Department of Metallurgical and Materials Engineering

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.

This study aims to investigate the mechanical behaviors of Ti–15 V–3Cr–3Al–3Sn (Ti-15333), a metastable β titanium alloy. Thermomechanical processing of the alloy sheet in its as-received state included solution treatment at 800°C, followed by varying aging temperatures from 450°C to 700°C. As the aging temperature rises, α precipitation increases with a fine, uniform distribution attained at 500°C to 550°C. Increased aging temperature and time causes precipitation not only at grain boundaries but also intra-granularly with morphology change from globular to lath. However, if the temperature goes above 600°C, the α phase starts to coarsen. Aged samples showed an upward trend in tensile strength that peaked and then declined, while % elongation showed an inverse trend, with optimal properties observed after 6 h aging at 500°C and 550°C. This is consistent with the idea that mechanical properties evolve alongside microstructure. The examination of the local fracture surface of aged tensile specimens using EBSD revealed a random texture. KAM value reflecting dislocation density rose with both aging temperature and duration, with its highest average reported for 6 h aging condition at 500°C and 550°C, aligning with peak level of α precipitation. The size, shape, and size distributions of α precipitates also had a pronounced impact on the fracture modes observed in the specimens. The likelihood of cleavage-like fractures occurring was higher in the under and peak-aged specimens when compared to the over-aged counterparts at 600°C and beyond. This tendency suggests that the over-aged specimens, featuring coarse α platelets, should exhibit decreased resistance to deformation and higher ductility, which is indeed the case. Investigation and analysis of different microstructures and mechanical properties obtained under different aging conditions can aid in summarizing the principles under laying deformation and fracture mechanisms of metastable β titanium alloy, potentially enhancing the service life of components and providing a foundation for alloy fabrication with customized microstructure.

Antimony-tin ternary alloys outperform graphite as negative electrodes for lithium-ion batteries, offering significantly higher gravimetric and volumetric capacities. These alloys, conductive without additives, can be electrodeposited without binders, enhancing cell design and capacity. Based on bath composition, Sn-rich or Sb-rich SbSnCu ternary alloys can be synthesized which undergoes active-active-inactive chemical buffering during lithiation. The Sb-rich alloys show better stress buffering due to more lithiation/delithiation peaks and better use of lithium-inactive copper. This study finds Sb-rich alloys exhibit superior structural stability and electrochemical performance, delivering 374 mAh g−1 at 200 mA g−1 after 100 cycles, while Sn-rich alloys show substantial capacity fading, retaining only 113 mAh g−1. The Sb-rich alloy maintains structural integrity, losing only 20 % capacity over the last 80 cycles, compared to 48 % loss in capacity during the 50–80th cycles in Sn-rich alloys.

Plant growth-promoting rhizobacteria (PGPR) are known to influence plant root cells to regulate and induce specific traits related to growth promotion and survival. In the present study, rhizobacteria from strawberry plants were screened for stress tolerance under osmotic stress in TSB-mediated PEG 6000 (−0.73 MPa) and tested their ability to produce proline, exopolysaccharide, and free amino acids, all of which induce and regulate stress tolerance. The rhizobacteria were also distinguished for the production of 1-aminocyclopropane-1-carboxylate deaminase (ACCd) for the mitigation of drought stress. Among the 111 rhizobacterial isolates, 41 isolates grow above threshold limit in osmotic stress tolerance, 33 of which exhibited stress tolerance. Further characterization screened 27 isolates as capable growth promoters. Among the experimentally screened rhizobacteria, the isolates SBU4 and SDK8 [Pseudomonas fluorescens (OP627557) (PGPR1) and Pseudomonas glycinae (OP627558) (PGPR2)] exhibited the most promising phyto-beneficial potential. Both the isolates grew exponentially well during the log phase, with increased growth in Log CFU ml−1 under experimentally produced osmotic stress. The drought stress tolerance test results of the SBU4 and SDK8 isolates revealed the presence of proline (1.93 μg mL−1, 2.05 μg mL−1), exopolysaccharide (2.19 mg mg−1 protein, 2.58 mg mg−1 protein), and free amino acid (11.47 μmol g−1, 13.32 μmol g−1) and positive growth in ACCd-enriched DF media, an ACCd assay (α-ketobutyrate (0.56 μmol/ml, 0.64 μmol/ml) and ammonia (0.37 μg mL−1, 0.53 μg mL−1)). The isolates SBU4 and SDK8 performed well in qualitative tests for P solubilization, N fixing ability, siderophore chelation, HCN, and ammonia production, as well as in assays involving P producers (94.57 μg mL−1 and 92.86 μg mL−1), siderophore units (52.14 % SU and 63.12 % SU), and IAA producers (74.63 μg mL−1 and 72.64 μg mL−1). These rhizobacterial isolates were optimized under various growth factors (pH, temperature, incubation) to achieve a relatively high log CFU ml−1. The isolates achieved maximum Log CFU mL−1 when cultured in either a specific range of pH or temperature or growth period (incubation) under standard test conditions. The growth of cultures on cross-streaked nutrient agar plates was tested for an efficient, effective consortium bioformulation that enhances growth and specific traits (drought stress mitigation) in strawberry plants.

The influence of LPBF-IN718 microstructure on hot corrosion behavior was studied at 650 °C. The H-IN718 with <001> oriented grains undergo more weight changes than V-IN718 with <110> oriented grains. Detailed characterization analysis corroborates that Rapp-Goto fluxing and sulphidation mechanisms are operating in LPBF-IN718 with 3SM. Further, GDOES confirms the sulphur gradient existence between Air/3SM and 3SM/H-IN718 interfaces. It suggests that <001> oriented grains serve as preferred sites for ion adsorption and reactions, which might facilitate the sulfur diffusion more effectively than other grain orientations. As a result, oxy-anions generated during sulfidation enhance the fluxing mechanism, thus increasing weight gain in H-IN718.

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.

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.

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets—by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials.

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.