Iron aluminides are promising structural materials for temperatures up to 700 degrees C. Among them, single-phase, B2ordered iron aluminides containing 35 - 50 at.% Al are of particular interest due to their exceptional oxidation and sulfidation resistance. However, their use is limited by low ductility, primarily caused by environmental embrittlement, especially in alloys with >40 at.% Al, and vacancy strengthening. Despite extensive research, the understanding of the combined effects of point defects, environmental embrittlement, and deformation mechanisms across the entire Al range has been lacking. The present study addresses this gap by systematically characterizing the composition- and temperature-dependent mechanical behavior of a series of alloys with 30 to 53 at.% Al from room temperature (RT) up to 700 degrees C. Particular emphasis was placed on achieving uniformly low impurity levels and establishing similar heat treatment conditions to reduce the strengthening effect of vacancies. Compression tests are employed to investigate the plastic deformation behavior and avoid premature failure. At RT, a minimum of hardness and offset yield strength is observed at 42 at.% Al, corresponding to the lowest vacancy and anti-site strengthening contribution. In the temperature range between 400 and 600 degrees C, a yield strength peak was observed for the alloys with 30 and 35 at.% Al related to the formation of D03-ordered domains (Fe-30Al) and the strengthening effect by thermal vacancies (Fe-35Al). The continuously decreasing strain hardening capability with increasing temperature and Al content is rationalized by thermally activated processes, increasing vacancy concentration and localized plastic deformation.

B2-ordered FeAl exhibits a wide homogeneity range (from 23.5 to 53at.% Al) and a strong sensitivity of its mechanical properties to point defects, but the individual roles of vacancies and anti-site atoms remain insufficiently clarified. In this work, we investigate how these defects influence hardness and reduced modulus across the full single-phase composition range of B2-ordered FeAl. For this purpose, diffusion couples between two Fe-Al alloys with 30 and 53at.% Al were prepared and mechanical properties were studied by nanoindentation. Diffusion bonding at 1000 °C followed by various heat treatments at 400-1000 °C is employed to systematically vary the concentration of quenched-in vacancies, while anti-site defect concentrations are primarily dictated by composition. Nanoindentation along the composition gradient reveals that low-vacancy states exhibit a non-monotonic hardness–composition behavior with a distinct minimum in hardness, whereas high-vacancy states show a monotonic increase in hardness with increasing Al content. Comparison of differently heat-treated conditions demonstrates that such trends can be explained by the combined effects of vacancies and anti-site defects, where vacancy hardening is known to be most pronounced near stoichiometric B2-ordered FeAl, while anti-site defect strengthening dominates on the Fe-rich side of the composition range. The composition dependence of the reduced modulus likewise changes with heat treatment, indicating that point defects also affect elastic stiffness. Together, these results provide a coherent experimental understanding of how vacancies and anti-site defects jointly control the mechanical response of B2-ordered FeAl and help rationalize previously reported anomalies in the composition-dependent properties.

Ta2Co7 is considered a potential candidate for a crystalline "mille-feuille" structure material, similar to Nb2Co7. However, the crystal structure of Ta2Co7 remains unknown, and several structures have been proposed. In addition, although it is known that Ta2Co7 forms via a peritectoid reaction, the invariant reaction temperature varies widely across studies, ranging from 950 degrees C to 1033 degrees C. In the present study, to elucidate the crystal structure and peritectoid reaction temperature of Ta2Co7, Co-Ta alloys were prepared with long isothermal heat treatments up to 10 000 h, and detailed experimental analyses were performed using electron-probe microanalysis, X-ray diffraction, and differential scanning calorimetry (DSC). Microstructural observation of long-term aged samples reveals that the phase previously reported as Ta2Co7 should be reinterpreted as beta-Co3Ta, with a stoichiometric composition of 25 at% Ta and a rhombohedral BaPb3-type (space group: 166, Rm) crystal structure. The formation of beta-Co3Ta is considered very slow and absent in the as-cast state. Detailed thermal analyses by DSC elucidate that the peritectoid reaction temperature of beta-Co3Ta is (995 +/- 1)degrees C.

The development of batteries with high energy density, short charging times and use of sustainable materials is critical for decarbonization. Magnesium (Mg)-based anodes for lithium (Li) metal batteries promote homogeneous Li plating, thereby avoiding the formation of Li dendrites that cause short circuits and battery failure. However, microstructural modifications induced by Li-alloying and their influence on battery operation remain elusive. Here, we unveil the previously unknown formation of an ordered B2 phase, which creates a conditional spinodal decomposition with the e̱ṯa̱-body-centered cubic phase. Chemical fluctuations characteristic of spinodal decomposition give rise to uniformly dispersed Li-rich e̱ṯa̱-BCC and Li-poor B2 continuous interconnected phases, with the former providing a fast diffusion pathway for Li diffusion towards the anode, hence decreasing the propensity for dendrite formation at elevated current density. This is achieved using Earth-abundant and inexpensive Mg.

