C54 Phase Research Articles

Topologically Close-Packed Frank-Kasper C15 Phase Intermetallic Ir Alloy Electrocatalysts Enables High-Performance Proton Exchange Membrane Water Electrolyzer.

Chemical synthesis of unconventional topologically close-packed intermetallic nanocrystals (NCs) remains a considerable challenge due to the limitation of large volume asymmetry between the components. Here, a series of unconventional intermetallic Frank-Kasper C15 phase Ir2M (M = rare earth metals La, Ce, Gd, Tb, Tm) NCs is successfully prepared via a molten-salt assisted reduction method as efficient electrocatalysts for hydrogen evolution reaction (HER). Compared to the disordered counterpart (A1-Ir2Ce), C15-Ir2Ce features higher Ir-Ce coordination number that leads to an electron-rich environment for Ir sites. The C15-Ir2Ce catalyst exhibits excellent and pH-universal HER activity and requires only 9, 16, and 27mV overpotentials to attain 10mA cm-2 in acidic, alkaline, and neutral electrolytes, respectively, representing one of the best HER electrocatalysts ever reported. In a proton exchange membrane water electrolyzer, the C15-Ir2Ce cathode achieves an industrial-scale current density of 1 A cm-2 with a remarkably low cell voltage of 1.7V at 80°C and can operate stably for 1000 h with a sluggish voltage decay rate of 50 µV h-1. Theoretical investigations reveal that the electron-rich Ir sites intensify the polarization of *H2O intermediate on C15-Ir2Ce, thus lowering the energy barrier of the water dissociation and facilitating the HER kinetics.

Adapting high-speed indentation mapping for investigating microstructure-property correlations in chromium carbide-nickel alloy coatings: Challenges and solutions

This study investigates the microstructural and mechanical characteristics of chromium carbide‑nickel rich alloy coatings produced through laser cladding, detonation spraying, and plasma spraying techniques. While all three processing techniques used the same feedstock powder, each method yields coatings with unique microstructures encompassing varying compositions and length scales. Employing Field Emission Scanning Electron Microscopy (FE-SEM) in combination with Energy Dispersive Spectroscopy (EDS) and nanoindentation mapping, local microstructure and mechanical properties were assessed at the micrometre length scale. Laser-clad coatings exhibit a typical metal matrix composite microstructure with high carbide content, distinct (MoNb)C2 phases, and Fe in the metallic matrix, while the thermal sprayed coatings showed carbides of varying sizes and metallic matrix with varying degrees of chromium dissolved in it. The instrumented indentation technique helped precisely record the microstructural features in all the coatings. Chromium carbide consistently emerges as the hardest phase across all coatings, with variations in metallic matrix hardness. Higher matrix hardness in thermal sprayed coatings correlates with increased Cr content, attributed to extended solid solubility of Cr and the presence of Mo and Nb. Energy dispersive spectroscopy and transmission electron microscopy provided clearer insight into the microstructure. This study highlights a direct one-to-one correlation between microstructure and mechanical properties mapped using instrumented indentation techniques in chromium carbide‑nickel rich coatings across different deposition methods.

Achieving room temperature hydrogen storage reversibility in Nb-rich alloys of the Nb-Cr-Mn system

