Strong and Ductile Non-equiatomic High-Entropy Alloys: Design, Processing, Microstructure, and Mechanical Properties
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We present a brief overview on recent developments in the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). The materials reviewed are mainly based on massive transition-metal solute solutions and exhibit a broad spectrum of microstructures and mechanical properties. Three relevant aspects of such non-equiatomic HEAs with excellent strength–ductility combination are addressed in detail, namely phase stability-guided design, controlled and inexpensive bulk metallurgical processing routes for appropriate microstructure and compositional homogeneity, and the resultant microstructure–property relations. In addition to the multiple principal substitutional elements used in these alloys, minor interstitial alloying elements are also considered. We show that various groups of strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual- or multiphase non-equiatomic compositional configurations with carefully designed phase instability. This design direction provides ample possibilities for joint activation of a number of strengthening and toughening mechanisms. Some potential research efforts which can be conducted in the future are also proposed.
Conventional alloy design over the past centuries has been constrained by the concept of one or two prevalent base elements. As a breakthrough of this restriction, the concept of high-entropy alloys (HEAs) containing multiple principal elements has drawn great attention over the last 13 years due to the numerous opportunities for investigations in the huge unexplored compositional space of multicomponent alloys.1, 2, 3, 4, 5, 6 A large number of studies in this field have been motivated by the original HEA concept, which suggested that achieving maximized configurational entropy using equiatomic ratios of multiple principal elements could stabilize single-phase massive solid-solution phases.1
However, an increasing number of studies have revealed that formation of single-phase solid solutions in HEAs shows weak dependence on maximization of the configurational entropy through equiatomic ratios of elements,7, 8, 9, 10 and it was even found that maximum entropy is not the most essential parameter when designing multicomponent alloys with superior properties.11,12 These findings encouraged efforts to relax the unnecessary restrictions on both the equiatomic ratio of multiple principal elements as well as the formation of single-phase solid solutions. In this context, non-equiatomic HEAs with single-, dual-, or multiphase structure have recently been proposed to explore the flexibility of HEA design and overcome the limitations of the original HEA design concept.4,13,14 Also, deviation from the equimolar composition rule facilitates identification of compositions which allow the often-brittle intermetallic phases to be avoided.
We aim herein to provide a brief overview on some recent developments of various strong and ductile non-equiatomic HEAs, placing specific attention on their compositional design, metallurgical processing routes, and microstructure–property relations. Some pending directions in this field, e.g., multifunctionalities of various non-equiatomic HEAs, are also pointed out.
Compositional Design of Strong and Ductile Non-equiatomic High-Entropy Alloys
Since the majority of the research effort in the field of HEAs during the last decade has focused on single-phase solid solutions, compositional design criteria for achieving single-phase solid solutions have been well explored; For instance, it was proposed that valence electron concentration (VEC) is a critical parameter determining the stability of single-phase FCC (VEC ≥ 8) and body-centered cubic (BCC) (VEC < 6.87) solid solutions.16 However, the limited hardening mechanisms available in single-phase HEAs, i.e., primarily dislocation interaction and solid-solution strengthening, restrict their strain-hardening capacity as well as the attainable strength–ductility combination.
