NiFe-based oxo-hydroxides are highly active for the oxygen evolution reaction but require complex synthesis and are poorly durable when deposited on foreign supports. Herein we demonstrate that easily processable, Earth-abundant and cheap Fe–Ni alloys spontaneously develop a highly active NiFe oxo-hydroxide surface, exsolved upon electrochemical activation. While the manufacturing process and the initial surface state of the alloys do not impact the oxygen evolution reaction performance, the growth/composition of the NiFe oxo-hydroxide surface layer depends on the alloying elements and initial atomic Fe/Ni ratio, hence driving oxygen evolution reaction activity. Whatever the initial Fe/Ni ratio of the Fe–Ni alloy (varying between 0.004 and 7.4), the best oxygen evolution reaction performance (beyond that of commercial IrO2) and durability was obtained for a surface Fe/Ni ratio between 0.2 and 0.4 and includes numerous active sites (high NiIII/NiII capacitive response) and high efficiency (high Fe/Ni ratio). This knowledge paves the way to active and durable Fe–Ni alloy oxygen-evolving electrodes for alkaline water electrolysers. NiFe-based oxo-hydroxides are active for the oxygen evolution reaction but suffer from complex synthesis and durability when deposited. Easily processable Fe–Ni alloys with a highly active oxo-hydroxide surface are now shown to pave the way for oxygen-evolving electrodes for alkaline water electrolysers.

The 6061 Al alloy in its T6 conditions is often considered a good candidate for applications requiring a good balance between strength and thermal conductivity. However, this alloy is often very difficult to process using laser powder bed fusion (PBF-LB) because of the development of hot cracks during fabrication. Here, we show that adding 2.3 wt% of Zr to the 6061 heritage alloy makes it processable by PBF-LB (suppression of hot cracks). Hot crack mitigation is attributed to grain refinement. However, the addition of 2.3 wt% of Zr greatly affects the microstructure and thus the mechanical and electrical/thermal properties of the Zr-modified 6061 alloy. Consequently, there is a need to design heat treatments to achieve a trade-off between yield strength and thermal conductivity. In this work, we designed two heat treatment sequences aiming at achieving such a trade-off: an adapted T6 sequence (550 degrees C/30 min + 180 degrees C/4 h) and direct ageing at 400 degrees C/4 h. On the basis of a multiscale microstructural study using optical microscopy, X-ray diffraction, and electron microscopy, we clarify the evo-lution of the microstructure induced by the designed heat treatments. The mechanical properties (hardness, tensile behavior) and thermal conductivity derived from electrical conductivity measurements are then discussed in light of the microstructural evolutions. The as-fabricated Zr-modified 6061 alloy shows a higher yield strength (370 MPa) than the heritage 6061 alloy in its T6 condition (260 MPa) but its thermal conductivity is found to be much lower (98 vs. 173 W/m.K). The two heat treatment sequences designed in this work enable the mechanical properties of the heritage 6061 alloy to be outperformed (yield strength of 350 and 460 MPa for the T6 and direct ageing heat treatment respectively) while maintaining an acceptable level of thermal conductivity (150 and 170 W/m.K for the T6 and direct ageing heat treatment respectively).

Austenitic Fe-13.50Mn-3.98Si-9.54Cr-4.51Ni shape memory alloyed with (0.18, 0.42 and 0.96 wt%) Ce was exposed in air at 800 ??C. Both 0.18 and 0.42 wt% Cerium additions yielded protective kinetics, while the 0.96 wt % Ce content did not change the oxidation resistance. Increased oxidation resistance was related to the growth of a fine-grained oxide layer, slightly wrinkled with a higher Cr content. Cerium also acted as a S scavenger, suppressing its interfacial segregation. In the Ce-free alloy, fast oxide growth occurred in regions where S segregated at the interface, leading to severe wrinkling of the layer, resulting in lower oxidation resistance.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Improvement in long-term oxidation resistance of AISI 304L by cold working Valérie Parry, Audrey Col, Céline Pascal

The relationship between alloy chemical composition and oxidation behavior of austenitic Fe-Mn-Si-Cr-Ni shape memory stainless steels (SMSSs) during exposure at 800 degrees C in air was investigated. Although high Si and Cr additions are beneficial for both the shape memory effect and oxidation resistance, the increase of these ferrite stabilizer elements in Fe-Mn-Si-Cr-Ni SMSSs makes the oxidation-induced ferrite stable at the early stages of the oxidation exposure, which in turn allows a faster diffusion of Mn, controlling the kinetics of oxidation. Our results suggested that the austenite stability should be maintained during the oxidation process for higher performance of these materials.

