Vol. 201 目录

ACTA

We examined fatigue-crack-growth behaviors of CoCrFeMnNi high-entropy alloys (HEAs) under as-fatigued and tensile-overloaded conditions using neutron-diffraction measurements coupled with diffraction peak-profile analyses. We applied both high-resolution transmission electron microscopy(HRTEM) and neutron-diffraction strain mapping for the complementary microstructure examinations. Immediately after a single tensile overload, the crack-growth-retardation period was obtained by enhancing the fatigue resistance, as compared to the as-fatigued condition. The combined mechanisms of the overload-induced larger plastic deformation, the enlarged compressive residual stresses and plastic-zone size, the crack-tip blunting ahead of the crack tip, and deformation twinning governed the pronounced macroscopic crack-growth-retardation behavior following the tensile overload. A remarkable fracture surface of highly-periodic serrated features along the crack-propagation direction was found in the crack-growth region immediately after the tensile overload. Moreover, a transition of plastic deformation from planar dislocation slip-dominated to twinning-dominated microstructures in the extended plastic zone was clearly observed at room temperature in the overloaded condition, in accordance with the simulated results by a finite element method (FEM). The above tensile overload-induced simultaneously combined effects in the coarse-grained CoCrFeMnNi shed light on the improvement of fatigue resistance for HEAs applications.

ACTA

The mechanisms controlling the fatigue response of an A357-T6 cast aluminium alloy under cyclic torsional loading are investigated. Surface crack monitoring coupled with Electron BackScattering Diffraction (EBSD) analysis is used to study crack initiation. Determination of S-N curve combined with interrupted in situ fatigue testing using synchrotron tomography allows the study of the propagation behaviour. It is observed that fractographic morphologies depend on the stress level. At intermediate-low stress levels (τ_max < 100 MPa), the grain structure controls the crack initiation and propagation periods. Cracks are usually nucleated in mode II from slip planes close to the specimen axis or perpendicular to it. Mode II crack growth dominates the early stages of crack propagation as mode III inward crack growth is rapidly decelerated. This behaviour leads to the formation of characteristic shallow surface cracks. Once the crack is long enough for the mode II driving force to be overtaken by mode III, propagation branches into  ~ 45° mode I.

ACTA

In-situ TEM observation and molecular dynamics simulation were used to investigate the evolution of irradiation-induced dislocation loops and their interactions with the pre-existing dislocation lines in molybdenum irradiated by 30 keV He⁺ at 673 K. The nucleation, annihilation, merging, size change, and types of dislocation loops were investigated in detail, and the hardening effect from dislocation loops was analyzed. With the increase of helium fluence, the volume number density of dislocation loops increased rapidly and then decreased slowly to a saturation value, while the average size of dislocation loops increased continuously until it reached an upper limit. Both 1/2<111> and <100> loops were formed, with a proportion of 60.2% to 39.8% at 3.95 × 10¹⁵ He⁺.cm⁻² (0.07 dpa). The pre-existing dislocation lines would suppress the nucleation and growth of the dislocation loops, while the irradiation-induced dislocation loops had a strong pinning effect on the dislocation lines and impeded their motion. Molecular dynamics simulations were used to clarify the atomistic mechanisms of some major features of the line-loop interactions observed in the in-situ experiment, which showed in a good agreement. The different interaction processes depended on the Burgers vector and plane of the dislocation loop, the motion and reaction kinetics of dislocation segments, and the external loading.

ACTA

Modification of twin boundaries has been deemed as a potential strategy to achieve high strength in combination with good ductility for many engineering alloys. Herein, we report a unique Li cluster segregation phenomenon in coherent contraction twin boundaries of Mg alloys. The structures at the atomic scale have been investigated by transmission electron microscopy. Meanwhile, the driving force for segregation and strengthening mechanisms have been elucidated by first-principles calculations and molecular dynamics simulations, respectively. Experimental results show that the segregated Li cluster with a hexagonal close-packed structure is coherent with a contraction twin. Subsequently, theoretical calculations reveal that the Li cluster is prone to occupy the interface vacancy in the contraction twin boundaries, following which it grows along the twin boundaries preferentially. Finally, both dynamics simulations and experimental observations demonstrate that the formation of Li cluster in contraction twin boundaries can not only directly pin dislocation movement analogous to precipitation strengthening, but also improve the critical shear strain of contraction twin boundaries, inhibiting their deformation or widening. The finding of cluster segregation in twin boundaries also occurs in other systems, which might open an avenue for twin boundary engineering to tune the mechanical properties of metals and ceramics.

ACTA

We developed a fabrication method of thin metal matrix composite films reinforced with ceramic nanoparticles. To this end, we deposited a 20 nm Ni / ultrathin Cr-oxide layer / 20 nm Ni tri-layer on a sapphire substrate, and annealed the sample at the temperature of 700°C. We found that after annealing for 10 min the Cr-oxide layer agglomerates into single- or bi-crystalline faceted Cr₂O₃ nanoparticles, with the orientation relationship of (11-1)_Ni||(0006)_Cr2O3 and [112]_Ni||[1-210]_Cr2O3 to the Ni matrix. We encountered a surprising vertical spread of the centers of mass of the Cr-oxide particles with respect to the initial position of the Cr-oxide layer. We developed a kinetic model of the evolution of particles shape and of their vertical drift, which enabled an estimate of the effective diffusion coefficient of the Ni-Cr₂O₃ interface (D_i=7.4·10⁻¹⁹m²/s). Though much slower than grain boundary diffusion in Ni, this interface diffusion enables fast transformation of ultra-thin Cr-oxide layer into an array of isolated Cr₂O₃ nanoparticles embedded in the Ni film. The insight into the kinetics of the system obtained in the present work paves the way for the design of particle reinforcement of thin metal films with a precise control over the spatial positions of the particles.

ACTA

向上滑动阅览英文摘要

ACTA

向上滑动阅览英文摘要

ACTA

向上滑动阅览英文摘要