Cette étude financée par SNECMA et le CNES a pour objectif de mieux comprendre et caractériser le résultats de la solidification de l'inconel 718. Cet objectif est motivé par le besoin grandissant de fiabilité sur les pièces brutes de fonderies utilisées dans l'assemblage des moteurs de fusée. En effet, lors de la solidification, la ségrégation chimique provoque la formation de phase fragile. C'est notamment le cas des phases de Laves. Ainsi, un four de fonderie sous vide et un moule instrumenté ont été mise au point afin de couler au laboratoire des plaques de géométrie variable. Le but étant d'obtenir plusieurs échantillons obtenu sous différente conditions de solidifications. Une méthode d'analyse quantitative au MEB a par la suite été mise au point pour analyser les 4 coulées exploitables. Elle a permis de quantifier la quantité d'intermétalliques en fonction de la vitesse de refroidissement. Mais aussi leur répartition vis à vis des joints de grains d'une part et l'impact de la désorientation entre chaque grain d'autre part. La ségrégation chimique a aussi pu être quantifiée selon ces mêmes paramètres.De travaux numériques ont été entrepris en parallèles avec tout d'abord la simulation macroscopique des coulées avec le logiciel Thercast. Ces simulations ont permis de connaître les conditions de solidification de l'ensemble des échantillons coulés et ont fourni des données d'entré pour les autres aspects numériques. Par exemple, avec un modèle élément fini et automate cellulaire, la formation de la structure granulaire a pu être modélisée. Enfin, la simulation de la microségrégation a permis de valider notre compréhension des phénomènes prédominant lors de la solidification de l'inconel 718. Ces simulations numériques apportent à la fois un éclairage sur les résultats expérimentaux mais aussi démontrent leurs possibilités dans l'optique de mettre en place un modèle global de solidification recouvrant ces trois échelles pour des pièces industrielles / The metallic alloy “Inconel 718” is frequently found in aerospace and aeronautic components. In this study, it is used in the case of the main engine turbopump of the Ariane 5 launcher. This case is obtained by investment casting. Thereby, a great focus is turned on as cast defect like detrimental interdendritic phase. For example, the Laves phase, about 1%, has poor mechanicals characteristics. Several samples was cast under vacuum at MINES ParisTech Cemef. Various superheat and cooling rate are tested. After specific preparation, the microstructure was observed by scanning electron microscopy (SEM). On a surface representative of solidification phenomenon, the NbC phase, the Laves phase and the Delta phase were quantified and chemical segregation measurements were performed. The results give a tendency of the detrimental phases to form at the grain boundaries. This phenomenon is stronger as the cooling speed is lower. Furthermore, the grains boundaries disorientations as an impact. This experimental information is used to model the sample solidification at three different scales. At the macroscopic scale with Thercast software, at an intermediate scale whit the simulation of the grain structure. These simulations were performed whit a model developed at Mines Paristech, CEMEF. The last scale is the simulation of the microsegregation. This model takes into account thermodynamic equilibrium, cooling rate and microstructure geometries. This numerical work improved the comprehension of the solidification phenomenon. A global solidification simulation of industrial piece is the aim of Cnes (French space agency) and Snecma who support this work.

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In new low-lead and lead-free brass alloys, it is not understood how the corrosion properties,such as dezincification, are related to material composition as well as annealing temperatureand duration. This study aims to fill this knowledge gap by mapping sixteen annealingconditions and three different brass alloy compositions to their respective microstructure anddezincification performance. It was found that high dezincification depth was a result ofannealing temperatures at 300°C – 400°C, which promoted precipitation of intermetallicAlAs-particles along grain boundaries, twins and lead particles as well as precipitation of β-phase along grain boundaries. Their presence was correlated to high micro additions ofaluminium or iron in the material composition. An additional compositional factorcontributing to precipitation of high amount of β-phase was low copper/zinc-ratio.

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Learner beliefs, which contribute to attitude and motivation, may affect language learning. It is therefore valuable to investigate the malleability of learner beliefs, and to determine whether potentially detrimental beliefs can be ameliorated. This study examines how instruction of the principles of Second Language Acquisition (SLA) affects students' beliefs about classroom activities and their beliefs about language learning in general. The 68 first-year German students at Brigham Young University who participated in this study were asked to rate the effectiveness of three activities typical of communicative language teaching: Dialogue activities, Peer Interview activities, and Information-gap activities. They were also asked to respond to 11 statements about language learning, seven of which were taken from the Beliefs About Language Learning Inventory(Horwitz, 1988). Students responded to the survey three times: once during the first week of the semester, again during the fourth week, and again during the eighth week. During the four weeks between the second and third surveys, students in the experimental group received seven treatment lessons based on some of the basic principles of SLA. A Repeated Measures ANCOVA and a Logistical Regression were used to determine the effects of the treatment, time, and a number of demographic variables. Results of this study show that the treatment did not have a significant effect on any of the beliefs that were measured. However, one language learning belief was significantly affected by time. A majority of the students who participated in this study agreed with the statement, “The instructor should teach the class in German.” After three weeks of class instruction, however, they agreed with this statement significantly stronger. The results of this study also show that many of the demographic variables, such as gender and previous language learning experience, had a significant effect on a number of the students' beliefs.