Novel Co 20 Cr 15 Fe 26 Mn 17 Ni 22 ultra-fine grained high-entropy alloy

International audienceIn a non-equiatomic HEA from CoCrFeMnNi family, so-called A3S® alloy (austenitic superstainless steel), exceptional mechanical properties are observed. High yield strength (800 MPa) associated with significant elongation to fracture (35 %) and a stable austenitic paramagnetic structure (down to LN2 temperature) may be obtained in hot forged materials. Mechanical resistance (YS) of A3S is at least 200 MPa higher than that measured in equiatomic alloy (EA) of the same family. These properties come from a nanostructure, easily formed in the material after classical hot thermomechanical treatment (forging). Yet, mechanical resistance of A3S decreases strongly after a post-forging high temperature annealing: in this state, identical behaviors of A3S and EA are noticed. Effects of thermal conditions of forging and recrystallization annealing have been investigated in A3S. Only slight effect of temperature of forging (between 900 and 1100°C) has been shown. Surprising effects of recrystallization annealing have been found. Low temperature (up to 600°C for 48h) treatment leads to recrystallization with formation of fine (1 µm) grains. At higher temperatures (700-1100°C), recrystallization is blocked: only recovery followed by grain growth and formation of numerous twins is observed. Moreover, in this state, very high density of dislocations is conserved: they present ordered configurations with alignments in {111} planes. Formation of twins has major effect on mechanical behavior of both A3S and EA. Their absence stabilizes nanostructures and leads to high YS values while decrease of YS is accompanied by twins presence. Relative difficulty to form these defects in A3S (as compared to EA) is explained by high value of stacking fault energy, evaluated from TEM measurements of dislocations dissociation. Moreover, strong dependence of SFE on temperature has been shown: its lower value at high temperatures is in agreement with numerous twins observed

In-situ TEM straining experiments in Cantor’s alloy at room and LN2 temperatures

Cantor’s single-phase equiatomic FeNiCoCrMn alloy is a “high-entropy” alloy (or HEA) which crystallizes in the face-centered cubic (fcc) crystal structure. Its mechanical properties include high strength, particularly at low temperatures, good ductility and a large number of slip systems [1], on which its plasticity largely depends. To have a better understanding of these properties, in situ TEM straining experiments were carried out at room and liquid nitrogen temperatures, with a straining holder that applies mechanical stress on the specimen (locally measured using dislocations’ curvature) to analyze dislocation movements. According to previous studies, the planar slip of dislocations is responsible for the first stages of plasticity and twinning starts afterwards [1] [2] [3]. The strengthening mechanisms are a result of the classical dislocation/obstacle (grain boundary, twinning) interaction, but also of the local lattice distortions that may impede moving dislocations. These interactions seem to affect both perfect and partial dislocations. Please click Additional Files below to see the full abstract

Low seroprevalence and low incidence of infection with "Toxoplasma gondii" (Nicolle et Manceaux, 1908) in pediatric hematopoietic cell transplantation donors and recipients : polish nationwide study

Czyzewski, Krzysztof, Fraczkiewicz, Jowita, Salamonowicz, Malgorzata, Pieczonka, Anna, Zajac-Spychala, Olga, Zaucha-Prazmo, Agnieszka, Gozdzik, Jolanta, Styczynski, Jan (2019): Low seroprevalence and low incidence of infection with Toxoplasma gondii (Nicolle et Manceaux, 1908) in pediatric hematopoietic cell transplantation donors and recipients: Polish nationwide study. Folia Parasitologica (019) 66: 1-6, DOI: 10.14411/fp.2019.019, URL: http://dx.doi.org/10.14411/fp.2019.01

Ségrégation d'équilibre du bismuth aux joints de flexion pure dans le cuivre : étude expérimentale et simulation par ordinateur

We studied the intergranular segregation of bismuth in copper by Auger electron spectroscopy, after "in situ" rupture of bicristalline samples of defined crystallographic characteristics (pure bending joints around the or axis) , of various disorientations). The kinetic and thermodynamic aspects of segregation in a defined joint ( 30 °) were examined; the relationship between the level of segregation and the angle of disorientation has been established. Some of the experimental data were compared with the results of simulations, by a method of molecular static, of the structure of a particular copper ( 37 °) joint containing bismuth atoms, in various concentrations: a reasonable agreement has been found.Nous avons étudié la ségrégation intergranulaire du bismuth dans le cuivre par la spectroscopie d'électrons Auger, après rupture "in situ" d'échantillons bicristallins de caractéristiques cristallographiques définies ( joints de flexion pure autour de l'axe ou , de désorientations diverses). Les aspects cinétiques et thermodynamiques de la ségrégation dans un joint défini ( 30° ) ont été examinés ; la relation entre le niveau de la ségrégation et l'angle de désorientation a été établie. Une partie des données expérimentales ont été confrontées aux résultats des simulations, par une méthode de statique moléculaire, de la structure d'un joint particulier de cuivre( 37° ) contenant des atomes de bismuth, en concentrations diverses : un accord raisonnable a été constaté

