The effect of microstructure and strain rate on the room-temperature (RT) and 700 degrees C compression deformation behavior of a powder metallurgy processed gamma-TiAl intermetallic alloy, Ti-45Al-2Nb-2Mn (at.%)-0.8 (vol%) TiB2, was investigated. Samples were heat-treated to obtain a duplex two-phase alpha(2)+gamma microstructure and two nearly fully-lamellar alpha(2)+gamma microstructures with different lamellar spacings and gamma-phase volume fractions. Compression experiments were performed to a minimum deformation of 10% true strain under strain rates of 10(-2), 10(-3), 10(-4), and 10(-5) s(-1). The compression strength, strain rate sensitivity, colony size, interlamellar spacing, and microhardness were dependent on microstructure. The nearly fully-lamellar microstructures exhibited higher compression strengths than the duplex microstructure for all the testing conditions. The strain rate sensitivity index (m), tended to increase with increasing temperature, and for the 700 degrees C deformation, m increased with increasing true strain. The apparent activation volumes, decreased with increasing true strain at 700 degrees C. Scanning electron microscopy observations showed that cracking preferentially occurred within the gamma phase and the extent of cracking increased with increased temperature, strain, and strain rate. Overall, the RT deformation was considered to be controlled by dislocation glide, while at elevated temperature, the likely thermally activated process controlling dislocation glide was associated with the forest junctions acting as pinning points.

According to a recent Hume-Rothery approach, the electron concentration, e/a, and the average radius can be used to identify the domain of stability of HEAs and to estimate the phases that may occur in the alloy. The present study investigates the influence of the electronic structure and the average radius on the hardness for a series of HEA alloys. The alloys investigated in this work all contained Co, Fe and Ni as base elements. To this base system one or more elements were added, including Al, Cr, Cu, Sn, Pd, Ru, Ti, and V in different proportions. For comparison, data on phases identified and hardness have been taken from a wide range of bibliography for other types of alloys in the systems Co Cr Fe Cu A B C D E F, with A, B, C, D, E, F = Al, Ti, V, Nb, Cu, Mo, Mn, B, Si, Y, Sc, Ru, Re, Gd, Dy, Ho, Lu, Tb, Er, Tm, La, W, Ta, Hf, Zr. In order to predict the occurrence of mainly fcc, bcc and hcp phases, the average atomic radius is preferable over to the average radius for a 12 nearest atoms neighbourhood. Based on this [e/a; radius] system, it is shown that the hardness of the HEA composition can be predicted. By using this classification, it is possible to determine compositions of HEA alloys with adequate range of hardness, density and phases present. The consequences of such predictions when modelling the structure and mechanical behaviour of HEAs is fundamental for their application. (C) 2021 Elsevier B.V. All rights reserved.

Ti-22Al-26Nb(at.%) and Ti-22Al-26Nb-5B(at.%) compacts were processed from gas atomized prealloyed powders by field assisted hot pressing (FAHP) for between 600 and 1200 s at temperatures between 725 and 1070 degrees C and the resulting microstructures were compared. The average powder diameters ranged between 50 and 100 mu m, where both the B-containing powders and resulting FAHP compacts contained elongated B-rich needles. The microstructure in the FAHP compacts depended on the processing temperature. For processing temperatures less than or equal to 960 degrees C, porosity was evident on the FAHP compact, where greater porosity was observed with decreasing temperature. Processing temperatures greater than or equal to 980 degrees C resulted in minimal porosity and three-phase microstructures containing a mixture of the orthorhombic, alpha(2), and body-centered-cubic (BCC) phases, where longer processing times led to microstructural coarsening. The microstructures of the FAHP compacts were compared to those from compacts consolidated at 1027 degrees C for 4 h using hot isostatic pressing (HIP). For both HIP and FAHP processing, the B-containing compacts exhibited finer prior-BCC grain sizes. Compositional analysis of the B-rich phase within the Ti-22Al-26Nb HIP compact suggested that it could be the B27 orthorhombic structure with a B2TiNb stoichiometry. Overall, FAHP was shown to be a viable powder metallurgy processing technique for intermetallic Ti2AlNb based intermetallic alloys. (C) 2020 Elsevier B.V. All rights reserved.

