Combinatorial synthesis of high entropy alloys: Introduction of a novel, single phase, body-centered-cubic FeMnCoCrAl solid solution
Graphical abstract
Introduction
High entropy alloys (HEA) introduced a decade ago have attracted a lot of interest in part due to the alloy design strategy that deviates from conventional guidelines [1], [2]. Rather than having one principal element (solvent) with minor alloying additions (solute) to obtain desired properties, HEAs are composed of multi-principal elements in equal or near equal atomic ratios [1], [2], [3], [4], [5]. The conventional HEA design concept intends to maximize the configurational entropy thereby decreasing the free energy and increasing the stability of the system [2], [4], [6], [7], [8]. Hence, the alloy is expected to stabilize into single phase alloys or solid-solutions with simple crystal structures rather than complex multiphase intermetallics [1], [4], [6], [7].
Being multi-component systems in near equi-molar ratios these alloys are positioned at the center of the multi-component phase diagram. Therefore, predictions of phase formation and stability at ambient and elevated temperatures using the existing thermodynamic databases such as Thermocalc®, which are generally based on extrapolated data stemming from available binary and ternary systems, often found to deviate significantly from experimental observations [1], [2], [6]. However, recently computational screening of thousands of potential equimolar HEA compositions have been reported [7], [9]. Nevertheless, reports have indicated that conventional HEA design approaches based on maximizing configurational entropy alone does not necessarily lead to single phase solid solution formation as multi-phase mixtures including intermetallic compounds have often been observed [4], [7], [8], [10], [11], [12].
Experimental approaches utilized to aid the design of HEAs such as rapid alloy prototyping (RAP) enable high-throughput screening along with their properties but is limited to five alloy compositions only [1], [6], [13]. Furthermore, the early success achieved by the design of non-equiatomic HEAs using RAP has increased the number of possible alloy combinations over which single phase solid solutions can be achieved tremendously [1], [3], [14], [15], [16]. With the rapid emergence of such novel multi-component HEAs and the limited information available about alloys positioned at the center of the phase diagram, efficient experimental approaches are therefore required for rapid screening of single phase HEAs.
In order to screen the structure formation at multiple length scales and to investigate the mechanical properties, we apply for the first time, the combinatorial thin film approach for HEAs. It includes combinatorial material's synthesis of thin-film composition-spreads using multi-target sputter-deposition [17]. Accordingly, HEAs of all possible compositions can be produced combined with high-throughput screening for phase formation and stability, supported and or guided by electronic structure calculations [17].
This paper therefore aims at rapid screening of the novel FeMnCoCrAl system over a widely varying non-equiatomic and equiatomic compositions along with their mechanical properties. Furthermore, the experimental phase formation data were compared to ab-initio calculations and the elastic modulus and hardness data obtained for the equiatomic thin film was compared to the bulk cast alloy. It has to be noted that the majority of single phase HEAs reported to have BCC structure are all constituted by refractory metals [18], [19], [20] and that the HEA introduced in this work for the first time, consists of transition metals and exhibits a single BCC solid solution over widely varying Al concentrations.
Section snippets
Theory
In order to predict the phase stability and to calculate the elastic properties of FeMnCoCrAl HEA configurations, exact muffin tin orbitals (EMTO) [21], [22], based on Green's functions [23] and full charge density [24], were employed. The generalized gradient approximation [25] within the EMTO approach was applied for the density functionals and ion cores were kept frozen. The integration in the Brillouin zone was carried out on a 13 × 13 × 13 k-mesh and the total energy convergence criterion
Experimental details
FeMnCoCrAl HEA thin films were grown at room temperature by combinatorial DC magnetron sputtering in an Ar atmosphere (3 mTorr), the schematic of which is shown in Fig. 1(a). Fe-Mn alloy (equiatomic composition), Co, Cr and Al as individual metal targets with ≥3 N purity were used on four individual magnetrons applying power densities of 7.5 W cm−2 for Fe-Mn and 4 W cm−2 for Co, Cr and Al respectively. All four magnetrons were powered simultaneously during co-deposition onto Si substrates for
Results and discussion
Fig. 1(b) shows the top surface view of the thin film HEA composition spread. It has to be noted that the equiatomic FeMnCoCr HEA has been reported [39] to result in multiple phase formation and hence the emphasis is placed along the Al concentration gradient at a constant Fe/Mn ratio of 0.84 (resulting from slightly higher sputter yield of Mn compared to Fe). Accordingly, EDX measurements were carried-out along the Al concentration gradient (top to bottom spots 1 to 5 in Fig. 1(b)). Fig. 1(c)
Summary
In summary, a novel, non-refractory, single BCC phase family of solid solution HEA namely, (FeMn)100-(x+y+z)Coy∼20-22Crz∼19-22Alx (x = ∼13–27 at.%) is introduced. The equiatomic HEA synthesized by both conventional casting and combinatorial sputtering exhibited identical structures and mechanical properties. The experimentally observed properties are also in good agreement with ab-initio predictions of equilibrium volume. The nanostructure of the HEA thin film, characterized at all length
Acknowledgement
M.A is grateful for the financial support provided by the “DAAD-IIT Master Sandwich program” for his stay at RWTH Aachen University, Germany.
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