Kinetics of vanadium carbonitride precipitation in steel: A computer model

We have constructed a computer model of the precipitation kinetics of vanadium carbonitride in steel that takes into account the composition evolution of the precipitates with time. The model takes advantage of the fast diffusion of nitrogen and carbon compared to niobium to derive the composition, size and rate of formation of the precipitates during their nucleation. A local equilibrium condition is used at the precipitate–matrix interface to derive the growth rate of each precipitate as a function of its size and the current matrix composition. Coarsening occurs naturally on account of the Gibbs–Thomson capillarity effect. For isothermal heat treatments, the calculations show that the precipitates nucleate as almost pure vanadium nitrides. They subsequently grow at the expense of solute nitrogen. When nitrogen is exhausted, the solute carbon precipitates and progressively transforms the nitrides into carbonitrides. The coarsening stage leads to a steady-state size distribution of niobium carbonitrides of the equilibrium composition

Market Efficiencies and Market Risks

Numerous papers constructed or simulated financial markets at an agent level, aiming to explain the non-stationarity of price processes. All such papers agree that the heterogeneity of agents and of pricing models creates a dynamics in terms of pricing models used that explains not only the non-stationarity of price processes, but also stylised facts such as bubbles and fat tails. However, all these results issue from very specific parametric set-ups, and even if multiple approaches confirm it, there is no proof of the aforementioned results outside of such specifications. By modeling agents as black boxes that receive information that they transform into an output information, information according to which they then act upon the financial market, we show that the diversity of agents is directly associated to the resulting quality of the information efficiency of the market : homogenous agents lead to good information propagation but poor information aggregation by the price, while heterogenous agents lead to good information aggregation but poor information propagation. This difference in quality of efficiency explains, outside of any parametric model, the dynamics of the number of different pricing models used within artificial stock markets.Market efficiency, group learning and evolutionary games.

New Prospects on Vines

In this paper, we present a new methodology based on vine copulas to estimate multivariate distributions in high dimensions, taking advantage of the diversity of vine copulas. Considering the huge number of vine copulas in dimension n, we introduce an efficient selection algorithm to build and select vine copulas with respect to any test T. Our methodology offers a great flexibility to practitioners to compute VaR associated to a portfolio in high dimension.Vine copulas, multivariate copulas, model selection, VaR.

An Econometric Study of Vine Copulas

We present a new recursive algorithm to construct vine copulas based on an underlying tree structure. This new structure is interesting to compute multivariate distributions for dependent random variables. We proove the asymptotic normality of the vine copula parameter estimator and show that all vine copula parameter estimators have comparable variance. Both results are crucial to motivate any econometrical work based on vine copulas. We provide an application of vine copulas to estimate the VaR of a portfolio, and show they offer significant improvement as compared to a benchmark estimator based on a GARCH model.Vines, multivariate copulas, risk management.

Atomic-scale study of low-temperature equilibria in iron-rich Al-C-Fe

The capability of the thermodynamic approach based on the independent point defect approximation to describe low-temperature phase equilibria is investigated and applied to the Al-C-Fe system. The method gives a reasonable description of the multicomponent and multisublattice Fe-rich corner and evidences numerous peculiarities concerning the ordered phases as well as the density-functional-theory (DFT) energy models. The study of Fe3Al(-C), revealing strong defect-induced instabilities, rules out the LDA, SLDA and GGA schemes and leaves (spin-polarized) SGGA as the only valid one. C stabilizes L12 Fe3Al with respect to D03, which justifies the fcc-type structure of the kappa Fe3AlC compound. The present work also helps in justifying the experimentally observed depletion of C in the kappa phase. Finally, a correct description of both Fe3C and kappa requires inclusion of interstitial carbon at low temperature, emphasizing the unexpected importance of interstitial defects in ordered phases