Phase Stability Of Alloys Research Articles

Atomic scale understanding of phase stability and decomposition of a nanocrystalline CrMnFeCoNi Cantor alloy

High entropy alloys (HEAs) provide superior mechanical and functional properties. However, these advantages may disappear when a metastable single-phase solid solution decomposes at low temperatures upon long-term annealing. Therefore, understanding the underlying phase separation mechanisms is important for the design of new HEAs with controlled properties. In the current work, the thermal stability of a nanocrystalline CrMnFeCoNi HEA was investigated at different annealing conditions using a combinatorial processing platform, involving fast and parallel synthesis of nanocrystalline thin films, short annealing time for a rapid phase evolution, and direct characterization by atom probe tomography. The microstructural features of the decomposed CrMnFeCoNi alloy as well as its decomposition process were analyzed in terms of elemental distributions at the near-atomic scale. The results show that the segregation of Ni and Mn to grain boundaries in the initial single-phase alloy is a prerequisite and is observed to be the only occurring physical process at the early stage of phase decomposition. When the concentrations of Ni and Mn reach a certain value, phase decomposition starts and a MnNi-rich phase forms at grain boundaries. Next, two Cr-rich phases form at the interface between the MnNi-rich phase and the matrix. Meanwhile, a FeCo-rich phase forms in the grain interior. Based on these observations, the underlying mechanisms involving nucleation, diffusivity as well as thermodynamic considerations were discussed.

Surface residual stress and phase stability in unstable β-type Ti–15Mo–5Zr–3Al alloy manufactured by laser and electron beam powder bed fusion technologies

The differences between the physicochemical properties of the laser and electron beam powder bed fusion (L- and EB-PBF) methods are yet to be explored further. In particular, the differences in the residual stress and phase stability of alloys with unstable phases remain unexplored. The present work is the first to systematically investigate how the heat source type and process parameters affect the surface residual stress and phase stability of an unstable β-type titanium alloy, Ti–15Mo–5Zr–3Al. The surface residual stress and β-phase behavior were studied using high-precision X-ray diffraction (HP-XRD). Significant differences were observed between the two methods. The L-PBF-made specimens exhibited tensile residual stresses of up to 400 MPa in the surface area. HP-XRD analysis revealed a stress-induced lattice distortion, interpreted as a transitional state between the β-phase and α”-phase. In contrast, the EB-PBF-made specimens showed no significant residual stress and had an undistorted β-phase coexisting with the hexagonal α-phase caused by elemental partitioning. This study provides new insights into the previously neglected effects of L-PBF and EB-PBF in unstable β-type titanium alloys.

Alloying effects on phase stability, mechanical properties, and deformation behavior of CoCrNi-based medium-entropy alloys at low temperatures

Alloying plays an important role in determining the phase stability and mechanical behavior of medium/high-entropy alloys (M/HEAs). In this work, the effects of Al, Ti, Mo, and W additions on the phase stability, strengthening behavior, and stacking fault energies of CoCrNi alloys are quantitatively investigated by using first-principles calculations. Our results reveal that the Al, Ti, and W additions enhance the structural stability of metastable face-centered cubic structures, whereas Mo is in favor of the formation of hexagonal close-packed structures at low temperatures. Through analyzing the elastic moduli and lattice mismatch based on a Labusch-type model, we show that the solute strengthening effect decreases in the order W > Mo > Ti > Al. The compositional dependence of intrinsic stacking fault energy (ISFE) and unstable stacking fault energy (USFE) of CoCrNi MEAs was calculated, and the results indicate that the Al, Ti, Mo and W additions significantly reduce the USFE, leading to a reduction in the energy barriers of dislocation slips. The alloying effects on the deformation behaviors of CoCrNi MEAs are discussed in terms of the ratio of ISFE to the energy barrier of dislocation slips. The present study not only sheds light on the fundamental understanding of phase stability and deformation mechanisms of M/HEAs but also provides useful guidelines for the alloy design of advanced M/HEAs with superior mechanical properties.

Magnetically altered phase stability in Fe-based alloys: Modeling and database development

Processing of materials under an externally applied magnetic field could enable exploitation of broader processing spaces, affecting the relative stability of phases. To properly predict how a material will react to an applied field, the magnetic properties must be incorporated into the free energy calculations. Previous works assumed alloy composition played a negligible effect on the magnetization term; however, this assumption is not valid for all systems. Here, we assess twelve binary iron systems to quantify the shifts of magnetic moment and Curie temperature with respect to alloy content. Descriptive magnetic property equations for these binary systems were assessed in conjunction with experimental data and prior descriptions obtained from literature. To showcase the impact of using the compositionally dependent magnetic property predictions, the austenite loops for Fe–Si and Fe–Mo were re-calculated under an applied magnetic field. Further, the magnetic property data summarized herein can also be used for future assessments or re-assessments of the iron alloy systems reported, in addition to the current goal of processing under magnetic fields.

