Phase Stability Of Alloys Research Articles

Phase stability of TiAl-based BCC high entropy alloys

The demand for materials that withstand harsh conditions in high-performance applications has increased drastically in recent years. Due to the predicted outstanding properties of high entropy alloys (HEAs) at elevated temperatures, this class of materials has attracted enormous scientific attention. This paper assesses the phase stability of TiAl-based HEAs, namely TiAlNbV–Mo and TiAlNbV–Mn systems, based on first-principles calculations using Density Functional Theory. We advocate that mixing energies of the HEAs with respect to their possible products offers a suitable way to make predictions about possible decomposition processes. Phase stability is first evaluated at 0 K, followed by the inclusion of the stabilizing effect of the configurational entropy at different temperatures. Additionally, also the effect of vibrational entropy is estimated within the harmonic Debye model. The predicted phase stabilities are discussed in light of existing experimental results showing microstructural evolution before and after heat treatments. Overall, TiAlNbV–Mo, exhibiting a body-centered cucic lattice, has been identified as a kinetically stabilized HEA, whereas TiAlNbV–Mn decomposes into a body-centered cubic phase and the hexagonal Laves phases.

Accelerating matrix/boundary precipitations to explore high-strength and high-ductile Co34Cr32Ni27Al3.5Ti3.5 multicomponent alloys through hot extrusion and annealing

• Hot extrusion reduces grain sizes and simultaneously accelerates precipitation. • Pre-existing dislocation slips and hierarchical precipitates cause high strength. • Interaction among dislocations, stacking faults, and twins ensures acceptable ductility. Annealing-regulated precipitation strengthening combined with cold-working is one of the most efficient strategies for resolving the conflict between strength and ductility in metals and alloys. However, precipitation control and grain refinement are mutually contradictory due to the excellent phase stability of multicomponent alloys. This work utilizes the high-temperature extrusion and annealing to optimize the microstructures and mechanical properties of the Co 34 Cr 32 Ni 27 Al 3.5 Ti 3.5 multicomponent alloy. Hot extrusion effectively reduces grain sizes and simultaneously accelerates the precipitation of coherent L1 2 nanoparticles inside the face-centered cubic (FCC) matrix and grain boundary precipitations (i.e., submicron Cr-rich particles and L1 2 -Ni 3 (Ti, Al) precipitates), resulting in strongly reciprocal interaction between dislocation slip and hierarchical-scale precipitates. Subsequent annealing regulates grain sizes, dislocations, twins, and precipitates, further allowing to tailor mechanical properties. The high yield strength is attributed to the coupled precipitation strengthening effects from nanoscale coherent L1 2 particles inside grains and submicron grain boundary precipitates under the support of pre-existing dislocations. The excellent ductility results from the synergistic activation of dislocations, stacking faults, and twins during plastic deformation. The present study provides a promising approach for regulating microstructures, especially defects, and enhancing the mechanical properties of multicomponent alloys.

Thermodynamic evaluation of the phase stability in mechanically alloyed AlCuxNiCoTi high-entropy alloys

This study investigates the phase stability of AlCuxNiCoTi high-entropy alloys with varying amounts of copper. The regular solution and Kohler’s models are used to thermodynamically evaluate the alloys and determine a symmetrical enthalpy across their compositions. The study calculates the Gibbs energy of mixing for a quinary alloy system using the extended Miedema theory and considering the change in enthalpy of mixing of the solid-solution and amorphous phases of 10 binary alloys. By comparing the experimental and predicted results, this study suggests that thermodynamic properties can help predict stable phases in multicomponent alloys comprising equiatomic and near-equiatomic elements. Furthermore, the structural instability of the solid-solution state resulting from differences in valence electron concentrations of the constituent elements in binary mixtures is discussed, highlighting the influence of alloying elements on the phase stability of multicomponent alloys.

Phase stability and possible superconductivity of new 4d and 5d transition metal high-entropy alloys

We report the synthesis of new equimolar high-entropy alloys formed from five or six 4d/5d transition metals that are each from a different Group of the Periodic Table. These include MoReRuRhPt and MoReRuIrPt, which appear to crystallise as single hexagonal close-packed phases and so are isostructural and isoelectronic to the high-pressure phase of the well-known high-entropy alloy CrMnFeCoNi. Other compositions crystallise as mixed phases, which we rationalise using the atomic size difference, δ, and the electronegativity difference, Δχ, of the constituent elements. Finally, we find signs of superconductivity in eight of these new systems.

