Dye tracer and time-lapse ground-penetrating radar (GPR) were used to image preferential flow paths in the shallow, unsaturated zone on hillslopes in two adjacent watersheds within the Susquehanna-Shale Hills Critical Zone Observatory (CZO). At each site we injected about 50 L of water mixed with brilliant blue dye (4 g/L) into a trench cut perpendicular to the slope (~1.0 m long by ~0.20 m wide by ~0.20 m deep) to create a line of infiltration. GPR (800 MHz antennae with constant offset) was used to monitor the movement of the dye tracer downslope on a 1.0 m x 2.0 m grid with a 0.05 m line spacing. The site was then excavated and the stained pathways photographed to document the dye movement. We saw a considerable difference in the pattern of shallow preferential flow between the two sites despite similar soil characteristics and slope position. Both sites showed dye penetrating down to saprolite (~0.40 m); however, lateral flow migration between the two sites was different. At the Missed Grouse field site, the lateral migration was ~0.55 m as an evenly dispersed plume, but at distance of 0.70 m a finger of dye was observed. At the Shale Hills field site, the total lateral flow was ~0.40 m, dye was barely visible until the excavation reached ~0.10 m, and there was more evidence of distinct fingering in the vertical direction. Based on laboratory and field experiments as well as processing of the radargrams, the following conclusions were drawn: 1) time-lapse GPR successfully delineated the extent of lateral flow, but the GPR resolution was insufficient to detect small fingers of dye; 2) there was not a distinct GPR reflection at the regolith-saprock boundary, but this interface could be estimated from the extent of signal attenuation; 3) the preliminary soil moisture conditions may explain differences in the extent of infiltration at the two sites; 4) rapid infiltration into the underlying saprock limited the extent of shallow lateral flow at both sites and can be seen using the mass balance calculation and the lateral extent of dye within the radargrams; and 5) variations in flow patterns were observed between sites with similar settings at Susquehanna-Shale Hills CZO. / Geology

Shallow groundwater flow in the critical zone of steep headwater mountain catchments is often assumed to mimic surface topography. However, groundwater flow is influenced by other variables, such as the elevation of the water table and subsurface hydraulic conductivity, which can result in temporal variations in both magnitude and direction of flow. In this study, I investigated the temporal variability of groundwater flow in the soil zone (solum) within the critical zone of a headwater catchment at the Hubbard Brook Experimental Forest in North Woodstock, NH. Groundwater levels were continuously monitored throughout several seasons (March 2019 to Jan 2020) in a network of wells comprising three hillslope transects within the upper hillslopes of the catchment. Five clusters of three wells per cluster were screened from 0.18 – 1.1 m depth at the base of the solum. Water levels were also monitored in five deeper wells, screened from 2.4 - 6.9 m depth within glacial sediments of the C horizon. I conducted 47 slug tests across the well network to determine hydraulic properties of the aquifer materials surrounding each well. In addition, our team conducted a large-scale auger investigation mapping soil horizon depths and thicknesses. Results show that the magnitude of hydraulic gradients and subsurface hydrologic fluxes varied at each site with respect to changing water-table elevation, having a maximum range of 0.12 m/m and 9.19 x 10-6 m/s, respectively. The direction of groundwater flow had an arithmetic mean deviating from surface topography by 2-10 degrees, and a total range that deviated from surface topography by as much as 51 degrees. During lower water table regimes, groundwater flow direction deviated from the ground surface, but under higher water table regimes, in response to recharge events, flow direction mimicked surface topography. Within most of the well clusters, there is an observable connection between the slope direction of the top of the C horizon and the direction of groundwater flow during lower water table regimes. Slug test results show the interquartile range of saturated hydraulic conductivity (Ksat) within the C horizon (1.5×10-7 to 9.8×10-7 m/s) is two orders of magnitude lower than the interquartile range of Ksat values within the solum (2.9×10-5 to 5.2×10-5 m/s). Thus, the C horizon is on average a confining unit relative to the solum that may constrict groundwater flow below the solum. Additionally, results from the larger scale auger investigation suggest that deviations in subsurface topography of the C horizon may be generalizable at the larger hillslope scale. Overall, these results indicate that 1) shallow groundwater flow direction and magnitude within this headwater catchment are dynamic and can deviate from surface topography, and 2) the subsurface topography of the C horizon can influence groundwater flow direction. These results imply that temporal dynamics of groundwater flow direction and magnitude should be considered when characterizing subsurface flow in critical zone studies. Additionally, knowledge of subsurface topography of confining units may provide constraints on the temporal variability of groundwater flow direction. / M.S. / Streams that originate at higher elevations (defined as headwater streams) are important drinking water sources and deliver water and nutrients to maintain freshwater ecosystems. Groundwater is a major source of water to these streams, but little is known about how groundwater flows in these areas. Scientists delineate watersheds (areas of land that drain water to the same point) using surface topography. This approach works well for surface water, but not as well for groundwater, as groundwater may not flow in the same direction as surface water. Thus, assuming that the ground-watershed is the same as the surface watershed can lead to errors in hydrologic studies. To obtain more accurate information about groundwater flow in headwater areas, I continuously measured groundwater levels in forest soils at the Hubbard Brook Experimental Forest in North Woodstock, NH. My main objective was to determine if there is variability in the direction and amount of groundwater flow. I also measured the characteristics of the soils to identify the thicknesses of soil units and the permeability of those units. I used these data to evaluate the relationship between groundwater flow direction, surface topography, and the permeability of soil units. Overall, I found that groundwater flow direction can differ significantly from surface topography, and groundwater flow direction was influenced by the groundwater levels. When groundwater levels were high (closer to the land surface), groundwater flow was generally in the same direction as surface topography. However, when groundwater levels were lower, flow direction typically followed the slope of the lowest permeability soil unit. These results suggest that scientists should not assume that groundwater flow follows the land surface topography and should directly measure groundwater levels to determine flow direction. In addition, results from this study show that characterizing soil permeability can help scientists make more accurate measurements of groundwater flow.