Recent developments in the design of body-centered cubic (BCC) multicomponent alloys via computational tools have demonstrated the possibility of obtaining alloys with excellent hydrogen storage behavior. In this work, we employ the CALPHAD (Calculation of Phase Diagrams) method to design Nb-rich alloys of the Nb-Cr-Mn system that present hydrogen storage reversibility at room temperature under moderate pressure conditions. We employ the valence electron concentration (VEC) factor as a compositional guide to select compositions with suitable thermodynamic properties. Using electric arc melting, we synthesize two alloys, namely Nb85Cr10Mn5 and Nb70Cr20Mn10, both forming predominant BCC solid solutions with VEC ∼ 5.2 and minor amounts of a eutectic microconstituent composed of BCC and Laves C14 phases. Both alloys are easily hydrogenated at room temperature without the need for an activation treatment. The Nb85Cr10Mn5 alloy reaches a storage capacity of 2.1 wt% of H (298 K; Peq∼ 20 bar) whereas the Nb70Cr20Mn10 alloy reaches a capacity of 1.4 wt% (298 K; Peq∼ 21 bar). Benefits in the storage kinetic performance are correlated with the BCC + C14 microstructure. Pressure-Composition-Temperature (PCT) diagrams show moderate values of equilibrium pressure for hydrogen storage reversibility at room temperature. Room temperature absorption/desorption cycling measurements demonstrated a reversible capacity of 1.2 wt% of H (Peq∼ 29 bar) for the Nb85Cr10Mn5 alloy and 0.8 wt% of H (Peq∼ 31 bar) for the Nb70Cr20Mn10 alloy after twenty cycles.

Magnetic Properties of High-Entropy Alloy FeCoNiTi.

We report the magnetic properties of the as-cast high-entropy alloy (HEA) FeCoNiTi, characterized by a dual phase comprising the face-centered cubic (fcc) and hexagonal C14 Laves phases. The HEA manifests three distinct ferromagnetic orderings at 1084, 214, and 168 K. The emergence of the 214 K transition is attributed to the influence of the C14 phase. The high-temperature ordering at 1084 K is associated with the fcc phase, which exhibits an additional ferromagnetic ordering at 168 K. The coercive field of the fcc phase attains 667 Oe at 400 K. Electronic structure calculations conducted for both phases substantiate the presence of ferromagnetic ground states. Comparative analyses between experimental and theoretical values are undertaken in the context of saturation magnetization. A comprehensive discussion is presented, delving into the origin of the relatively high coercive field observed at high temperatures in the fcc phase.

Nuclei engineering for even halide distribution in stable perovskite/silicon tandem solar cells.

Wide-bandgap (WBG) absorbers in tandem configurations suffer from poor crystallinity and weak texture, which leads to severe mixed halide-cation ion migration and phase segregation during practical operation. We control WBG film growth insensitive to compositions by nucleating the 3C phase before any formation of bromine-rich aggregates and 2H phases. The resultant WBG absorbers show improved crystallinity and strong texture with suppressed nonradiative recombination and enhanced resistance to various aging stresses. Perovskite/silicon tandem solar cells achieve power conversion efficiencies of 29.4% (28.8% assessed by a third party) in a 25-square centimeter active area and 32.5% in a 1-square centimeter active area. These solar cells retained 98.3 and 90% of the original efficiency after 1301 and 800 hours of operation at 25° and 50°C, respectively, at the maximum power point (AM 1.5G illumination, full spectrum, 1-sun) when encapsulated.

Microstructure evolution and assessment of creep behaviour of SiC nanoparticles dispersed squeeze-cast Mg-5.0Al-2.0Ca-0.3Mn alloy

The microstructural modification and creep behaviour of Mg-5.0Al-2.0Ca-0.3Mn (AXM520) alloy with SiC nanoparticles (SiCnp) dispersion have been investigated. The concentrations of the SiCnp are varied from 0.5 to 3.0 (wt.%), and the squeeze-cast nanocomposites (NCs) are abbreviated as NC0·5SiC, NC1·0SiC, NC2·0SiC and NC3·0SiC. The microstructures of the AXM520 alloy and NCs consist of a primary solid solution (α-Mg), a eutectic of α-Mg and (Mg,Al)2Ca (C36), and an Al8Mn5 phase. Additionally, the SiC phase is also present in the NCs. The continuous network of the C36 phase in the NCs is fragmented and becomes discontinuous with the increase in the fraction of the SiCnp. All the NCs revealed improved creep performance compared to the AXM520 alloy. The creep rate of the NCs decreases with the increase in the SiCnp content. The NC2·0SiC exhibited an improvement in creep resistance by 73.2 % compared to the alloy. However, the creep resistance deteriorated since the amount of the nanoparticles was further increased in the NC3·0SiC, leading to agglomeration. The stress exponents vary from 5.0 to 6.7, and activation energies vary from 89.8 to 101.8 kJ/mol, implying that the creep in the materials is controlled by the climb of dislocation assisted by the pipe diffusion. The pile-ups of dislocations took place around the C36 phase and near the SiCnp. The additional strengthening owing to the presence of the SiCnp in the NCs was responsible for their improved creep performance compared to the AXM520 alloy.

