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The present article advances the idea of compositional flexibility. The intent is to provide an avenue for value creation from waste alloys in cases where costs of separating co-mingled alloy flows are prohibitively high. A compositionally flexible alloy has the potential to be produced solely from varying and intermingled flows of end-of-life alloys. While the alloy composition may vary from day-to-day or from heat-to-heat, the intent is that it will still satisfy base performance targets. Using FeNiCr based high entropy alloys as inspiration, we exploited Bayesian Optimization to define a number of example compositionally complex alloys that display strengths within specified limits (150-250 MPa, 200-350 MPa and 300-350 MPa). The concept was motivated by the surprising number of high entropy alloys reported in the literature that display a similarity of properties despite their wide dispersion of composition. We propose a simple measure of compositional flexibility by combining the ranges over which each elemental addition is permitted to vary and suggest methods of alloy specification. A series of 68 prototype compositions were produced and characterized to demonstrate the concept and approach. We show that our model alloys could conceivably be produced using multiple different combinations of existing Ni alloys in combination with 316 stainless steel. Our hope is that the concept will find application to facilitate alloy recovery where it might not otherwise be feasible. [ABSTRACT FROM AUTHOR]

In the present study, the evolution of microstructure, texture and high-temperature mechanical behavior of the additively manufactured (AM) eutectic high entropy alloys (EHEA) AlCoCrFeNi 2.1 have been investigated. The material was manufactured through laser engineered net shaping (LENS) process. The microstructural investigation revealed constituent phases consisting of dendritic and eutectic features, with the phases having ordered FCC (L1 2) and BCC crystal structures. The phase fraction of L1 2 was more across the build (X) face and that of the BCC was more along build plane (Z face). In both the phases, nickel constituted the base element with L1 2 phase being deficient in Al and the BCC deficient in Cr and rich in Al. The stability of L1 2 phase was attributed to Co, Cr, and Fe with near equiatomic distribution. Kurdjumov-Sachs (KS)orientation relationship was followed between the ordered L1 2 and BCC phases along and across the build-up direction. Strain partitioning was observed more in the BCC phase than in the L1 2 phase during uniaxial compression at all temperatures. Difference in hardness was observed along Z and X directions which further resulted in yield anisotropy during compression test at room temperature. High-temperature compression tests at temperatures 400 °C, 600 °C, 700 °C, and 800 °C revealed that the yield strength increased from room temperature till 400 °C and then started decreasing till 800 °C. Image 1 • Phase evolution in additively manufactured high entropy alloy AlCoCrFeNi 2.1 • High-temperature mechanical behaviour of the AM eutectic HEA. • Orientation relationship between the phases. [ABSTRACT FROM AUTHOR]

Abstract The microstructure of the Al x CoCrFeNiTi y high entropy alloy (HEA) system was studied using X-ray diffraction, scanning and transmission electron microscopy. A microstructural evolution from single-phase FCC to FCC + BCC + B2 occurred with increasing Al content. The addition of a comparatively small amount of Ti led to the formation of a Fe-Cr sigma phase. The corrosion characteristics of the alloy system were studied across different compositions, with such an alloy system exhibiting a high resistance to general corrosion, superior to stainless steel 304L in 0.6 M NaCl. Cyclic potentiodynamic polarisation suggested that the HEAs studied underwent pitting corrosion following breakdown. From exposure testing, it was seen that very fine pitting, although not extensive in nature, was the principle form of corrosion for Al x CoCrFeNiTi y after prolonged immersion. There was little evidence of microgalvanic corrosion or selective dissolution of a particular phase observed, despite the heterogeneous microstructure and significant elemental segregation in the alloys studied. The composition of the surface films formed upon the Al x CoCrFeNiTi y alloys were elaborated by X-ray photoelectron spectroscopy, which provided new and further insights regarding the surface films of such alloys. The study herein contributes to an emerging understanding of the corrosion characteristics of high entropy alloys. Graphical abstract Unlabelled Image Highlights • The microstructural evolution of Al x CoCrFeNiTi y CCAs is investigated in details using XRD, SEM and TEM. • Good corrosion resistance of these CCAs in 0.6 M NaCl is revealed, with fine pitting as the main type of corrosion. • The composition of the surface films was elaborated by XPS which provided new insights on the corrosion mechanisms of CCAs. [ABSTRACT FROM AUTHOR]

• Computational design of a multi-dimensional Al-Co-Cr-Fe-Ni-Ti alloy space with γ + γ' phase field at 800 °C. • Novel high entropy alloys with high volume fraction of γ'-phase with an extended range of (Al+Ti)-concentrations (up to 20 at%) is proposed. • Critical concentration limits of Fe/Cr/Co is defined as a function of Al/Ti for high entropy alloys with γ + γ' phase field is proposed. • High entropy alloys with γ + γ' structure showed excellent strength/coarsening resistance at 800 °C. A multi-dimensional Al-Co-Cr-Fe-Ni-Ti alloy space with a Ni 3 (Al, Ti)-type ordered (γ') phase in a disordered face‐centered cubic matrix phase (γ) without detrimental intermetallic phases at 800 °C was obtained by integrating calculated-phase diagrams and a computational framework. The solubility limits of alloying elements in the compositional space were defined for the γ-γ' structure at 800 °C. The present model was calibrated with high entropy superalloys (HESAs) from the literature and experimentally validated by the synthesis of HESAs with an extended range of Al (10.5–15 at%) and (Al+Ti)-concentrations (up to 20 at%). The Ni 51 Co 18 Fe 5 Cr 10 Al 12 Ti 4 HESA developed using the present approach exhibited a γ'-volume fraction of 63%, γ'-dissolution temperature of 1188 °C and showed excellent coarsening resistance and strength retention at 800 °C. The current approach aids the high-throughput design of potential high-performance Al-Co-Cr-Fe-Ni-Ti HESAs for high-temperature structural applications without the need for expensive alloying elements. [ABSTRACT FROM AUTHOR]