Optimisation d'alliage Fe40AlB (effet du manganèse sur la microstructure et les propriétés mécaniques)

Malgré des diverses intéressantes propriétés des alliages FeAl, leur faible ductilité à température ambiante limite fortement leur application industrielle. Jusqu à présent, seul le dopage au bore permet de supprimer la fragilité intergranulaire des alliages. La recherche des meilleures propriétés mécaniques nous a conduit à étudier les effets des différentes additions ternaires. Parmi une série des additions ternaires étudiées, seule l addition du manganèse semble être prometteuse. En présence de 2% Mn et par rapport à l alliage binaire, une nette amélioration de l allongement à la rupture à température ambiante est observée pour une limite d élasticité préservée à toutes les températures étudiées (20 à 900 C). Cependant, un tel effet est limité aux faibles teneurs en manganèse : en présence de 5% Mn, une précipitation apparaît et fragilise l alliage. L effet ductilisant du manganèse a été attribué à des modifications de la structure des défauts cristallins, qui se manifestent par une baisse de la concentration des lacunes thermiques et une diminution du paramètre d ordre. Ces propriétés d alliage ternaire FeAlMn ont pour conséquence une amélioration du comportement en conditions d un traitement thermomécanique. En effet, nous avons pu montrer qu un raffinement de la taille de grains de Fe40Al2MnB par recristallisation dynamique à hautes températures et à grande vitesse de déformation, est possible.In spite of various interesting properties of the iron - aluminium alloys, their low ductility at room temperature strongly limits their industrial application. Until now, only doping with boron allows to remove the intergranular brittleness of these alloys. The research of the best mechanical properties led us to study the effects of the various ternary additions. Only the addition of manganese seems to be promising among a series of the studied ternary additions. In the presence of 2% Mn and compared to the binary alloy, a clear improvement of the elongation to fracture at room temperature is observed for a yield stress preserved at all the studied temperatures (20 to 900 C). However, this effect is limited to the low manganese contents: in the presence of 5% Mn, a precipitation appears and weakens alloy. The ductiling effect of manganese was attributed to modifications of the structure of the crystalline defects, which appear by a fall of the concentration of the thermal vacancies and a reduction of the long range order parameter. FeAlMn alloys have also been shown to present a good behavior in conditions of a thermomechanical treatment. A refinement of the grains size of Fe40Al2MnB by dynamic recrystallization at high temperatures and high strain rate could be obtained in this work.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Tenue mécanique d'interfaces modèles dans les alliages Fe-40%at. Al (ordonnés B2)

Le développement des alliages FeAl est fortement limité par leur fragilité intergranulaire à température ambiante, fragilité supposée due à une faible cohésion intrinsèque des interfaces. Jusqu'à présent, seul le dopage en bore permettait de renforcer ces joints dits faibles. Nous avons caractérisé la tenue mécanique d'interfaces de flexion d'axe dans des alliages binaires ou dopés en bore. Les résultats indiquent que le bore supprimerait la rupture intergranulaire en facilitant le transfert du glissement aux interfaces. De plus, contrairement à ce qui est communément admis, toutes les interfaces, même binaires, présentent une haute cohésion, suffisante pour éviter la rupture intergranulaire. Les interfaces de flexion d'axe sont donc spéciales au regard de la fissuration, le plan du joint jouant un rôle fondamental dans sa tenue. L'existence de ces interfaces de haute cohésion permettrait d'améliorer les propriétés des alliages FeAl via une "ingénierie des interfaces".Use of FeAl alloys is still severely limited by their room temperature intergranular brittleness, assumed to be due to a low intrinsic cohesion of the interfaces. Until now, doping with boron was the only way to reinforce these weak boundaries. The fracture behaviour of some special boundaries, with symmetric tilt boundary, pure or doped with boron, was characterized. Results indicate that boron would suppress intergranular fracture by improving slip transfert across the grain boundaries. Then, and on the contrary to the common knowledge, boundaries of B-free bicrystals exhibit a cohesion high enough to avoid intergranular fracture. tilt boundaries are thus special boundaries regarding fissuration resistance, with grain boundary plane playing a major role. The existence of such highly cohesive interfaces opens a new way to efficiently modify macroscopic behaviour of B-free FeAl through a "grain boundary engineering" approach.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Nickel-induced strengthening of boron-doped FeAl (B2) alloys