First principles study of Ti-Zr-Ta alloy phase stability and elastic properties

The effects of Ta and Zr content on the stability, elastic properties and electronic structure of Ti-Zr-Ta alloy phase were studied by first principles calculation method based on density functional theory. Moreover, Ti-Zr-Ta alloy was fabricated by spark plasma sintering (SPS) using spherical Ti powder,Ta powder, and Ti-Zr-Ta alloy powder produced by the plasma rotation electrode process (PREP). Afterward, the effects of sintering temperature, Zr and Ta content on the microstructure and mechanical properties of the alloy samples were investigated. The results showed that sintering temperature, Ta and Zr content were the key factors which affected the densification. When the sintering temperature was raised, the relative density and mechanical properties of Ti-Zr-Ta alloy were significantly increased. The first- principles calculation also indicated that Ti-1Zr-Ta possesses the lowest Young’s modulus and the best ductility, showing great potential of biomedical applications which agrees with the results of experimental results of alloy preparation.

Short-range order and compositional phase stability in refractory high-entropy alloys via first-principles theory and atomistic modeling: NbMoTa, NbMoTaW, and VNbMoTaW

Using an all-electron, first-principles, Landau-type theory, we study the nature of short-range order and compositional phase stability in equiatomic refractory high-entropy alloys, NbMoTa, NbMoTaW, and VNbMoTaW. We also investigate selected binary subsystems to provide insight into the physical mechanisms driving order. Our approach examines the short-range order of the solid solutions directly, infers disorder/order transitions, and also extracts parameters suitable for atomistic modeling of diffusional phase transformations. We find a hierarchy of relationships between the chemical species in these materials which promote ordering tendencies. The most dominant is a relative atomic size difference between the $3d$ element, V, and the other $4d$ and $5d$ elements which drives a $B32$-like order. For systems where V is not present, ordering is dominated by the difference in filling of valence states; pairs of elements that are isoelectronic remain weakly correlated to low temperatures, while pairs with a valence difference present $B2$-like order. Our estimated order-disorder transition temperature in VNbMoTaW is sufficiently high for us to suggest that SRO in this material may be experimentally observable.

Phase stability and magnetic properties of B2 and L10 PtMn1-XFeX alloys: a cluster expansion and ab initio approach

MnPt-based alloys are promising new materials for magnetic recording and spintronic applications due to their interesting magnetic properties. The underlying thermodynamic properties are still unclear, despite numerous efforts to improve ferromagnetic stability through alloying. In this work, the phase stability of B2 and L10 PtMn1-xFex alloys is investigated using a combination of density functional theory (DFT) and Cluster Expansion (CE). CE model accuracy is evaluated against DFT data, where cross-validation scores were found to be reasonable. The CE generated 16 and 27 new structures on B2 and L10 PtMn1-xFex, respectively. The ground state line predicted 6 stable structures with negative formation energies for both B2 and L10 PtMn1-xFex alloys, suggesting thermodynamic stability. Furthermore, the magnetic strength of the ground state structures is evaluated using DFT.

Role of strain rate in phase stability and deformation mechanism of non-equiatomic Fe38-xMn30Co15Cr15Ni2Gdx high-entropy alloy

Excellent combination of strength and ductility makes metastable high entropy alloys (HEAs) stand out from multi-principal elements alloys. In this study, micro-alloying Gd element is effectively solid solutionized in metastable single FCC phase FeMnCoCrNi HEA system, to bring the FCC phase stability decreased that causes a pronounced FCC → HCP phase transformation during compression. In order to decouple the temperature effect from the influence of dynamic strain rate on phase transformation, a wide strain rate range is adopted from 10 −3 –10 −1 s −1 to 1000–5000 s −1 . The results show that the increased strain rate and adiabatic temperature rise both can restrain the phase transformation. Under low strain rates (10 −3 –10 −1 s −1 ), phase transformation is activated in all tested samples and gradually suppressed with the increase of strain rates, because the rate-induced shear bands inhibit the growth of the HCP phase. The reduction of strain hardening during phase transformation is related to the increase of local thermal activation volume due to disentangling of dislocations. Under dynamic strain rates (1000–5000 s −1 ), the phase transformation is completely substituted by deformation twinning due to the significant adiabatic temperature rise. The present work provides insights into the influence of strain rate on phase stability and deformation mechanism in a wide strain rate range. • Solid solution of Gd decreases the FCC phase stability bringing pronounced FCC → HCP phase transformation during quasi-state compression. • Phase transformation related dislocation dissociation enlarges the thermally activated volume causing the decrease in strain hardening rate. • Under low strain rates (10 −3 –10 −1 s −1 ), the increased strain rates activate shear bands earlier that hinder the growth of the HCP phase, it is a rate-related effect on phase transformation. • Under high strain rates (1000–5000 s −1 ), adiabatic temperature rise enhances the phase stability which is a thermo-related effect, co-works with the rate-related effect, making the phase transformation completely suppressed.