In the present research, the microstructure and oxidation behavior of an Fe-8.26Mn-5.25Si-12.80Cr-5.81Ni-11.84Co shape memory stainless steel (SMSS) was studied at 800 degrees C in air for up to 120 h. Phase changes and oxidation mechanism were discussed based on microscopy analyses, thermogravimetric measurements and thermodynamic simulations. The results show that oxidation exposure promotes the formation of the sigma, chi and ferrite phases in the metallic substrate. The oxidation behavior follows a parabolic law, with the kinetics of oxidation being controlled by the Mn2O3 oxide growth in the first hours, and by Mn3O4 and MnCr2O4 spinel growth after 24 h of exposure.

The effect of a pre-straining on oxidation resistance of a low stacking fault energy austenitic stainless steel, Fe-17.5Cr-8.1Ni, was investigated at 830 degrees C in O-2 for 12 days. A decrease of the oxidation rate has been observed compared to the as-received material. For the first hours of oxidation, the martensitic deformed zone was transformed into numerous fine grains supplying a network of Cr fast diffusion paths. The high density of oxide nucleation sites related to the reverted martensite resulted in a fine-grained oxide scale which has limited the transport of scale-forming elements and moderated the long-term Cr consumption.

AISI 441 sheets were oxidized from 4 to 24 h at 800 degrees C in wet atmosphere. Micrometric oxide nodules containing Ti, Nb, Cr and Mn were observed to form on the passive scale. The internal microstructure of these objects was investigated using STEM-EDX and FIB-SEM tomography. Experimental results reveal a complex microstructure linked with the presence of Si and Nb and their competition for interfacial oxidation. Eventually, the chemical compositions of the nodule and of the oxidation affected zone in the nodule vicinity are discussed in relation with thermodynamic calculations investigating the stability of the different oxides.

As machine tool coating specifications become increasingly stringent, the fabrication of protective titanium aluminum nitride (Ti-Al-N) films by physical vapor deposition (PVD) is progressively more demanding. Nanostructural modification through the incorporation of metal dopants can enhance coating mechanical properties. However, dopant selection and their near-atomic-scale role in performance optimization is limited. Here, yttrium was alloyed in multilayered Ti-Al-N films to tune microstructures, microchemistries, and properties, including mechanical characteristics, adhesion, wear resistance, and resilience to oxidation. By regulating processing parameters, the multilayer period (Λ) and Y content could be adjusted, which, in turn, permitted tailoring of grain nucleation and secondary phase formation. With the composition fixed at x = 0.024 in (Ti0.6Al0.4)1–x Y x N and Λ increased from 5.5 to 24 nm, the microstructure transformed from acicular grains with 〈111〉 preferred orientation to equiaxed grains with 〈200〉 texture, while the hardness (40.8 ± 2.8 GPa to 29.7 ± 4.9 GPa) and Young’s modulus (490 ± 47 GPa to 424 ± 50 GPa) concomitantly deteriorated. Alternately, when Λ = 5.5 nm and x in (Ti0.6Al0.4)1–x Y x N was raised from 0 to 0.024, the hardness was enhanced (28.7 ± 7.3 GPa to 40.8 ± 2.8 GPa) while adhesion and wear resistance were not compromised. The Ti-Al-N adopted a rock-salt type structure with Y displacing either Ti or Al and stabilizing a secondary wurtzite phase. Moreover, Y effectively retarded coating oxidation at 1073 K (800 °C) in air by inhibiting grain boundary oxygen diffusion.

