It is known that Ni-based superalloys experience microstructural rafting during high temperature straining. This paper presents a comparative study of the different rafting models by means of finite ...element modeling and experimentation investigation. It was found that uniaxial elastic loading does not alter the misfit or misfit isotropy of the structure, but causes repartitioning of the misfit into elastic strains in the γ and γ' phases. This causes an elastic strain energy anisotropy and hydrostatic stress anisotropy among the vertical and horizontal γ phase channels. Both anisotropies predict a raft structure that is contrary to the experimental observation. Through a carefully designed pre-treatment at 750 °C and under a uniaxial stress of 750 MPa, an anisotropic dislocation structure was created on the {001} γ'/γ interfaces without altering the cuboidal morphology of the γ' phase. This allows the evaluation of the influence of dislocations on the rafting behavior of the alloy. It was found that rafting occurred in the pre-treated samples containing anisotropic dislocation structures during thermal exposure without applied stress, instead of isotropic corsening, and that the raft structures conform to the expectations base don the anistropic dislocation strucutres. This demonstrates that an anisotropic dislocation structure on the {001} γ'/γ interfaces is a direct cause of rafting. This is attributted to the fact that dislocations on γ'/γ interfaces help to relax the local misfit strains, causing some to expand at the expense of others. At the same time, the formation of the anisotropic dislocation structure is a direct result of the anisotropy of elastic strains induced by the applied bia stess in during the pre-treatment. These explain the complex interplay of the elastic and plastic models reported the literature.
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•The elastic stretching alters the partitioning of elastic strains in two phases, whereas the misfit remains unchanged.•The elastic strain energy and hydrostatic stress anisotropies are ruled out to be responsible for rafting.•The anisotropic dislocation on γ'/γ interfaces wtith γ' phase cuboid morphology unchanged were created by pre-straining.•The dislocation anisotropy is effective in causing rafting in Ni-based superalloys by causing misfit anisotropy.
Critics decry cryptocurrency mining as a huge waste of energy, while proponents insist on claiming that it is a green industry. Is Bitcoin mining really worth the energy it consumes? The high power ...consumption of cryptocurrency mining has become the latest global flashpoint. In this paper, we define the Mining Domestic Production (MDP) as a method to account for the final outcome of the Bitcoin mining industry’s production activities in a certain period time, calculate the carbon emission per unit output value of the Bitcoin mining industry in China, and compare it with three other traditional industries. The results show that Bitcoin mining does not always have the highest when compared with others. The contribution of this paper is that we give a new perspective on thinking whether Bitcoin mining is more efficient to make more profit, in terms of the same amount of carbon emissions per unit compared to other industries. Moreover, it could even be argued that Bitcoin may present an opportunity for some developing countries to build out their electrical capacity and generate revenue.
Detwinning is an important plastic deformation mechanism that can significantly affect the mechanical properties of twin-structured metals. Although many detwinning mechanisms have been proposed for ...pure metals, it is unclear whether such a deformation model is valid for nanocrystalline alloys because of the lack of direct evidence. Here, the atomicscale detwinning deformation process of a nanocrystalline AuAg alloy with an average grain size of ∼15 nm was investigated
in situ
. The results show that there are three types of detwinning mechanisms in nanocrystalline AuAg alloys. The first type of detwinning results from grain boundary migration. The second type of detwinning occurs through combined layer-by-layer thinning and incoherent twin boundary migration. The last one occurs through incoherent twin boundary migration, which results from the collective motion of partial dislocations in an array.
In situ atomic-scale bending tests of twin-structured Ni nanowires were realised using a homemade deformation device. The results showed that the plastic deformation mechanism in twin-structured Ni ...nanowires depended on the deformation stage. At the early stages of bending deformation, the plasticity of twin-structured Ni nanowires was controlled by dislocations interacting with the twin boundaries or parallel to them. With increasing bending strain, both dislocation and face-centred cubic–body-centred tetragonal phase transition occurred. At very high bending strain, grain boundaries resulting from the lattice distortion/collapse were formed. This study details the deformation mechanisms of the twin-structured Ni nanowires under bending deformation, which advances the basic understanding of the plasticity mechanisms in metals.
