Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.The authors are thankful for the financial support from the Technology Strategy Board (TSB) through the TSB/101177 Project and to the Engineering and Physical Sciences Research Council (EPSRC) through the EP/J013749/1 and EP/K031422/1 Projects

Grain refinement of Al-Si alloys by Nb-B inoculation. Part II: application to commercial alloys

This article has been made available through the Brunel Open Access Publishing Fund.The potency of Nb-B inoculation for the refinement of Al-Si cast alloy has been demonstrated in Part I of this work by the systematic analysis of binary Al-xSi alloys (where x = 1-10 wt.%). In Part II of this work the effect of Nb-B inoculation on commercial Al-Si alloys is assessed. Specifically, hypo-eutectic alloys such as LM24 (A380) and LM25 (A356) as well as near-eutectic LM6 (A413) Al-Si alloys are considered. The aim is to quantify the grain refinement and detect possible interaction with alloying elements commonly present in Al cast alloys, such as Mg, Fe, Cu, Mn and Zn. The in-depth analysis of the alloys solidified under wide range of cooling rates indicates that Nb-B inoculation does not only lead to a much finer microstructural features but also makes the final grain size far less sensitive to the cooling rate employed to solidify the material. Finally, the mechanism essential for the grain refinement of commercial Al cast alloys by Nb-B inoculation is determined on the base of SEM and thermal analysis results. It is found that in-situ formed Al3Nb and NbB2 intermetallic particles (forming from the interaction of Al alloy/Nb powder/KBF4 flux) are the heterogeneous nuclei responsible for the grain refining of Al cast alloys.The Technology Strategy Board (TSB) through the TSB/101177 Project and to the Engineering and Physical Sciences Research Council (EPSRC) through the EP/J013749/1 Project

Hydrogen charging of carbon and low alloy steel by electrochemical methods

Atomic hydrogen can be the result of different processes like electroplating, chemical and electrochemical pickling treatments, in welding or by cathodic processes in corrosive fluids. Moreover, adsorption of atomic hydrogen can affect materials in contact with high pressure gaseous hydrogen. Once entered the material, atomic hydrogen interacts with the metal structure and may produce a "damage" of various forms, such as Hydrogen Induced Cracking (HIC), delayed fracture, blistering and hydrogen embrittlement. In particular, when H2S is present ("sour service"), metallic materials, such as carbon and low alloy steels, may suffer hydrogen damage and hydrogen embrittlement. Sour service materials must be used in compliance with international accepted standards, used worldwide in oil and gas activities, when fluids are classified as sour. The present study has been carried out in order to set up an electrochemical method to charge with hydrogen two typical pipeline materials, carbon and low alloy steels. The reason of the use of an electrochemical method is to avoid any critical conditions from the point of view of preparation, safety and disposal. Hydrogen content in the specimens was measured by two different methods: hot glycerol bath and Inert Gas Fusion (IGF) analysis. Hydrogen content in the specimens is about 0.6-2 ppm; mechanical performances were assessed by means of J integral tests: a pronounced decrease of fracture toughness was observed for H charged specimens.{GRAPHIACAL ABSTRACT

Effect of deer density on tick infestation of rodents and the hazard of tick-borne encephalitis. II: Population and infection models

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Modelling farm-to-farm disease transmission through personnel movements:From visits to contacts, and back

AbstractInfectious diseases in livestock can be transmitted through fomites: objects able to convey infectious agents. Between-farm spread of infections through fomites is mostly due to indirect contacts generated by on-farm visits of personnel that can carry pathogens on their clothes, equipment, or vehicles. However, data on farm visitors are often difficult to obtain because of the heterogeneity of their nature and privacy issues. Thus, models simulating disease spread between farms usually rely on strong assumptions about the contribution of indirect contacts on infection spread. By using data on veterinarian on-farm visits in a dairy farm system, we built a simple simulation model to assess the role of indirect contacts on epidemic dynamics compared to cattle movements (i.e. direct contacts). We showed that including in the simulation model only specific subsets of the information available on indirect contacts could lead to outputs widely different from those obtained with the full-information model. Then, we provided a simple preferential attachment algorithm based on the probability to observe consecutive on-farm visits from the same operator that allows overcoming the information gaps. Our results suggest the importance of detailed data and a deeper understanding of visit dynamics for the prevention and control of livestock diseases.</jats:p