Phase stability of novel HfNbTaTiVZr refractory high entropy alloy under ion irradiation

HfNbTaTiVZr refractory high entropy alloy (RHEA) fabricated by arc melting shows the single-phase body-centered cubic (BCC) structure. Radiation-induced amorphization (RIA) is observed at 2 displacements per atom (dpa) at 298 K while the specimen remains its crystal structure at 423 K. The critical amorphization temperature (Tc) is likely within 298–423 K. RIA is mainly attributed to the accumulation of Frenkel pairs caused by elastic collisions under ion irradiation and the large atomic size mismatch in the studied RHEA facilitates the amorphization process. RIA of RHEA is for the first time reported and the finding challenges the current understanding of phase stability of RHEAs upon irradiation.

A systematic analysis of phase stability in refractory high entropy alloys utilizing linear and non-linear cluster expansion models

The phase segregation behavior of three key refractory high entropy alloys (NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr) is studied using first-principles calculations. Several linear and non-linear methods are utilized to generate surrogate models for three key refractory high entropy alloys (NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr) via the cluster expansion formalism. The characteristics of each of the generated models is explored and the regression methods are compared. Finally, these surrogate models are utilized to generate Monte Carlo trajectories in order to explore the link between phase segregation and previously documented mechanical degradation in these materials.Phase segregation and intermetallic phases documented in the experimental literature are reproduced in all three high entropy alloys. NbTiVZr forms vanadium and zirconium clusters at lower temperatures (250 K) which disperse into the single-phase matrix by 1000 K. HfNbTaTiZr forms HfZr, NbTa, and possibly TiZr intermetallic phases at lower temperatures (250 K). Unlike the other HEAs studied here, HfNbTaTiZr does not lose short-range ordering in the solid state until around 3500 K, which is above its melting temperature. AlHfNbTaTiZr forms NbTa and AlHfTiZr phases at lower temperatures (250 K), which are not observed at higher temperatures (1000 K).

Elastic energy of multi-component solid solutions and strain origins of phase stability in high-entropy alloys

The elastic energy of mixing for multi-component solid solutions is derived by generalizing Eshelby's sphere-in-hole model. By surveying the dependence of the elastic energy on the chemical composition and lattice misfit, we derive a lattice strain coefficient λ*. Studying several high-entropy alloys and superalloys, we propose that most solid solution multi-component alloys are stable when λ*<0.16, generalizing the Hume-Rothery atomic-size rule for binary alloys. We also reveal that the polydispersity index δ, frequently used for describing strain in multi-component alloys, directly represents the elastic energy (e) with e=qδ2, q being an elastic constant. Furthermore, the effects of (i) the number and (ii) the atomic-size distribution of constituting elements on the phase stability of high-entropy alloys were quantified. The present derivations and discussions open for richer considerations of elastic effects in high-entropy alloys, offering immediate support for quantitative assessments of their thermodynamic properties and studying related strengthening mechanisms.

Phase Stability and Precipitation in L12-Strengthened CoCrNi Medium-Entropy Alloys at Intermediate Temperatures

Understanding phase stability and precipitation at intermediate temperatures is crucial for tailoring microstructures and mechanical properties of L12-strengthened multicomponent alloys. In this study, the precipitate type, morphology, and distribution of (CoCrNi)100−2x(AlTi)x (x = 3, 5, and 7 at.%) medium-entropy alloys (MEAs) at 600-900 °C were systematically investigated through a combination of scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, electron backscatter diffraction, and thermodynamic calculations. Our results reveal that the Al and Ti additions promote the destabilization of supersaturated fcc into L12 and σ phases, and the dominating phases of the MEAs change from fcc + L12 to fcc + L12 + σ and to L12 + σ + L21 phases as the Al and Ti concentrations increase. In addition, increasing the temperature leads to a change of precipitate morphology from lamellar to granular microstructures. The effects of alloying additions and aging temperature on the phase stability, precipitation behavior, and mechanical properties of the MEAs were discussed from the thermodynamic and kinetic points of view.

