Abstract
Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk ...quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m
2
, far above the SFE regime for twinning (<~50 mJ/m
2
) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.
It remains challenging to achieve attractive mechanical strength and ductility in commercial 7xxx alloys fabricated by wire-arc additive manufacturing (WAAM). Here, we combine WAAM processing with ...solution and artificial aging treatments to functionalise their inherent age-hardening capabilities via microstructure controlling. The as-deposit WAAM 7055 alloy shows a heterogeneous microstructure with a bimodal grain size distribution, and the primary metallurgical defects are spherical gas pores with an inter-layer distribution. Upon the optimised T6 temper (470°C/2 h for solution treatment and 120°C/65 h for artificial aging), nano-sized platelike η' precipitates are generated, and the tensile strength boosts from ∼226 MPa to ∼562 MPa. Evident yielding with a uniform plastic deformation is also mediated by the T6 temper, with the yield strength > 500 MPa and the fracture strain > 5%, which is comparable to conventional wrought 7xxx alloys. This work provides insight into the microstructure tuning for commercial 7xxx alloys via WAAM for achieving excellent mechanical properties.
•A novel precipitation-hardened CoNiV medium entropy alloy with outstanding tensile properties is developed.•The brittle intermetallic phase κ not only contributes to high tensile strengths by ...shearing, but also enhances the flow stress to approach the critical value for the onset of deformation twins.•Deformation twins in turn assist further work hardening to improve the deformability.•The degradation of mechanical properties caused by intermetallic phase embrittlement can be recovered.
The degradation of ductility is obvious in low-temperature annealed face centered cubic (FCC) based alloys under bulk quasi-static tensile experiments due to intermetallic phase embrittlement. Here, we demonstrate a novel approach to overcome this loss of deformability by realizing brittle intermetallic phase (κ) shearing and deformation twins in an equiatomic CoNiV medium entropy alloy (MEA) with high stacking fault energy. The brittle κ phase not only contributes to high true ultimate tensile strengths (∼1800→2000 MPa) by shearing, but also enhances the flow stress to approach the critical values for the onset of deformation twins (∼1660-1750 MPa) in this MEA. Such shearing and twins in turn assist further work hardening and strengthening mechanisms that improve the deformability of MEA (uniform elongation ∼25→27%). As a result, the degradation of ductility caused by intermetallic phase embrittlement in this MEA can be recovered. The combination of deformable intermetallic phase and high stress deformation twins proposes a so far untapped strengthening mechanism, for enabling the design of FCC based alloys with improved mechanical properties.
Metallic alloy design for room temperature applications typically aims at avoiding undesired brittle intermetallic phases. In transition metal alloys, the sigma phase is particularly known as a ...harmful phase leading to serious embrittlement. Here, we develop a novel strategy that utilizes displacive transformation and heterogeneous structures to mitigate the embrittlement of sigma phase particles in high-entropy alloys (HEAs). A careful study of the deformation behavior reveals that the displacive transformation from face-centered cubic (FCC) to hexagonal close packed (HCP) phase can effectively suppress the propagation of microcracks originated in these brittle sigma particles (310±52 nm) and contributes to high work hardening behavior during tensile deformation. This is achieved by tuning the stacking fault energy of the FCC matrix by reducing the Ni content to promote transformation induce plasticity (TRIP) around the sigma phase in a non-equiatomic Fe34Mn20Co20Cr20Ni6 (at. %) HEA. Such TRIP effect can be optimized in various heterogeneous structures with bimodal grain sizes via simple cold-rolling (∼60%) and subsequent annealing (30 min at 700 or 800 °C). The heterogeneously structured HEAs containing brittle sigma particles exhibit ultimate tensile strengths as high as ∼1.2 GPa while maintaining a ductility up to ∼50%. This is mainly attributed to the transformation induced stress-relaxation around the regions containing brittle sigma particles. The insights provide a new design strategy of combining TRIP effect and heterogeneous structures for developing strong and ductile alloys.
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Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is ...simultaneously achieving an ultra‐low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi‐component body‐centered cubic (BCC) biomedical high‐entropy alloy (Bio‐HEA), which is composed of non‐toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio‐HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress‐induced martensite transformation with extremely low trigger stress. The transformation‐induced plasticity and work hardening capacity are achieved via the stress‐induced martensite transformation. Additionally, the hierarchical stress‐induced martensite twin structure and crystalline‐to‐amorphous phase transformation provide robust toughening mechanisms in the Bio‐HEA. The cytotoxicity test confirms that this Bio‐HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra‐low Young's modulus, excellent biocompatibility, and decent ductility.
