Search

Pathways for ductility Alloys containing multiple elements can be very strong but often suffer from poor ductility. F. Wang et al. found that different mechanisms accommodated plasticity in a molybdenum-niobium-titanium multiprincipal element alloy (see the Perspective by Cairney). Instead of so-called “screw” dislocations, deformation is accommodated by multiple pathways that include “edge” dislocations and activation of crystallographic slip planes. These results offer a design paradigm for developing new high-strength alloys. Science , this issue p. 95 ; see also p. 37

Complex, concentrated alloys (CCAs) containing refractory principal elements and Al can have microstructures with disordered body-centered cubic (BCC) and ordered B2 phases that are remarkably similar to superalloys. The simple idea of copying superalloy microstructures in these BCC-based refractory alloys introduces a set of audacious challenges. BCC metals and B2 compounds are often brittle at room temperature; the B2 phase is unstable in refractory-Al binaries and reasons for its stability in higher-order systems are a mystery; and many refractory metals and alloys display catastrophic oxidation. Here we discuss opportunities and challenges to develop RSAs as new high temperature structural materials.

A phenomenon of “phase inversion”, presumably the first ever experimental evidence in metallic alloys, is shown in a refractory high entropy alloy (RHEA), Al0.5NbTa0.8Ti1.5V0.2Zr. Phase inversion in crystalline solid systems is driven by the differences in elastic modulus of the two phases. Quenching from a high temperature single phase field, the RHEA exhibits a co-continuous mixture of a disordered BCC and an ordered B2 phase, that upon isothermal annealing at 600 °C develops via spinodal decomposition into a continuous B2 matrix with discrete cuboidal BCC precipitates aligned along the 〈001〉 directions. Longer term annealing at 600 °C results in the development of necking constrictions along the B2 channels, eventually pinching-off these channels and making the BCC phase continuous with discrete B2 precipitates. This inversion process can be related to the simultaneous operation of two processes: (i) spheroidization of the initially discrete cuboidal BCC precipitates driven by a reduction in the total interface energy and (ii) an increase in the stiffness of the B2 phase, relative to the BCC phase, due to chemical composition changes during annealing, forcing the B2 regions to become discrete driven by the reduction in the total elastic strain energy.

The microstructure, phase composition, and mechanical properties of NbTiZr, TaTiZr, Re0.3NbTiZr, and Re0.3TaTiZr are reported. The alloys were produced by vacuum arc melting and hot isostatically pressed (HIP’d) at 1400 °C for 3 h under 276 MPa hydrostatic pressure of high-purity argon prior to testing. NbTiZr had a single-phase BCC crystal structure, while TaTiZr had a Ti- and Zr-rich BCC matrix phase and Ta-rich nanometer-sized BCC precipitates, at volume fractions of 0.49 and 0.51, respectively. Re0.3NbTiZr consisted of a BCC matrix phase and Re-rich precipitates with a FCC crystal structure and the volume fraction of 0.14. The microstructure of Re0.3TaTiZr consisted of a Zr-rich BCC matrix phase and coarse, Re and Ta rich, BCC particles, which volume fraction was 0.47. NbTiZr and TaTiZr had a room temperature (RT) yield stress of 920 MPa and 1670 MPa, respectively. While, 10 at.% Re additions increased the RT yield stress to 1220 MPa in Re0.3NbTiZr and 1715 MPa in Re0.3TaTiZr. Re also considerably improved the RT ductility of TaTiZr, from about 2.5% to 10% of true strain. The positive strengthening effect from the Re additions was retained at high (800–1200 °C) temperatures.

International audience; Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, Tm) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000 °C and 1200 °C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 Tm, so that the alloys with higher Tm retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000 °C is probably due to dissolution of secondary phases.

Room temperature strength of nine multi-principle element body-centered cubic alloys are predicted using atomistic potentials and a solution-hardening model. The complex solute dislocation interactions are estimated using Embedded Atom Method potentials. At least six of these alloys are known to form single-phase solid solutions on annealing. Assuming that strength is rate limited by screw motion by kink migration, the interaction energies are incorporated into Suzuki's model for kink-solute interactions and evaluated at room temperature. The method predicts room temperature yield strength across all the alloys with a standard error of 13%.