Exploring CsPbI3 – FAI alloys: Introducing low-dimensional Cs2FAPb2I7 absorber for efficient and stable perovskite solar cells

Among inorganic lead halide perovskites, cesium lead triiodide (CsPbI3) is considered the most promising for photovoltaic applications. The material is a good absorber due to its narrow bandgap enabling efficient light harvesting; however, the low stability of the “black” photoactive phases of CsPbI3 at low temperatures represents a major challenge for the practical application of this material. Herein, we report a new family of promising perovskite-like 2D absorber materials produced by alloying CsPbI3 with formamidinium iodide (FAI). Using a set of complementary experimental techniques in combination with the density functional theory calculations we have found that the introduction of FAI as overstoichiometric additive to CsPbI3 favors the formation of perovskite-like phases with reduced dimensionality and improved stability. The optimal CsPbI3⋅xFAI (x = 0.5, 0.6) (~Cs2FAPb2I7) material formulations do not undergo any phase transitions and do not show any signs of degradation upon continuous exposure to light for more than 1200 h. The observed high phase stability of the optimal CsPbI3-FAI alloys has been explained through DFT calculations by using criteria for thermodynamic stability. The perovskite solar cells based on Cs2FAPb2I7 absorber material delivered light power conversion efficiency of ca. 13% and retained ~ 80% of the initial efficiency after 1200 h of continuous light soaking. The obtained results set new efficiency and stability records for the Cs-rich low dimensional perovskite-like absorber materials. Further exploration of the newly emerging family of 2D materials stemming from the CsPbI3-FAI and similar alloys will pave the way to the development of a new generation of efficient and stable perovskite solar cells.

Interstitially strengthened metastable FeCoCr-based medium-entropy alloys with both high strength and large ductility

We investigated the effect of interstitial solutes on the phase stability and tensile properties of metastable FeCoCr-based medium-entropy alloys (MEAs). Thermodynamic calculations indicate that the interstitial carbon atom acts as an austenite stabilizer and suppresses the thermally induced martensite formation. With the benefit from interstitial strengthening, carbon-doped FeCoCr-based MEAs have demonstrated an enhanced tensile strength as compared with the undoped counterpart. Originated from the phase metastability and low stacking-fault energy, the martensitic transformation can be activated upon the plastic deformation, leading to the dynamic microstructural refinement. In this way, the significantly improved strength with a maintained tensile ductility can be achieved in the developed MEAs. Our findings have demonstrated that the incorporation of interstitial solutes into metastable alloys contributes to the development of high-performance alloys with a superior strength-ductility synergy.

Structural and magnetic transitions of CoFeMnNiAl high-entropy alloys caused by composition and annealing

Tailoring magnetic behavior by composition and annealing is an effective way. The phase transition from the face-centered-cubic (FCC) to B2 structure influenced by the Al content on the CoFeMnNiAlx alloys leads to the enhanced magnetization. However, the excessive Al addition in the B2-structured alloys impairs the magnetization. The phase transformation caused by annealing also indicates that the saturation magnetization of the alloy depends on the volume fraction of the B2 phase, and a higher content of the B2 phase is beneficial to the large saturation magnetization. Ab initio calculations are used to explain the magnetic behavior of the present HEAs and the effects of phase structures on magnetic characteristics. The phase stability of the CoFeMnNiAlx alloy is thoroughly studied by both the existing phase-formation empirical criteria and annealing. The existing empirical criteria used to predict the phase formation are not universal. However, combining the ΔHmix-δ relation and the φ value, whether the single disordered solid-solution phase is formed can be preliminarily predicted.

New mechanism and criterion for forming multi-component solid-solution alloys

Some existing criteria for forming single-phase multicomponent solid-solution alloys (MCSSAs) are assessed, and a new criterion based on the topology of atomic packing is propounded. A new mechanism concerning the development of MCSSAs is posited, where the multicomponent effect in surface layer, reducing the surface free energy of nanocrystalline nucleus, plays a significant role. More reasonable interpretation regarding the phase stability of CoCrFeMnNi alloy annealed at different temperatures is provided.

Mechanical Properties and Phase Stability of WTaMoNbTi Refractory High-Entropy Alloy at Elevated Temperatures

Mechanical Properties and Phase Stability of WTaMoNbTi Refractory High-Entropy Alloy at Elevated Temperatures

The effect of quench rate on the β-α″ martensitic transformation in Ti–Nb alloys

While Ti–Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the β-α″ martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for α″ phase to be thermally-induced. To resolve these issues, the β-α″ phase transformation is studied in a Ti–24Nb (at.%) alloy subjected to two different water quench conditions. It is demonstrated that only the fastest quench rates (≳500 °C/s) result in the formation of α″ phase, and these materials exhibit a reversible thermally-induced β-α″ transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a β+ω microstructure, and these samples do not exhibit a thermally-induced β-α″ transformation, even when cooled to −168 °C. Despite this, room temperature mechanical testing results indicate that α″ can be induced by stresses as low as 200 MPa. To explain these results, it is proposed that quenching stresses play a role in the competition between α″ and athermal ω phases at rapid quench rates. The present results are then reconciled with the body of literature and a broader understanding of phase stability in metastable β-titanium alloys.