Student Number : 0201438A - PhD thesis - School of Geosciences - Faculty of Science / This project focuses on the Upper Critical and Lower Main Zones in the eastern Bushveld Complex, South Africa. Lithological and stratigraphic information show that there are distinct differences at this level between the eastern and western limbs of the complex. Geochemical studies are centred on the Merensky and Bastard Cyclic Units in which the platiniferous Merensky Reef occurs. A major geochemical hiatus occurs in the Bushveld Complex at the level of the platiniferous Merensky Reef, close to the Critical/Main Zone boundary. The origin of this hiatus and its relation to mineralisation has not been fully resolved. Geochemical parameters are investigated that allow minerals in the Merensky and Bastard Cyclic Units to be classified as originating from either Critical or Main Zone magmas. Modelling of element ratios (Ni/Y, Cr/Ni, Cr/Co, Y/Co, Cr/V, Co/V and Cr/MgO) demonstrates the varying reliability of using ratios as geochemcial tools to constrain magma influxes within a chamber. However, it is shown that the Cr/MgO ratio is effective in determining real differences across the Critical/Main Zone boundary that are independent of lithology. In addition, initial Sr isotope ratios for plagioclase are significantly different in Critical and Main Zone rocks. Geochemical data through the Merensky and Bastard Cyclic Units indicate that orthopyroxene that originated from magma with composition like that of the Critical Zone magma sometimes occurs together with plagioclase that originated from Main Zone magma. In detail, in the pyroxenite at the base of the Merensky Unit, both plagioclase and orthopyroxene display Critical Zone signatures, but in the overlying part of the Merensky Cyclic Unit, plagioclase increasingly shows a Main Zone signature, whereas orthopyroxene continues to display a Critical Zone signature. Similarly, in the Bastard pyroxenite, Sr isotopes and absolute Sr in plagioclase display a range of values from Main Zone to Critical Zone, but orthopyroxene consistently displays Critical Zone affinity. These observations of mineral disequilibrium clearly show that the two major minerals in the Merensky and Bastard Cyclic Units were formed from two different, but coexisting, magmas. A model that accounts for this disequilibrium is proposed here. It invokes the influx of Main Zone magma at the level of the base of the Merensky unit that dispalced the Critical Zone magma upward, but the two magmas did not mix. The latter continued to crystallise orthopyroxene which sank through the Main Zone influx, due to its density contrast. These crystals collected on the crystal pile to form the Merensky pyroxenite. The Main Zone magma, into which the cumulus Critical Zone orthopyroxene accumulated, crystallised interstitial plagioclase that had a Main Zone Sr isotopic ratio. Whole-rock, major element geochemical data show that a variable proportion of the plagioclase in both the Merensky and Bastard pyroxenites is cumulus. It is inferred to have accumulated with orthopyroxene and has a Critical Zone initial Sr isotope ratio. Thus the two pyroxenites now yield a mixed Sr isotopic signature of Critical Zone cumulus and Main Zone intercumulus and possibly cumulus plagioclase that varies along strike. Above the two pyroxenites, the Sr signature of the norites and anorthosites of both cyclic units is dominated by cumulus plagioclase from the Main Zone magma. It is concluded that the variations in initial Sr isotope ratios do not result from mixing of magmas, but result from accumulation of orthopyroxene and plagioclase from a higher, isotopically distinct layer of magma into an underlying layer. The Merensky and Bastard Cyclic Units therefore display features of Critical or Main Zone magma characteristics depending upon which chemical parameter is considered. These cycles are therefore classified as a Transitional Unit.