Revisiting the thermodynamic properties of the ZrCr2 Laves phases by combined approach using experimental and simulation methods

A comprehensive study of ZrCr2 Laves phases is important for both fundamental research and technological applications. The ZrCr2 compound is a typical precipitate observed in Zr-based alloys including the newly developed Cr-coated Zr cladding known as Accident Tolerant Fuel cladding. The brittle behavior of the Laves phases and the low melting point at the interface zone between the precipitates and the Zr matrix are considered as governing the thermomechanical behaviors of these newly developed nuclear fuel claddings for normal operating conditions but more dramatically in case of Loss Of Coolant Accident (LOCA) conditions. The aim of the present study is to investigate the various ZrCr2 Laves polymorphs and their thermodynamic features particularly where conflicts exist in the literature. Based on diffraction experiments of annealed samples, it has been established that the transformation from the C14 high-temperature form to the C15 low-temperature form is of displacive nature without the C36 polymorph forming as an intermediate phase. The stable and metastable domains of C14 and C15 and their thermodynamic properties have been determined. The specific heat of both C14 and C15 types was measured over a wide range of temperatures from 2 to 1063 K by coupling relaxation calorimetry and DSC. The experimental data were fitted using a modified Einstein model. The room temperature entropies of the C14 and C15 phases were evaluated. The enthalpy of formation was measured for the first time using drop solution calorimetry in liquid Al at 1173 K in a Tian–Calvet calorimeter. The experimental value is in good agreement with Density Functional Theory (DFT) calculations. Finally, all these new results are discussed regarding the abundant literature on the ZrCr2 Laves phases.

SiC nanoparticles additions to squeeze-cast Mg-5.0Al-2.0Ca-0.3Mn alloy: An evaluation of microstructure and mechanical properties

The influences of the SiC nanoparticles (SiCnp) content on the microstructural modification and mechanical properties of the squeeze-cast Mg-5.0Al-2.0Ca-0.3Mn (AXM520) alloy have been investigated. The concentrations of the SiCnp are varied from 0.5 to 3.0 (wt%), and the nanocomposites (NCs) are abbreviated as NC0.5SiC, NC1.0SiC, NC2.0SiC and NC3.0SiC. The as-cast microstructures of the AXM520 alloy and NCs consist of an α-Mg phase, a eutectic of α-Mg and (Mg,Al)2Ca (C36), and an Al8Mn5 phase. Additionally, the SiC phase is also present in the NCs. The continuous network of the C36 phase is fragmented and becomes discontinuous as the content of the SiCnp increases in the NCs. All the NCs exhibit superior tensile and compressive properties than the AXM520 alloy. The NC2.0SiC exhibits the best tensile properties among the NCs employed. The YS of the NCs improves with the increase in the SiCnp content up to 3.0 (wt%). The UTS and %El of the NCs increase up to 2.0SiCnp (wt%), and beyond that, the same declines owing to the agglomeration of the nanoparticles. The discontinuous network of the C36 phase in the NCs inhibits crack propagation, leading to their improved %El. All the NCs exhibit higher strain-hardening exponent (n) and strain-hardening rate (SHR) compared to the AXM520 alloy. The superior ‘n’ and SHR exhibited by the NCs were attributed to the grain refinement and dislocations generation. The strengthening from CTE mismatch contributes the most to the overall strengthening of the NCs. The ‘Zhang and Chen’ model is modified by introducing an agglomeration factor, and the predicted YS of the NCs matches pretty well with the experimentally obtained values.