The practical applications of Laves alloys are hindered by their intrinsic brittleness at ambient temperature. Dislocation lock structures are expected to play a critical role in governing the plasticity of these alloys; however, their atomic-scale nature remains poorly understood. Here, combining conventional transmission electron microscopy analyses, atomic-resolution scanning transmission electron microscopy images, and atomistic simulations, we unveiled the types, configurations, and formation mechanisms of dislocation locks in a Nb-rich (35 at.% Nb) C15 NbCr2 Laves phase alloy. Moreover, complex defect structures within the cores of dislocation locks, including antisite-like columns of Nb and vacancy-like columns of Cr, were experimentally identified at the atomic scale and rationalized by atomistic simulations. The dislocation locks may serve as preferential sites for crack initiation due to the localized stress concentration. Such stress concentration-induced cracking was experimentally confirmed by the observed intersection of slip planes. Furthermore, the formation mechanisms of four types of dislocation locks and their potentially distinct influences on crack initiation were discussed. The discovery of these lock structures and their influence on crack formation may offer new insights into defect engineering in Laves phases, paving the way for improved strategies to mitigate their intrinsic brittleness.

Understanding the transformation mechanisms between different Laves phase polytypes is critical for tuning the performance of Laves phase-containing alloys. In this work, we report the atomic-scale configuration at the incoherent boundary between C36 and C15 polytypes in a heat-treated NbCr2 Laves phase alloy. The observed polymorphic transformation is generally governed by the synchroshear mechanism; however, its propagation along the thickening direction requires the repetitive stacking of two distinct structural motifs. In the case of dislocation insertion along a single direction (Type I), the two motifs are characterized by a row of antisite-like column of Nb (Motif 1) and a row of missing atomic column of Cr (Motif 2) defects, respectively. When twinning occurs simultaneously on conjugate {111} planes (Type II), the motifs evolve into a complex missing column-associated structure (Motif 3) and a mu-phase-like unit (Motif 4). These findings challenge the conventional view of a singular incoherent interface structure between C36 and C15 phases. Moreover, the identified interface defects, such as antisite-like atomic columns and vacancy-like atomic columns, may play a critical role in mediating diffusion-controlled phase transformation across the interface, providing new insights into the kinetics of polytype transformation in Laves phases.

The cubic C15 CaAl2 Laves phase is an important brittle intermetallic precipitate in ternary Mg–Al–Ca structural alloys. Although knowledge of the mechanical properties of the co-existing phases is essential for the design of improved alloys, the fracture toughness of the C15 CaAl2 intermetallic has not yet been studied experimentally due to limitations posed by macroscale testing of defect-free specimens. Here, miniaturised testing techniques like micropillar splitting and microcantilever bending methods are used to experimentally determine the fracture toughness of the CaAl2 Laves phase. It is found that the toughness value of 1 MPa·√m obtained from pillar splitting with a sharp cube corner geometry is largely insensitive to sample heat treatment, the ion beam used during fabrication, micropillar diameter, and surface orientation. From correlative nanoindentation and electron channelling contrast imaging supported by electron backscatter diffraction, fracture is observed to take place mostly on 011 planes. Atomistic fracture simulations on a model C15 NbCr2 Laves phase showed that the preference of 011 cleavage planes over the more energetically favourable 111 planes is due to lattice trapping and kinetics controlling fracture planes in complex crystal structures, which may provide insights into the experimental results for CaAl2. Using rectangular microcantilever bending tests where the notch plane was misoriented to the closest possible 112 cleavage plane by 8° and the closest 001, 011, and 111 planes by > 20°, a toughness of 2 MPa·√m was determined along with the electron microscopy observation of significant deviations of the crack path, demonstrating that preferential crystallographic cleavage planes determine the toughness in this material. Further investigation using pentagonal microcantilevers with precise alignment of the notch with the cleavage planes revealed similar fracture toughness values for different low-index planes. The results presented here are the first detailed experimental study of fracture toughness of the C15 CaAl2 Laves phase and can be understood in terms of crack plane and crack front-dependent fracture toughness.