The spalling/cracking behaviour, at room temperature, of thermally grown oxide scales under tensile stress was investigated using SEM in-situ tensile testing for two austenitic stainless steels with close composition except their S content. A correlation between damage patterns, microstructure, mechanical and adhesion properties of the oxide scales is proposed. The difference in microstructure evolution during oxidation between the two steels is explained in relation with the volume fraction of MnS inclusions in the substrate (i.e. S content). Although a direct effect of S content on the oxide scale adhesion is not evidenced, the metal/oxide toughness seems strongly affected by oxides features such as scale thickness, Fe content and location of internal oxides (SiO2 along the metal/scale interface or at the grain boundaries of the underneath substrate).

Two austenitic stainless steels, AISI 304L and AISI 303, were submitted to cyclic oxidation and to static mechanical loading after isothermal oxidation at 1000 degrees C. Alloy 303 contains ten times more S than 304L and some Mn addition. During the steel process, it formed manganese sulfides that lead to the formation of a less resistant oxide scale. Both alloys showed similar behavior during thermal cycling but breakaway oxidation and intensive spallation occurred much sooner for alloy 303 than for alloy 304L. A correlation could be drawn between tensile test on preoxidized samples, isothermal and cyclic oxidation. (C) 2015 Elsevier Ltd. All rights reserved.

Most industrial heat-resistant stainless steels contain silicon as a minor constituent. At high temperature, the internal formation of amorphous silica reduces oxidation rates but decreases the metal/oxide interface toughness. Tensile testing experiments performed on AISI 304L previously oxidized in synthetic air for 50 h at 900 or 1000 °C showed a relation between the silica morphology and location and the crack patterns. A micromechanical modeling using cohesive zone models to describe interfaces fracture behavior is proposed to investigate relevant parameters controlling the silica/alloy interface debonding. Calculations carried out using the finite elements method have shown that location of silica inclusions and silica/metal interface toughness are key parameters determining the cracks pattern morphology and the critical strain at failure.

Oxidation tests of AISI 304L in breakaway conditions (850 degrees C in O-2) were performed up to 312 h. The evolution of the oxidation affected zone microstructure was investigated using a combination of cornposional/elemental (TEM, Raman spectroscopy) and structural (EBSD) mapping techniques as well as thermodynamic calculations. The formation of a dense and continuous Cr2O3 healing layer at the border of the internal oxidation zone happens along the grain boundaries and is linked to their more efficient Cr supply. The propagation of the oxidation front is related to a local conversion of Cr2O3 in less protective FeCr2O4. 2016 Elsevier Ltd. All rights reserved.

•Chromium volatilisation from AISI 441 at 800°C in O2+5%H2O was measured and was shown to depend on gas velocity.•Preoxidation of samples before volatilisation measurements could reduce volatilisation rate by a factor 10.•Preoxidation at 250°C formed an iron oxide scale, making a physical barrier between the steel and the gas phase.•For preoxidation at 850°C in CO2/CO, the reduction of the volatilisation rate was the result of the modification of chromia electronic properties from p to n.

This work focuses on the effect of alloyed sulphur as MnS inclusions in austenitic stainless steels. AISI 304L and AISI 303 were oxidized at 1000 degrees C in synthetic air. The high sulphur grade, AISI 303, presents a breakaway oxidation with formation of nodules with an inner part composed of alternated layers of Fe-rich and Cr-rich oxides. During oxidation, MnS inclusions near the metal/oxide interface are partially dissociated. Manganese is incorporated in the oxide. The sulphur is trapped in Cr,Mn-oxysulphide aggregates which formation leads to a local decrease of the Cr concentration and consequently to the formation of non-protective Fe-rich oxide. (C) 2015 Elsevier Ltd. All rights reserved.

A nanonet is a nanostructured network composed of randomly oriented nanowires. Si nanonets, allowing electrical conduction even under an oxidizing atmosphere, are successfully fabricated by Céline Ternon and co-workers. In article number 1500172, it is demonstrated that stable Si nanowire–nanowire junctions, insensitive to oxidation, can be formed by low temperature processing.