The effect of grain boundaries and precipitates on the mechanical behavior of the refractory compositionally complex alloy NbMoCrTiAl

Refractory compositionally complex alloys (CCAs) have been found to exhibit promising high-temperature properties. However, the brittleness of most CCAs at room temperature limits their application. To understand the influence of microstructure on the deformation behavior of these alloys, we conducted micromechanical experiments, including nanoindentation and micropillar compression tests, on a representative NbMoCrTiAl alloy at room temperature. The indentation at the grain boundaries showed increased shear stress and hardness compared to the matrix (ordered B2 crystal structure) due to the presence of local intermetallic precipitates (C14 and A15 phases). Although the NbMoCrTiAl alloy exhibits no ductility at the millimeter scale as shown in previous studies, the micropillars at the grain boundaries, which were decorated with precipitates, demonstrated higher yield strength and significant plastic strain >30% at room temperature. Numerous slip lines were observed, while cracks and fracture occurred at higher levels of deformation. The grain boundaries with precipitates increased the probability of crack initiation and propagation, but they did not necessarily lead to catastrophic failure of the pillars.

On the Microstructure and Properties of Complex Concentrated bcc Solid Solution and Tetragonal D8m M5Si3 Silicide Phases in a Refractory Complex Concentrated Alloy

In this work, the refractory complex concentrated alloy (RCCA) 3.5Al–4Cr–6Ge–1Hf–5Mo–36Nb–22Si–1.5Sn–20Ti–1W (at.%) was studied in the as cast and heat treated conditions (100 h or 200 h at 1500 °C). There was strong macrosegregation of Si in the 0.6 kg button/ingot of the cast alloy, in which A2 solid solution, D8m βNb5Si3, C14-NbCr2 Laves phase and Tiss and a ternary eutectic of the A2, D8m and C14 phases were formed. The partitioning of Ti in the as cast and heat treated microstructure and its relationships with other solutes was shown to be important for the properties of the A2 solid solution and the D8m βNb5Si3, which were the stable phases at 1500 °C. The near surface microstructure of the alloy was contaminated with oxygen after heat treatment under flowing Ar. For the aforementioned phases, it was shown, for the first time, that there are relationships between solutes, between solutes and the parameters VEC, Δχ and δ, between the said parameters, and between parameters and phase properties. For the contaminated with oxygen solid solution and silicide, trends in relationships between solutes, between solutes and oxygen content and between the aforementioned parameters and oxygen content also were shown for the first time. The nano-hardness and Young’s modulus of the A2 solid solution and the D8m βNb5Si3 of the as cast and heat-treated alloy were measured using nanoindentation. Changes of nano-hardness and Young’s modulus of the A2 solid solution and D8m βNb5Si3 per solute addition for this multiphase RCCA were discussed. The nano-hardness and Young’s modulus of the solid solution and the βNb5Si3, respectively, were 9.5 ± 0.2 GPa and 177.4 ± 5.5 GPa, and 17.55 ± 0.5 GPa and 250.27 ± 6.3 GPa after 200 h at 1500 °C. The aforementioned relationships and properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. Implications of synergy and entanglement for the design of metallic ultra-high temperature materials were emphasised.

The effect of carbide decomposition and carbon loss on the steady state composition of WC-Cr3C2-Ni composite coatings at 900 °C.

