Determining the Dependence of Grain Boundary Mobility on Misorientation in High Purity Aluminum with Zirconium Additions

Aluminum alloys are used frequently for applications within the aerospace industry, creating a demand for finely tuned aluminum alloys that maximize a material property of interest (strength, toughness, etc.) and minimize weight. In order to formulate more complex alloys, it is important to understand how alloying elements affect the kinetics of grain growth in the solvent system. This study analyzes the effect of small concentrations of zirconium in high purity aluminum on grain growth during primary recrystallization by empirically determining the grain boundary mobility via measuring grain boundary velocity as well as stored energy within a sample and correlating grain boundary misorientation to mobility data. Grain boundary velocity is measured by annealing single crystal samples of deformed high purity aluminum with Zr additions in a box furnace and completing orientation imaging microscopy (OIM) scans that use electron back scattered diffraction (EBSD) patterns to index a lattice and create an inverse pole figure (IPF). This inverse pole figure assigns colors to orientations of crystalline grains and allows for the tracking of grain boundaries after subsequent heat treatments as well as for the acquisition of the misorientation at any given grain interface. TSL software allows for analysis of the EBSD data which can calculate the average subgrain size and average misorientation within a region to provide a stored energy term. This value is used in tandem with micro hardness measurements to estimate stored energy. With a measured grain boundary velocity and stored energy, it is possible to calculate grain boundary mobility and correlate mobility with grain boundary misorientation.Grain boundary Mobility is a useful parameter to metallurgists as a predictor of grain size after deformation and heat treatments. However, grain boundary mobility has a variety of variables that are subject to change with composition, and is thus difficult to calculate. As such, it is necessary to experimentally determine the grain boundary mobility in unexplored alloy compositions for modeling as well as processing. Alloying metals for use in industry requires knowledge of how alloying elements will alter the processing parameters used to generate a desired set of properties. Thus by determining the bulk grain boundary mobility of high purity aluminum samples with Zirconium additions via heat treatment, this work validates the combined use of EBSD and microhardness as a useful means of collecting data that replicates previous results obtained in the literature.Stored energy results obtained in this work align well with values in the literature obtained by Huang and Humphreys as well as values obtained from microhardness by Taheri as well as Huang, Tao and Lu. The dependence of grain boundary mobility on misorientation is also seen to correspond well with results in the past by Taheri, Gottstein and Rollett with boundaries near 40° having higher mobility than random HAGBs. Additionally, data found in this study aligns with results predicted by the effect of preferential Zr segregation observed by Taheri. However, the less prominent shift of peak mobility from 40° to higher misorientation as anneal temperature increases is in contrast with previous results, calling into question if there is a true difference in observed mobility peaks between 38° and 40°.M.S., Materials Engineering -- Drexel University, 201

Dynamique Hydro-Erosive Actuelle Des Bassins Versants Endoreiques De La Region De Niamey (Sud-Ouest Du Niger)

La généralisation du ravinement et la baisse de la productivité des terres sont quelques-unes des principales contraintes qui assaillent l’utilisation des sols au Sahel. Pour gérer efficacement ces sols, une évaluation des processus hydro-érosifs est nécessaire. Ce travail a ainsi pour objectif d’analyser la dynamique hydro-érosive sur un site expérimental installé depuis 2004 dans la région de Niamey (Niger). Sur ce site, le dispositif de mesure est composé de huit parcelles de mesures de ruissellement et d’érosion, des stations hydrométriques à l’exutoire des deux bassins versants endoréiques et de plusieurs piquets destinés aux suivis morpho-dynamiques des ravines. Après une décennie d’observation, les ruissellements mesurés sur les parcelles se caractérisent par une dynamique saisonnière croissante sur les surfaces encroûtées (croûtes biologique et d’érosion) et décroissante sur les surfaces cultivées. Le coefficient de ruissellement varie de 5 % sur la surface cultivée à 58 % sur la croûte d’érosion. Au cours des 10 années de mesure, ce coefficient a connu une forte croissance en particulier sur la jachère (+ 80 %) et sur la surface cultivée (+ 300 %), traduisant ainsi la dégradation des sols. A l’échelle des bassins versants, l’augmentation du coefficient de ruissellement s’accompagne d’une érosion aréolaire qui décape le sol à une vitesse moyenne de 5 mm/an et d’une érosion linéaire active, de l’ordre de 4 m3/an au niveau des ravines suivies. Les transferts sédimentaires qui en résultent agissent sur le fonctionnement morpho-sédimentaire des cours d’eau. Des aménagements antiérosifs sont nécessaires pour dissiper les processus hydro-érosifs et préserver les services écosystémiques des sols des bassins. Widespread gullying and the declining land productivity are some of the main constraints plaguing land use in the Sahel. In order to effectively manage these soils, it has become necessary to assess the hydro-erosive processes. The current study, thus, aims at analyzing the hydro-erosion dynamics on an experimental site installed since 2004 in the Niamey region (Niger Republic). On this site, the measurement device is made up of eight runoff and erosion measurement plots, hydrometric stations at the outlet of the two endorheic watersheds and several stakes intended for morpho-dynamic monitoring of the gullies. After a decade of observation, the runoff measured on the plots is characterized by an increasing seasonal dynamic on encrusted surfaces (biological and erosion crusts) and a decreasing one on cultivated surfaces. The runoff coefficient varies from 5% on the cultivated area to 58% on the erosion crust. Over the 10 years of measurement, this coefficient has greatly increased, especially on the fallow (+ 80%) and on cultivated area (+ 300%); this increase consequently reflects soil degradation. At the watershed scale, the increase in the runoff coefficient is accompanied by area erosion (or the erosion of the area out of the basin) which strikes the soil at an average speed of 5 mm/year and active linear erosion measuring 4 m3/year as observed at the monitored (the ongoing investigated) gullies. The resulting sediment transfers act on the morpho-sedimentary functioning of rivers. Anti-erosion facilities are necessary to dissipate the hydro-erosive processes and preserve the ecosystem services of the soil in the basin