Large scale, atomistic simulations of the core structure and mobility of ½[111] screw, edge and mixed dislocations in ternary multicomponent alloys (e.g. High Entropy alloys), NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5, are presented. The core structure of ½[111] screw dislocations continuously varies from compact to 3-fold with decreasing Nb content. The screw dislocation core structures in NbTiZr and Nb1.5TiZr0.5 are calculated using Embedded Atom Potentials (Johnson-Zhou) and compared with first-principles calculations of the screw dislocation in a quasi-random structure. In both simulations the dislocation core spreads on different (110) glide planes as the composition varies along the dislocation line in stoichiometric NbTiZr. The Nb-rich composition Nb1.5TiZr0.5 shows a compact core with very little core structure variation along the dislocation line in both First Principles and atomistic simulations. The screw dislocation deposits interstitial and vacancy dipole debris as it moves under stress. Average solute-dislocation core interaction energies in NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5 are derived from the average interatomic potential derived for each of the three systems. The interaction energies are used to determine the critical stress for the motion of ½[111] screw dislocations in the three systems as a function of temperature using the Suzuki model of kink migration controlled mobility developed for concentrated BCC random alloys. This analysis shows that the relatively high barrier for kink migration caused by fluctuations in solute concentration along the screw dislocation line and the dipole dragging stress associated with the screw dislocation motion results in a shallow fall-off of critical stress with temperature in these alloys as compared to simple BCC metals. Finally, the screw dislocation to edge and mixed dislocation critical stress ratio in NbTiZr are shown to be low, ∼2 at 5 K, in contrast to simple BCC metals, where it could be as high as 100–1000.

Equilibrium phase diagrams for ten MeX(NbTiZr)100-X alloy systems, where Me is Al, Cr, Fe, Hf, Mo, Re, Si, Ta, V or W and X ranges from 0 to 25 at% were calculated using the PanNb2018a database recently developed by CompuTherm, LLC. By the type of phases and sequence of formation, these alloy systems can be divided into two groups. The first group of quaternary alloys, containing Al, Hf, Mo, Ta, or V has a single-phase BCC region below the solidus line over the entire X range and can be considered as potential candidates for the development of single-phase high entropy alloys. The second group consists of the quaternary alloy systems in which the fourth element (Cr, Fe, Re, Si or W) has limited solubility in BCC NbTiZr, which leads to the formation of an additional phase below the solidus and above the solubility limit. These quaternary alloy systems can be used for the development of precipitation or dispersoid strengthened complex concentrated alloys. To verify CALPHAD calculations three representative alloys, Cr10Nb30Ti30Zr30, Ta10Nb30Ti30Zr30 and Re10Nb30Ti30Zr30, were prepared by arc melting. The alloy densities were 6.56, 7.81 and 7.85 g/cm3, respectively. The phase compositions of the produced alloys agreed satisfactorily with CALPHAD calculations. Mechanical properties were also studied and compared with those of NbTiZr. At room temperature (RT) all the alloys showed high hardness exceeding 350 Hv and high compression yield stress exceeding 1000 MPa. RT compression ductility of Re and Ta containing alloys was above 50%, but Cr-containing alloy showed low ductility of 5%. With an increase in temperature ≥800 °C, compression strength decreased more rapidly for the Cr-containing alloy, which at 1200 °C became softer than NbTiZr. The least temperature dependence of the strength was observed for the Ta-containing alloy, which became the strongest at 1200 °C.

Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict whether a material will be brittle or ductile. A phenomenological correlation by Pugh between metal brittleness and its shear modulus to bulk modulus ratio was established more than 60 years ago. Nearly four decades later Pettifor conducted a quantum mechanical analysis of bond hybridization in a series of intermetallics and derived a separate ductility criterion based on the difference between two single-crystal elastic constants, C12–C44. In this paper, we discover the link between these two criteria and show that they are identical for materials with cubic crystal structures.

This data article presents a compilation of mechanical properties of 630 multi-principal element alloys (MPEAs). Built upon recently published MPEA databases, this article includes updated records from previous reviews (with minor error corrections) along with new data from articles that were published since 2019. The extracted properties include reported composition, processing method, microstructure, density, hardness, yield strength, ultimate tensile strength (or maximum compression strength), elongation (or maximum compression strain), and Young’s modulus. Additionally, descriptors (e.g. grain size) not included in previous reviews were also extracted for articles that reported them. The database is hosted and continually updated on an open data platform, Citrination. To promote interpretation, some data are graphically presented., Measurement(s) alloy • oxygen content • carbon content • nitrogen content • yield strength • elongation • ultimate strength • hardness • density • grain size • observed phases Technology Type(s) digital curation Factor Type(s) alloy processing method • test temperature • test type • alloy composition Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12804137