Short-range order and phase stability of CrCoNi explored with machine learning potentials

We present an analysis of temperature-dependent atomic short-range ordering and phase stability of the face-centered cubic CrCoNi medium-entropy alloy employing a combination of ab initio calculations and on-lattice machine learning interatomic potentials. Temperature-dependent properties are studied with canonical Monte Carlo simulations. At around 975 K a phase transition into an ordered $\mathrm{Cr}{(\mathrm{Ni},\mathrm{Co})}_{2}$ phase (${\mathrm{MoPt}}_{2}$-type) is found. This hitherto not reported state has an ordering energy twice as large than the ordered structures previously suggested. We show that magnetism is not responsible for the observed chemical ordering.

Effect of addition of Nb on phase stability of equiatomic CrFeMoV alloy

Multiprincipal alloys with better high temperature properties and irradiation resistance are being explored as potential candidates for nuclear applications. This work is aimed at investigating the effect of Nb addition on the phase stability of CrFeMoV alloy. Microstructure of the as cast CrFeMoNbV alloy showed a dendritic morphology. Microsegregation in the cast alloy has been understood on the basis of partition coefficient and difference in melting point of the constituent elements. Structural analysis through Precession Electron Diffraction (PED) and 3D Automated Diffraction Tomography (ADT-3D) showed the formation of HCP intermetallic Laves phase of Fe2Nb type in a BCC matrix. The formation of this dual phase structure has been understood through in depth empirical calculations and CALPHAD simulations. Addition of Nb is shown not only to increase the lattice strain through increase in atomic size difference, but also increases the enthalpy contribution, which favors the formation of intermetallic Laves phase.

Local lattice distortions, phase stability, and mechanical properties of NbMoTaWHfx alloys: A combined theoretical and experimental study

Refractory high-entropy alloys (RHEAs) are of growing interest due to their potentially superior mechanical performances at elevated temperatures. Inherent lattice distortions are believed to be a major contributor to strength in solid solution phases of RHEAs. Here, we investigate the NbMoTaWHfx alloy series (x = 0, 0.27, 0.57, 0.92, 1.33, 1.82) using first-principles simulations, thermodynamic modeling, and experimental techniques. The first-principles results suggest that Hf alloying is an effective means to enhance atomic-scale lattice distortions in BCC NbMoTaWHfx solid solutions. X-ray diffraction on prepared as-cast samples shows that the alloys with Hf content x≤0.92 are single phase BCC alloys, whereas a dual BCC phase microstructure is observed for x = 1.33 and 1.82. Elemental mappings from scanning electron microscopy for the dual-phase alloys are checked with predictions from thermodynamic modeling for equilibrium and non-equilibrium solidifications. Room-temperature compressive mechanical tests reveal that yield and ultimate strengths increase strongly with the addition of Hf and saturate for x>0.92, whereas the compressive plasticity is slightly improved by Hf but remains limited. We predict the compositional effects on poly-crystal elastic moduli for the constituent BCC phases and the dual-phase composites and find a linear behavior between modulus-normalized yield strength and lattice distortion on average.

Phase stability of a light-weight AlNb2TiV refractory high-entropy alloy at high temperature

• The phase stability of a new low-weight AlNb 2 TiV RHEA was firstly investigated. • A15-Nb 3 Al and σ -Nb 2 Al precipitates formed from B2 matrix when annealing at 1100 °C. • Rod-like Nb 3 Al phase original from GBs grow along parallel orientation. AlNb 2 TiV, a newly proposed low-weight refractory high-entropy alloy (LW-RHEA), shows good mechanical properties at high temperatures. However, the thermodynamic phase stability is still not clear. Here, we evaluated the phase stability of AlNb 2 TiV at the temperature of 700–1100 °C. The LW-RHEA is decomposed into B2, A15-Nb 3 Al and σ -Nb 2 Al phases after annealing at 1100 °C. Rod-like A15-Nb 3 Al precipitates from GBs grow along parallel orientation. Chain-like σ -Nb 2 Al particles precipitate along GBs.

