ACTA

Tailoring nanoscale defect structures for desirable deformation behaviors is crucial to designing and optimizing the mechanical properties of alloys. Distinguishing from the predominant toughening mechanisms (e.g., mechanical twinning and deformation-induced phase transformation), here we report an unusual stacking-fault-mediated deformation in equiatomic FeNiCoCr high-entropy alloy (HEA) by controllably introducing helium nanobubbles with high pressures of ~2.5-4.7 gigapascals. Using in situ transmission electron microscopy nanomechanical testing, we demonstrate that highly pressurized helium nanobubbles can not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Through atomistic simulations, we reveal that high helium pressures contribute to reducing the nucleation energy of partial dislocations at the nanobubbles surface, which enhances dislocation nucleation rates and offers sustainable stacking fault sources for retaining ductility. Our results provide a novel design strategy for tuning deformation mechanisms of HEAs via introducing highly pressurized helium nanobubbles, which may open up avenues towards the facile tailoring of mechanical responses in micro/nanoscale HEA components.

ACTA

Alumina forming Ni-base superalloys are essential due to their oxidation resistance at elevated temperatures. A two-step procedure allows to assess the outward growth of the oxide scale from the resulting oxide ridges that form at 1100°C and cap the alumina grain boundaries. Employing undoped 50Ni50Al (at%) as reference, the impact of reactive elements on the diffusion processes, here Zr and Hf, is quantified using atom probe tomography. Unexpectedly, we find that up to one monolayer of Ni may co-decorate the alumina grain boundaries. Additionally, a decrease in Al-, and Ni-diffusivity of two orders of magnitude is observed owing to the reactive element effect. We employ density functional theory calculations to better understand the role of aliovalent cations, here Ni(II), Zr(IV), and Hf(IV) in the α−alumina scale. The calculations show that Ni may not only decorate the alumina grain boundaries but also facilitates transport of electrons as well as oxygen vacancies. Thereby oxide scale growth becomes enhanced. In turn, the dual impact of reactive elements, i.e. to annihilate oxygen vacancies and to remove impurity states in the band gap, explains the reduced scale growth rate.

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ACTA

A statistical continuum model is developed in the present study which uses microstructural information for a two-phase Ti-6Al-4V alloy to simulate the microstructure evolution during hot isothermal compression. Current models of microstructure prediction often use finite element methods. However, the statistical continuum model developed here does not rely onfinite element methods and thus can be performed much faster and with less computational resources without sacrificing prediction accuracy. This approach uses two-point statistics to describe the microstructural information related to phase distribution, grain size and phase/grain morphology. This formulation further utilizes the macroscopic strain-rate tensor to predict the deformation. The deformation is then used to calculate the resulting two-point statistics of the deformed microstructure. Finally, the simulated two-point statistics is compared to that experimentally obtained from scanning electron micrograph in backscatter electron mode i.e. SEM (BSE) images of the deformed microstructures.

ACTA

向上滑动阅览英文摘要

The present study investigated the hydrogen-related fracture behavior of specimens with different stress concentration factors through microstructure observation, finite element (FE) simulation, and digital image correlation (DIC) analysis. The alloy studied was a simple model alloy (Fe-0.2C binary alloy) with fully martensite structure. When the hydrogen content was large (2.21 mass ppm (121 at ppm)), the crack initiation and propagation occurred along the prior austenite grain boundaries. Through the FE simulations, we found that the crack initiation sites corresponded to the region with high stress and high hydrogen content. Although the stress concentration factors were different, the stress level and the hydrogen content at the crack initiation sites were almost the same, indicating that the hydrogen-related intergranular fracture originated from stress-controlled decohesion at the prior austenite grain boundaries. For the specimen with small hydrogen content (0.41 mass ppm (22.5 at ppm)), the quasi-cleavage cracks formed at the surface of the notch root and propagated along the {011} planes. The FE simulations revealed that the plastic strains were maximum at the initiation sites of the quasi-cleavage cracks. Moreover, we confirmed that hydrogen enhanced the local plastic deformation by DIC analysis. As the local values of maximum principal stress, plastic strain, and hydrogen content at the initiation sites of the quasi-cleavage cracks were different depending on the stress concentration factor, the critical quantitative condition for the initiation of quasi-cleavage cracking was not simple compared to that of the case of intergranular cracking.

ACTA

向上滑动阅览英文摘要

Martensitically transforming polycrystalline Titanium-Niobium (TiNb) alloys exhibit tailorable coefficients of thermal expansions (CTEs) between large positive and negative values via deformation in martensite due to their anisotropic crystallographic CTE. However, the effects of Nb content, temperature, and deformation level on the crystal level and polycrystalline CTE anisotropy are not known, which limit the utilization of this unique feature in applications. In this study, TiNb alloys with 15 to 24 at.% Nb were systematically investigated using in-situ high energy synchrotron X-ray and iterative tension experiments to examine the evolution of crystalline CTE anisotropy, texture, and polycrystalline CTE. Analysis showed that crystalline CTE anisotropy and polycrystalline CTE values at a given temperature are the highest in Ti76Nb24 alloys, reaching +/-40 ppmK⁻¹ at 90 °C. Lower Nb content allows for higher maximum temperatures at which CTE anisotropy is observed, but at the expense of maximum CTE achieved. Similarly, increasing deformation level reduces crystalline CTE anisotropy. Ti76Nb24 experiences a swift and massive texture evolution with deformation, resulting in CTE change along tensile axis from 8 ppmK⁻¹ to -30 ppmK⁻¹ after only 5% strain. Lower Nb alloys exhibit lower CTEs and less pronounced texture evolution at a given strain, culminating in a failure to manifest any apparent change in CTE for Ti75Nb15 polycrystals. This was attributed to dislocation plasticity accompanying martensite reorientation in lower Nb alloys. The analysis of in-situ texture and tension experiments revealed that there is a direct correlation between applied strain level, texture evolution, and maximum negative CTE achieved in different TiNb alloys.

Vol. 210，15 May. 2021, 116842