The Alkemade theorem goes back to a very fundamental paper on the graphical description of thermodynamic equilibrium problems from 1893 (van Rijn van Alkemade in Z Phys Chem 11: 289-327, 1893). It is one of the most helpful implements for the construction of the liquidus surface of ternary phase diagrams. In its original form, it allows to find the direction of falling or increasing temperature along the monovariant reaction lines forming the boundaries of the primary crystallization fields. The theorem is valid for systems with any number of phases; however, its geometrical construction rule is only defined for the case of stoichiometric phases and it is not clear how to apply the theorem in the case of phases with extended homogeneity ranges. Some examples from a ternary, transition-metal-based system containing phases with large homogeneity ranges are presented, and the usefulness and limits of applicability of the theorem are discussed.

The Cr–Ti system was investigated by several experimental methods and first-principles calculations. The thermodynamic activity of the body-centered cubic solid solution was measured by Knudsen effusion mass spectrometry. The stability of all three polymorphic structures of the Laves phase ( C 14, C 15, and C 36) was determined by differential thermal analysis, and the equilibrium tie-lines with the solid solution were obtained by combining results from diffusion couples and equilibrated alloys. The enthalpy of formation of the Laves phases with the corresponding end-members were calculated using density functional theory and the obtained values were integrated in the models. The experimental and computed data available in the literature was reviewed and the binary system was assessed by the Calphad method. The present evaluation results in an improved thermodynamic description, which can describe the experimentally observed activity in a large temperature range. The temperatures of the invariant reactions between the C 15 and the C 36 phase with the Cr-rich and the Ti-rich bcc solid solution were significantly modified. The difference of the temperature of transformation between the C 15 and the C 36 polytypes on both sides of the Laves phase is much smaller than reported previously.

This work comments on a recent publication by Hajra et al. (Mater. Design 236 (2023) 112483), which claims to have presented compelling experimental and theoretical evidence in favour of the existence of an equilibrium C14-NbCr2 high-temperature Laves phase in the Cr-Nb system. In the present comment, evidence and conclusions reported in the paper of Hajra et al. are critically put into context of insight from previous works. From this it is concluded here, that the evidence in favour of an equilibrium C14-NbCr2 high-temperature Laves phase is, by far, not that compelling as claimed by Hajra et al.. Instead, the most direct evidence presented in the literature does not support the existence of an equilibrium C14-NbCr2 high-temperature Laves phase. Alternative interpretations of Hajra et al.’s evidence and conclusions are offered, and it is elaborated, which true gaps in knowledge exist concerning the Laves phases in the Cr-Nb system.

Molybdenum is one of the major alloying elements in TiAl-based alloys for high-temperature structural applications developed over the last decades. This is due to its stabilising effect on the (beta Ti) phase at high-temperatures, which crucially improves the hot-workability of such alloys. For further alloy development, an exact knowledge of the phase equilibria in the Ti-Al-Mo system is essential. However, there are still uncertainties and inconsistencies related to the ternary phase diagram. In this study, a series of ternary alloys was produced and heat-treated between 700 and 1300 degrees C. Composition, structure, and thermal stability of the phases were analysed and the resulting phase equilibria were established by applying a combination of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), differential scanning calorimetry (DSC), and high-energy X-ray diffraction (HEXRD). From these results, a series of partial isothermal sections is obtained illustrating the phase equilibria in the Ti-rich part of the Ti-Al-Mo system.

Brittle topologically close-packed precipitates form in many advanced alloys. Due to their complex structures, little is known about their plasticity. Here, a strategy is presented to understand and tailor the deformability of these complex phases by considering the Nb-Co µ-phase as an archetypal material. The plasticity of the Nb-Co µ-phase is controlled by the Laves phase building block that forms parts of its unit cell. It is found that between the bulk C15-NbCo2 Laves and Nb-Co µ-phases, the interplanar spacing and local stiffness of the Laves phase building block change, leading to a strong reduction in hardness and stiffness, as well as a transition from synchroshear to crystallographic slip. Furthermore, as the composition changes from Nb6 Co7 to Nb7 Co6 , the Co atoms in the triple layer are substituted such that the triple layer of the Laves phase building block becomes a slab of pure Nb, resulting in inhomogeneous changes in elasticity and a transition from crystallographic slip to a glide-and-shuffle mechanism. These findings open opportunities to purposefully tailor the plasticity of these topologically close-packed phases in the bulk by manipulating the interplanar spacing and local shear modulus of the fundamental crystal building blocks at the atomic scale.