Phase Stability of AlCoTiZn High-Entropy Alloy Prepared by Mechanical Alloying

Phase Stability of AlCoTiZn High-Entropy Alloy Prepared by Mechanical Alloying

Effect of Annealing Temperatures on Phase Stability, Mechanical Properties, and High-Temperature Steam Corrosion Resistance of (FeNi)67Cr15Mn10Al5Ti3 Alloy

The effect of different annealing temperatures on the phase stability and mechanical properties of (FeNi)67Cr15Mn10Al5Ti3 high-entropy alloys (HEAs) was studied. The phase stability was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscattering diffraction (EBSD). The mechanical properties of the alloy were characterized by hardness and tensile tests. Furthermore, the heat-resistant corrosion properties of the (FeNi)67Cr15Mn10Al5Ti3 alloy after annealing at 800 °C was tested under high-temperature steam. The results indicated that HEAs exposed to different annealing temperatures always exhibited the face-centered cubic (FCC) phase. With rising annealing temperature, the dendrite structure of the alloys in the as-cast condition gradually disappeared, with recrystallization and precipitation of larger grains. The tensile strength of the alloy first increased and then decreased with the rising annealing temperature, the hardness and yield strength of the alloy decreased slightly, and the tensile elongation varied greatly. These findings can be used as a basis for improving the phase stability and mechanical properties of a Cr-Fe-Ni-Mn-HEA system with unequal atomic ratios. The heat and corrosion resistance of the alloy at 360 °C and 400 °C was better than that of Zr-4 alloy.

Design of multicomponent alloys with C14 laves phase structure for hydrogen storage assisted by computational thermodynamic

Design methods with predictive properties modeling are paramount tools to explore the vast compositional field of multicomponent alloys. The applicability of an alloy as a hydrogen storage media is governed by its thermodynamic properties, which can be represented by pressure-composition-temperature (PCT) diagrams. Therefore, the prediction of PCT diagrams for multicomponent alloys is fundamental to design alloys with optimized properties for hydrogen storage applications. In this work, a strategy to design C14-type Laves phase multicomponent alloys for hydrogen storage assisted by computational thermodynamic is presented. Since electronic and geometrical factors play an important role in the formation and stability of multicomponent Laves phase, valence electron concentration (VEC), atomic radius ratio (rA/rB), and atomic size mismatch (δ) are initially considered to screen a high number of compositions and find alloy systems prone to form Laves phase structure. Then, CALPHAD method was employed to investigate the phase stability of alloys of the (Ti, Zr or Nb)(V, Cr, Mn, Fe, Co, Ni, Cu, or Zn)2 system, resulting in 440 alloys prone to solidify as C14 Laves phase structure. In addition, we present a thermodynamic model, which allowed calculating the PCT diagrams of the suggested C14 Laves phase alloys based solely on the alloy's composition. For these calculations, the entropy and enthalpy of hydrogen solution in the C14 Laves phase were modelled considering that hydrogen solid solution occurs only at the A2B2-type interstitial sites of the C14 Laves phase structure. Therefore, the room temperature PCI diagrams of 440 C14 Laves phase multicomponent alloys were calculated. Experimental pressure-composition-isotherm (PCI) diagrams of six C14 Laves phase alloys were compared against the calculated ones. Although the PCI curve shapes were not perfectly predicted for some alloys, the order of magnitude of the equilibrium pressure for all the tested alloys were well predicted. The results show that C14 Laves phase multicomponent alloys within a wide range of equilibrium pressure at room temperature can be obtained, being promising candidates for different hydrogen storage applications, such as room temperature tanks, hybrid tanks and Ni-metal hydrides batteries.

Phase stability of Ti-containing high-entropy alloys with a bcc or hcp structure

Solid-solution-strengthened alloys with a hexagonal close-packed (hcp) structure are used as high-temperature structural materials; for example, hcp and body-centered cubic (bcc) two phase Ti alloys are used in aircraft jet engines. The temperature limit of Ti alloys is ~873 K because of deterioration of their creep properties and oxidation resistance at higher temperatures. Greater solid-solution strengthening will be beneficial to improve the strength of Ti alloys at temperatures greater than 873 K. High-entropy alloys (HEAs) are expected to exhibit superior mechanical properties because of the sluggish diffusion of their constituent elements and their large lattice distortion. Hcp HEAs are expected to result in improved high-temperature mechanical properties. However, making the hcp structure stable in HEAs is difficult. To understand the phase stability of Ti-containing HEAs, the present study focuses on two alloys: Ti34Al33V33 and Ti34Zr33Hf33. Al and V are used in commercial Ti alloys. In Ti34Al33V33, a bcc structure is stable at high temperatures and a hcp phase precipitates during cooling. Hcp-structured Ru, Sc, Co, and Zr were selected as additional elements to investigate the change in phase stability of a bcc phase of TiAlV. In Ti34Zr33Hf33, a hcp structure is stable at room temperature. Ru with a hcp structure, Al with a face-centered cubic structure, and V with a bcc structure were selected as additional elements to investigate the phase stability of TiZrHf. Phase stability is discussed on the basis of binary phase diagrams, the formation energy of phases, and the valence electron concentration of the constituent elements.

