boron (B) plus free carbon (C) – on the Spark Plasma Sintering treatment of silicon carbide. The results so obtained clearly show that free boron plus free carbon additions lead to the higher densification rates. This favourable behaviour with regards to the densification kinetics is accompanied by the absence of any abnormal grain growth. At the opposite, boron carbide additions do not significantly raise the densification kinetic after SPS treatment of SiC in comparison to pure silicon carbide. In this case, TEM investigations point out the formation of a borosilicate vitreous phase due to the dissolution process of B4C in contact with a native superficial silica layer surrounding the SiC grains. The resulting liquid phase leads to an abnormal grain growth coupled with undensifying process

Fluid dynamic simulation of CrO2(OH)2 volatilization and gas phase evolution during the oxidation of a chromia forming alloy

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The nitrogen effect on the oxidation behaviour of Ti6242S titanium-based alloy: contribution of atom probe tomography

International audienceAt high temperatures under oxidizing environments, titanium-based alloys form an oxide scale and dissolve large amount of oxygen in their metallic matrix. Oxygen dissolution is a cause of embrittlement. Nitrogen is a secondary oxidant, which also dissolves in titanium during oxidation in air. Oxidation experiments of Ti-6Al-2Sn-4Zr-2Mo-0.1Si titanium-based alloy at 650 °C for 1000 h in synthetic air (20%O 2 - 80%N 2 ) and in a mixture of 20%O 2 -80%Ar, showed that nitrogen reduces both oxide scale growth and oxygen dissolution. Atom probe tomography revealed that nitrogen effect is due to the formation of an interfacial layer of nitride Ti 2 N but also to the formation of a nitrogen rich a-Ti-based solid solution, which both act as difiusion barriers for oxygen because of their low oxygen solubility

The role of nitrogen in the oxidation behaviour of a Ti6242S alloy: a nanoscale investigation by atom probe tomography

International audienceWhen used at high temperature in air, titanium-based alloys form an oxide scale at their surface but also dissolve large amount of oxygen in their metallic matrix and this is a cause of embrittlement. Nitrogen is a secondary oxidant, which also dissolves in and embrittles the alloy. Oxidation experiments of Ti-6Al-2Sn-4Zr-2Mo-0.1Si titanium-based alloy, for 10 0 0 h at 650 °C in synthetic air (N 2-20%O 2) and in a mixture of Ar-20%O 2 , showed that nitrogen decreases both oxide scale growth and oxygen dissolution. Atom probe tomography was used to investigate the alloy/oxide interface. The results revealed that the nitrogen effect is due to the formation of interfacial layers of oxynitrides and nitride (Ti 2 N), but also to the formation of a nitrogen rich α-titanium-based solid solution, which all act as diffusion barriers for oxygen, because of their low oxygen solubility. A comparison between the experimental results and thermodynamic calculations is also reported