Ultrafine-grained High-entropy Alloy Research Articles

Activation of deformation twinning in ultrafine-grained high-entropy alloys via tailoring stacking fault energy and critical twinning stress

In this study, we propose a new material design approach to activate deformation twinning in ultrafine-grained (UFG) AlCoCrCuFeNiTi-based high-entropy alloys (HEAs) by considering both thermodynamic and microstructural aspects. Stacking fault energy of the solid-solution phase is reduced by tailoring composition with increasing Co/Ni ratio and diminishing the concentration of Al and Ti through precipitation. Furthermore, grain size is adjusted to enable the flow stress to surpass the critical twinning stress. It results in the activation of nano-twinning in the UFG HEAs, making the alloys exhibit high strain-hardening after yielding from 1.6 to 2.3 GPa.

A novel ultrafine-grained high entropy alloy with excellent combination of mechanical and soft magnetic properties

An ultrafine-grained FeCoNiCu0.2Si0.2 high entropy alloy (HEA) was designed and produced via melt-spinning. This HEA has Cu-rich cell-like grain structure with the size of 200–500 nm and Si-rich network-like intergranular structure with the width of 10–80 nm, leading to high yield strength and ultimate tensile strength of 820 MPa and 1100 MPa, respectively, as well as good plasticity. Moreover, it also shows high saturation magnetization, low coercivity and high Curie temperature. This ultrafine-grained HEA exhibits excellent combination of mechanical properties and soft magnetic properties among HEAs. Our work provides a novel route for designing high-strength soft magnetic HEAs, which is beneficial for the development of HEAs with soft magnetism and broadening their applications.

Deformation behavior of nanocrystalline and ultrafine-grained CoCrCuFeNi high-entropy alloys

Abstract

Ultrahigh cryogenic strength and exceptional ductility in ultrafine-grained CoCrFeMnNi high-entropy alloy with fully recrystallized structure

Similar to the conventional alloys, attaining excellent combination of high strength and ductility is still challenging in the high-entropy alloys (HEAs). In this work, a fully recrystallized ultrafine-grained (UFG) microstructure was introduced into the single-phase CoCrFeMnNi HEA via cold rolling and annealing process. The UFG HEA displays ultrahigh yield and ultimate tensile strengths of 1.24 GPa and 1.46 GPa, respectively, and excellent ductility with a uniform elongation of 41% at 77 K. The exceptional strength-ductility combination in the UFG CoCrFeMnNi HEA surpasses those of all existing HEAs and other advanced engineering alloys. Such unparalleled mechanical properties stem from the fully recrystallized ultrafine grains that significantly strengthen the material and simultaneously provide high strain-hardening capability and the activations of multiple deformation mechanisms consisting of prominent dislocation activities, profuse stacking faults, and deformation twins. The fully recrystallized UFG microstructures do not only shed light on the design and development of HEAs with superior mechanical properties, but also enrich the potential applications of HEAs at the cryogenic environment.

Fatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation

Bulk ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained (CG) counterpart were investigated under fully reversed cyclic deformation. The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However, no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.

Microstructure and properties of a CoCrFeNiMn high-entropy alloy processed by equal-channel angular pressing

A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673K and the results show that the strength increases gradually with increasing straining up to ~ 1GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073K for 60min and it is shown that the hardness increases slightly due to precipitation to 773K and then decreases to 1073K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle σ-phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773K produces a very high yield strength of ~ 1015MPa and an ultimate strength of ~ 1080MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.

Enhanced strength and ductility of bulk CoCrFeMnNi high entropy alloy having fully recrystallized ultrafine-grained structure

A high efficient magnetic levitation melting technique was reported for fabricating bulk equiatomic CoCrFeMnNi high-entropy alloy (HEA) ingot with a diameter of 110mm. The bulk ingot can be either forged or rolled. In particular, fully recrystallized ultrafine-grained (UFG) HEA with a minimum grain size of 503±181nm was successfully obtained through a simple cold rolling and annealing process. The tensile properties of the HEA were studied by changing the grain size from the UFG regime to the coarse-grained regime. The UFG HEA exhibited a good balance of strength and ductility due to the low stacking fault energy. The linear Hall-Petch relationship was well fitted when the grain sizes range from 503nm to 88.9μm.