Microstructure and compression properties of (Hf0.73Ta0.27)100-XMoX alloys, where X = 0, 5, 16, 21 and 30 at. %, are reported. The alloys were prepared by vacuum arc melting and hot isostatically pressed at 207 MPa for 3 h at 1400 °C. The alloys with 0, 5, 16 and 21 at. % Mo contained two phases, BCC and HCP, while the alloy with 30 at. % Mo contained three phases, BCC, HCP and cubic Laves (C15). The Hf73Ta27, Hf69Ta26Mo5 and Hf61Ta23Mo16 alloys experienced eutectoid transformation with the formation of a mixture of Hf-rich HCP and Ta-rich BCC phases. The volume fraction of the eutectoid regions decreased from 55% to 0% with increasing Mo from 0 to 21 at.%. The alloy with 30% Mo (Hf51Ta19Mo30) experienced eutectoid transformation with the formation of a mixture of the Mo-rich cubic Laves (C15) phase and Hf-rich HCP phase. The volume fraction of the eutectoidally transformed regions in Hf51Ta19Mo30 was ~20%. Among the studied alloys, Hf73Ta27 had the highest room temperature yield stress of 1738 MPa. The yield stress continuously decreased with increasing Mo concentration to 1468 MPa at 16% Mo and then increased to 1672 MPa at 30% Mo. The room temperature ductility strongly depended on the amount of Mo. The maximum true fracture strain of 0.137 was achieved in Hf69Ta26Mo5, but it rapidly decreased to 0.027 in Hf51Ta19Mo30. All the studied alloys were ductile at 1000–1400 °C and no fracture was observed after 0.7 true strain. The yield stress increased almost linearly with increasing the amount of Mo at these high temperatures.

The 1200 °C oxidation behaviors of CrNb, CrNbTi and CrNbTaTi are reported. All alloys formed some combination of BCC and Laves phases. However, additions of Ti produced a unique inflection point where the microstructure inverts to a Ti-rich BCC matrix with Laves phase precipitates. Further additions of Ta to CrNbTi were found to increase the overall phase fraction of BCC phase. The oxidation kinetics for CrNb and CrNbTi were similar and nearly parabolic throughout the duration of exposure. CrNbTaTi exhibited inferior oxidation kinetics and mixed mode behavior throughout oxidation testing. All alloys formed intricate multi-phase oxide structures, consisting of a combination of both simple and complex oxides. The resulting microstructures are characterized using a variety of analytical techniques and the results are discussed relative to modeling predictions using ab initio density functional theory (DFT).

International audience; This data article presents the compilation of mechanical properties for 370 high entropy alloys (HEAs) and complex concentrated alloys (CCAs) reported in the period from 2004 to 2016. The data sheet includes alloy composition, type of microstructures, density, hardness, type of tests to measure the room temperature mechanical properties, yield strength, elongation, ultimate strength and Young׳s modulus. For 27 refractory HEAs (RHEAs), the yield stress and elongation are given as a function of the testing temperature. The data are stored in a database provided in Supplementary materials, and for practical use they are tabulated in the present paper. The database was used in recent publications by Miracle and Senkov [1], Gorsse et al. [2] and Senkov et al. [3].

The inertia friction welding behavior of similar (LSHR to LSHR or Mar-M247 to Mar-M247) and dissimilar (LSHR to Mar-M247) alloy pairs was determined. The relationships between the IFW process parameters and welding response, such as the kinetics and efficiency of the transformation of the mechanical energy of the flywheel into heat of joining samples, weld duration, kinetics of flash formation, and bond quality were identified. Conditions resulting in sound welding were determined. It was found that the mechanistic behavior of each of the alloys during dissimilar welding can be approximated by the behavior of the respective alloy during similar welding. By contrast, the microstructure, microhardness and the thickness of the heat affected zone depended on the type of the alloy pair used during IFW.

Refractory high-entropy alloys (RHEAs) have recently attracted much attention, primarily due to their mechanical properties at elevated temperatures. However, the equilibrium phase-stability of these alloy systems is not well established. The present investigation focuses on the phase stability of Al0.5NbTa0.8Ti1.5V0.2Zr RHEA at temperatures ranging from 600 to 1200 °C. The detailed phase characterization involves coupling of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The stable phases present at these temperatures are (i) 1200 °C—body-centered cubic (BCC) matrix with nano-B2 precipitates; (ii) 1000 °C and 800 °C—a BCC matrix phase with Al–Zr rich hexagonal closed packed intermetallic precipitates and, (iii) 600 °C—a BCC + B2 microstructure, comprising a continuous BCC matrix with discrete B2 precipitates. These results highlight the substantial changes in phase stability as a function of temperature in RHEAs, and high-entropy alloys in general, and also the importance of accounting for these changes especially while designing alloys for high temperature applications.