During the SARS-CoV-2 (COIVD-19) outbreak, China repeatedly stressed that the response to the pandemic required action at all levels of government, including the issuance of Pandemic Bonds to help ...the country return to work and production. However, studies on the effectiveness of Pandemic Bonds during that period are rare. Starting with China’s national financial bond market data after COVID-19 in 2020, this paper focuses on the correlation between the Credit Spreads of the relevant bonds and the corresponding bond market rate of return, based on the Copula model. The empirical analysis is also carried out for multiple dimensional groupings such as enterprises, industries, provinces, and bond maturities. The results show that there is a significant positive correlation between the Credit Spreads of Pandemic Bonds and market returns. In addition, the market correlation is higher for Pandemic Bonds issued in Hubei Province, which is at the center of the 2020 pandemic, and the shorter the maturity of the Pandemic Bond issued, the stronger the relationship with market returns. Finally, this paper provides recommendations for financial regulators and policy makers to consider in their decisions on how to build a more resilient financial system under heavy economic, fiscal, and social pressures.
Although extensive simulations and experimental investigations have been carried out, the plastic deformation mechanism of body-centered-cubic (BCC) metals is still unclear. With our home-made ...device, the in situ tensile tests of single crystal tantalum (Ta) nanoplates with a lateral dimension of ∼200 nm in width and ∼100 nm in thickness were conducted inside a transmission electron microscope. We discovered an unusual ambient temperature (below ∼60°C) ultra-large elongation which could be as large as 63% on Ta nanoplates. The in situ observations revealed that the continuous and homogeneous dislocation nucleation and fast dislocation escape lead to the ultra-large elongation in BCC Ta nanoplates. Besides commonly believed screw dislocations, a large amount of mixed dislocation with b=
1
2
<
111
>
were also found during the tensile loading, indicating the dislocation process can be significantly influenced by the small sizes of BCC metals. These results provide basic understanding of plastic deformation in BCC metallic nanomaterials.
Metals usually have three crystal structures: face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal-close packed (hcp) structures. Typically, metals exhibit only one of these structures ...at room temperature. Mechanical processing can cause phase transition in metals, however, metals that exhibit all the three crystal structures have rarely been approached, even when hydrostatic pressure or shock conditions are applied. Here, through in situ observation of the atomic-scale bending and tensile process of ∼5 nm-sized Ag nanowires (NWs), we show that bending is an effective method to facilitate fcc-structured Ag to access all the above-mentioned structures. The process of transitioning the fcc structure into a bcc structure, then into an hcp structure, and finally into a re-oriented fcc structure under bending has been witnessed in its entirety. This re-oriented fcc structure is twin-related to the matrix, which leads to twin nucleation without the need for partial dislocation activities. The results of this study advance our understanding of the deformation mechanism of small-sized fcc metals.
Ni-based superalloys rely on high volume fractions of L12 ordered γ′ phase precipitates for strength against creep deformation at elevated temperatures. However, under certain conditions dislocations ...existing in the FCC γ matrix may enter the L12 γ′ phase in pairs. The shear motion of different combinations of dislocation pairs creates different planar defects in the γ′ phase, including anti-phase boundaries (APB) or stacking faults. The formation of an APB requires the shear distortion associated with an a/2〈110〉 dislocation and the formation of a stacking fault requires the shear distortion associated with a k〈112〉 dislocation. Given that the native dislocations in FCC structure are a/2〈110〉, the formation mechanism of k〈112〉 dislocations remains to be clarified. Different mechanisms have been suggested for the formation of stacking faults in the γ′ phase in the literature. In this study, the shearing motions of various partial dislocation pairs and the planar defects formed in the γ′ phase were investigated by means of transmission electron microscopy and the mechanisms were analyzed in terms of their crystallographic and energetic implications.
•Creation of k〈112〉 partials from native a/2〈110〉 dislocations is clarified.•All possible dislocation pairs among m〈110〉 and n〈112〉 are examined.•Formation mechanisms of anti-phase boundaries and stacking faults are clarified.•Criterion for annihilation of anti-phase boundaries is established.
Stochastic Gradient Langevin Dynamics (SGLD) is believed to preserve differential privacy as its intrinsic attribute since it obtain randomness from posterior sampling and natural noise. In this ...paper, we propose Differentially Private General Stochastic Gradient Langevin Dynamics (DP-GSGLD), a novel variant of SGLD which realizes gradient estimation in parameter updating through Bayesian sampling. We introduce the technique of parameter clipping and prove that DP-GSGLD satisfies the property of Differential Privacy (DP). We conduct experiments on several image datasets for defending against gradient attack that is commonly appeared in the scenario of federated learning. The results demonstrate that DP-GSGLD can decrease the time for model training and achieve higher accuracy under the same privacy level.
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