The steady-state composition and microstructure of 42 wt% WC-42 wt% Cr3C2–16 wt% Ni coatings sprayed under “low”, “medium” and “high” thermal input conditions was explored following treatment at 900 °C for up to 30 days. None of the coatings achieved thermodynamic equilibrium compositions. Attempts at compensating for mechanisms of bulk carbon loss and selective carbon loss from the dissolution phases alone, were both unsuccessful in predicting the steady-state compositions based on equilibrium phase diagrams. The low thermal input HVOF coating underwent recovery of the dissolved Cr3C2 phase to form an interconnected, two-phase Cr3C2 + Ni-alloy matrix surrounding the retained WC grains, which did not achieve thermodynamic equilibrium with the precipitating phases. The high carbide dissolution plasma ArH2 coating generated a homogeneous distribution of metallic and carbon elements such that the metal: carbon ratio exceeded 1:1, favouring the formation of (W,Cr)2C. The remaining material formed (Cr,W)3C2 within a Ni(Cr,W) alloy binder. Chromium stabilised the (W,Cr)2C phase such that it did not decompose into W + WC, as predicted in the WC system. The plasma ArHe response was a mix of these mechanisms. A distinctive feature of the heat-treated coatings was the lack of significant grain growth, with submicron carbide grains retained out to 30 days at 900 °C.

Phase diagram and physical properties of pyrocarbonate CaC2O5: first-principles calculations

Understanding the physical properties of Ca-carbonate at high pressure is crucial for comprehending the carbon cycle in the Earth’s interior. Recently, a new pyrocarbonate, CaC2O5, has been discovered. In this work, we update the phase diagram of CaC2O5. The structure, elastic, and seismic properties of CaC2O5 polymorphs under lower mantle pressure are studied using first-principles calculations, and compared with those of orthocarbonate Ca2CO4. Our findings reveal that CaC2O5 may have four stable phases under lower mantle temperature and pressure conditions, namely the I4̅2d, Fdd2, Pc, and C2 phases. The carbon may be stored in the new pyrocarbonate CaC2O5 polymorphs. The thermodynamic properties of CaC2O5- I4̅2d are obtained by quasi-harmonic approximation. This work contributes to our understanding of the behavior of Ca-carbonate in the Earth’s lower mantle conditions.

Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of Ba0.6Sr0.4MnO3.

Ba-rich compositions in the BaxSr1-xMnO3 (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond x = 0.5, they only form in hexagonal polymorphs. Here, the 3C phase boundary is pushed to Ba0.6Sr0.4MnO3 for the thin films. Using regular pulsed laser deposition (rPLD), 3C Ba0.6Sr0.4MnO3 could be epitaxially stabilized on DyScO3 (101)o substrates by using a 0.1% O2/99.9% N2 gas mixture. However, the 3C phase was mixed with the 4H polymorph for films 24 nm thick and above, and the films were relatively rough. To improve flatness and phase purity, changes in growth kinetics were investigated and interval PLD (iPLD) was especially effective. In iPLD, deposition is interrupted after completion of approximately one monolayer, and the deposit is annealed for a specific period of time before repeating. Both film flatness and, more importantly, the volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase films. The results imply that iPLD improves the persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

Phase stability and thermo-physical properties of Laves-Co<sub>2</sub>(Hf Ta) alloys

There exists still the controversy over the stable structure of Laves-phase Co<sub>2</sub>(Hf Ta) alloys with the C14, C15 or C36 structures. In this study, the stability, electronic and thermodynamic properties of Laves-phase Co<sub>2</sub>(Hf Ta) are investigated. In order to fully understand the influence of magnetic state and temperature on phase stability, we systematically study the free energy change at finite temperature, elastic stability, and phonon dispersion. The low Curie temperature can be estimated, which suggests that the Co<sub>2</sub>(Hf Ta) alloys possess the paramagnetic state in a wide temperature range. Results indicate that the lattice vibration and electronic excitation have an important effect on the phase stability. The ground state of Co<sub>2</sub>Hf compound has a C14-type structure, while the ground state of Co<sub>2</sub>Ta has a C36-type structure, without the effect of temperature. After doping Hf with different concentrations (0.25, 0.50, 0.75) into Co<sub>2</sub>Ta, the most stable structure still possesses the C36-type structure. After considering the contribution of vibration entropy and electron entropy, the relatively stable structures of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta undergo the C36 and C14 phase transition, respectively. In addition, the thermodynamic properties, including Debye temperature, heat capacity, and vibration entropy, which vary with pressure and temperature, are studied. The electronic properties of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta compounds are analyzed by the charge difference and density of states. The similar electronic density of states between different phases suggest that the Lave phases have the similar stability. The Hf-Co bonding with a certain direction is revealed. Our results are of great significance in understanding the structure and properties of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta compounds.