Process efficiency is a crucial parameter for inertia friction welding (IFW). A new method is developed to determine the efficiency by comparing the workpiece torque used to heat and deform the joi...

Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

Phase composition, microstructure and mechanical properties of six complex concentrated alloys, NbTiZr, NbTiZrV, NbTiZrVMo, NbTiZrVTa, NbTiZrVCr, and NbTiZrVAl0.24, are reported. The alloy density was in the range from 6.34 g/cm3 to 8.43 g/cm3. After homogenization annealing at 1400 °C or 1200 °C, only the ternary NbTiZr alloy had a single-phase BCC crystal structure. NbTiZrV and NbTiZrVTa produced predominantly single-phase BCC structure with clusters of fine V-rich precipitates inside NbTiZrV grains and fine, Zr-rich grain boundary precipitates in NbTiZrVTa. The other three alloys contained BCC and Laves phases, and NbTiZrVMo also contained a second BCC phase. After compression deformation at 1000 °C, noticeable changes in the phase composition were found in NbTiZrV, NbTiZrVTa and NbTiZrVAl0.24, while the phase composition of other alloys retained unchanged. NbTiZr, NbTiZrV and NbTiZrVTa showed good ductility and noticeable work hardening rate at room temperature, while the ductility of other alloys rapidly decreased with increasing the amount of Laves phase. The RT yield stress varied from 975 MPa for NbTiZ to 1510 MPa for NbTiZrVMo. At 1000 °C, all the alloys showed excellent compression ductility; however only NbTiZrVCr and NbTiZrVMo were stronger than NbTiZr. The results indicate that increasing the composition complexity of refractory near-equiatomic alloys by increasing the number of the alloying elements does not necessary result in a simpler phase structure and/or better mechanical properties. The type of the alloying elements seems to be more important than their number. Additions of V, Al or Ta decrease while additions of Cr or Mo increase the 1000 °C strength of NbTiZr.

CALPHAD databases have traditionally been developed for investigation of single-principal component alloys. With the advent of batch processing capability, engineering teams have proposed using these models to systematically explore compositional space for multiprincipal element systems. However, the uncertainty of phase equilibria predictions outside of traditional compositional bounds has yet to be evaluated. This study assesses the current capabilities of commercially available CALPHAD databases to predict phase equilibria within ternary phase space as a function of the number of full binary system descriptions contained within the thermodynamic databases, the spatial location in compositional space relative to subsystem descriptions, and the specific database used. A strong correlation was observed between the fraction of subsystem descriptions available for the free energy calculation and the accuracy of phase predictions in undefined ternary space. The accuracy of equilibria predictions degraded with increased compositional extrapolation from defined subsystems.

The microstructure and tensile properties of HfNbTaTiZr after cold working and annealing were investigated. Cold work was introduced by axial compression followed by rolling resulting in a total thickness reduction of 89 pct without any evidence of cracking. The cold-worked material retained a single-phase microstructure and had a room temperature tensile yield stress σ0.2 = 1438 MPa, peak true stress σp = 1495 MPa, and true fracture strain ef = 5 pct. Annealing at 800 °C for up to 256 hours resulted in the precipitation of Nb and Ta rich particles with a BCC crystal structure inside a Hf-and-Zr-enriched BCC matrix. The second phase particles nucleated heterogeneously inside deformation bands and slip lines and coarsened during annealing. Analysis of the coarsening behavior suggested that kinetics were controlled by the diffusion of Nb and Ta. In the two-phase material, σ0.2 and σp decreased from 1159 to 1071 MPa and from 1174 to 1074 MPa, respectively, with an increase in particle diameter from 0.18 to 0.72 μm, while ef remained between 5 and 8 pct. Full recrystallization and normal grain growth, with the activation energy of 238 kJ/mol and activation volume of 5.3 to 9.6 m3/mol, occurred during annealing above 1000 °C. After heat treatment at this temperature, the alloy was characterized by a single-phase BCC structure with σ0.2 = 1110 to 1115 MPa, σp = 1160 to 1195 MPa, and ef = 12 to 19 pct with the maximum values attained after annealing for 1 hour.