High Entropy Alloys (HEA) can be classified in three domains according to their e/a and r values, with e/a, the number of itinerant valence electrons and r the average radius for a 12 nearest atoms neighborhood. The phase composition, thermal stability and possible phase transformations of a series of HEA alloys, CoCrzFeNi-XY (with X and Y = Al, Cu, Pd, Ru, Ti and z = 0 or 1), selected according to their e/a ratio were investigated in cast conditions (TO), after 3 h homogenization at 1100 degrees C (TI) and after 3 h annealing at 700 degrees C (T3). When observing the behavior of the different Domains of HEAs as classified by electronic structure it is observed that for the alloys from Domain I which contain fcc structures, the microstructure transforms from multi-to almost single-phase under homogenization (T1). In Domain III alloys containing cubic (bcc and/or B2) structures, very small multi-structural changes are observed. Alloys in Domain II have a mixed structure, i.e. several different structures in the diffraction pattern, which changes during heat treatments. (C) 2020 Elsevier B.V. All rights reserved.

High entropy CrxAlFeCoNi alloys with x = 0, 0.5, 1.0 and 1.5 were synthesized using arc-melting and sintering preparation techniques. Three crystal structures (fcc, bcc and sigma) were observed using XRD technique, while EDX measurements showed the presence of up to three chemically different phases (FeCr-rich phase with fcc structure, AINi-rich phase with bcc structure and Cr-rich phase with bcc and/or sigma structures). The reasons for the observed phase coexistence were addressed to total energy electronic structure calculations using KKR-CPA method accounting for chemical disorder effects. Such theoretical analysis confirmed that the multi-phase system was energetically more favorable than the single-phase one. Furthermore, DSC measurements allowed to identify two phase transitions in melted samples, unlike sintered ones due to high-temperature nitrogen corrosion. This process turned out to be highly selective, resulting in the formation of the scales consisting of AlnNm-phases at the cost of total Al loss in the HEA alloy. Crown Copyright (C) 2019 Published by Elsevier B.V. All rights reserved.

The structural and magnetic properties of CoCrFeyNi and CoCrFeNi-Pdx alloys earlier investigated experimentally by x-ray and neutron diffraction techniques and magnetometry have been theoretically reproduced using two complementary approaches for electronic structure calculations, i.e. the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) and implemented in the ab initio framework of density functional theory and the Vienna ab initio simulation package (VASP) for supercell models of high-entropy alloy (HEA) structures. The comparison between experimental results and calculations of the lattice constants by both calculation methods indicate that the structure of CoCrFeyNi is well described by ordered fcc configurations. The values of local magnetic moments on Fe, Co, Cr, and Ni atoms depend not only on the Pd concentration but on chemical disordering. In the case of the CoCrFeNi-Pdx alloys, the KKR-CPA and the VASP calculations of disordered configurations reproduce the experimental values at 5 K up to equimolar composition and at 300 K above. The experimental values above the equimolar composition at 5 K are not satisfactorily reproduced by any of the calculations. The divergence between the experimental and calculated values is related to the variation of the ferromagnetic to paramagnetic transition temperature as a function of palladium content and to the existence of several phases, FeCoCr-rich above room temperature and FeCrPd-rich below, observed by diffraction and detected by microscopy and atom probe investigations. VASP calculations of a FeCrPd-rich phase effectively reproduced both the lattice constant and magnetization of the alloy above equimolar composition. An important conclusion of this work is that the combined analysis of the electronic, structural, and magnetic properties plays an important role in understanding the complexity of magnetic HEAs.

The structure of CoCrFeyNi (y = 0, 0.8 and 1.2) and CoCrFeNi-Pd-x (x = 0.0, 0.5, 0.8, 1.0, 1.2 and 1.5) High Entropy Alloys has been investigated by neutron and standard X-ray as well as by high-energy X-ray diffraction techniques. The alloys were produced by arc melting and afterwards heat treated under several different conditions. It has been concluded that the CoCrFeNi alloy in as-cast condition is, contrary to what is claimed in the literature, not single-phase but consists of at least two different phases, both of fcc type. The difference in lattice constant between the two phases is close to 0.001 angstrom. Diffraction patterns measured by X-ray and neutron diffraction have shown that the structure of the alloy is not affected by 3 h heat treatment up to 1100 degrees C. Changing the amount of Fe has no drastic effect on alloy structure. The Pd-containing alloys have also all been found not to be single-phase but to consist of at least four different phases, all being of fcc type. The lattice constants for all phases increase with Pd content. The relative amounts of the different phases depend on Pd concentration. Furthermore, heat treatments of 3 h duration at different temperatures have a significant effect on the alloy phase composition. It is suggested that HEAs should be considered as multicomponent alloys presenting "simple" diffraction patterns, e.g. consisting of one or several lattices of fcc, hcp or bcc type with very close lattice parameters. (C) 2016 Elsevier B.V. All rights reserved.

