Nene, Saurabh

Alloying traditionally enhances material properties by adding small amounts of secondary elements to a primary base. Over the past decade and a half, a revolutionary strategy has emerged: combining multiple principal elements in high concentrations to create high-entropy alloys (HEAs). This approach unlocks a vast, largely unexplored compositional space, leading to the discovery of alloys with exceptional properties. This work provides a comprehensive review of recent advancements in HEA research, focusing on metallurgical aspects, highlighting key findings, and identifying future research directions. We reviewed the design philosophy of conventional alloys and its transition to HEAs, emphasizing metallurgical differences and similarities. Key topics include the fundamentals of high-entropy alloys design for variety of HEAs, such as equiatomic and non-equiatomic, eutectic, metastable, refractory, lightweight, spinodal HEAs, and high-entropy superalloys. The review explores microstructural features of HEAs, including as-cast wrought and additively manufactured morphologies, texture development, precipitation, and dispersion-based microstructures. The design of HEAs involves understanding and manipulating these microstructural characteristics to achieve desirable properties. Metallurgical properties of HEAs are summarised, including tensile and compressive properties, fatigue properties, creep, superplastic, corrosion behavior and weldability. Typical deformation mechanisms such as slip, twinning, twinning-induced plasticity, transformation-induced plasticity, and precipitation-assisted deformation, which are found to be mainly active in HEAs, are also discussed. Despite the progress made, the potential of HEAs is far from fully realized. Future directions conceive the concept of high-entropy conventional alloys (HECAs), which merge the high-entropy effect in HEAs with the conventional alloy design approach, thereby introducing high entropy phases in the conventional alloy matrix or vice-versa. Thus, the novel concept of HECA provides a future pathway to materials design and foster enhanced metallurgical properties utilizing the HEA concept for practical applications. To explore the extensive compositional and microstructural possibilities of HEAs and HECAs, high-throughput experimental techniques and computational methods are essential. This review aims to serve as a valuable resource for new researchers and provides insights to guide future investigations in the field of high-entropy alloys and high-entropy conventional alloys, with a strong emphasis on their metallurgical aspects.

Strength and ductility vary inversely due to a rapid decrease in dislocation storage capacity with the pronounced increase in work hardening rate at the expense of ductility in most conventional and recently designed advanced ductile materials. This limitation can be overcome by creating heterogenous or hierarchical microstructures containing not only presence but also different length scales of twins, bands, phases synergistically. In line of that, here we present Fe44Mn20Cr15Ni7.5Co6Si7.5 (all in at.%) compositionally complex alloy (M-CCA), which showed an exceptional increase in strength and ductility simultaneously as a result of hierarchical microstructuring that forms after conventional thermo-mechanical treatment. The increase in strength-ductility synergy is attributed to the occurrence of hetero-deformation induced (HDI) strengthening in early stage of deformation whereas planar slip assisted hetero-deformation banding, and deformation twinning in later stages of deformation during plastic straining. Hence, hierarchical microstructuring in M-CCA resulted in exceptional work hardening ability which is needed for structural integrity and manufacturing applications under tensile loads to suppress sudden failures during service.

Lightweight alloys are known to improve the fuel efficiency of the structural components due to high strength-to-weight ratio, however, they lack formability at room temperature. This major limitation of poor formability is most of the time overcome by post-fabrication processing and treatments thereby increasing their cost exponentially. We present a novel Ti50V16Zr16Nb10Al5Mo3 (all in at. %) complex concentrated alloy (Ti-CCA) designed based on the combination of valence electron concentration theory and the high entropy approach. The optimal selection of constituent elements has led to a density of 5.63 gm/cc for Ti-CCA after suction casting (SC). SC Ti-CCA displayed exceptional room temperature strength (UTS ∼ 1.25 GPa) and ductility (ε ∼ 35 %) with a yield strength (YS) of ∼ 1.1 GPa (Specific YS = 191 MPa/gm/cc) without any post-processing treatments. The exceptional YS in Ti-CCA is attributed to hetero grain size microstructure, whereas enormous strength-ductility synergy is due to the concurrent occurrence of slip and deformation band formation in the early stages of deformation followed by prolonged necking event due to delayed void nucleation and growth. The proposed philosophy of Ti-CCA design overcomes the conventional notion of strength-ductility trade-off in such alloy systems by retaining their inherent characteristics.

In this study, a novel Ni46.8Fe23Co10V7(Al, Si)6.6 compositionally complex alloy (Ni-CCA) has been designed by merging the CALPHAD approach with the theoretical concepts (enthalpy of mixing, atomic radius mismatch parameter, valence electron concentration (VEC), and pair sigma forming elements (PSFE)). The theoretical analysis and the CALPHAD modeling predict the formation of a single FCC phase at room temperature along with the absence of TCP phases in the designed Ni-CCA. Subsequently, the pseudo-binary phase diagram obtained from Thermo-Calc through the latest HEA database predicts the presence of newer strengthening ordered phases containing Ni-Al-Si at elevated temperatures in Ni-CCA. Microstructural characterization of as-cast Ni-CCA displayed the formation of γ-FCC phase dominated microstructure containing a minor fraction of BCC phase at room temperature whereas high-temperature compression depicted synergistic precipitation of Ni-Al-Si containing L12 type precipitate and dynamic recovery/recrystallization events during deformation leading to a marginal drop in yield strength (YS) at 800 °C. Moreover, the formation of necklace microstructure in a deformed specimen confirms the occurrence of dynamic recrystallization (DRX) in novel Ni-CCA.

Transformation induced plasticity (TRIP) leads to enhancements in ductility in low stacking fault energy (SFE) alloys, however to achieve an unconventional increase in strength simultaneously, there must be barriers to dislocation motion. While stacking faults (SFs) contribute to strengthening by impeding dislocation motion, the contribution of SF strengthening to work hardening during deformation is not well understood; as compared to dislocation slip, twinning induced plasticity (TWIP) and TRIP. Thus, we used in-situ neutron diffraction to correlate SF strengthening to work hardening behavior in a low SFE Fe40Mn20Cr15Co20Si5 (at%) high entropy alloy, SFE ~ 6.31 mJ m−2. Cooperative activation of multiple mechanisms was indicated by increases in SF strengthening and γ-f.c.c. → ε-h.c.p. transformation leading to a simultaneous increase in strength and ductility. The present study demonstrates the application of in-situ, neutron or X-ray, diffraction techniques to correlating SF strengthening to work hardening.

Cu-based alloys are known for their high corrosion resistance but with a lack of processing ability and vice versa. Here we present a novel alloy design concept realizing Cu-rich high entropy conventional alloy (HECA) having exceptional corrosion resistance (Ecorr = - 0.27 V, icorr = 2.83 × 10−6 A/cm2) and work hardenability (strength = 413 MPa, total elongation = 28 %) synergy simultaneously in as-fabricated state. This cocktailing effect in HECA (Cu-12Fe-8Mn-7.5Co-2.5Cr, all in at%) is attributed to the formation of Fe-Mn-Co-Cr containing high entropy phase (HEP) in the Cu-rich matrix. A pronounced two-phase hardening by hetero-deformation induced (HDI) strengthening effect at the HEP/Cu matrix resulted in excellent work hardenability whereas, exceptional corrosion resistance was attributed to delayed corrosion kinetics owing to the preferential corrosion of the Cu-rich phase over HEP in HECA. Thus, Cu-rich HECA overcomes the corrosion-strength ductility dilemma in Cu-rich alloys when compared with classical binary Cu-Fe alloys in an as-fabricated state.