An AlMo0.5NbTa0.5TiZr baseline alloy was shown earlier to have good high temperature strength but poor ductility below 600 °C due to coarse intermetallic grain boundary particles and a continuous ordered B2 matrix phase. Systematic composition changes intended to remove the deleterious microstructural features and to improve mechanical properties were explored in the present work. The baseline alloy and the new alloys studied here, AlMo0.5NbTa0.5TiZr0.5, AlNbTa0.5TiZr0.5, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, all had an ordered B2 matrix crystal structure. Additionally, coherent BCC nanoscale precipitates were present at a high volume fraction inside the B2 matrix grains in AlMo0.5NbTa0.5TiZr, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, and/or coarse, grain-boundary particles existed in AlMo0.5NbTa0.5TiZr and AlMo0.5NbTa0.5TiZr0.5. The mechanical properties were assessed with microhardness and compression testing at 25 °C and 1000 °C. Al0.5Mo0.5NbTa0.5TiZr showed the highest hardness (Hv = 6.4 GPa) and strength (σ0.2 = 2350 MPa) at 25 °C and modest strength (σ0.2 = 579 MPa) at 1000 °C. AlMo0.5NbTa0.5TiZr0.5 had the highest strength (σ0.2 = 935 MPa) at 1000 °C, but was brittle at 25 °C. High-temperature deformation produced a desirable microstructure in Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr alloys consisting of a continuous BCC phase and discontinuous B2 nano-precipitates. The relationships between the composition, microstructure, and properties were identified and discussed. Keywords: High entropy alloy, Superalloy, Microstructure, Mechanical properties

Solution hardening in body centered cubic alloys as a function of temperature can be well described by the Suzuki model. This model includes significant strengthening from the dragging of edge dislocation dipoles produced by kink collisions along the screw dislocations. However, the model does not consider diffusion-controlled processes. Here we estimate a critical temperature Tcr above which diffusional effects become important, resulting in the collapse of edge dipoles. Loss of edge dipoles leads to a significant decrease in yield strength above Tcr and marks the temperature limit for structural applications of these single phase body centered cubic alloys.

The microstructures and mechanical properties in compression (25 °C, 1000 °C and 1200 °C) of CrNb, CrNbTi and CrNbTaTi are reported. The CrNb alloy consisted of a Cr2Nb Laves phase matrix and Nb-rich BCC precipitates. After additions of Ti (CrNbTi alloy) the microstructure inverts to a Ti-rich BCC matrix, with Laves phase precipitates inside the matrix phase. Further additions of Ta to CrNbTi refined and decreased the volume fraction of the Laves phase particles in the CrNbTaTi alloy. CrNb exhibited limited ductility at 25 °C and 1000 °C, but became very ductile at 1200 °C and offered the highest compressive strength at all studied temperatures. CrNbTi and CrNbTaTi had better ductility, but suffered from a reduction in compressive strength.

Room temperature tensile and compressive properties of refractory complex, concentrated alloys (RCCAs) have been analyzed to explore correlations capable of anticipating ductility in this growing class of structural materials. The Pugh-Pettifor and Rice-Thomson ductility criteria are generally satisfied by the current dataset, but the dataset does not cover a sufficiently wide range to provide a critical evaluation. A previously suggested ductility criterion based on the average valence electron concentration (VEC) of an alloy, i.e. abrupt ductile to brittle transition at VEC = 4.5, is not supported in the present study. A revised VEC criterion is suggested, but the data supporting this trend is sparse and additional work is needed. These three ductility criteria are evaluated using only single-phase RCCA data. A new ductility criterion based on the RCCA yield strength (σy) is suggested from the present work, using both single-phase and multi-phase data. RCCAs with commercially relevant tensile ductility have σy ≤ 1200 MPa, and alloys with compressive ductility of ≥30% (analyzed here to provide useful tensile ductility) have σy ≤ 1500 MPa. Strengthening models to predict σy in RCCAs may thus be used to anticipate alloys that may also possess useable levels of ductility.

The relationships between the inertia friction welding process parameters (initial angular velocity, ωₒ, and moment of inertia, I, of the flywheel and the axial compression force, P) and welding behavior (welding time, sample upset and flash formation, efficiency and kinetics of friction-induced sample heating) of dissimilar superalloys were determined. The results showed that the initial kinetic energy of the flywheel, Eo = Iωₒ2/2, should not be considered as a key parameter for the design of the IFW process. Only a fraction of this energy (denoted as ‘sample energy’, ES), which is process-parameters dependent, is used to heat the workpieces at the weld interface. Together with I and P, ES controls the upset length, the duration of the welding process, temperature profiles near the weld interface and the weld quality. Optimization of the inertia friction welding for better quality, reduced energy consumption and materials saving requires information about the process efficiency η = ES/Eo and its dependence on the process parameters. Critical sample energy per unit surface area (≈ 79 MJ/m2) was identified above which good quality metallurgical bonds were produced between LSHR and Mar-M247 alloys.