The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanisms behind the composition-property relationships are unclear. One reason is that few systematic and accurate determinations of their composition-dependent structure on atomic level have been made. In this paper some results on the structure obtained by X-ray and neutron diffraction of the CoCrFeNi alloy, to which Pd, Sn and Cu have been added in different amounts, are reported. The investigations make it obvious that none of the alloys is completely homogeneous, as has earlier been suggested, and that they do not form a perfect solid solution.

The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanism behind the composition-property relationship is unclear. One reason is that few systematic and accurate determinations of the composition-dependent structure on the atomic level and of the physical properties have been made. In this paper we report on the structure and physical properties of CoCrFeNi and CoCrFeNi-X, (X=Pd, Sn, Ru) alloys of equimolar composition using different experimental techniques (microscopy, neutron and X-ray diffraction, atom probe tomography, Mössbauer spectroscopy, calorimetry). The results show that i) the alloys are not completely homogeneous as is generally suggested in existing literature; ii) they do not form a perfect solid solution; iii) their structure is not single phase, even not either fcc or bcc.

The structure of a family of high entropy alloys based on the composition CoCrFeNi, to which Pd and Sn have been added, is presented. The results stem from combined investigations by atom probe tomography as well as by scanning and transmission electron microscopy on samples produced by arc melting. Although CoCrFeNi is of fcc structure, the sample is not homogeneous on atomic scale. The addition of Pd as a fth element retains the fcc lattice with the indication of the coexistence of at least two additional phases. The addition of Sn changes the general structure considerably.

A new generation of steels has been designed, which on transformation at low temperature (200–350 °C), leads to a nano-scale microstructure, known as NANOBAIN. The microstructure consists of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes (20–40 nm). These advanced steels present the highest strength/toughness combinations ever recorded in bainitic steels. Their properties are mainly a consequence of the formation of nanoscale bainitic ferrite plates at very low temperatures. Transmission electron microscopy observations have shown that plastic relaxation in the austenite adjacent to the bainite plates may control the final size of the bainitic ferrite plates. The dislocation debris generated in this process resists the advance of the bainitic ferrite–austenite interface, the resistance being greatest for strong austenite. The yield strength of the austenite must then feature in any assessment of plate size. In this scenario, the plates are expected to become thicker at high temperatures because the yield strength of the austenite will then be lower. The goal of this study is to evaluate the influence of yield strength of austenite to the nanoscale structural refinement of advanced bainitic steels. In this sense, in situ measurements of austenite strength before bainite formation using a deformation dilatometer Bähr 805D have been performed in a medium carbon high silicon steel transforming at intermediate temperatures (325–400 °C) to a submicron structure of bainite and in a high carbon high silicon steel transforming at low temperatures (200–350 °C) to nanostructured bainite. The role of the transformation driving force on the bainite plate thickness will be also discussed.

Since the discovery of bainite, research over many decades has revealed a substantial amount of information about the mechanism of the bainite transformation in steels. Elements of the theory are now routinely being used in many parts of the world in the design of novel alloys and in the interpretation of a variety of experimental data. However, current experimental and theoretical understanding is limiting technological progress. The purpose of this atom probe tomography study was to track atom distributions during the bainite reaction in a nanocrystalline steel. The results are providing new experimental evidence on subjects critically relevant to the understanding of the atomic mechanisms controlling bainitic ferrite formation, such as the incomplete transformation phenomenon, the carbon supersaturation of ferrite, and the plastic accommodation of the surrounding austenite.

