Single-phase Face-centered Cubic Structure Research Articles

Design of novel high entropy alloys based on the end-of-life recycling rate and element lifetime for cryogenic applications

The equiatomic face-centred cubic (FCC) CoNiCrFeMn alloy, known as the Cantor alloy, is renowned for its high ductility under extreme conditions, such as cryogenic temperatures. Despite this, it suffers from low hardness and yield strength (YS) and includes elements with significant supply concerns. This study introduces novel non-equiatomic CoNiCrFeMn alloys, designed using machine learning (ML)-assisted-high-throughput atomistic simulations to enhance sustainability and mechanical properties such as hardness and YS while maintaining the alloy's single-phase FCC structure. We incorporated two critical sustainability indicators, end-of-life recycling rate (EOL−RR) and lifetime (τ), to guide the alloy design process. Mean-flow stress (MFS), measured from the yield point to 15% strain during tensile tests, was used to assess and predict the mechanical properties. Two alloys, with cobalt contents of 12.5% and 21.5%, were developed and analysed. The Co12.5 alloy showed better sustainability (44% improvement in τ with almost the same mechanical properties), while the Co21.5 alloy exhibited better mechanical properties (20% improvement in MFS with almost the same sustainability) as the equiatomic system. Their stable FCC microstructures were confirmed through CALPHAD modelling and X-ray diffraction (XRD) analysis of vacuum arc melted, as-cast samples. The results highlight the potential of integrating sustainability metrics into high-performance alloy design.

Hydrogen-Stabilized ScYNdGd Medium-Entropy Alloy for Hydrogen Storage.

The research on the functional properties of medium- and high-entropy alloys (MEAs and HEAs) has been in the spotlight recently. Many significant discoveries have been made lately in hydrogen-based economy-related research where these alloys may be utilized in all of its key sectors: water electrolysis, hydrogen storage, and fuel cell applications. Despite the rapid development of MEAs and HEAs with the ability to reversibly absorb hydrogen, the research is limited to transition-metal-based alloys that crystallize in body-centered cubic solid solution or Laves phase structures. To date, no study has been devoted to the hydrogenation of rare-earth-element (REE)-based MEAs or HEAs, as well as to the alloys crystallizing in face-centered-cubic (FCC) or hexagonal-close-packed structures. Here, we elucidate the formation and hydrogen storage properties of REE-based ScYNdGd MEA. More specifically, we present the astounding stabilization of the single-phase FCC structure induced by the hydrogen absorption process. Moreover, the measured unprecedented high storage capacity of 2.5 H/M has been observed after hydrogenation conducted under mild conditions that proceeded without any phase transformation in the material. The studied MEA can be facilely activated, even after a long passivation time. The results of complementary measurements showed that the hydrogen desorption process proceeds in two steps. In the first, hydrogen is released from octahedral interstitial sites at relatively low temperatures. In the second, high-temperature process, it is associated with the desorption of hydrogen atoms stored in tetrahedral sites. The presented results may impact future research of a novel group of REE-based MEAs and HEAs with adaptable hydrogen storage properties and a broad scope of possible applications.

Effects of Al or Mo Addition on Microstructure and Mechanical Properties of Fe-Rich Nonequiatomic FeCrCoMnNi High-Entropy Alloy

