Exceptional enhancement of mechanical properties in high-entropy alloys via thermodynamically guided local chemical ordering

Abstract

Understanding the local chemical ordering propensity in random solid solutions, and tailoring its strength, can guide the design and discovery of complex, paradigm-shifting multicomponent alloys. First, we present a simple thermodynamic framework, based solely on binary enthalpies of mixing, to select optimal alloying elements to control the nature and extent of chemical ordering in high-entropy alloys (HEAs). Next, we couple high-resolution electron microscopy, atom probe tomography, hybrid Monte-Carlo, special quasirandom structures, and density functional theory calculations to demonstrate how controlled additions of Al and Ti and subsequent annealing drive chemical ordering in nearly random equiatomic face-centered cubic CoFeNi solid solution. We establish that short-range ordered domains, the precursors of long-range ordered precipitates, inform mechanical properties. Specifically, a progressively increasing local order boosts the tensile yield strengths of the parent CoFeNi alloy by a factor of four while also substantially improving ductility, which breaks the so-called strength-ductility paradox. Finally, we validate the generality of our approach by predicting and demonstrating that controlled additions of Al, which has large negative enthalpies of mixing with the constituent elements of another nearly random body-centered cubic refractory NbTaTi HEA, also introduces chemical ordering and enhances mechanical properties.

Keywords: chemical ordering; high-entropy alloys; thermodynamics.

Fig. 1.

Microstructural characterization of CoFeNi using TEM and APT. (A) SAEDP recorded from [101] FCC zone axis showing only fundamental reflections, indicating absence of ordering or secondary phases. (B) Intensity profile along the red dotted line in A confirms single-phase microstructure. APT ion maps of (C) Fe and (D) Ni show homogeneous distribution. (E) No statistically significant deviation is observed between the frequency distribution of Ni obtained from APT and the standard binomial distribution.

Fig. 2.

(A–D) Microstructure of Al_0.25CoFeNi from TEM and APT. (A) SAEDP recorded from [101] FCC zone axis shows only fundamental reflections, indicating the absence of ordering or secondary phases. (B) Frequency distribution of Al and Ni obtained from APT when compared with binomial distributions reveals statistically significant deviations. (C) APT ion map of Al shows uniform distribution. (D) Al–Ni-rich clustered-ordering domains are revealed via cluster analysis and are colored differently. (E) Pair distribution functions (PDFs) of Ni–Ni, Al–Al, and Ni–Al pairs from 216-atom SQS structure for perfectly random Al_0.27CoFeNi alloy. (F) PDFs from 216-atom SQS structure for short-range ordered Al_0.27CoFeNi alloy annealed at 1,200 °C. (G) Warren–Cowley SRO parameters (α_i-j) of various i and j pairs for 1NN shell. The SRO parameters for the perfectly random solid solution and the alloy with SRO at 1,200 °C are shown in sky blue and in red, respectively.

Fig. 3.

Microstructural characterization of Al_0.3CoFeNi using TEM and APT. (A) SAEDP recorded from [101] FCC zone axis shows faint superlattice reflections along with fundamental reflections, which indicate ordering. (B) Intensity profile along the red dotted line in (A) confirms ordering. (C) Dark-field TEM micrograph recorded from $(10\overrightarrow{1})$ reflection reveals the ordered domains. HR-STEM micrographs show ordered (D) and disordered (E) structures. (F) Frequency distribution of Al and Ni obtained from APT, compared with the binomial distribution, shows significant statistical deviation. (G) Al–Ni-rich clusters, obtained from APT.

Fig. 4.

Microstructural characterization of Al_0.3Ti_0.2CoFeNi using TEM and APT. (A) SAEDP recorded along [101] FCC zone axis shows both sharp superlattice and fundamental reflections, indicating ordering. (B) Intensity profile along the red dotted line shown in A confirms ordering. (C) Dark-field TEM micrograph recorded from $(1\overrightarrow{1}0)$ reflection reveals ordered domains. HR-STEM micrographs in D and E show the ordered and disordered structures, respectively. (F and G), respectively, are APT ion map of Fe, and Ti 11 at.% iso-concentration surface. The latter shows Ti-rich LRO domains. (H) Proximity histogram showing the elemental partitioning between FCC matrix and LRO domains. Error bars are colored gray.

Fig. 5.

Tensile deformation plots for CoFeNi, Al_0.25CoFeNi, Al_0.3CoFeNi, and Al_0.3Ti_0.2CoFeNi. (A) Engineering stress–strain curves. (B) True stress–strain curves. (C) Work-hardening rate plots.

Fig. 6.

NbTaTi and Al_0.5NbTaTi microstructural characterization using TEM and APT. (A) SAEDP recorded along [001] BCC zone axis showing only fundamental reflections, indicating absence of ordering and/or secondary phases. (B) Intensity profile along the red dotted line in A confirms single phase. (C) Ti and Nb APT ion maps show very early stages of Ti clustering. (D) SAEDP recorded along [001] BCC zone axis showing superlattice reflections, indicating presence of ordering. (E) Intensity profile along the red dotted line in D confirms ordering. (F) Dark-field TEM micrograph recorded from {001} reflection revealing the ordered domains. (G) APT ion map of Al and Ti showing early stages of compositional partitioning. (H) Proximity histogram constructed across Ti 30.4 at.% iso-concentration surface shows elemental partitioning between BCC matrix and LRO (B2) domains. Error bars are colored gray. (I) Al–Ti-rich clustered-ordering domains obtained from APT.