Ni-65wt%Nb alloy by aluminothermic reduction process

Ni–Nb alloys have been used as master-alloy in the production of Ni-based superalloys containing niobium. In the case of INCONEL 718, the most used superalloy, the niobium content is in the range 5.0–5.5 wt%. The aim of this work is to present results related to the production of Ni-65wt%Nb alloy by aluminothermic reduction process. Two different reactant mixtures were used: Nb₂O₅ + Ni + Al and Nb₂O₅ + NiO + Al. For each type of mixture the Al excess was varied from 0% to 15% over the stoichiometric amount. All the alloys were produced in a bulk form with Nb and Ni composition according to specifications. The best results came from the experiments involving mixtures of Nb₂O₅ + NiO + Al, where an overall metallic yield of 86% was obtained. The alloys produced from Nb₂O₅ + Ni + Al mixtures showed low metallic yields and high oxygen contents associated to the presence of a niobium oxide-phase in the microstructure.     

Lactic acid bacteria biodiversity in Italian marinated seafood salad and their interactions on the growth of Listeria monocytogenes

The aim of this research was to study the biodiversity of lactic acid bacteria (LAB) in marinated seafood salad (pH 5.0) and their interaction on the growth of Listeria monocytogenes. LAB were highly present in the samples considered in this study, reaching values of 8.0 log cfu/g at the end of product’s shelf-life. A high biodiversity in terms of LAB species and strains was detected by means of RAPD–PCR within the 171 bacterial isolates collected. Among them Lactobacillus curvatus, Lactobacillus sanfranciscensis and Enterococcus were present in all the salad batches considered. Three challenge tests against L. monocytogenes were carried out in order to assess the growth of this pathogen in the presence of dominant populations of LAB. L. monocytogenes tended to decrease in time, suggesting that a stable concentration of LAB inhibits the development of this pathogenic micro-organism.     

Tool-supported requirements prioritization: Comparing the AHP and CBRank methods

Requirements prioritization aims at identifying the most important requirements for a software system, a crucial step when planning for system releases and deciding which requirements to implement in each release. Several prioritization methods and supporting tools have been proposed so far. How to evaluate their properties, with the aim of supporting the selection of the most appropriate method for a specific project, is considered a relevant question.In this paper, we present an empirical study aiming at evaluating two state-of-the art tool-supported requirements prioritization methods, AHP and CBRank. We focus on three measures: the ease of use, the time-consumption and the accuracy. The experiment has been conducted with 23 experienced subjects on a set of 20 requirements from a real project. Results indicate that for the first two characteristics CBRank overcomes AHP, while for the accuracy AHP performs better than CBRank, even if the resulting ranks from the two methods are very similar. The majority of the users found CBRank the “overall best” method.     

Heat rejection pressure optimization for a carbon dioxide split system: An experimental study

Recent studies indicate carbon dioxide (R744) as a valid alternative to classical substances such as HFCs used in vapour compression plants. However a transcritical refrigeration cycle is needed because the critical temperature of carbon dioxide is usually near the ambient temperature. Consequently the carbon dioxide refrigerator performances are significantly influenced by the heat rejection pressure. In this paper an experimental investigation on working optimization for a “split-system” to cool air in residential applications is presented: by varying the heat rejection pressure an optimum working condition has been found at different ambient temperatures. Furthermore a simplified model to predict the optimum heat rejection pressure is shown and a comparison with experimental results is carried out. Both the model validation and the experimental results suggest that the heat rejection pressure optimization is an convenient method to improve the performance of a carbon dioxide split system. Finally an algorithm based on the aforementioned model has been proposed in order to control an electronic back pressure valve by means of a PLC.     

Diffusion bonding of SU 263

Using a specially constructed apparatus, diffusion bonding of SU 263 alloy was studied in the temperature range of 1123–1323 K and compressive stress of 90% of its yield strength at the corresponding temperatures to determine the relative importance of the process parameters, the mechanism(s) responsible for bonding and the joint characteristics. Bond quality was assessed by optical metallography and lap shear testing. The mechanism of bonding was evaluated by grain growth equation. The experimental results were compared with a model developed by Pilling [Pilling, J., 1988. The kinetics of isostatic diffusion bonding in superplastic materials. Mater. Sci. Eng. 100, 137–144] in which the void closure by creep flow and diffusion are considered. Quantified EPMA line scan analysis was carried out to confirm the bonding mechanism and to determine the composition at the interface.