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Journal Club for January 2019: Nanoprecipitation strengthening in high entropy alloys

Xiaoyan Li's picture

Nanoprecipitation strengthening in high entropy alloys

Xiaoyan Li, Jianguo Li

Center for Advanced Mechanics and Materials, Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China

Introduction

    High-entropy alloys (HEAs) are a class of new metallic materials that have revolutionized alloy design over the past ten years. HEAs generally contain five or more principal elements with the atomic concentration of each element ranging between 5 and 35% [1,2]. The name of HEA just originates from the high configurational entropy of random mixing of at least five principle elements with near/equiatomic ratios. As exemplified by a typical ternary phase diagram in Fig. 1a, the HEA’s concept [1,2] makes the researchers watch the center of the phase diagram, rather than focus on the corners occupying small portions corresponding to the conventional alloys. Such novel alloy design opens up new avenues for exploring more alloys with special microstructures and properties in much wider compositional space.

    Over the past ten years, HEAs have attracted the worldwide attention [3-30]. Generally, HEAs have four core effects [2,11]: (1) high entropy, (2) sluggish diffusion, (3) severe lattice distortion, and (4) cocktail effects. Here we introduce these four core effects separately. The high entropy effect means that the alloys have high configurational entropy (or mixing entropy) due to the co-existence of multiple principal elements with near/equiatomic ratios. The high entropy effects tend to stabilize the solid-solution phases by reducing the Gibbs free energy of mixing and to expand the solution limits between the elements [11]. The thermodynamic rules have been used to understand the interplay between mixing entropy and phase selection in HEAs. In fact, except the mixing entropy, the phase distribution and their stability in HEAs are also associated with the difference in the atomic size and the valence electron concertation [3-11]. The sluggish diffusion effect means that the atomic diffusion coefficients in HEAs are lower than those in the pure metals and stainless steels. Such effect can be used to explain the formation of nano-sized precipitations in HEAs [11]. The severe lattice distortion effect means that the lattices in HEAs are distorted due to the difference in the atomic size of multiple principal elements (Fig. 1b). Compared with one dominant element alloys or pure metals, such lattice distortion effect induces the decreasing of dislocation mobility in HEAs, which to some extent contributes to high strengths of HEAs. Such effect is also related to the tensile brittleness of HEAs. The cocktail effect means that the unexpected properties can be obtained after mixing many elements, which could not be obtained from any one independent element [11]. It implies that the alloy properties can be greatly adjusted by changing composition or alloying [11].

    Due to four core effects mentioned above, HEAs exhibited unique combinations of properties that are not attainable in the conventional alloys, including ultra-high strength and fracture toughness [12,13] (Fig. 1c), even excellent properties/performance at elevated temperatures [4,14-18] (Fig. 1d), high thermal stability, good superconductivity, remarkable corrosion resistance, attractive tribological properties, good creep resistance and excellent irradiation tolerance [11,19-22].

Fig 1. (a) Schematic illustration of a typical ternary phase diagram. It shows the difference between conventional alloys and HEAs [10]. (b) Schematic illustration of severe lattice distortion. It is caused by randomly distributed atoms with different sizes occupying the lattice site in HEAs [11]. (c) Ashby map showing fracture toughness as a function of yield strength for HEAs in relation to a wide range of material systems [12]. (d) Temperature dependence of compressive yield strength in some BCC HEAs. For comparison, some conventional high temperature superalloys also are included. It shows that BCC HEAs have more stable high temperature properties [4].

Recent advances in nanoprecipitation strengthening in HEAs

    In the early stage of the development of HEAs, the researchers dedicated to seek single-phase solid-solution alloys, because the intermetallics are thought to be brittle and to be likely to degenerate the properties of HEAs. However, the fact that in most engineering alloys, the secondary phases contribute significantly to the alloy properties, that is true in HEAs. The recent studies have reported that some HEAs that can overcome the strength-ductility trade-off contain two or more phases [28,29]. In HEAs, various strengthening mechanisms, such as solid-solution strengthening, phase transformation strengthening and precipitation strengthening, have been used to achieve their ultra-high strengths. The recent experimental studies [23-30] showed that the precipitation strengthening is the most effective manner to enhance the strength of HEAs and to simultaneously sustain good ductility. In the following section, we show some recent progress in nano-sized precipitation strengthening in HEAs. Generally, the solid-solution phases reported in HEAs include five types of phases, including disordered face-centered cubic (FCC, A1), disordered body-centered cubic (BCC, A2), ordered FCC (L12), ordered BCC (B2), and hexagonal close-packed (HCP, A3). The ordered L12 phases are an important class of strengthening phases in FCC-based alloys, which are ductile and coherent with FCC matrices.