Refractory complex concentrated alloys (RCCAs) represent an emerging class of materials that have displayed significant potential for increased mechanical performance at extreme temperatures over conventional refractory alloys and Ni-base superalloys. While limited work has been conducted in this area, several RCCAs have shown remarkable levels of oxidation resistance in comparison to the majority of uncoated commercial refractory alloys. However, the oxidation mechanisms in these types of alloys have not been well understood. This study aims to develop the foundation for understanding RCCA oxidation mechanisms through the systematic evaluation of NbTiZrV and NbTiZrCr, which display vastly different oxidation behaviors. In both cases, the predominant mechanism appeared to be internal oxidation, where a mixture of simple and complex oxides was observed after prolonged oxidation times. This resulted in extended linear oxidation kinetics for the NbTiZrV alloy and linear to near-parabolic oxidation kinetics for the NbTiZrCr alloy. Under this mindset, novel design strategies for promoting increased oxidation resistance in both RCCAs and dilute refractory alloys based on sluggish complex oxides are proposed.

The effect of microstructure and soft impingement on the kinetics of static dissolution of γ′ precipitates during supersolvus solution treatment of the powder-metallurgy nickel-base superalloy LSHR were determined experimentally and interpreted in the context of a simple diffusion model. The starting material had either a fine duplex structure of γ grains and γ′ precipitates (each of which were ~2 μm in diameter) or a structure comprising ~2-μm-diameter γ′ precipitates lying within large (~15 μm) γ grains. Using a direct-resistance-heated Gleeble® machine, the temperature of each sample was first equilibrated at a fixed subsolvus temperature (at which the microstructure was also stabilized) and then quickly raised to and held for a predetermined time in the single-phase γ field. The dissolution of γ′ was found to be approximately five times as fast for the duplex structure in comparison to the kinetics for the intragranular precipitates. The results were interpreted using numerical simulations of dissolution based on the Whelan model modified to account for a distribution of precipitates (rather than a single, isolated spherical particle) and soft impingement of diffusion fields. Using independently-measured values for diffusivity, precipitate composition, and phase equilibria as input, simulation predictions showed good agreement with the observations in terms of the temporal variation of volume fraction and average particle radius. By this means, insight into the retarding influence of soft impingement and the accelerating effect of pipe diffusion (due to stored dislocations) on the rate of dissolution was obtained

The radial temperature gradient developed via direct-resistance heating of round-bar hot-torsion specimens in a Gleeble® machine and its effect on the interpretation of plastic-flow behavior were established using a suite of experimental, analytical, and numerical-simulation tools. Observations of the microstructure variation developed within a γ′-strengthened nickel-base superalloy were used to infer the temperature gradient as well as differences between the temperature at the outer diameter and that indicated by thermocouples welded to the surface. At temperatures of the order of 1375 K (1102 °C), the radial variation of temperature was typically ~20 K (~20 °C). Such variations were in agreement with an analytical heat-conduction model based on the balance of input thermal energy and radiation heat loss at the free surface. Using a constitutive model for LSHR, the effect of the radial temperature gradient on plastic flow during hot torsion was assessed via numerical integration of the torque as a function of radial position for such cases as well as that corresponding to a uniformly-heated sample. These calculations revealed that the torque generated in the non-uniform case is almost identical to that developed in a sample uniformly preheated to a temperature corresponding to that experienced at a fractional radial location of 0.8 in the former case.

Refractory high-entropy alloys present attractive mechanical properties, i.e., high yield strength and fracture toughness, making them potential candidates for structural applications. Understandings of atomic and electronic interactions are important to reveal the origins for the formation of high-entropy alloys and their structure−dominated mechanical properties, thus enabling the development of a predictive approach for rapidly designing advanced materials. Here, we report the atomic and electronic basis for the valence−electron-concentration-categorized principles and the observed serration behavior in high-entropy alloys and high-entropy metallic glass, including MoNbTaW, MoNbVW, MoTaVW, HfNbTiZr, and Vitreloy-1 MG (Zr41Ti14Cu12.5Ni10Be22.5). We find that the yield strengths of high-entropy alloys and high-entropy metallic glass are a power-law function of the electron-work function, which is dominated by local atomic arrangements. Further, a reliance on the bonding-charge density provides a groundbreaking insight into the nature of loosely bonded spots in materials. The presence of strongly bonded clusters and weakly bonded glue atoms imply a serrated deformation of high-entropy alloys, resulting in intermittent avalanches of defects movement. A cluster-and-glue model of atomic arrangements explains the yield strength and mechanical response of high entropy alloys. Inspired by metallic glass, a team led by William Yi Wang at China’s Northwestern Polytechnical University and collaborators in the United States of America used molecular dynamics to build different atomic arrangements of refractory high entropy alloys consisting of four or more elements. Depending on atomic size and the periodic table group of each atom, some atoms organized into clusters while others glued the clusters together. Chemical bonds broke and formed with plastic deformation as the alloys went from one atomic arrangement to another via the glue atoms, causing defect avalanches explaining the serrated mechanical response of high entropy alloys. Taking into account atomic arrangement may thus help us predict the properties of high entropy alloys.