Long-term effect of forest and landscape restoration practices on soil organic carbon stock in semi-arid Burkina Faso

In semi-arid areas, forest and landscape restoration (FLR) practices are being implemented to reverse the land degradation process. The objective of this study was to investigate the long-term effect of FLR practices on soil organic carbon stock (SOCs) under different land uses in the semi-arid region of Burkina Faso. The study was conducted on degraded land under rehabilitation practices for 45, 27, 18 and 11 years, which were compared to similar land without specific rehabilitation measures. The soil was collected in 2018 in 35 sampling plots of 30 m x 30 m. Soil analysis concerned bulk density, soil particle size, soil pH, soil organic carbon content, and respiratory activity of microorganisms. SOCs increased by 150%, 98% and 29% over 0-10 cm depth in 45-, 27- and 11-year of FLR practices, and decreased by 6% in 18-year of FLR practices compared to their respective control. SOCs were not linearly increased with the duration of the implementation of FLR practices because the variation of SOCs depends on several other parameters such as soil texture, and types of combination of FLR practices. The highest SOCs were recorded for 27 years (9.5 t.ha-1) and 45 years (8.5 t.ha-1) of FLR practices. This study revealed the importance of including Assisted Natural Regeneration (ANR) as one of the combined FLR practices, for improvement of SOCs

Impact of Drought and Land – Use Changes on Surface – Water Quality and Quantity: The Sahelian Paradox

International audienceAfrique de l'Ouest a connu des conditions de sécheresse depuis la fin des années 1960. Cette tendance a été particulièrement évident dans le Sahel, mais semble avoir atténué dans la dernière décennie dans les régions orientales et centrales de cette région. D'autre part, la pluviométrie annuelle reste très faible dans la partie ouest du Sahel [ 1 ]. Une diminution correspondante a également été observé dans le débit moyen annuel des fleuves Sénégal et Niger, qui sont le plus grand dans la région et principalement alimenté par l'eau provenant de régions tropicales humides. Toutefois, le pourcentage de diminution du débit annuel moyen était presque deux fois plus grande que la diminution des précipitations [ 2 ] pour la période 1970-2010. Des tendances similaires ont été observées sur des réseaux hydrographiques plus petits. En revanche, même si le Sahel et la plupart de l'Afrique de l'Ouest ont connu la sécheresse aussi importante au cours des 40 dernières années, les coefficients de ruissellement et des débits d'eau ont augmenté dans la plupart des régions du Sahel. Ce phénomène a été appelé «Le Sahel Paradox" après l'augmentation de la nappe phréatique au Niger depuis les années 1960, a été nommé le paradoxe de Niamey et attribués à des changements importants dans l'utilisation des terres. Le les (Afrique multidisciplinaire de la mousson d'analyse) programmes AMMA HAPEX-Sahel (hydrologique et Expérience atmosphérique pilote) et ont fourni, parmi de nombreux résultats complets, les mesures de valeur portant sur les variations spatiales et temporelles de la teneur en eau du sol sahélienne ainsi que de l'infiltration de l'eau à travers les couches profondes du sol de la zone non saturée. Le but de ce chapitre est de fournir un aperçu du comportement hydrologique en Afrique de l'Ouest basée sur le point, locale, méso et échelles régionales observations

Observed long-term land cover vs climate impacts on the West African hydrological cycle: lessons for the future ? [P-3330-65]

West Africa has experienced a long lasting, severe drought as from 1970, which seems to be attenuating since 2000. It has induced major changes in living conditions and resources over the region. In the same period, marked changes of land use and land cover have been observed: land clearing for agriculture, driven by high demographic growth rates, and ecosystem evolutions driven by the rainfall deficit. Depending on the region, the combined effects of these climate and environmental changes have induced contrasted impacts on the hydrological cycle. In the Sahel, runoff and river discharges have increased despite the rainfall reduction (“less rain, more water”, the so-called "Sahelian paradox "). Soil crusting and erosion have increased the runoff capacity of the watersheds so that it outperformed the rainfall deficit. Conversely, in the more humid Guinean and Sudanian regions to the South, the opposite (and expected) “less rain, less water” behavior is observed, but the signature of land cover changes can hardly be detected in the hydrological records. These observations over the past 50 years suggest that the hydrological response to climate change can not be analyzed irrespective of other concurrent changes, and primarily ecosystem dynamics and land cover changes. There is no consensus on future rainfall trend over West Africa in IPCC projections, although a higher occurrence of extreme events (rainstorms, dry spells) is expected. An increase in the need for arable land and water resources is expected as well, driven by economic development and demographic growth. Based on past long-term observations on the AMMA-CATCH observatory, we explore in this work various future combinations of climate vs environmental drivers, and we infer the expected resulting trends on water resources, along the west African eco-climatic gradient. (Texte intégral