Non-Equiatomic High-Entropy Alloys: Compositionally Complex Alloys with High Strength and Ductility
What are non-equiatomic high-entropy alloys?
Conventional alloy design over the past centuries has been constrained by the concept of one or two prevalent base element. As a breakthrough of this restriction, the concept of high-entropy alloys (HEAs) containing multi-principal elements has drawn great attention over the last 15 years due to the numerous opportunities for investigations in the huge unexplored compositional space of multi-component alloys.
A large number of studies in this field have been motivated by the original HEA concept which suggested that achieving maximized configurational entropy via the equiatomic ratios of multiple principal elements could stabilize single-phase massive solid solution phases.
However, an increasing number of studies revealed that formation of single-phase solid solutions in HEAs shows a weak dependence on maximization of the configurational entropy through equiatomic ratios of elements, and it was even found that maximum entropy is not the most essential parameter when aiming at the design of multi-component alloys with superior properties. These findings have recently encouraged efforts to relax both, the unnecessary restrictions on the equiatomic ratio of multiple principal elements and also on the formation of single-phase solid solutions. In this context, non-equiatomic HEAs with single-, dual- or multi-phase structures have been proposed recently to explore the flexibility of HEAs design and overcome the limitations of the original HEA design concept. Also, deviating from the equimolar composition rule facilitates identifying compositions which allow avoiding the often brittle intermetallic phases.
What is the thermodynamic background for designing non-equiatomic high-entropy alloys?
Thermodynamic investigations of non-equiatomic HEAs showed that the configurational entropy curve of these alloys is rather flat, indicating that a wide range of compositions aside from the equiatomic configuration assume similar entropy values. As schematically illustrated in the Figure, compared to the conventional alloys with one or two principal elements plus minor alloying components, and equiatomic HEAs with equimolar ratios of all alloy elements, the non-equiatomic HEAs greatly expand the compositional space that can be probed. Indeed, recent studies have revealed that outstanding mechanical properties exceeding those of equiatomic HEAs can be achieved by non-equiatomic alloys. As one of the possible pathways, a novel type of transformation-induced plasticity-assisted dual-phase (TRIP-DP) HEAs was developed. The two constituent phases in the alloy, i.e., the face-centered cubic (FCC) matrix and the hexagonal close-packed (HCP) phase, are compositionally equivalent and thus can be both referred to as high-entropy phases. This leads to a significantly improved strength-ductility combination compared to the corresponding equiatomic HEAs mainly due to the combination of massive solid-solution strengthening and the TRIP effect. The above findings indicate clearly that expanding the HEA design concept to non-equiatomic compositions has a great potential in pursuing more compositional opportunities for designing novel materials with exceptional properties.
Processing of bulk non-equiatomic high-entropy alloys
For the 3d transition metal HEAs, well-established bulk metallurgical processes are available to synthesize high-quality alloy sheets when the thermomechanical processing parameters are controlled
in a proper way. In our group we use a vacuum induction furnace to melt and cast the various transition metal HEAs. Except for the traditional casting setups, the recently developed combinatorial
approach, referred to as rapid alloy prototyping (RAP), can also be employed towards a rapid trend screening of suited alloy compositions. The RAP technique enables synthesis of five different alloys
with tuned compositions of an alloy system in one operation by using a set of five copper molds which can be moved stepwise inside the furnace.
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) may suggest single- or dual-phase structures. Following casting, alloy plates are cut from the cast blocks and hot-rolled at 900 ℃ with a 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 the hot-rolled HEAs show still some retained compositional inhomogeneity. The hot-rolled alloy sheets are thus homogenized at 1200 ℃ for more than 2 h followed by water-quenching. The homogenization time should be extended according to the dimensions of the alloys sheets, i.e., the larger the alloy sheets, the longer the homogenization time. The homogenized HEA sheets generally show homogeneous distribution of the multiple principal elements and no cracks or pores. For HEAs containing a high amount of Mn (e.g. >10 at. %), however, there would be a few inclusions enriched in Mn which are very hard to be removed even with long-term homogenization.
Interstitally alloyed non-equiatomic high-entropy alloys
Further progress along HEAs and dual phase (DP)-HEAs design pathway lead next to a new type of interstitially alloyed DP-HEA (DP-iHEA). The new interstially alloyed DP-iHEA showed further substantially improved strength while maintaining the excellent ductility of the DP-HEAs. This behavior was attributed to the fact that the DP-iHEAs unify all known strengthening mechanisms in one bulk material, e.g., massive interstitial and substitutional solid solution, the TRIP effect, twinning-induced plasticity (TWIP), multiple phases, nano-precipitates, the evolution of complex dislocation substructures due to the low stacking fault energy and grain boundaries.