It has been widely assumed, but never proven, that the efficiency of the inertia friction welding (IFW) process is independent of process parameters and is relatively high, i.e., ~70 to 95 pct. In the present work, the effect of IFW parameters on process efficiency was established. For this purpose, a series of IFW trials was conducted for the solid-state joining of two dissimilar nickel-base superalloys (LSHR and Mar-M247) using various combinations of initial kinetic energy (i.e., the total weld energy, E o), initial flywheel angular velocity (ω o), flywheel moment of inertia (I), and axial compression force (P). The kinetics of the conversion of the welding energy to heating of the faying sample surfaces (i.e., the sample energy) vs parasitic losses to the welding machine itself were determined by measuring the friction torque on the sample surfaces (M S) and in the machine bearings (M M). It was found that the rotating parts of the welding machine can consume a significant fraction of the total energy. Specifically, the parasitic losses ranged from 28 to 80 pct of the total weld energy. The losses increased (and the corresponding IFW process efficiency decreased) as P increased (at constant I and E o), I decreased (at constant P and E o), and E o (or ω o) increased (at constant P and I). The results of this work thus provide guidelines for selecting process parameters which minimize energy losses and increase process efficiency during IFW.

The flow-stress behavior at hot-working temperatures and strain rates of the powder-metallurgy superalloy LSHR was determined under nominally-isothermal and transient-heating conditions. Two conventional methods, compression of right-circular cylinders and torsion of thin-walled tubes, were used for isothermal tests. A direct-resistance-heating technique utilizing torsion of round-bar specimens in a Gleeble® machine was applied for both isothermal and transient-heating conditions. When expressed in terms of effective stress and strain, baseline data determined by the two conventional methods showed good agreement. With the aid of a flow-localization analysis to assess the confounding influence of axial (and radial) temperature gradients on deformation uniformity, the flow stresses determined from nominally-isothermal Gleeble® torsion tests were shown to be broadly similar to those from the conventional tests. With regard to transient phenomena, Gleeble® tests were also useful in quantifying the effect of rapid heating and short soak time on the observed higher flow stress associated with a metastable microstructure. The present work also introduces two new test techniques using direct-resistance-heated torsion specimens. One involves continuous heating under constant-torque conditions, and the other comprises testing an individual specimen at a series of temperatures and strain rates. Using a single specimen, the former method enabled the determination of the apparent activation energy for plastic flow, which was similar to that determined from the series of isothermal tests; the latter provided a low-cost, high-throughput approach to quantify the flow behavior.

High entropy alloys (HEAs) are barely 12 years old. The field has stimulated new ideas and has inspired the exploration of the vast composition space offered by multi-principal element alloys (MPEAs). Here we present a critical review of this field, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment include definition of terms; thermodynamic analysis of complex, concentrated alloys (CCAs); taxonomy of current alloy families; microstructures; mechanical properties; potential applications; and future efforts. Based on detailed analyses, the following major results emerge. Although classical thermodynamic concepts are unchanged, trends in MPEAs can be different than in simpler alloys. Common thermodynamic perceptions can be misleading and new trends are described. From a strong focus on 3d transition metal alloys, there are now seven distinct CCA families. A new theme of designing alloy families by selecting elements to achieve a specific, intended purpose is starting to emerge. A comprehensive microstructural assessment is performed using three datasets: experimental data drawn from 408 different alloys and two computational datasets generated using the CALculated PHAse Diagram (CALPHAD) method. Each dataset emphasizes different elements and shows different microstructural trends. Trends in these three datasets are all predicted by a ‘structure in – structure out’ (SISO) analysis developed here that uses the weighted fractions of the constituent element crystal structures in each dataset. A total of 13 distinct multi-principal element single-phase fields are found in this microstructural assessment. Relationships between composition, microstructure and properties are established for 3d transition metal MPEAs, including the roles of Al, Cr and Cu. Critical evaluation shows that commercial austenitic stainless steels and nickel alloys with 3 or more principal elements are MPEAs, as well as some established functional materials. Mechanical properties of 3d transition metal CCAs are equivalent to commercial austenitic stainless steels and nickel alloys, while some refractory metal CCAs show potential to extend the service strength and/or temperature of nickel superalloys. Detailed analyses of microstructures and properties allow two major HEA hypotheses to be resolved. Although the ‘entropy effect’ is not supported by the present data, it has nevertheless made an enduring contribution by inspiring a clearer understanding of the importance of configurational entropy on phase stability. The ‘sluggish diffusion’ hypothesis is also not supported by available data, but it motivates re-evaluation of a classical concept of metallic diffusion. Building on recent published work, the CCA field has expanded to include materials with metallic, ionic or covalent bonding. It also includes microstructures with any number of phases and any type of phases. Finally, the MPEA field is shown to include both structural and functional materials applications. A significant number of future efforts are recommended, with an emphasis on developing high-throughput experiments and computations for structural materials. The review concludes with a brief description of major accomplishments of the field and insights gained from the first 12 years of research. The field has lost none of its potency and continues to pose new questions and offer new possibilities. The vast range of complex compositions and microstructures remains the most compelling motivation for future studies.

