Several nickel-based superalloys were nitrided at 400 degrees C for 4 h in a plasma-assisted low pressure gaseous nitriding set-up. Composed of an austenitic gamma phase matrix with Ni-3(Al,Ti,Nb)-type gamma' precipitates, the studied superalloys include the polycrystalline Udimet (R) 720Li (U720), the directionally solidified MarM200-DS and the single crystal Rene N4 and MC2 superalloys. The nitrided samples were characterized by scanning electronic microscopy, energy dispersive spectroscopy and glow discharge optical emission spectroscopy to determine the nitriding behaviour of the gamma' precipitates. The nitrogen incorporation is confirmed in the gamma phase, up to 25-30 at.%, corresponding to the expanded gamma(N) phase, similar to the one obtained in low temperature nitriding of alloys with a pure austenitic structure. However various behaviours of the gamma' precipitates were observed in the different alloys. Various complementary experiments have shown that the gamma' nitriding is not dependent on the size of the gamma' precipitates, on the gamma' crystalline orientation nor on the gamma/gamma' interface coherency. The main influent parameter is the intrinsic gamma' composition, that is the Al substitution by solute elements like Ti or Nb: the closer to pure Ni3Al is the gamma' composition, the lower is the nitrogen incorporation. This influence of the composition on the nitriding efficiency is discussed through thermodynamic and diffusion kinetic considerations. However the gamma' nitriding is thought to be favoured by the propagation of dislocations in the gamma' precipitates thanks to the high compressive stress induced by the surrounding gamma(N) phase expansion. So, the gamma' chemical composition controls the gamma' nitriding ability as it influences the shear strength of the gamma' precipitates. (C) 2018 Elsevier B.V. All rights reserved.

Thanks to their good fatigue and creep properties at high temperature, nickel based superalloys are widely used in pressurized water heat exchangers or in the hottest sections of aeroengines or industrial gas turbines. Superalloys typically have a matrix with an austenitic face centered cubic (FCC) crystal structure called γ phase. In addition to the hardening by substitution solid solution of the γ phase, some Ni-based superalloys gain their impressive mechanical properties from the presence of a high fraction of ordered L12 face-centered cubic γ′ precipitates (Ni3(Ti,Al,Ta) type). However, increasing their resistance to very aggressive conditions (wear, fretting, oxidation, corrosion, higher temperatures and pressures, higher mechanical solicitations...) is still challenging and plasma nitriding could be a potential efficient surface treatment to enhance the life time and to fulfill the expected conditions of operations of the next generation gas turbines. In previous studies [1] and [2], plasma assisted nidriding of various Ni-based superalloys, polycrystals (Haynes230, Udimet720Li, N18) and the single crystal MC2, presenting from 100 vol.% to 30 vol.% of γ phase, has been performed at moderate temperature (400°C) to shed light on the specific response of the different phases (, ’) under nitriding. Macroscopic characterizations, via glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD), have evidenced the formation of nitrided layers with ~25-30 at.% N at the surface and thickness varying from ~3 μm to ~7 μm depending on alloys. In all alloys, formation CrN was revealed by XRD. The γ phase was also shown to be nitrided as in austenitic stainless steels to form the expanded austenitic FCC phase γN, a nitrogen insertion solid solution, with ~25 at.% N. On the other hand, ’ precipitates was found to exhibit different behaviour depending on alloys: in the studied polycrystals, ’ seem nitrided in similar proportion than the matrix γ, while, in case of MC2, ’ seem not or poorly nitrided. The present work is focused on characterization of the structural and chemical modifications induced by plasma nitriding at the submicrometric/nanometric scales. Characterization by SEM and TEM, coupled with HAADFSTEM, EELS and EDS have thus been performed to confirm the macroscopic results, but also to determine the distribution and the size of the CrN formed and the chemical and structural changes of nitrided ’ precipitates due to nitrogen incorporation. All these results should provide important information especially to explain the different nitriding responses of the Ni based superalloys observed at the macroscopic scale.

The mechanical and magnetic properties of a nitrided austenitic stainless steel are studied using a combinatorial approach. Plasma nitriding of a [100]-oriented 316L single crystal is carried out using a loose shadow mask to produce an in-plane lateral gradient of nitrogen concentration that extends up to 100 μm. The local mechanical and magnetic properties across the gradually nitrided area are resolved by nanoindentation and the polar magneto-optic Kerr effect, respectively. The hardness, reduced Young's modulus and remanence qualitatively depict the nitrogen profile, suggesting that the nitrogen concentration is a central effect for these observed dependencies. Conversely, the coercivity exhibits a non-monotonic behaviour due to the interplay between magnetic anisotropy and the strength of the induced ferromagnetism. Fingerprints of the expected transition from a nitrogen supersaturated solid solution to a multiphase nature of expanded austenite are evidenced along the gradually nitrided area.