Advanced high strength steels for automotive applications were designed to achieve a carbide-free bainitic microstructure after conventional thermo-mechanical processing and a continuous annealing treatment. The microstructure obtained consists of ferrite laths interwoven with thin films of untransformed retained austenite. The sufficiently tough matrix and the control of the heterogeneity in the microstructure will allow an optimum combination of strength, ductility, and formability to be achieved. The designed steels reached far higher uniform elongations than that in commercial dual phase steels and martensitic steels with the same range of ultimate tensile strengths. Their formability was found to be appropriate for the production of final parts after cold-stamping or cold-forming. On the other hand, the yield strength/ultimate tensile strengths ratio was found to remain roughly constant (similar to 0.7). The reduction of area value did not seem to change as a function of overaging temperature, but the V-bending angle and the hole expansion ratio (cut-edge stretching ability) decreased significantly at the bainite holding temperature increases. (C) 2013 Elsevier Ltd. All rights reserved.

El objetivo de este trabajo se centra en el estudio de los mecanismos de descomposición isotérmica de la austenita en perlita en un acero de composición 0,44C-0,73 Mn. En particular, se estudia cómo afecta la temperatura de austenización (Tγ), y por tanto, el tamaño de grano austenítico de partida (TGA), en la cinética de formación de la perlita. Se ha comprobado que existe una relación directa entre Tγ y el tamaño medio de las colonias perlíticas. Por el contario, el espaciado interlaminar perlítico no depende de Tγ, sino que está controlado por la temperatura de descomposición isotérmica (T), de la austenita en perlita. Finalmente se ha observado que la cinética de formación de perlita se ve acelerada para el caso de TGA pequeños y cuando el mecanismo predominante en el rango de temperaturas de trabajo es la difusión en volumen del carbono.

The carbon supersaturation of bainitic ferrite was investigated by means of atom probe tomography in three steels with different carbon and silicon contents, to elucidate the effect of transformation temperature and the reaction velocity on the mechanisms controlling bainite formation with and without the interference of cementite precipitation. Results indicated no difference in the growth mechanism over the temperature range investigated. These results provide new evidence that the bainite transformation is essentially martensitic in nature. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

With the increasing demand for high performance engine or suspension components, bainitic steels are receiving interest as potential replacement of their quench and tempered counterparts. Indeed, for a number of mechanical components, ferrite pearlite microstructures are no longer sufficient in terms of mechanical properties. Bainitic steel grades allow production of hot-rolled bars or forged components exhibiting a homogeneous bainitic microstructure and achieving UTS up to 1200 MPa without the need for additional heat-treatments [1]. During tempering, these V-microalloyed bainitic steels exhibit unusual yield strength variations, with a very pronounced increase around 250-300 °C followed by the better known secondary hardening peak for temperatures around 600-650 °C. Indeed, after tempering at 250-300 °C, some of these steels exhibit an increase in yield strength of up to 200 MPa, concurrent with an increase in impact toughness of up to 25%. This, however, goes unnoticed if hardness measurements are used to characterize tempering. In the following, results are presented for three different bainitic steel grades, and the origins of the changes in mechanical properties are discussed.

The goal of this paper is to determine the austenite-to-pearlite isothermal decomposition mechanisms in a 0.44C-0.73 Mn steel. More precisely, the role of austenitizing temperature (T-gamma), and hence the austenitic grain size (AGS), in the kinetics of pearlite formation has been studied. Results allow us to conclude that the average size of pearlitic colonies is increased as the AGS is increased. On the other hand, it appears that the interlamellar spacing of the pearlite does not depend on the T-gamma but is controlled by the isothermal decomposition temperature of austenite (T). Finally, it was found that formation of pearlite is triggered for small AGS values and isothermal decomposition temperature regimes where the predominant controlling mechanism is due to carbon volume diffusion.

Much recent work has been devoted to characterize the :microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125-350 degrees C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B-s-M-s temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2-1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to similar to 3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features. (c) 2012 Elsevier B.V. All rights reserved.

The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

A displacive transformation involves the motion of a glissile interface. As in work hardening, its motion can be halted by defects such as dislocations, stacking faults or twins in the austenite. The defects are created when the shape deformation accompanying bainite growth is accommodated by plastic relaxation of the surrounding austenite. The growing plate stops when it collides with the austenite grain boundary. Because transformation from strong austenite leads to fine plates, alloys can be designed such that the bainite transformation is suppressed to low temperatures (125–350°C), leading to a nanoscale bainitic microstructure. Complementary high-resolution transmission electron microscopy and atom probe tomography have provided new experimental evidence on the accommodation of transformation strain, a subject critically relevant to understanding the atomic mechanisms controlling bainitic ferrite growth.