This paper reports the microstructural evolution and phase stability in a newly developed low-density Al10Nb15Ta5Ti30Zr40 refractory high entropy alloy (RHEA) at different temperatures. This alloy composition was adapted from the composition of the B2 phase in a two-phase B2+BCC mixture at 1000°C in the refractory high entropy superalloy Al0.25NbTaTiZr. After homogenizing at a high-temperature, followed by fast or slow cooling to room temperature, this alloy exhibited a nano-scale mixture of co-continuous BCC and B2 phases, resembling a spinodally decomposed microstructure with concurrent ordering. Interestingly, this novel nano-scale BCC+B2 microstructure exhibits excellent room temperature compressive yield strength (~1075MPa) and ductility (true strain at failure ~ 0.55). Annealing at 600°C and 750°C resulted in the formation of additional ordered omega type AlZr2 phase in this alloy. The experimentally observed phase evolution is in fair agreement with CALPHAD predictions.

An analytic model describing substitutional solid solution strengthening in body-centered cubic (BCC) complex, concentrated alloys (CCAs) based on a/2 screw dislocation mobility is developed and presented. At lower temperatures it is assumed that dipole dragging is the rate limiting strengthening mechanism, and as the temperature increases this changes to a jog dragging mechanism. While both mechanisms are contained in Suzuki's classical model for substitutional solid solution strengthening developed for BCC alloys significant modifications are required to apply such a model to the refractory CCA's. First, two approaches for incorporating the net effects of the complex solute chemistry are developed and contrasted using a ternary NbTaTi alloy and four quaternary alloys, MoNbTaTi, WNbTaTi, CrMoNbTi and CrMoTaTi . Our model is extended to higher temperatures and applied to analyze experimental yield data as a function of temperature (0.15–0.7TL, where TL is the absolute liquidus temperature) and strain rate (10−5 to 10−2/s) in a range of BCC chemically complex alloys including stoichiometric NbTiZr and HfNbTaTiZr. We find that the model, based on just these two mechanisms, gives excellent agreement with experimental yield data in both these BCC CCAs, NbTiZr and HfNbTaTiZr. The experimental yield data of these two BCC CCA's as well as NbTaTi and four quaternaries are also analyzed using Maresca's edge dislocation model developed for these types of alloys. The screw model presented is useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

This study reports the microstructure, phase composition, mechanical properties and oxidation behavior of NbTaTi and NbTaZr ternary equiatomic alloys. These ternary components, together with earlier reported NbTiZr and MoNbTi, are commonly observed in refractory high entropy alloys (RHEAs) with a single-phase BCC or multi-phase crystal structures. Therefore, the reported information about these ternary alloys is thought to be useful for the development of advanced RHEAs. While NbTiZr and MoNbTi are single-phase BCC structures, NbTaTi and NbTaZr are multi-phase alloys. NbTaTi consists of a BCC matrix and discontinuous, Ti-rich precipitates with BCC and FCC crystal structures. NbTaZr consists of a co-continuous spinodal mixture of two BCC phases, one of which is rich in Ta and Nb and another is rich in Zr, and it also contains a Zr-rich HCP phase in the form of coarse grain-boundary particles. NbTaTi has compression yield stress of 724 MPa at 25 °C, 268 MPa at 800 °C and 129 MPa at 1200 °C. NbTaZr is stronger, with the compression yield stress of 1027 MPa at 25 °C, 530 MPa at 800 °C and 183 MPa at 1200 °C. Both alloys show rapid oxidation with formation of complex oxides during holding at 1200 °C.