The anisotropic lattice rotation of individual grains induced by plasma nitriding of 316L austenitic stainless steel has been analyzed with the aim of identifying correlations between the initial grain’s orientation and the rotation behavior. Due to the quite large nitriding-induced strains (up to 20%), the Taylor–Bishop–Hill model has been chosen for the simulation of the lattice rotations. The model predicts the overall rotations, both amplitude and direction, reasonably well over the entire stereographic triangle. The magnitude of the rotations is in agreement with the level of deformation induced by insertion of nitrogen atoms into an austenitic lattice. With regard to plasticity, parallels between the nitriding process and tensile elongation along the normal surface can be drawn.

The impact of plasma nitriding at 400 °C on the monotonic mechanical behaviour of 316L austenitic stainless steel at room temperature has been investigated. It is shown that the residual stresses in the nitrided layer lead to a tension–compression anisotropy whose magnitude depends on the residual stresses intensity and extension (i.e. thickness of the nitrided layer). Using the stress differential technique, average residual stresses in the nitrided layer ranging from −1.5 up to −3 GPa were measured. The local mechanical behaviour of the nitrided layer has also been investigated through SEM in situ tensile tests. A quasi-brittle mechanical behaviour of the nitrided layer is observed with first evidences of crack initiation for plastic strains below 1%, whatever the nitrided layer. By increasing the total applied strain up to 20%, a progressive segmentation of the layer occurs. The crack initiation location mainly depends on the local nitrided thickness.

A three-dimensional atomic characterization of AISI 304L plasma nitrided at 400°C has been carried out with atom probe tomography (APT). While only a single phase, usually called γN phase or expanded austenite, can be detected by the X-ray diffraction, APT reveals the formation of nanometric MN (M=Cr, Fe) precipitates. Preferential precipitation of MN at planar defects has been observed. These results suggest that even at moderate nitriding temperatures the diffusion of Cr takes place resulting in slow but nevertheless detectable precipitation kinetics.

Ni-based superalloys are composed of a matrix with an austenitic face-centered cubic (FCC) crystal structure (γ phase) which is generally strengthened by the presence of a high fraction of ordered FCC precipitates γ′ (Ni3(Ti,Al) type) and/or γ″ (Ni3Nb type). Plasma assisted nitriding of various Ni-based superalloys, either polycrystals (Haynes®230, Inconel®718, Udimet®720Li, N18) or single crystals (MC2 and MC-NG), presenting from 100vol.% to 30vol.% of γ phase, has been performed at moderate temperature (400°C) to shed light on the specific responses of the different (δ, γ′, γ″) precipitates under nitriding. Characterizations by glow discharge optical emission spectroscopy and scanning electron microscopy have evidenced the formation of nitrided layers with thicknesses varying from ~1μm to ~8μm depending on alloys and treatment duration. In all alloys, X-ray diffraction on nitrided samples confirms that the γ phase is nitrided, as in austenitic stainless steels, to form the expanded austenitic FCC phase γN, a nitrogen insertion solid solution, with a mean concentration of ~25at.% N. CrN nitrides were also identified. The incorporation of nitrogen within the precipitates appeared to depend on the alloy microstructure: for polycrystals, regardless of their proportion, the δ, γ′ or γ″ precipitates seem nitrided in similar proportion than the matrix γ although the exact nature (nitrides or expanded δ, γ′, γ″ phases) of the obtained nitrided phases was not confirmed. On the contrary, in the single crystal MC2, the γ′ precipitates accommodate a much lower nitrogen amount, in the range 3–5at.%. Whatever the materials, the surface swelling induced by the nitrogen uptake is only relative to the total incorporated nitrogen quantity and is roughly independent on the nature of the nitrided phases.

Ferromagnetic single crystalline [100], [110], and [111]-oriented expanded austenite is obtained by plasma nitriding of paramagnetic 316L austenitic stainless steel single crystals at either 300 or 400 °C. After nitriding at 400 °C, the [100] direction appears to constitute the magnetic easy axis due to the interplay between a large lattice expansion and the expected decomposition of the expanded austenite, which results in Fe- and Ni-enriched areas. However, a complex combination of uniaxial (i.e., twofold) and biaxial (i.e., fourfold) in-plane magnetic anisotropies is encountered. It is suggested that the former is related to residual stress-induced effects while the latter is associated to the in-plane projections of the cubic lattice symmetry. Increasing the processing temperature strengthens the biaxial in-plane anisotropy in detriment of the uniaxial contribution, in agreement with a more homogeneous structure of expanded austenite with lower residual stresses. In contrast to polycrystalline expanded austenite, single crystalline expanded austenite exhibits its magnetic easy axes along basic directions.