    We designed and fabricated non-equiatomic system Al0.5Cr0.9FeNi2.5V0.2 by adding more Ni element and controlling the Ni/Al ratio [28]. In such HEA system, high-content Ni3Al-type L12 phases have been introduced by the spinodal decomposition. The high-resolution STEM image in Fig. 2a shows a distinctive nanostructure consisting of a disordered FCC matrix and ordered L12 phases with diffuse coherent interfaces. The atom probe tomography (APT) results in Fig. 2b-c further reveals 3D morphologies of the ordered L12 nanoprecipitates and disordered FCC matrix. It is observed that the interconnected disordered FCC matrix serves as the frame and is filled with ordered L12 phases. These L12 nanophases can effectively block dislocation motion by providing strong diffuse attractive obstacles and by creating antiphase boundaries on the slip planes of dislocations. It leads to a yield strength increase of ~1.5 GPa relative to the HEA without precipitation, achieving one of the highest tensile strength (1.9 GPa) among all bulk HEAs reported previously. Moreover, the diffuse low-misfit coherent FCC-L12 interfaces can minimize the elastic strain accumulation and hence prevent crack initiation at these interfaces, which contributes to good ductility.

Fig 2. (a) High-resolution STEM image revealing the nanostructure in the grains consisting of disordered FCC matrix and ordered L12 phases with diffuse coherent interfaces. (b-c) APT data showing 3D morphologies of the ordered L12 nanoprecipitates and disordered FCC matrix.

    Most recently, Yang et al. [29] introduced another multi-component intermetallic nanostructured L12 phases to strengthen the FCC HEAs by controlling the order-disorder phase transformation and elemental partition. The TEM image in Fig. 3a shows that the near-spherical multicomponent L12 phases (about 30~50 nm) are relatively uniformly distributed inside the FCC matrix. The high-resolution TEM image in Fig. 3b further reveals that the multicomponent L12 phases are perfectly coherent with the matrix. The introduction of these L12 nanostructures results in both cross-slip and rearrangement of high-density dislocations during plastic deformation, which further induces the formation of micro-bands under the large strains (Fig. 3c). These micro-bands can generate a large increase in back stress hardening and further stabilize the uniform plasticity. These work-hardening behaviors lead to both high strength and good ductility of HEAs.

Fig 3. (a) TEM image showing the nanostructured L12 phases inside the FCC matrix. (b) High-resolution TEM image showing the coherent interfaces between the L12 phases and matrix. (c) Dynamic evolution of the deformation substructures of HEA with increasing tensile strain. It reveals the influence of L12 phases on dislocation slips and main plasticity mechanisms (HDDWs: high density dislocation walls; MBs: micro-bands). [29]

Outlook

    There have been only 15 years since the HEAs’s concept was proposed independently by Yeh and Cantor et al. [1,2]. In spite of the rapid developments in experiments and simulations for HEAs in the recent years, our fundamental understanding on the connections between alloy composition/internal microstructures and mechanical properties of HEAs is still in its infancy. There remain many important open questions related to design, fabrication and mechanics of HEAs that deserve further research efforts. As a very brief review, we here highlight three common questions for further discussion.

(1) So far, the definition of HEA has been not universally agreed. What’s the intrinsic feature of HEAs?

(2) Is it necessary for HEAs to pursuit the single solid-solution during design and fabrication? If the HEAs contain multiple phases, how do these phases arrange/interact to achieve excellent mechanical properties?

(3) Whether/How can one design/control the size, shape and distribution of nanoprecipitations to optimize the mechanical properties (such as strength and ductility) of HEAs?

References

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