International audienceThe effect of nickel additions on yield stress temperature dependence as well as on deformation mechanisms was investigated for Fe–Al alloys containing 40% Al. A set of different nickel containing alloys (up to 10 at.%), B-doped or B-free, was deformed in compression up to 950 °C. Subsequent dislocation structures were examined. It was shown that the addition of nickel strongly decreases the solid state solubility of boron in FeAl (B2) alloys. On the microscopic scale, nickel was confirmed to clearly decrease the dissociation width of the 111 superdislocations. From a mechanical point of view, it was shown to induce a large and linear strengthening at low temperatures (<400 °C). Owing to that specific hardening, the yield stress anomaly, usually occurring in such materials, was observed to be blurred out, but was nonetheless revealed by a zero strain rate sensitivity

Renforcement d'alliages fer-aluminium ordonnés B2 (influence d'additions (Ni, B) et de la microstructure)

? Nous étudions les effets d'additions (Ni et B) et de la microstructure sur le comportement mécanique d'alliages fer-aluminium contenant 40 % at. Al.Au niveau macroscopique, nous montrons que le nickel renforce les alliages FeAl sur toute la gamme de température, mais qu'il conduit simultanément à une diminution de la contrainte de clivage, accentuant ainsi la fragilité à l'ambiante de ces alliages. Nous confirmons l'intérêt du raffinement de la taille des grains, par métallurgie des poudres, pour améliorer la limite d'élasticité et la résistance à la rupture. Nous montrons alors le caractère additif de l'effet du nickel sur la limite d'élasticité par rapport au renforcement de type "Hall et Petch". Nous montrons aussi que les phénomènes de durcissement (nickel ou taille de grains) provoquent le masquage de l'anomalie de la limite d'élasticité que ces alliages présentent habituellement.L'étude des mécanismes de déformation à l'échelle des dislocations nous permet de préciser que le durcissement en solution solide (DSS) dû au nickel à basse température ne peut être expliqué par les théories classiques de DSS mais plus vraisemblablement par un effet du nickel sur la contrainte de Peierls. En outre, nous montrons que nos résultats microscopiques et macroscopiques concernant l'anomalie peuvent être pris en compte par le modèle de traînage de tubes d'APB. Enfin, nous mettons en relation l'observation en dynamique (microscopie en transmission in situ) d'un mécanisme de multiplication des superdislocations, avec la tendance au clivage des alliages contenant du nickel.? We study the effects of additions (Ni and B) and microstructure on the mechanical behaviour of 40 at. % Al iron-aluminium alloys.From a macroscopic point of view, we show that nickel reinforces FeAl alloys over the whole temperature range, but that it simultaneously leads to emphasize the room temperature brittleness of these alloys through a cleavage stress decrease. We confirm powder metallurgy grain refining interest to enhance yield stress as well as fracture resistance. We show that nickel-induced yield stress effect is additive to "Hall-Petch" one. Also, we point out that the strengthening phenomena (nickel or grain size) cause the yield stress anomaly, which these alloys usually present, to be hidden.Through a dislocation structures analysis of deformed materials we precise that low temperature nickel-induced solid solution hardening (SSH) can not be explained on the basis of classical SSH theories but more probably through nickel influence upon the Peierls stress. Moreover, we show that the APB tubes dragging model may be compatible with our microscopic and macroscopic results about the anomaly. Eventually, we put into relation a dynamic superdislocations multiplication process observation (in situ transmission microscopy) with the nickel-containing alloys tendency to cleavage.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Suzuki effect on \0 0 1\ stacking faults in boron-doped FeAl intermetallics

International audienceThree dimensional atom probe (3DAP) investigations of boron doped (400 ppm) FeAl40Ni3.8 samples aged at 400 °C for 1800 h, combined with electron microscopy observations, reveal that \001\ planar faults are boron enriched and aluminium depleted. The structure of such high energy defect is discussed and is thought to be stabilised by the segregation of boron. \textcopyright 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Effects of Solidification Conditions on Microstructure and Properties of High-Entropy Alloys from the CoCrFeMnNi Family

International audienceAlloys from the CoCrFeMnNi family remain the most studied austenitic high-entropy alloys. In this study, four alloys, i.e., Cantor alloy, A3S (modified nonequiatomic Cantor composition), both “pure” or doped with carbon (200 wt. ppm) and niobium (1.3 wt.%), were investigated. Firstly, alloys were induction cast using a cold-crucible method. The obtained ingots were molten, and rapidly solidified by melt-spinning at two cooling rates to obtain “ribbons”, typical of such processing. The effects of the solidification rate and the presence of carbon and niobium on the microstructure and hardness were studied. All the studied alloys show an fcc structure. The lattice parameter of the fcc phase increases with the increasing cooling rate, and with the addition of niobium and carbon, which confirms at least a partial presence of these elements in solid solution. Yet, TEM observations revealed the formation of nanometric NbC precipitates. The microstructure of melt-spun ribbons consists of equiaxed grains of a few micrometers in size. The higher cooling rate led to a small decrease in the grain size and a slight increase in hardness. Moreover, the hardness of doped alloys can be further improved by annealing (500°C for 24 h) through NbC precipitation