In this work, a Fe-rich nonequiatomic Fe40Cr15Co15Mn10Ni20 high-entropy alloy was successfully prepared based on phase analysis and cost reduction. Fe40Cr15Co15Mn10Ni20 high-entropy alloy with a single-phase face-centered cubic (FCC) structure was strengthened by the addition of 11 at.% Al or 10 at.% Mo, and the variations of phase and mechanical properties of the strengthened alloys were subsequently investigated. It has been found that the addition of 11 at.% Al led to the formation of FCC and body-centered cubic (BCC) dual-phase structure in the Fe40Cr15Co10Mn4Ni20Al11 alloy, while its yield strength (σ0.2) and tensile strength increased from 158 ± 4 MPa and 420 ± 20 MPa to 218 ± 7 MPa and 507 ± 16 MPa, respectively, as compared to the single-phase FCC structure Fe40Cr15Co15Mn10Ni20 alloy. The addition of 10 at.% Mo introduced intermetallic compounds of μ and σ phases, which resulted in improved yield strength of 246 ± 15 MPa for the Fe40Cr15Co10Mn5Ni20Mo10 alloy. However, the alloy exhibited premature brittle fracture due to the existence of a large number of intermetallic compounds, which led to deteriorated tensile strength of 346 ± 15 MPa. The findings of this work suggest that the introduced secondary phases by the addition of Al and Mo can effectively strengthen the high-entropy alloy; however, the number of intermetallic compounds should be controlled to achieve a combination of high strength and good ductility, which provides a reference for the follow-up study of nonequiatomic high-entropy alloys.

Tribological behavior of boronized Al0.1CoCrFeNi high-entropy alloys under dry and lubricated conditions

High-entropy alloys (HEAs) with a single-phase face-centered cubic (FCC) structure generally have excellent plasticity. However, its limited room temperature strength and hardness result in lower wear resistance, which hinders tribological and engineering applications. In this paper, the pack-boronizing method was used to improve the tribological properties of Al0·1CoCrFeNi HEAs with a single-phase FCC structure. The boronizing treatment was carried out at 900 °C for 2, 4, 6, and 8 h, respectively. The structure and phase formation of boronized alloys were studied. Moreover, the reciprocating sliding wear behavior of boronized alloys against Si3N4 balls in dry and deionized water was investigated. A boronized layer with a double-layer structure was formed on the HEA surface, with a total thickness of about 17.3–58.5 μm. The wear mechanism of boronized alloy was transformed from the abrasive and delamination wear to the polishing effect with increasing the boronizing time in air. In contrast, the wear mechanism of boronized alloys in the deionized water gradually changed from the two-body to three-body wear with increasing the boronizing time. Meanwhile, the wear mechanism of the Si3N4 counterface was also transformed to three-body wear gradually.

Strength and ductility of CrFeCoNiMo alloy with hierarchical microstructures

Single-phase multi-principal-element alloys (MPEAs) with face-centered cubic (FCC) structure generally exhibit low yield strength but superb ductility and strain hardening capability. In this work, we demonstrate that enhancing yield strength while retaining good ductility of single phase FCC MPEAs can be achieved by developing hierarchical microstructures. A non-equiatomic Cr20Fe6Co34Ni34Mo6 alloy with single-phase FCC structure was fabricated and immediately cold-rolled with ∼70% thickness reduction after a liquid nitrogen bath. The cold-rolled sample was then annealed at various temperatures in a range of 675–1150 °C for various periods. As annealing temperature exceeds 800 °C, annealed samples exhibit coarse-grained microstructure and low yield strength, while high strain hardening rate and good ductility. As annealing temperature is lower than 800 °C, annealed samples develop hierarchical microstructures that comprise high density of annealing nano-twins in recrystallized fine grains (grain size ∼ 1 μm) and stable dislocation walls in non-fully recrystallized fine grains. The addition of Mo in the system is found to be very effective in retarding the recrystallization and grain growth, promoting formation of recrystallized fine grains. Mechanical tensile testing reveals that such kind of hierarchical microstructure enhances yield strength of single phase FCC MPEAs (1.1 GPa) while retains good ductility (uniform elongation of ∼29%) and high ultimate tensile strength (1.3 GPa). Grain boundaries, twin boundaries and dislocation walls act as strong barriers for dislocation motion, enhancing yield strength and strain hardening capacity. Dislocation walls also act as sources for dislocations and deformation twins at large deformation stages, retaining a good ductility. Engineering such hierarchical microstructures should thus be an efficient strategy in enhancing mechanical properties of FCC MPEAs with low or medium stacking fault energies.