Plasma nitriding of MC2, a single crystalline (γ/γ′) Ni-based superalloy, was performed at 400 °C for 1 and 4 h. Owing to its monomodal γ′ particle size distribution, MC2 was used as a model material in an attempt to investigate the behavior of precipitates during the nitriding of a γ matrix. The nitrogen profiles, the morphology, and the nature of the phases in the nitrided layer were characterized by glow discharge optical emission spectroscopy, scanning electron microscopy, and X-ray diffraction. The γ matrix appeared to be nitrided similarly to the γ solid solution in austenitic stainless steels with the development of an expanded γN phase. The amount of nitrogen in the γ matrix varies from ~30 at.% at the surface till ~20 at.% at the interface with the non nitrided matrix. On the contrary, the γ′ precipitates accommodate no more than few at.% of nitrogen. This disparity modifies the morphology and the average γ channel width increased by ~25 %.

Correlations between the grain orientations and elastic properties of plasma-nitrided polycrystalline 316L austenitic stainless steel are investigated. The grain orientations (hkl) in a delimited area were obtained from electron backscatter diffraction and related to hardness (Hhkl) and elastic modulus (Ehkl) maps obtained from large nanoindentation matrices. The influence of nitrogen concentration on the local mechanical properties has been studied by repeating these indentation matrices in the same area after successive partial removals of the nitrided layer. This nanoindentation tomography allowed the orientation, the shape and the surroundings of individual grains to be taken into account. The results show that plasma nitriding leads to a complete reversal of the elastic behaviour anisotropy: while the non-nitrided 316L austenitic stainless steel shows the typical elastic anisotropy of face-centred-cubic-type metals with a maximum of Ehkl for the 〈111〉 oriented grains, the maximum of Ehkl is observed for the 〈001〉 oriented grains in the nitrided layer. A similar anisotropy reversal is observed for the hardness Hhkl. These observations are discussed on the basis of the microstructural changes induced by the nitrogen incorporation.

The nature of the near-surface γN phase produced by low-temperature (∼400°C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γN phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe4N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe4N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γN phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.

Swelling of 316L austenitic stainless steel plasma nitrided at 400°C under floating potential has been investigated using electron back scattered diffraction and white-light interferometry. Swelling of individual grains strongly depends on their crystallographic orientation, similarly to the thickness of the nitrided layer. After 1 h of treatment, swelling is maximum for the 〈001〉 oriented grains and minimum for the 〈111〉 oriented grains. After 8 and 33 h of nitriding, the maximum of swelling is observed in the grains having their normal direction at about 15° from the 〈001〉 orientation. These results are discussed on the basis of plastic strain after comparison with calculated swellings of the 〈001〉 and 〈111〉 oriented grains, using the thickness of the nitrided layer deduced from the trapping–detrapping diffusion model and a rough estimation of the plastic strain. The satisfactory agreement between experimental and calculated swellings supports the idea that swelling results from the lattice expansion due to the incorporation of nitrogen plus an elastic strain and a plastic strain. For individual grains of the 316L matrix, nitriding leads to a tensile-like elongation of high magnitude (around 20%) and it might be the origin of the lattice rotations which were previously observed after nitriding.

Xenon and krypton have been implanted into muscovite mica at room temperature and at liquid nitrogen temperature. The behavior of the implanted Xe and Kr was followed by low-temperature transmission electron microscopy and energy dispersive x-ray analysis. An electron diffraction pattern of diffuse bands is observed at room temperature due to the presence of fluid rare gas and to noncrystalline mica. Visible cavities with diameters 10–300 nm formed in the Xe-implanted mica. Visible cavities in room-temperature Kr-implanted mica ranged from 5–50 nm in diameter. The gas pressures at room temperature in the cavities are estimated, assuming all of the implanted gas precipitated in cavities to be ∼10 MPa for Xe and ∼20 MPa for Kr. These pressures are considerably lower than found for rare gases implanted in metals and ceramics, but sufficient to liquefy the rare gases at room temperature. The Xe and Kr were observed by dark-field microscopy to form fcc crystalline solids within the cavities at temperatures below their triple points, with lattice parameters of a (xe) = 0.630 ± 0.0015 nm and a (Kr) = 0.565 ± 0.005 nm. The solid Xe within bubbles was unstable under the electron beam of the transmission electron microscope at temperatures above 80 K, while the solid Kr within bubbles was unstable at temperatures as low as 35 K. The crystalline mica matrix undergoes a transformation from a crystalline structure to an amorphous structure as a result of implantation.