Effect of Al addition on Nb-rich phase precipitation behavior in ferritic stainless steel

The precipitation behavior of Nb-rich phase in Al-free and 1Al Nb-bearing ferritic stainless steels (FSSs) at 700 °C were investigated. The results show that Nb-bearing carbides and Laves phases with mutual transformation relationships are precipitated in both Al-free and 1Al Nb-bearing FSSs at the early stage of aging. At this stage, the coarsening rate of Laves phase is higher than that of Nb2C phase, and the addition of Al can promote the precipitation of Laves phase and weaken the precipitation of (Nb, Ti)2C phase. As the aging time extended to 60 min, the Nb-rich phase in the two experimental steels was mainly Laves phase, and the intragranular Laves phase showed feather like aggregation distribution. On the one hand, the added Al can be enriched in the Laves phase to produce a tailing morphology, on the other hand, it can reduce the diffusion rate of Nb atoms in the matrix, thereby weakening the aggregation state of the intragranular Laves phase. During the coarsening process of Laves phase, the addition of Al can reduce the coarsening rate of Laves phase by 7.45 times.

The Effect of Double Solution Treatment on Changes in the Microstructure of Austenitic Manganese Steel

R esearch has been carried out to eliminate of the carbide phase (FeMn)3C at the grain boundaries through a double solution treatment. The process begins, which is heating the steel to a temperature of 1000°C with a holding time of 100 minutes, then hot rolling is carried out by 28%. Furthermore, the steel reheated from room temperature to 1100°C with a holding time of 20 minutes, followed by a double solution treatment process, which is heating the steel gradually from room temperature to 730°C with a holding time of 30 minutes, then heating is continued to a temperature of 1000°C with a holding time of 15 minutes, and quenching by water. SEM mapping shows steel contains Fe, Mn, C, S, Si and Ni elements. XRD characterization showed that the double solution treatment process did not change the constituent phases in the form of Fe3O4, γ-Mn, γ-Fe, α-Fe and MnO2, but reduce the crystalline size from 0.0247 to 0.0223 μm. Based on optical microscope and SEM observations, it can be seen that the double solution treatment process was also able to reduce the grain size from 5.33 to 1.30 μm and remove the carbide (FeMn)3C phase at the grain boundaries, as well as successfully change the morphology from cleavage areas to dimple areas.

Structural Stability of Titanium-Based High-Entropy Alloys Assessed Based on Changes in Grain Size and Hardness

The thermal stability of the grain structure and mechanical properties of the high-entropy two-phase TiCoCrFeMn alloy produced by powder metallurgy, assessed based on microhardness measurements, was analyzed in this work. For this purpose, material obtained via sintering using the U-FAST method was subjected to long-term heating at a temperature of 1000 °C for up to 1000 h in an argon atmosphere. For homogenization times of 1, 10, 20, 50, 100, and 1000 h, grain size changes in the identified phase components of the matrix were assessed, and microhardness measurements were conducted using the Vickers method. It has been shown that the changes in the analyzed parameters are closely correlated with non-monotonic modifications in the chemical composition. It was found that the tested alloy achieved structural stability after 100 h of annealing. A stable grain size was obtained in the BCC solid solution of approximately 2 µm and the two-phase BCC+C14 mixture of roughly 0.4 µm. Long-term heating for up to 1000 h caused the grain structure to grow to 2.7 µm and 0.7 µm, respectively, with a simultaneous decrease in hardness from 1065 HV to 1000 HV. The chromium and titanium diffusion coefficient values responsible for forming the BCC solid solution and the Laves C14 phase, including the material matrix, were also determined at this level to be DCr = 1.28 × 10−19 (m2·s−1) and DTi = 1.04 × 10−19 (m2·s−1), demonstrating the sluggish diffusion effect typical of high-entropy alloys.