Influence of Hydrogen and Low Temperature on Pipeline Steels Mechanical Behaviour

Abstract In the presence of H2S, metallic materials, such as carbon and low alloy steels, may suffer hydrogen damage and hydrogen embrittlement. Gas transporting pipes in low temperature environment, during the shutdown and the subsequent re-starting operations, are exposed to very low temperatures (T=-40 °C). In the presence of high H2S content in the gas, the risk of brittle failure can be increased due to the effect of hydrogen on steel toughness. In this paper the influence of hydrogen and low temperature on mechanical properties of two pipeline materials, F22 low alloy and X65 micro-alloyed steels, is studied. Steels have been hydrogen charged by means of an electrochemical method: diffusible hydrogen content of steels is in the range 0.6 to 2 ppm. Charpy and J-R curves tests were carried out in the range from room temperature to T=-120 °C. Hydrogen affects mechanical properties of the tested materials, mainly reducing fracture toughness in J integral tests, while little influence has been observed in CV tests. Fracture surface examination confirms the results of mechanical testing

Effetto dell’intermittenza della corrente di protezione nei sistemi di protezione catodica dell’acciaio

The paper reports the results of a preliminary research study on the effects of protection current intermittence on the cathodic protection conditions of buried or immersed pipelines. It is well known that cathodic protection has chemical and electrochemical beneficial effects on carbon steel, which do not disappear instantly in correspondence to the current interruption. According to this premise, laboratory tests of intermittent cathodic protection in which the current has been interrupted cyclically were carried out on carbon steel specimens in soil-simulating solution. The effects of the duration of the current-off time, the number of on-off cycles, and of the applied current density have been studied. Potential has been monitored weekly and the residual corrosion rate was calculated by means of a simple electrochemical model

La realcalinización y la extracción electroquímica de los cloruros en las construcciones de hormigón armado

Realkalisation and electrochemical chloride removal techniques, developed for rehabiliting carbonated and chloride-containing structures, are presented. Electrolysis and electromigration mechanisms and consequences as well as electrochemical conditions at the reinforcement surface are discussed and compared with cathodic protection ones. Furthermore, possible side effects are commented

A Comprehensive Investigation on the Effects of Surface Finishing on the Resistance of Stainless Steel to Localized Corrosion

The present research investigates the influence of surface roughness imparted by cold surface finishing processes on the localized corrosion resistance of stainless steel. Five different alloys were studied: ferritic AISI 430, martensitic AISI 430F, austenitic AISI 303, AISI 304L, and AISI 316L. It was demonstrated that the grinding process, executed on previously cold drawn bars, leads to an improvement in corrosion resistance according to the results obtained with electrochemical tests, namely, potentiostatic and potentiodynamic tests in chloride-rich environments, the salt spray test, and long-term exposure in urban and marine atmospheres. This allowed us to establish a trend among the different alloys regarding the resistance to pitting corrosion, which was assessed according to pitting potentials, critical chloride contents, and pitting initiation time. All the tests confirmed that surface finishing, as well as alloy chemical composition, is an important factor in controlling the corrosion resistance of stainless steel

Considerations on the effect of solutal on the grain size of castings from superheated melts

© 2017 The Authors. The amount of solutal present in an alloy affects the grain size of the cast metal as solute is rejected at the solidification front. This is normally quantified using the so called growth restriction factor Q. This work presents some considerations about the effect of solutal on the final cast structure with a focus on the nature of the alloy system, the effect of non-equilibrium solidification conditions and the effect of superheating of the molten metal