1. Five types of recrystallized fractions, 12.5 %, 45.7 %, 77.4 %, 87.0 % and 100 %, were obtained after cold rolling and annealing at 600 °C for 1 h, 3 h, 6 h, 12 h and 120 h in Al_0.1CoCrFeNi high-entropy alloy (HEA). All partially recrystallized structures were comprised of non-recrystallized region with strong {111} <112> texture and recrystallized region with a nearly similar scale of fine grain (~1.22 – 1.57 μm). The partially recrystallized specimens exhibited a good strength-ductility combination, i.e., yield strength increased from ~547.1 MPa to ~881.0 MPa with decreasing recrystallized fractions, while ductility still remained above ~20 %. Based on the law of mixtures, the calculated results of the contributions from multiple strengthening mechanisms to yield strength closely matched the experimental data. It is found that increasing recrystallized fraction leads to enhancing contribution of grain boundary strengthening but weakening contribution of dislocation strengthening and synergistic strengthening. TEM results revealed that plastic deformation in partially recrystallized specimens was dominated by high-density dislocations and deformation twins in grain interior, as well as dislocation planar slip and pile-up near heterogeneous interface; for comparison, dislocation cells and more deformation twins were formed in fully recrystallized specimens.

2. A multi-phase fine-grained microstructure was engineered through cold rolling and annealing treatment in Al_0.3CoCrFeNi high-entropy alloy (HEA) to obtain an enhanced fatigue property. This alloy consists of FCC matrix phase with an average grain size of ~ 1.13 ± 0.84 μm, and B2 and σ precipitates with sub-micron scale. Their phase fractions are ~ 90 %, ~ 9 % and ~ 1 %, respectively. Stress‒life (S‒N) data indicate that fatigue endurance limit and ratio in terms of stress amplitude are ~ 281 MPa and ~ 0.3, based on stress ratio of 0.1. Good fatigue property is related to high ultimate tensile strength derived from good work-hardening ability and fine microstructure. In addition, activation of deformation twins, blunting of microcracks and deflection of fatigue cracks associated with precipitates are beneficial for enhancement of fatigue resistance. The above results provide theoretical guidance on how to enhance fatigue resistance of HEAs by regulating microstructures.

3. We engineer a Al_0.3CoCrFeNi high-entropy alloy (HEA) with hierarchically heterogeneous microstructure by cold rolling and annealing treatment, which includes heterogeneous grains, annealing and deformation twins, residual dislocations and B2 precipitates with different morphologies, sizes and distributions. Stress-life (S–N) tests and characterization techniques including scanning electron microscope (SEM) and transmission electron microscope (TEM) were carried out to investigate fatigue properties as well as corresponding mechanisms. It is found that this HEA possesses good strength-ductility combination (i.e., yield strength of ~870 MPa, ultimate tensile strength of ~1060 MPa and ductility of ~26 %) and fatigue resistance with fatigue ratio of ~0.46 under stress ratio of –1. This fatigue ratio exceeds those of most reported HEAs. High strength renders the fatigue deformation mainly occurs in deformation twin regions decorated with B2 precipitates. Surface damage morphologies indicate that fatigue cracks initiate from persistent slip band-like shear bands. In addition, microstructural hierarchy results in the deflected fatigue crack propagation path, which is beneficial for the enhancement of fatigue resistance. Present results offer the guidance on future design for high fatigue-resistant HEAs by manipulating heterogeneous microstructure.

4. The fatigue property of metallic glass (MG) is unexpectedly poor, which limits its application as structural material. The key problem of shear band formation under cyclic stress level lower than yield strength was clarified by free volume theory; then, the shear band-mediated crack initiation and propagation mechanisms were analyzed based on quasi-in situ cyclic method (loading…unloading…observing…); to enhance the fatigue property of MG, we proposed a new strategy through tailoring the microstructure to suppress the shear band formation and then to impede the fatigue crack initiation, in contrast to the previous way of proliferating shear bands.