When designing the composition of strong and ductile non-equiatomic dual- or multiphase HEAs, it is also essential to note that the multiple principal elements selected should be distributed uniformly in the microstructure, or at least partition in such a way that all of the coexisting phases have a high solid-solution effect and high mixing entropy. This was not achieved in previous studies, where dual- or multiphase structures were rather formed by elemental segregation, generally leading to undesired brittle intermetallic compounds.3 From this point of view, deformation-driven partial athermal martensitic transformation without associated chemical gradients across phase boundaries is likely to be a well-suited approach for producing dual- or multiphase HEAs with uniformly distributed multiple principal elements. The TRIP-DP effect explained above for addressing the strength–ductility trade-off can also be introduced into other types of HEAs such as the refractory metal TiNbTaZrHf system via a “d-electron alloy design” approach where athermal transitions between the BCC and the HCP phases are conceivable.24
Furthermore, minor interstitial element fractions can also be introduced into strong and ductile non-equiatomic dual- or multiphase HEAs to further improve their mechanical properties. We added carbon as interstitial element into a TRIP-DP-HEA in the pursuit of two main trends25: (i) that addition of interstitial carbon leads to a slight increase in stacking fault energy and hence phase stability, enabling tuning of the FCC matrix phase stability to a critical point so as to trigger the TWIP effect while maintaining the TRIP effect, thereby further improving the alloy’s strain-hardening ability; (ii) that HEAs can benefit profoundly from interstitial solid-solution strengthening with its huge local distortions instead of only the established massive substitutional solid-solution strengthening provided by its multiple principal elements. Thus-prepared interstitial HEA (referred to as iHEA) was indeed characterized by a combination of various strengthening mechanisms including interstitial and substitutional solid solution, TWIP, TRIP, nanoprecipitates, dislocation interactions, stacking faults, and grain boundaries, leading to twice the tensile strength compared with the equiatomic Co20Cr20Fe20Mn20Ni20 reference HEA while maintaining identical ductility.25
Processing of Strong and Ductile Bulk Non-equiatomic High-Entropy Alloys
In as-cast condition, the multiple principal elements are typically not homogeneously distributed in the bulk HEAs with their coarse dendritic microstructure owing to classical Scheil segregation, although x-ray diffraction (XRD) analysis may suggest single- or dual-phase structures.27 Following casting, alloy plates are cut from the cast blocks and hot-rolled at 900°C with total thickness reduction of 50% to remove the dendritic microstructure and possible inherited casting defects. The hot-rolling temperature can be adjusted to higher values depending on the specific alloy compositions. Often, even such hot-rolled HEAs still show some retained compositional inhomogeneity. The hot-rolled alloy sheets are thus homogenized at 1200°C for more than 2 h followed by water quenching. The homogenization time should be extended according to the dimensions of the alloy sheets; i.e., the larger the alloy sheet, the longer the homogenization time. Such homogenized HEA sheets generally show homogeneous distribution of the multiple principal elements and no cracks or pores. However, for HEAs containing high amount of Mn (e.g., >10 at.%), there would be a few inclusions enriched in Mn, which are very hard to remove even by long-term homogenization.
Since homogenized HEA sheets exhibit huge grain size (>30 μm), cold-rolling and annealing processes are generally required to refine the grains to achieve better mechanical properties. We cold-rolled homogenized alloy sheets to total thickness reduction of ~60% and annealed them at 900°C for different time periods. Annealing was conducted to obtain full recrystallization of the microstructure and to control the grain sizes, hence times and temperatures were adjusted in each case according to the targeted grain sizes and textures. Long-term annealing treatments (e.g., 500 days) at intermediate temperatures (e.g., 700°C and 500°C) can lead to chemical segregations even for the single-phase equiatomic CoCrFeMnNi HEA.8
Processing routes are not only decisive for the grain size and phase fraction corresponding to a targeted specific composition, but are also critical with respect to the compositional homogeneity state, which also has significant effects on the mechanical behavior. We applied various processing routes including hot-rolling, homogenization, cold-rolling, and recrystallization annealing on cast alloys to obtain samples in different compositional homogeneity states.27 In the case of coarse grains (~300 µm) obtained for as-cast alloys without homogenization treatment, the ductility and strain hardening of the material were significantly reduced due to the compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe-enriched regions with lower stacking fault energy, promoting early stress–strain localization.27 The grain-refined structure (~4 µm) with compositional heterogeneity obtained for annealed alloys without any preceding homogenization treatment was characterized by almost total loss of work hardening. This effect was attributed to large local shear strains due to inhomogeneous planar slip.27 These findings demonstrate the importance of processing routes for the development of strong and ductile HEAs.