The uniaxial compressive deformation characteristics of single-phase Nb2Co7 were investigated by an electron backscatter diffraction analysis focused on microstructural evolution and kink formation in order to determine whether this monoclinic intermetallic phase is a novel crystalline "mille-feuille" structured (MFS) material. During uniaxial compressive deformation, Nb2Co7 does not behave brittle but shows high plasticity. In some microstructure regions, kink-like structures are observed, showing no delamination. In kink-free regions, the frequency of boundaries with rotation angles of 60 & DEG;, 120 & DEG;, and 180 & DEG;, which correspond to changes in the monoclinic stacking vector of Nb2Co7 layers between adjacent close-packed (CP) layers, increases significantly after the compression test. The interfaces in the kink-like structures are low -angle boundaries with rotation axes within basal (001) planes. The rotation angles and axes of interfaces in the kink-like structures take various values, suggesting that the origin of the kink-like structures is not twinning, but in fact, these structures are a result of deformation kinking. Deformation of single-phase Nb2Co7 is considered to occur due to dislocation glide on basal (001) planes and kink formation, which is regarded as playing an indispensable role in the plasticity of Nb2Co7 during compression. It can therefore be concluded that Nb2Co7 is a novel crystalline MFS material.

Within the search for new improved high-temperature materials for gas turbine applications with higher energy saving potential and improved greenhouse gas balance, new Ti-Al-W based alloys have recently been discussed. A basic prerequisite for targeted alloy development is the precise knowledge of the phases and phase relationships that determine the microstructure and mechanical behavior of the material. However, there is not much known about the phase equilibria in the application- and manufacturing-relevant temperature range between 800 and 1300 & DEG;C. No information on ternary intermetallic compounds or possible ordering of the cubic (& beta;Ti,W) solid solution is reported in the literature. In the present investigation, the Ti-rich part of the Ti-Al-W system between 800 and 1300 & DEG;C was studied. Ten different alloys were heat-treated and quenched samples were characterized by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM). Based on these results, a series of partial isothermal sections was established. The investigations show that there is no ternary intermetallic compound in this system and the (W) solid solution forms equilibria with the binary Ti-Al phases.

The mechanism of solid-state dendrite formation in high-aluminum Fe-Al alloys is not clear. Applying an in-situ observation technique, the real-time formation and growth of FeAl solid-state dendrites during the eutectoid decomposition of the high-temperature phase Fe5Al8 is visualized. In-situ experiments by HT-CSLM reveal that proeutectoid FeAl usually does not preferentially nucleate at grain boundaries regardless of rapid or slow cooling conditions. The critical radii for generating morphological instability are 1.2 μm and 0.9 μm for slow and rapid cooling, respectively. The morphology after both slow and rapid cooling exhibits dendrites, while there are differences in the size and critical instability radius Rc, which are attributed to the different supersaturation S and the number of protrusions l. The combination of crystallographic and thermodynamic analysis indicates that solid-state dendrites only exist on the hypoeutectoid side in high-aluminum Fe-Al alloys. A large number of lattice defects in the parent phase provides an additional driving force for nucleation, leading to coherent nucleation from the interior of the parent phase grains based on the orientation relationship {3¯30}Fe5Al8//{1¯10}FeAl, <111¯>Fe5Al8//<111¯>FeAl. The maximum release of misfit strain energy leads to the preferential growth of the primary arm of the nucleus along <111¯> {1¯10}. During the rapid cooling process, a large supersaturation is induced in the matrix, driving the Al atoms to undergo unstable uphill diffusion and causing variations in the concentration gradient as well as generating constitutional undercooling, ultimately leading to morphological instability and the growth of secondary arms.

Precise knowledge of the phase equilibria in the Ti-Al-Nb system between 700 and 900 °C is of crucial importance for the urgently needed improvement of TiAl-based turbine materials already in industrial use to achieve further energy savings. As a result of the occurrence of the two ternary intermetallic phases ω o (“Ti 4 NbAl 3 ”) and O (“Ti 2 NbAl”), which form in the solid state just in the range of the application-relevant temperatures, the phase relations are very complex and not well studied. In the present investigation, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 700, 800, and 900 °C were determined by a systematic study of 15 ternary alloys, one solid-solid diffusion couple, and three liquid-solid diffusion couples. Using scanning electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) investigations, type and composition of phases as well as phase transitions were determined. With these results, the phase equilibria were established. A focus of the investigations is on the homogeneity ranges of the two ternary phases ω o and O, which both are stable up to temperatures above 900 °C. Based on the compositions measured for the ω o phase and its crystal structure type, a new formula (Ti,Nb) 2 Al is suggested. The results also indicate that the phase field of the ω o phase is split into two parts at 900 °C because of the growing phase field of the ordered (βTi,Nb) o phase.