Phase Stability and Microhardness of the AlxCr2-xCoFeNi High-Entropy Alloys

Phase Stability and Microhardness of the AlxCr2-xCoFeNi High-Entropy Alloys

Enhanced phase stability of a BCC structure system high-entropy alloys by decreasing elastic distortion energy

Lightweight BCC high-entropy alloys exhibit outstanding specific strength. However, these alloys usually have poor phase stability, causing brittle precipitates. These precipitates would worsen the plasticity. In order to improve the alloy performance, it is important to improve its phase stability. In this paper, the phase stability of two kinds of BCC high-entropy alloys at 400°C, 500°C and 600°C is investigated. The contributions of mixing enthalpy and elastic distortion energy to their phase stability are discussed. Besides mixing enthalpy, elastic distortion energy also plays a significant role in phase stability. Our results prove that decreasing elastic distortion energy is an effective way to improve phase stability of typical BCC HEAs.

B2 ordering in body-centered-cubic AlNbTiV refractory high-entropy alloys

The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder.

Alloying and magnetic disordering effects on the martensitic transformation and elastic property of Co2VGa Heusler alloy: A first-principles study

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the alloying and magnetic disordering effects on the phase stability and elastic property of L21- and D022-Co2VGa Heusler alloy are systematically investigated. It is shown that at the ground ferromagnetic (FM) state, Co2VGa alloy possesses the L21 structure and correspondingly, its shear modulus, Young modulus, and Debye temperature of the phase are all larger than those of the D022 phase. When the FM state transiting to the paramagnetic (PM) one, both C′(=(C11-C12)/2) and C44 of the L21 phase decrease whereas the elastic anisotropy increases, and they do so as well with increasing the number of valence electrons per atom (e/a) in the off-stoichiometric ternary and doped quarternary alloys with the FM ordering. As a result, at 0 K the magnetic disordered Co2VGa alloy with the disordering degree (y) larger than 0.2 can show the martensitic transformation (MT) from L21 to D022, and the MT is also obtained in the FM Co2V(Ga1-xZx) alloys with Z = Co (x⩾0.2), V and Ge (x⩾0.4), Si (x⩾0.5), and Sb (x⩾0.3). Accompanying the MT, their volume tends to decrease, and these L21 alloys possessing a bigger volume than those with the same e/a ratios are also relatively more prone to the MT. In the Z = V alloys, both greater magnetocaloric and magnetostrain effects may be expected during the MT as well. Both the minority Co d and V d states around the Fermi level should be responsible for the composition and magnetic ordering dependence of the phase stability of Co2VGa alloy.

Hydrogen storage properties of new A3B2-type TiZrNbCrFe high-entropy alloy

Crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFe high-entropy alloy (HEA) were studied. The selected alloy, which had a A3B2-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen) was designed to produce a hydride with a hydrogen-to-metal atomic ratio (H/M) higher than those for the AB2- and AB-type alloys. The phase stability of alloy was investigated through thermodynamic calculations by the CALPHAD method. The alloy after arc melting showed the dominant presence of a solid solution C14 Laves phase (98.4%) with a minor proportion of a disordered BCC phase (1.6%). Hydrogen storage properties investigated at different temperatures revealed that the alloy was able to reversibly absorb and fully desorb 1.9 wt% of hydrogen at 473 K. During the hydrogenation, the initial C14 and BCC crystal structures were fully converted into the C14 and FCC hydrides, respectively. The H/M value was 1.32 which is higher than the value of 1 reported for the AB2- and AB-type HEAs. The present results show that good hydrogen storage capacity and reversibility at moderate temperatures can be attained in HEAs with new configurations such as A3B2/A3B2H7.

Alloy design by tailoring phase stability in commercial Ti alloys

The mechanical characteristics and the operative deformation mechanisms of a metallic alloy can be optimised by explicitly controlling phase stability. Here an integrated thermoelastic and pseudoelastic model is presented to evaluate the β stability in Ti alloys. The energy landscape of β→α′/α″ martensitic transformation was expressed in terms of the dilatational and transformational strain energy, the Gibbs free energy change, the external mechanical work as well as the internal frictional resistance. To test the model, new alloys were developed by tailoring two base alloys, Ti–6Al–4V and Ti–6Al–7Nb, with the addition of β-stabilising element Mo. The alloys exhibited versatile mechanical behaviours with enhanced plasticity. Martensitic nucleation and growth was fundamentally dominated by the competition between elastic strain energy and chemical driving force, where the latter term tends to lower the transformational energy barrier. The model incorporates thermodynamics and micromechanics to quantitatively investigate the threshold energy for operating transformation-induced plasticity and further guides alloy design.