A new theory of the solid-state growth of embryos during nucleation: the fundamental role of interfacial mobility

One of the major challenges in nucleation theory is to explain the kinetic pathway allowing multicomponent precipitates to grow until they reach stability. This problem is particularly challenging when the supersaturation is low, so that the critical size of nucleation is large and requires the condensation of thousands of atoms. A new theory is proposed to explain the growth of embryos before they reach the critical size of nucleation. This theory is not a substitute of the classical nucleation theory, but a complement aiming to understand the kinetic pathway allowing unstable embryos to grow at the expense of their neighbours. The theory stands on the strong interactions between embryos. The latter may exchange atoms via impingement and coarsening, which are possible when there are no concentration gradients between the embryos. This condition is supposed to be met during the unstable growth regime of nucleation considering that the growth is limited by the interface during that period. Assuming that the embryos behave in a collective manner when they are grouped in a cloud, we show that the growth velocity of the most active embryos will be limited only by their interfacial mobility and the available driving force.     

Strain incompatibility and its influence on grain coarsening during cyclic deformation of ARB copper

It is well known that the characteristic length scale in ultra-fine grained and nanocrystalline metals has a significant effect on the mechanical behaviour. The inhibited ability to accommodate imposed strain with conventional dislocation mechanism has led to the activation of unconventional deformation mechanisms. For one, grain coarsening at shear bands has been observed to occur within metals with sub-micron grain size upon cyclic deformation. Such grain coarsening is often linked to the observed cyclic softening behaviour. The purpose of this study was to investigate the relationship between strain localisation associated with shear banding and the observed deformation-induced grain coarsening in ultra-fine grained metals. The investigation was carried out using ultra-fine grained, oxygen-free high conductivity copper processed by accumulative roll-bonding. A close relationship between strain localisation and deformation-induced grain coarsening was revealed. As strain localisation is not only found at shear bands, but also at other places whereby heterogeneous microstructure or geometric discontinuity is present, hence the present study bears a general significance. Such strain localisation sites may also include a hard constituent embedded in a relatively ductile matrix, micro-crack tips and artificial notches. The stress concentration at these sites provides a high input of strain energy for grain boundary motion leading to grain coarsening. Furthermore, when the grain size is very small, the stress gradient leading away from the stress concentration sites is also believed to increase the driving force for grain boundary migration within the affected regions.     

The azimuthal dependent oxidation process on Cu(110) by energetic oxygen molecules

We report a study on the oxidation process induced by a hyperthermal oxygen molecular beam (HOMB) on Cu(110) using X-ray photoemission spectroscopy in conjunction with a synchrotron radiation source. The oxidation process induced by energetic O₂ beams on Cu(110), depending on the azimuthal angle of incidence, suggests that the –Cu–O– added row structure has a role in inhibiting adsorption as a steric obstacle for incident O₂ molecules.     

Computational design of precipitation strengthened austenitic heat-resistant steels

A new genetic alloy design approach based on thermodynamic and kinetic principles is presented to calculate the optimal composition of MX carbonitrides precipitation strengthened austenitic heat-resistant steels. Taking the coarsening of the MX carbonitrides as the process controlling the life time for steels in high temperature use, the high temperature strength is calculated as a function of steel chemistry, service temperature and time. New steel compositions for different service conditions are found yielding optimal combinations of strength and stability of the strengthening precipitation for specific applications such as fire-resistant steels (short-time property guarantee) and creep-resistant steels (long-time property guarantee). Using the same modelling approach, the high temperature strength and lifetime of existing commercial austenitic creep-resistant steels were also calculated and a good qualitative agreement with reported experimental results was obtained. According to the evaluation parameter employed, the newly defined steel compositions may have higher and more stable precipitation strengthening factors than existing high-temperature precipitate-strengthened austenite steels.     

An improved theory for temperature-dependent Arrhenius parameters in mesoscale surface diffusion

Experiments on metals typically show an abrupt change in the Arrhenius behavior of surface self-diffusion at temperatures near 60-75% of the bulk melting point. To explain this phenomenon, we propose based on correlational evidence that the most common mechanism for surface self-diffusion is one in which adatoms dominate low-temperature transport, while surface vacancies dominate at high temperatures. The high-temperature dominance of vacancies results from their substantially higher entropy of diffusion, which is a consequence of the large vibrational displacements of surface atoms relative to the bulk. This phenomenon may also explain the Arrhenius behavior on some non-metal surfaces.     

Tolerance of aggressive and non-aggressive isolates of Ceratocystis ulmi to organotin fungicides

The effect of triorganotin compounds, R₃SnX, on the growth of three wild strains of Ceratocystis ulmi (C. ulmi) fungus, two aggressive and one non-aggressive strains, was evaluated in shake culture. In all cases, the triphenyltins were the more effective organotins for the inhibition of C. ulmi in vitro. The anionic group, X, did not have a significant role in the inhibition, suggesting that the species involved in the inhibition is the triphenyltin moiety (Ph₃Sn⁺) or the hydrated triphenyltin moiety (Ph₃Sn(H₂O)⁺₂). It is further suggested that the triphenyltin species Ph₃SnOH and Ph₃SnOAc are the preferred compounds for the control of Dutch elm disease. The tolerance of aggressive isolates to fungitoxins appears to depend more on the nature of the fungicide than on the type of fungus.     

改进交叉评价DEA在模糊综合评价中的应用

借鉴对抗型交叉评价的思想,首先利用对抗型交叉评价DEA(数据包络分析)模型对模糊综合评价的量化指标进行评价,三角模糊化后将其作为模糊综合评价量化指标的输入与非量化指标数据合成进行二次评价,以此建构了一种基于对抗型交叉评价DEA的模糊综合评价方法.方法可从根本上解决已有评价方法中模糊量化结果的不确定性问题,使客观数据与主观因素并存的多属性决策更加可靠.最后,通过算例说明了方法的应用.     

Coarsening of the phase structure in immiscible polymer blends. Coalescence or ostwald ripening?

The results of theories of the Ostwald ripening and coalescence applied to molten quiescent polymer blends with dispersed phase structure were analyzed. From a comparison of predictions of the theories with available experimental results follows that coarsening of the phase structure in quiescent polymer blends with medium or high interfacial tension is induced by the coalescence. Both the mechanisms play a role in coarsening of the phase structure in blends with low interfacial tensions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 181–187, 1999     

High-order multigrid strategies for hybrid high-order discretizations of elliptic equations

This study compares various multigrid strategies for the fast solution of elliptic equations discretized by the hybrid high-order method. Combinations of $h$-, $p$-, and $hp$-coarsening strategies are considered, combined with diverse intergrid transfer operators. Comparisons are made experimentally on 2D and 3D test cases, with structured and unstructured meshes, and with nested and non-nested hierarchies. Advantages and drawbacks of each strategy are discussed for each case to establish simplified guidelines for the optimization of the time to solution.