Laves phase precipitation and its transformation kinetics in the ferritic stainless steel type AISI 441

Electrolytic extraction followed by XRD analysis of precipitates after isothermal annealing of an AISI 441 ferritic stainless steel between 600 and 850 ◦C produced the time–temperature–precipitation (TTP) diagram for the Fe2Nb-Laves phase. The TTP diagram shows two classical C noses, the first one between 750 and 825 ◦C and the second one, estimated to be close to 650–675 ◦C. TEM analyses show two independent nucleation mechanisms, i.e. at about 600 ◦C nucleation takes place on dislocations but above 750 ◦C, grain boundary nucleation is dominant. The measured solvus temperature of 875 ◦C for Fe2Nb in this steel is 50 ◦C higher than predicted by Thermo-Calc®. Kinetic modelling at 800 ◦C agreed well with the experimental results for a surface energy of 0.435 J m−2 and an initial particle center-to-center separation distance on grain boundaries of about 0.2 m.Columbus Stainless, South Africa and the Department of Materials Science and Metallurgical Engineering of the University of Pretoria.http://www.elsevier.com/locate/mseahb2016Materials Science and Metallurgical Engineerin

Laves phase embrittlement of the ferritic stainless steel type AISI 441

The effect of Laves phase (Fe2Nb) formation on the Charpy impact toughness of the ferritic stainless steel type AISI 441 was investigated. The steel exhibited good room temperature toughness after solution treatment of 30 min at 850 ◦C, but above and below this treatment temperature the room temperature impact toughness decreased sharply. In as received and already brittle specimens where different volume fractions of Laves phase were introduced through isothermal equilibration at various temperatures below 850 ◦C, it was observed that a decrease in the Laves phase volume fraction with increasing annealing temperature towards 850 ◦C (Thermo-Calc® predicted a Laves phase solvus temperature of 825 ◦C versus a later measured solvus of 875 ◦C for this steel) corresponded to an increase in the impact toughness of the steel. On the other hand, annealing at various temperatures above 900 ◦C where no Fe2Nb exists, grain growth was found to also have a very negative influence on the steel’s room temperature impact properties. Through deliberate prior grain growth and by varying the cooling rate after solution treatment, it was found that where both a large grain size and Fe2Nb are present, it appears that the grain size of the two is the dominant embrittling mechanism. It was possible to qualitatively relate the impact strength results to current models on the effects of grain size and brittle grain boundary precipitates on the brittle fracture of ferritic materials. Finally, both the presence of Fe2Nb and grain growth, therefore, have a significant influence on the impact properties of the type AISI 441 stainless steels, which leads to a relatively narrow processing “window” in the final hot rolling temperature followed by rapid cooling rates in the manufacturing process.http://www.elsevier.com/locate/mseahb2016Materials Science and Metallurgical Engineerin

25 Years of Self-organized Criticality: Concepts and Controversies

Introduced by the late Per Bak and his colleagues, self-organized criticality (SOC) has been one of the most stimulating concepts to come out of statistical mechanics and condensed matter theory in the last few decades, and has played a significant role in the development of complexity science. SOC, and more generally fractals and power laws, have attracted much comment, ranging from the very positive to the polemical. The other papers (Aschwanden et al. in Space Sci. Rev., 2014, this issue; McAteer et al. in Space Sci. Rev., 2015, this issue; Sharma et al. in Space Sci. Rev. 2015, in preparation) in this special issue showcase the considerable body of observations in solar, magnetospheric and fusion plasma inspired by the SOC idea, and expose the fertile role the new paradigm has played in approaches to modeling and understanding multiscale plasma instabilities. This very broad impact, and the necessary process of adapting a scientific hypothesis to the conditions of a given physical system, has meant that SOC as studied in these fields has sometimes differed significantly from the definition originally given by its creators. In Bak’s own field of theoretical physics there are significant observational and theoretical open questions, even 25 years on (Pruessner 2012). One aim of the present review is to address the dichotomy between the great reception SOC has received in some areas, and its shortcomings, as they became manifest in the controversies it triggered. Our article tries to clear up what we think are misunderstandings of SOC in fields more remote from its origins in statistical mechanics, condensed matter and dynamical systems by revisiting Bak, Tang and Wiesenfeld’s original papers

Construction of malaria gene expression network using partial correlations

No abstract available