Microstructure and Mechanical Properties of Non-equiatomic High-Entropy Alloys
Furthermore, the homogenized dual-phase non-equiatomic Fe50Mn30Co10Cr10 alloy (Fig. 6; #6) shows much higher ultimate strength compared with the various single-phase alloys (#1–4) and the dual-phase Co20Cr20Fe34Mn20Ni6 alloy (#5), while maintaining total elongation above 50%. Interestingly, the alloy with the same composition but refined FCC matrix grains (#7) exhibits further significant joint increase of strength and ductility. This is ascribed to the substantially improved work-hardening ability of the alloy due to the well-tuned phase stability via adjustment of grain size and phase fractions.4,13 With addition of interstitial element carbon into the dual-phase microstructure, the grain-refined Fe49.5Mn30Co10Cr10C0.5 alloy (#8) shows further increased ultimate strength up to nearly 1 GPa with total elongation of ~60%. These superior mechanical properties are attributed to the joint activity of various strengthening mechanisms including interstitial and substitutional solid solution, TWIP, TRIP, nanoprecipitates, dislocation interactions, stacking faults, and grain boundaries.25
To further clarify the mechanisms responsible for the above microstructure–property relations, Fig. 7 provides an overview of the various deformation mechanisms in different multicomponent HEAs presented in Fig. 6. From the single-phase Fe40Mn27Ni26Co5Cr2 and Fe35Mn45Co10Cr10 alloys towards the single-phase Co20Cr20Fe20Mn20Ni20 and Fe40Mn40Co10Cr10 alloys, a TWIP effect has been introduced. Then, the presence of phase boundaries (dual-phase structure) and the TRIP effect are included in the dual-phase Co20Cr20Fe34Mn20Ni6 and Fe50Mn30Co10Cr10 alloys. Proceeding further to the carbon-containing Fe49.5Mn30Co10Cr10C0.5 alloy, interstitial solid solution and nanoprecipitate strengthening effects are additionally utilized, thereby unifying all known strengthening mechanisms in one material. Indeed, these multiple deformation mechanisms enable significant improvement of the strain-hardening capacity and strength–ductility combination. This clearly shows that tuning deformation mechanisms via composition adjustment is key to the design of strong and ductile non-equiatomic HEAs. Therefore, we suggest that introduction of multiple deformation mechanisms which are activated gradually or, respectively, sequentially during mechanical and/or thermal loading is a key route for future development of new HEAs with advanced mechanical properties. Such a concept can be realized by shifting from single-phase equiatomic to dual- or multiphase non-equiatomic compositional configurations.
Summary and Outlook
We have presented a brief overview on the design, processing, microstructure, and mechanical properties of non-equiatomic HEAs. As the most widely studied HEA family, we mainly focused on 3d transition-metal HEAs, which exhibit a broad spectrum of microstructures and mechanical behavior. We showed that various strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual- or multiphase non-equiatomic compositions. The non-equiatomic HEA concept provides possibilities for the unification of various strengthening and toughening mechanisms, enabling significant improvement of strain-hardening capacity and strength–ductility combinations. To design strong and ductile non-equiatomic transition-metal HEAs, the intrinsic stacking fault energies and/or the free energy differences between the FCC and HCP phases for various compositions can be probed in an effort to tune the phase stabilities by adjusting the compositions. Proper processing routes toward homogeneously distributed multiple principal elements and appropriate grain sizes are also important for achieving targeted strength–ductility combinations, especially for dual-phase non-equiatomic HEAs with TRIP effect.
The strength and ductility of the various non-equiatomic HEAs at low and elevated temperatures are still unknown, and new (non-equiatomic) HEAs with excellent strength–ductility combinations at low and elevated temperatures can be designed and studied.
For the widely studied transition-metal HEAs with their good strength–ductility combinations, other properties such as the resistance to hydrogen-induced degradation,28,29 corrosion resistance, fatigue behavior, and magnetic performance can also be explored in an effort to find superior combinations of properties, i.e., multifunctionalities, to justify their relatively high cost compared with established high-strength and austenitic stainless steels. Also, following the design approach associated with the stacking fault energies and/or the free energies, a shape-memory effect could be introduced into non-equiatomic HEAs.
Other than the transition-metal family, non-equiatomic HEAs containing refractory elements such as Ti, Nb, Ta, Zr, Hf, V, Mo, and W30 can also be developed towards high-performance refractory HEAs as high-temperature load-bearing structures for application in the aerospace industry.
Considerable research work can also be conducted to screen the effects associated with different minor interstitial element fractions (C, N, B, O, etc.) to further improve the performance of the various non-equiatomic HEAs and understand the corresponding mechanisms.
Open access funding provided by Max Planck Society. The authors would like to acknowledge the financial support by the European Research Council under the EU’s 7th Framework Programme (FP7/2007–2013) and the contributions of Konda Gokuldoss Pradeep, Cemal Cem Tasan, Yun Deng, Mengji Yao, Hauke Springer, Fritz Körmann, Blazej Grabowski, and Jörg Neugebauer.
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