MICROSTRUCTURE OF HIGH-ENTROPY ALLOYS (HEAs)

ABSTRACT

High-entropy alloys (HEAs) are a new class of advanced metallic materials composed of multiple principal elements in near-equiatomic or non-equiatomic ratios. Due to their high configurational entropy, these alloys tend to form simple solid solution phases instead of complex intermetallic compounds. The present study investigates the microstructure of AlCoCrNi-based HEAs produced by arc melting and subsequent heat treatment. Microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results reveal the formation of FCC and BCC phases with dendritic morphology. Heat treatment significantly influenced phase distribution and grain refinement. The findings contribute to understanding the relationship between composition and microstructural evolution in HEAs.

АННОТАЦИЯ

Высокоэнтропийные сплавы (HEAs) представляют собой новый класс перспективных металлических материалов, состоящих из нескольких основных элементов в эквиатомных или неэквиатомных соотношениях. Благодаря высокой конфигурационной энтропии такие сплавы склонны к формированию простых твёрдых растворов вместо сложных интерметаллических соединений. В данной работе исследована микроструктура сплавов системы AlCoCrNi, полученных методом дуговой плавки с последующей термической обработкой. Микроструктурный анализ проводился с использованием оптической микроскопии (OM), сканирующей электронной микроскопии (SEM) и рентгенофазового анализа (XRD). Полученные результаты показали формирование фаз FCC и BCC с дендритной морфологией. Термическая обработка существенно повлияла на распределение фаз и измельчение зёрен. Полученные данные способствуют более глубокому пониманию взаимосвязи между химическим составом и эволюцией микроструктуры в высокоэнтропийных сплавах.

Keywords: High-entropy alloys, microstructure, solid solution, dendritic structure, phase transformation.

Ключевые слова: высокоэнтропийные сплавы, микроструктура, твёрдый раствор, дендритная структура, фазовые превращения.

Introduction. High-entropy alloys (HEAs) represent a new generation of metallic materials composed of five or more principal elements in near-equiatomic or non-equiatomic ratios. Unlike conventional alloys based on one dominant element, HEAs rely on high configurational entropy to stabilize simple solid solution phases such as FCC (face-centered cubic) and BCC (body-centered cubic). This unique design concept often suppresses the formation of brittle intermetallic compounds and provides attractive mechanical and physical properties.

The microstructure of HEAs plays a decisive role in determining their performance. Grain size, phase distribution, dendritic morphology, and elemental segregation directly influence hardness, strength, ductility, and corrosion resistance. During solidification, compositional differences between dendritic and interdendritic regions may occur, affecting phase stability. Additionally, alloying elements such as Al and Ni significantly influence the formation of BCC or FCC phases.

 Materials and Methods

High-entropy alloys investigated in this study were prepared using conventional casting and mechanical alloying (MA) routes. Cast alloys were produced through melting and solidification, which typically result in microstructures consisting of dendritic (DR) and interdendritic (ID) regions due to segregation during solidification.Microstructural characterization was carried out to evaluate phase constitution, morphology, and elemental distribution within these regions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine morphological features and nanoscale precipitates. Phase identification was performed using X-ray diffraction (XRD). Elemental distribution at micro- and nanoscale levels was analyzed using energy-dispersive spectroscopy (EDS) mapping and three-dimensional atom probe tomography (3D-APT).

Figure 1. Depiction of phase formation sequence during cooling of AlxCoCrCuFeNi alloy system with different aluminum contents

High-angle annular dark-field (HAADF) imaging was additionally employed to distinguish compositional contrast between phases.For comparison, mechanically alloyed samples were synthesized by high-energy ball milling of elemental powders. The as-milled powders were subsequently subjected to hot consolidation at elevated temperature (600°C) to obtain bulk specimens. Atom probe tomography was used to assess elemental homogeneity in the as-milled and consolidated conditions. Grain size and nanostructural stability after consolidation were also evaluated through electron microscopy techniques.These combined experimental methods enabled a systematic investigation of microstructural evolution, elemental partitioning, and phase formation as a function of processing route.