Simultaneously achieving an ultra‐low Young's modulus, an excellent biocompatibility, and an acceptable ductility possess significant challenges in traditional biomedical alloys. This work presents a generic solution to an ever‐lasting challenge in metal materials design: i.e., achieving low Young's modulus analogous to the human bone while maintaining commendable tensile ductility as well as excellent biocompatibility in a biomedical high‐entropy alloy (Bio‐HEA).
Here, we investigate the interplay effects of Si and Nb alloying on the oxidation behaviors of the refractory high entropy alloys (RHEAs) at 1000 ℃. The alloying of Nb and Si promotes the formation ...of dendritic microstructures, suppresses the transition from parabolic to linear oxidation kinetics and significantly improves the oxidation resistance. Severe cracking of oxide scales is also reduced via Nb and Si alloying. According to the scanning transmission electron microscopy analysis, ZrO2 and TiO2 are initially formed during oxidation, and oxidation proceeds with the inward diffusion of oxygen, preferentially through the interdendritic region and BCC grain boundaries.
•The alloying of Nb and Si significantly improvs the oxidation resistance of RHEA.•The transition from parabolic to linear kinetics occurs via single Si-alloying.•The Nb-Si-alloyed RHEA shows the lowest parabolic oxidation kinetics constant.•Single Si-alloying leads to severe cracking of oxide scale formed on RHEA.
A novel phenomenon of twin boundary-assisted dual-nanoprecipitation in a selective-laser-melted (SLM) Al alloy is systematically investigated. The dual-nanoprecipitation of Mg-Zn-rich η’ and ...Al-Sc-Zr-rich L12 phases is observed at a twin boundary. This twin boundary can be decomposed into incoherent twin boundaries with 9R phases, whereas provide nucleation sites for the dual-nanoprecipitation during in-situ heating at 120 °C. Such dual-nanoprecipitation is primarily driven by the Gibbs-free energy difference between the incoherent twin boundaries with 9R phases and the adjacent matrix. The results offer primary guidance to design a novel SLM Al alloy with dense twins and precipitations.
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•The Mg-Zn-rich η’ and Al-Sc-Zr-rich L12 phases are formed at a twin boundary of selective-laser-melted Al alloy after aging.•The incoherent twin boundary segments with 9R phases are preferential sites for the precipitation of η’ and L12 phases.•The formation mechanism of twin boundary-assisted η’ and L12 phases is investigated by in-situ heating experiments.
Despite achieving notable strength and wear resistance in precipitation-strengthened Cantor high-entropy alloy (HEA), the inherent brittleness of precipitates leads to a loss of ductility. In this ...study, we propose a novel approach to address this issue by introducing nanosized M23C6-combined with Cr2N, termed dual-nanoprecipitation, and heterogeneous structure in a C and N co-doped interstitial HEA (iHEA). The abundant nanoscale M23C6 and Cr2N particles serve not only as reinforcing agents to enhance strength and wear resistance but also hinder the recrystallization process, resulting in a heterogeneous structure containing recrystallized and non-recrystallized zones. This heterogeneity triggers additional strengthening and strain hardening mechanisms, thereby enhancing the deformability of iHEA. Furthermore, the heterogeneous structure effectively mitigates strain localization on the sliding surface during wear, thus improving tribological properties. By overcoming the challenge of poor ductility while maintaining exceptional strength and wear resistance, the dual-nanoprecipitation-induced heterogeneous structure offers promising avenues for the development of alloys with superior strength-ductility synergy and wear resistance.
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Here, we study the hydride formation in a metastable Ti–33Zr–22Hf–11Ta (at.%) refractory high entropy alloy (RHEA). Deviating to non-equiatomic compositions of RHEAs promotes the formation of ...transformation-induced plasticity where the body-centered cubic phase transforms to hexagonal close-packed (HCP) phase. It is found that the phase transformation capability assists the hydride formation due to the low solubility of hydrogen within the HCP phase. In this study, hydrogen is charged via electrochemical polishing and the corresponding phase transformation is activated in the metastable RHEAs. The newly formed HCP phase interacts with hydrogen to form a face-centered cubic hydride verified by electron energy loss spectroscopy. This work provides a primary exploration of the formation of compositionally complex metal hydrides in the metastable RHEAs, which are potential candidates for future hydrogen storage material design.
