Phase Stability Of High Entropy Alloys Research Articles

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.

Influence of Cu Addition on Phase Stability of High-Entropy Alloys

In this study, the effect of Cu addition on the phase stability of AlCuXNiCoTi (X = 0.5, 1.0, 1.5, and 2.0) high-entropy alloys was analyzed using the regular solution and Kohler’s models to obtain an enthalpy that is symmetrical with respect to the alloy composition. The change in Gibbs energy of mixing of a quinary alloy system was calculated based on the extended Miedema theory by considering the change in enthalpy of mixing of the solid-solution and amorphous phases of 10 binary alloys. The comparison between the experimental and prediction results suggested the possibility of using the thermodynamic properties to predict the stable phases in multicomponent alloys consisting of equiatomic and near-equiatomic elements. In this paper, the structural instability of the solid-solution state arising from the difference between the valence electron concentrations of the constituent elements of the binary mixture are discussed in the aspect of the influence of alloying elements on phase stability of multicomponent alloys.

Microstructure and mechanical properties of Cr-rich Co-Cr-Fe-Ni high entropy alloys designed by valence electron concentration

Cr element is considered to have strong solid solution strengthening effect in high entropy alloys (HEAs). However, the formation of brittle σ phase is enhanced with the increase of Cr content, which is harmful to the ductility of alloys. The valence electron concentration (VEC) is highly correlated with phase stability of HEAs, which is used to predict the formation of σ phase. The microstructure and mechanical behavior of Cr-rich non-equal molar Co-Cr-Fe-Ni HEAs were investigated. Results indicate the formation of σ phase could be suppressed with the high VEC, while lattice parameter tends to increase as Cr content increases, which enhances the metastability of Co-Cr-Fe-Ni system to accelerate grain-boundary precipitates of σ phase. Minor precipitates of σ phase strengthen the HEAs and do not cause serious embrittlement. Deformation-induced martensitic transformation (FCC →HCP) occurs during the tensile deformation of alloys with the VEC of 8.0 and 7.9, and the transformation induced plasticity effect is the main reason for good combination of strength and ductility.

Phase stability of single phase Al0.12CrNiFeCo high entropy alloy upon irradiation

Phase stability of NiCoFeCr and Al0.12NiCoFeCr single phase high entropy alloys (HEAs) was studied under 3 MeV Ni2+ irradiation at 500 °C to a fluence of 1 × 1017/cm2 reaching a peak dose of ~100 displacements per atom (dpa). Transmission electron microscopy (TEM) diffraction pattern and scanning transmission electron microscopy-energy dispersive X-Ray spectroscopy (STEM-EDS) were utilized to detect any second phases that formed in the irradiated regions of the specimens. While the single phase of NiCoFeCr alloy has remained stable under the irradiation, the phase in Al0.12NiCoFeCr alloy decomposed into FCC matrix phase, and Ni3Al type nanoprecipitates with L12 ordered structure. On the other hand, in Al0.12NiCoFeCr alloy, the non-irradiated region of the TEM specimen (annealed for 10 h at 500 °C) and the as-cast alloy annealed for 490 h at 500 °C exhibited no such precipitates similar to those observed in the irradiated region. Based upon the thermodynamic stability of these alloys, we propose that radiation-enhanced diffusion under the irradiation accelerated the formation of the Ni3Al type equilibrium phase. Our study clearly demonstrates that the phase stability of HEAs under irradiation should be considered as one of the deciding factors in the materials design and selection for nuclear applications.

Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys

Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system.

Anomalous High-Temperature Strengthening of an Al &lt;sub&gt;0.5&lt;/sub&gt;CoCrCuFeNi Dual-Phase High-Entropy Alloy

Mechanical response of an Al0.5CoCrCuFeNi high-entropy alloy was investigated through uniaxial compression tests, the 0.2 % offset yield strength decreases with increasing temperature up to 773 K. However, at temperatures above 773 K, two anomalous yield strength hardening regions peaked at 873 K and 1073 K were observed. Microstructure analyses reveal an enhanced L12 ordering with the increase of compression temperatures and the subsequent precipitation of B2 phase at 973 K. Meanwhile, transmission electron microscopy observations of the deformation structure reveal that reveal that plastic deformation is carried primarily by dislocation at room temperature, but changes to deformation twinning at temperatures higher than 873 K. At temperature higher than 1073 K, a mix of dislocations and deformation twins is observed. These findings suggest that the yield strength anomaly is strongly related to the phase stability of high-entropy alloys at high deformation temperatures.

Deformation stimulated precipitation of a single-phase CoCrFeMnNi high entropy alloy

We report transmission electron microscope characterization of phase decomposition in a single-phase CoCrFeMnNi high entropy alloy (HEA) subjected to cold rolling and subsequent annealing at intermediate temperatures. It is found that plastic deformation plays a key role in phase destabilization of the HEA during intermediate annealing. The formation of Cr-rich precipitates in recrystallized grains and recovered samples suggests that dislocations generated by cold rolling is one of the critical factor leading to the destabilization of the stable HEA. Atomic-scale characterization of the Cr-rich precipitates reveals a complex crystallographic structure which is comprised of one-dimensional pentagons with selective atomic occupation. This study indicates that the phase stability of HEAs is very fragile at intermediate temperatures at which the multicomponent solid solutions stabilized by high configurational entropy may break down by local chemical disturbance at dislocations and grain boundaries.

Statistical physics of multicomponent alloys using KKR-CPA

We apply variational principles from statistical physics and the Landau theory of phase transitions to multicomponent alloys using the multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent potential approximation (CPA). This theory is a multicomponent generalization of the $S^{(2)}$ theory of binary alloys developed by G. M. Stocks, J. B. Staunton, D. D. Johnson and others. It is highly relevant to the chemical phase stability of high-entropy alloys as it predicts the kind and size of finite-temperature chemical fluctuations. In doing so it includes effects of rearranging charge and other electronics due to changing site occupancies. When chemical fluctuations grow without bound an absolute instability occurs and a second-order order-disorder phase transition may be inferred. The S$^{(2)}$ theory is predicated on the fluctuation-dissipation theorem; thus we derive the linear response of the CPA medium to perturbations in site-dependent chemical potentials in great detail. The theory lends itself to a natural interpretation in terms of competing effects: entropy driving disorder and favorable pair interactions driving atomic ordering. To further clarify interpretation we present results for representative ternary alloys CuAgAu, NiPdPt, RhPdAg, and CoNiCu within a frozen charge (or band-only) approximation. These results include the so-called Onsager mean field correction that extends the temperature range for which the theory is valid.