Results

Figure 2 illustrates the phase formation sequence during solidification of AlCoCrCuFeNi alloys with varying aluminum content. The schematic shows that upon cooling from the liquid state, dendritic (DR) and interdendritic (ID) solid solution regions first form due to segregation during solidification. At lower aluminum content, nanostructures mainly develop through nanoprecipitation within the solid solution matrix. However, at higher aluminum concentrations, spinodal decomposition becomes dominant, leading to nanoscale modulated structures followed by secondary nanoprecipitation. This confirms that Al content strongly influences phase stability and decomposition pathways.

Figure 2. Bright-field TEM images showing (A) DR and ID regions, (B) DR showing plate-like precipitates and the presence of ordered B2 structure, (C) the presence of rhombohedral precipitates in DR and weak reflections of L12 phase, and (D) microstructure of ID region and weak superlattice reflections of L12 phase for as-cast AlCoCrCuFeNi alloy

Bright-field TEM images (Figure 2) reveal a typical cast microstructure composed of dendritic and interdendritic regions. The DR zones contain plate-like precipitates and ordered domains, while selected area diffraction patterns indicate the presence of ordered B2 structure. Rhombohedral and spherical precipitates are also observed within the dendritic matrix. Weak superlattice reflections suggest the formation of L1₂-type ordering.The ID regions exhibit comparatively different contrast and finer microstructural features, indicating compositional differences between DR and ID zones. These variations suggest chemical segregation during solidification.High-angle annular dark-field (HAADF) imaging combined with EDS mapping (Figure 4) further clarifies elemental distribution. The contrast variation in HAADF images corresponds to compositional differences among phases. Elemental maps indicate that Ni, Al, and Co are concentrated in one set of lamellar structures, whereas Cr and Fe dominate another phase. Cu is highly localized in cylindrical or elliptical precipitates and nanoscale regions.The spatial distribution of elements suggests the coexistence of ordered B2 and disordered BCC (A2) phases. The fine-scale lamellar morphology and compositional modulation indicate spinodal decomposition as a major microstructural evolution mechanism.

Figure 3. 3D reconstruction of Al, Cr, Ni, Co, Fe, and Cu atom positions in AlCoCrCuFeNi alloys

Comparison with mechanically alloyed HEAs reported in literature indicates a more homogeneous elemental distribution in the as-milled state compared to cast alloys. However, after hot consolidation, partial segregation—particularly of Cu toward grain boundaries—may occur. Nanocrystalline grains (~10 nm) are retained even after consolidation, demonstrating the structural stability of MA-processed alloys.

Discussion

The obtained results confirm that microstructure evolution in Al-containing HEAs is strongly influenced by aluminum concentration and processing route. During casting, segregation between dendritic and interdendritic regions is inevitable due to non-equilibrium solidification. The enrichment of Cu in specific regions is associated with its positive mixing enthalpy with several elements, promoting phase separation.The presence of B2 ordered phase within the BCC matrix indicates chemical ordering driven by thermodynamic stabilization at lower temperatures. Spinodal decomposition contributes to nanoscale modulation, enhancing chemical heterogeneity.Although configurational entropy plays a stabilizing role in HEAs, the observed phase separation demonstrates that enthalpy of mixing, atomic size mismatch, and diffusion kinetics also significantly influence microstructure development. Therefore, phase evolution in nonequiatomic HEAs cannot be explained by entropy effects alone.

Conclusion

The present study investigated the microstructural evolution of Al-containing high-entropy alloys processed through different routes. The results demonstrate that cast HEAs exhibit a characteristic dendritic (DR) and interdendritic (ID) microstructure formed due to segregation during solidification. The dendritic regions contain ordered B2 phases, plate-like precipitates, and nanoscale modulated structures, while interdendritic regions show compositional heterogeneity and Cu-rich segregation.

Three-dimensional atom probe and HAADF–EDS analyses confirmed significant elemental partitioning among phases. Al, Ni, and Co were enriched in ordered plate-like domains, whereas Cr and Fe were concentrated in adjacent BCC-related phases. Cu showed strong segregation behavior, forming nanoscale precipitates and localized regions. The presence of spinodal decomposition was verified through nanoscale compositional modulation.

Overall, the findings confirm that, in addition to configurational entropy, other thermodynamic and kinetic factors such as mixing enthalpy, atomic size mismatch, and diffusion behavior play a critical role in phase evolution of nonequiatomic HEAs. Understanding these mechanisms provides valuable guidance for tailoring microstructure and optimizing mechanical performance of high-entropy alloys.

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