Study on Stability and Elastic Properties of β-TiX (X=Nb, Ta) Alloys From First-Principles Calculations

In this article, the phase stability, elastic properties, and electronic structure of the β-TiX (X = Nb, Ta) alloy body-centered cubic (bcc) structure were systematically studied with the aid of first-principles calculations. The results show that the phase stability and elastic properties of the β-TiX alloys are closely related to the contents of alloying element X. For β-TiX alloys, the contents of Nb and Ta that satisfy their mechanical stability are 10% and 13%, respectively; at room temperature, both β-TiNb and β-TiTa alloys can reach a thermodynamically stable state when the content of Nb or Ta is 25%. In terms of elastic properties, the content of alloying element X is positively correlated with the elastic constant, Young’s modulus, and shear modulus of the β-TiX alloys. The elastic modulus reaches its minimum when the X content is 25%, and the smallest direction of Young’s modulus appears in the <111> direction. The calculation results of the electronic structure show that the bonding strength between the Ti atom and X atom increases with the content of alloying element X, which leads to improvement of phase stability and elastic modulus.

Magnetochemical effects on phase stability and vacancy formation in fcc Fe-Ni alloys

We investigate phase stability and vacancy formation in fcc Fe-Ni alloys over a broad composition-temperature range, via a density functional theory parametrized effective interaction model, which includes explicitly spin and chemical variables. On-lattice Monte Carlo simulations based on this model are used to predict the temperature evolution of the magnetochemical phase. The experimental composition-dependent Curie and chemical order-disorder transition temperatures are successfully predicted. We point out a significant effect of chemical and magnetic orders on the magnetic and chemical transitions, respectively. The resulting phase diagram shows a magnetically driven phase separation around 10--40% Ni and 570--700 K, between ferromagnetic and paramagnetic solid solutions, in agreement with experimental observations. We compute vacancy formation magnetic free energy as a function of temperature and alloy composition. We identify opposite magnetic and chemical disordering effects on vacancy formation in the alloys with 50% and 75% Ni. We find that thermal magnetic effects on vacancy formation are much larger in concentrated Fe-Ni alloys than in fcc Fe and Ni due to a stronger magnetic interaction.

Phase prediction and effect of intrinsic residual strain on phase stability in high-entropy alloys with machine learning

The phase formation and stability of high-entropy alloys (HEAs) are crucial to their properties, but the efficient prediction of them remains challenging due to the associated vastness of the composition space. In the present work, we study the formation and stability of solid-solution (SS) phases with a dataset consisting of 656 HEAs using machine learning (ML). We demonstrate the independence of nine physical parameters by self-organizing map (SOM) algorithm and rank them by feature importance methods, discovering that the root mean square residual strain (εrmsm s) is the most critical parameter. Based on the theoretical calculations and parallel coordinates plot (PCP) technique, we find that εrms can be used to quantitatively predict the stability of SS phases. Moreover, we apply the support vector machine, gradient boosting decision tree, multi-layer perceptron, and logistic regression algorithms to predict the formation of SS phases with testing accuracy values of 95.22 %, 94.78 %, 90.87%, and 89.57 %, respectively. Our results provide a perspective on the stability of SS phases from the viewpoint of intrinsic residual strain and show the support vector machine could be a better algorithm to predict the phase formation of HEAs.

X-ray diffraction study of phase transformation dynamics of Fe and Fe-Si alloys along the shock Hugoniot using an x-ray free electron laser

The x-ray free electron laser (XFEL) enables probing a highly compressed material response at the subnanosecond timescale. We exploit the ultrafast XFEL pulse to combine reflection x-ray diffraction and laser-driven shock compression to perform a study of the phase transformation and stability in Fe and Fe-Si alloys. Our approach enables us to observe that solid-solid phase transformations occur in Fe and Fe-${\mathrm{Si}}_{8.5\phantom{\rule{0.28em}{0ex}}\mathrm{wt}\phantom{\rule{0.16em}{0ex}}%}$ in $\ensuremath{\le}130$ ps at $\ensuremath{\sim}130$ GPa; no transformation is observed in Fe-${\mathrm{Si}}_{16\phantom{\rule{0.28em}{0ex}}\mathrm{wt}\phantom{\rule{0.16em}{0ex}}%}$ up to 110 GPa. Density functional theory calculations predict similar phase relations.