Hydrogen storage properties of Zr-based multicomponent alloys with C14-Laves phase structure derived from the Zr–Cr–Mn–Fe–Ni system

This study focused on assessing the crystal structure and hydrogen storage properties of two multicomponent alloys having the Zr33(CrMnFeNi)67 and Zr33Cr22Mn15Fe25Ni5 compositions. Both alloys were designed by thermodynamic calculations derived from the TiZrCrMnFeNi system after removing the titanium element from the original composition and keeping the high fraction of zirconium to explore new Zr-based compositions with a strong tendency to crystallize as a C14 Laves phase. The arc-melted Zr-based alloys exhibited a significant presence of C14 Laves phases and small amounts of NiZr intermetallic phase. Hydrogen storage properties investigated through pressure-composition-temperature (PCT) absorption and desorption isotherms revealed that the Zr33Cr22Mn15Fe25Ni5 alloy could absorb 1.8% (H/M = 1.2) of hydrogen with fast kinetics at room temperature after a simple heat activation, while the Zr33(CrMnFeNi)67 alloy could reversibly absorb 1.6% (H/M = 1) with equally fast kinetics. Zr-rich alloys exhibited a low hydrogen equilibrium pressure in the PCT isotherms. After full hydrogenation at room temperature, the initial metallic C14 Laves phases were converted to the respective Laves phase hydrides in both cases. Under cycling, the fractions of the secondary NiZr and the C14 phase change as the samples become more activated. The microstructural analysis, before and after the cycling, showed a very homogeneous microstructure and good distribution of the elements in both alloys.

High-temperature phase stability and phase transformations of Niobium-Chromium Laves phase: Experimental and first-principles calculation studies

The high-temperature phase stability of the NbCr2 Laves phase has not yet been established beyond disagreement. The well accepted C14 phase field has been eliminated from the Nb-Cr phase diagram in a recent study that claimed that the NbCr2 alloy shows the C15 structure up to melting. The present study was conducted to address these ambiguities. The experimental investigations show the presence of C14, C15, and C36 structures in the as-cast alloy. Differential scanning calorimetry studies of the as-cast alloy show the transformation of the eutectic mixture of bcc + NbCr2 to single-phase NbCr2 at 1338 K. An in-situ X-ray diffraction (XRD) study of as-cast alloy shows a phase transformation of C36 → C14 below 1273 K. Density functional theory (DFT) calculations confirm two phase transformations: (a) a metastable transformation of C36 → C14 at 783 K and (b) an equilibrium transformation of C15 → C14 at 1971 K, which was validated by high temperature DTA and TEM studies. Further, extended DFT calculations on supercells with four compositions of NbCr1.9, NbCr2, NbCr2.1, and NbCr2.2 also corroborated the presence of the C14 phase, and consequently, the Nb-Cr phase diagram is modified. This finding resolves the controversy regarding the existence of an equilibrium C14 phase field before melting.

Pressure modulated charge transfer and phonon interactions drive phase transitions in uranium–aluminum laves phases

Lanthanide AB2 intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A–A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl2 is the only known actinide Laves phase to show a pressure-controllable C15 → C36 transition. Here, we apply first principles calculations to determine the origin of the C15 → C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression–induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U–U bonding environment. The opposite phenomena is observed in C36: varied U–U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl2.