The changes in anisotropic hardness and indentation modulus Induced by plasma nitriding at 400 degrees C of a 316L polycrystalline austenitic stainless steel are analyzed The dependence of hardness and elastic modulus modifications on the crystallographic orientation is investigated through instrumented indentation and electron backscattering diffraction Both hardness and indentation modulus exhibit an inverted anisotropy compared to the untreated 316L, likely associated with the presence of the N atoms in interstitial sites (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved

The microstructure of a muscovite mica exposed to a rare gas ion beam has been studied by transmission electron microscopy. The investigation of damage without implantation was carried out using argon and helium ions of sufficient energy to traverse the 100–150 nm mica specimens. For 340 keV Ar++ irradiation, amorphization of mica occurred at a fluence as low as 3.5 × 1014 ions · cm−2, which corresponds to 0.29 dpa. Muscovite can be amorphized using 80 keV helium ions, but this requires a much higher fluence and damage production of 4.6 × 10−6 ions · cm−2 and 0.60 dpa, respectively. Since helium irradiation results principally in ionization energy loss, it indicates that amorphization of muscovite results mainly from nuclear interactions. Complete amorphization of muscovite mica is found to take place for all ions at approximately the same amount of nuclear energy transfer to energetic primary knock-on atoms, assuming a recoil energy greater than 500 eV. This suggests that amorphization occurs directly in dense displacement cascades. A significant amount of helium, 100 ppm, can be implanted into muscovite mica without destroying the crystal structure.

Hard and tough nanocomposite coatings consisting of hard TiN nanograins embedded in a soft metallic intergranular phase of Ni have been produced using ion beam assisted deposition. The chemical composition has been obtained by Rutherford Backscattering and the microstructural properties: phases, grain size, and texture of the coatings have been investigated by X-Ray Diffraction. In the composition range 0–22.5at.% Ni, δ-TiN is the only crystalline phase and Ni appears as an X Ray amorphous phase. The hardness increases up to a maximum of 41GPa at ~7at.% Ni which corresponds to a TiN crystallite size of ~8nm and a Ni intergranular phase thickness of roughly 1 monolayer. It is shown that the hardness enhancement in TiN–Ni nanocomposite coatings is not correlated with residual stresses, but rather with the intrinsic properties of the nanostructure. An important improvement in wear resistance is obtained for the coatings exhibiting the highest toughness and not the highest hardness. These results show that ion assisted processing is an effective tool for producing dense TiN–Ni nanocomposite coatings and tailoring their structure and mechanical properties.

Thin Mg films were magnetron sputter deposited on fused quartz substrates and subsequently irradiated by molecular ions extracted from plasmas generated in water vapour or a mixture of water vapour and argon using pulsed negative 0.5-1 kV bias voltage. The depth profiles of H and O atoms have been analyzed using Nuclear Reaction Analysis and Secondary Ion Mass Spectrometry techniques, respectively. It is shown that energetic incident molecular ions are broken into elements upon entrance into the solid; hydrogen is separated and stored in the bulk of Mg film. The role of grain boundaries in controlling the H-transport kinetics for nanocrystalline Mg films is discussed.

Crystallographic structure and lattice rotation of the ‘expanded’ austenite produced by means of low temperature plasma nitriding is investigated. The microstructure of the nitrogen enriched layer has been investigated by means of X-ray diffraction (XRD) while the lattice rotations in the nitrided zone were assessed by electron backscattered diffraction (EBSD). The nitrogen depth profiles have been determined by means of glow discharge optical emission spectroscopy and nuclear reaction analysis. XRD shows the presence of the ‘expanded’ austenite or γN phase in all the nitrided samples characterized by average larger lattice spacing in relation to non-nitrided steel matrix. EBSD investigation demonstrates that in addition to the lattice expansion nitrogen incorporation into the stainless steel matrix induces significant lattice rotations. The amount and direction of these crystallographic plane rotations are function of the initial orientation. An unusual evolution of the 220 γN line as a function of the nitriding duration is observed together with an anomalously high intensity ratio of the 111 and 200 γN and matrix lines. The XRD results are interpreted on the basis of the lattice rotations of diffracting planes, nitrogen concentration gradient, nitrogen diffusion anisotropy and residual stress. It shows that these rotations are a pertaining feature for the understanding of the γN microstructure.