It is important to understand the processes that influence the critical zone (CZ) evolution to ensure its sustainability. This thesis reports on laboratory and field experiments designed to measure the behavior of biogenic and lithogenic chemical species and their interaction in the CZ from column to pedon to catchment scales. We postulated that interactions between organic matter and rock-derived metals drive coupled processes of carbon stabilization and chemical weathering and denudation in the Jemez River Basin Critical Zone Observatory (JRB-CZO). First, we observed that secondary mineral coatings (Al and Fe (oxy)hydroxides) on primary silicate surfaces play a major role in sequestering aromatic and "humified" dissolved organic matter (DOM) into sorbate form, significantly retarding their subsurface transport. Further, reinfusion to OM-reacted-porous-media of a different DOM source resulted in exchange reactions consistent with a zonal model of OM adsorption at mineral surfaces. This dissertation also aimed to examine the influence of water and DOM fluxes on the CZ weathering processes. Rare earth elements (REE) were selected because of their coherent trends in reactivity toward organic ligands common to soils. Specifically, trends in REE fractionation were explored for their utility to inform on biogeochemical weathering processes in forested terrain in the JRB-CZO. Mineral weathering mechanisms are expected to differentially influence REE release, fractionation, and transport and the relative importance of such processes should be reflected in REE signatures of bulk soil, pore and surface waters. Our studies showed: (1) REE depletion trends with depth in bulk soils are correlated with topographically-induced variation in water and dissolved organic carbon (DOC) flux (reflected in negative correlations between total water and C fluxes) and solid phase REE concentrations measured at the same depths; (2) REE and DOC concentrations in stream waters were strongly correlated during snowmelt periods of high discharge, consistent with REE complexation and mobilization in association with organic ligands during shallow subsurface flow; (3) preferential sequestration of Eu occurs during formation of secondary Mn(IV)-oxides, explaining patterns of Eu enrichment in bulk soils; and (4) the incremental increase in positive Ce-anomalies with depth in bulk soils are apparently controlled by adsorption/co-precipitation with secondary Fe-(oxy)hydroxide minerals.

High latitude and altitude ecosystems are currently undergoing rapid and unprecedented warming in response to anthropogenically induced climate change. Subarctic, alpine regions are particularly vulnerable to increases in air temperature and changing precipitation regimes, which have caused cascading hydrological and ecological impacts. In addition to changing flow regimes, thawing permafrost, and declining glaciers, widespread changes in vegetation composition, density and distribution have been observed across northern regions. Specifically, treeline is advancing with increasing latitude and altitude and shrubs are increasing in height, extent, and density. Despite widespread documentation of this northern greening, few field-based studies have evaluated the hydrological implications of these changes. Quantification of total evapotranspiration (ET) across a range of vegetation gradients is essential for predicting water yield, yet challenging in cold alpine catchments due to heterogeneous land cover. Direct field-level measurements of transpiration (T) and evaporative partitioning across subarctic, alpine ecosystems and species are rare, yet essential to assess sensitivities and hydrological response to changing climate drivers. This thesis presents six years of surface energy balance components and ET dynamics and two years of sap flux measurements and critical zone stable water isotope sampling at three sites along an elevational gradient in a subarctic, alpine catchment near Whitehorse, Yukon Territory, Canada. These sites span a gradient of thermal and vegetation regimes, providing a space-for-time comparison for future ecosystem shifts: 1) a low-elevation boreal white spruce forest (~12-20 m), 2) a mid-elevation subalpine taiga comprised of tall, dense willow (Salix) and birch (Betula) shrubs (~1-3 m) and 3) a high-elevation subalpine taiga with short, sparse shrub cover (< 0.75 m) and moss, lichen, and bare rock. We utilize both mass flux measurements and stable water isotopes to evaluate the timing, magnitude, sensitivities, and sources of plant water uptake across these vegetation covers. Total ET decreased and interannual variability increased with elevation, with mean May to September ET totals of 349 (±3) mm at the forest, 249 (±10) mm at the tall, dense shrub site, and 240 (± 26) mm at the short, sparse shrub site. The shrub sites exhibited similar ET losses over 6 years despite differences in shrub height and abundance, although daily rates were higher at the tall shrub site in the peak growing season. From May to September, ET:R ratios were the highest and most variable at the forest (2.19 ± 0.37) and similar at the tall, dense shrub (1.22 ± 0.09) and short, sparse shrub (1.14 ± 0.05) sites. In the mid-growing season, mean T rates were greater at the dense shrub site (2.0 ± 0.75 mm d-1) than the forest (1.47 ± 0.52 mm d-1). During this time, T:ET was lower at the forest (0.48) than at the tall, dense shrub site (0.80). During the growing season between the two years, 2020 was considerably wetter and cooler than 2019. At the tall shrub site, during the mid-growing season (July 1-Aug 15), T dropped considerably in 2020 (-26%), as T was suppressed during the short, wet growing season. In contrast, T at the forest was only moderately suppressed (-3%) between years in this same period. Evapotranspiration was more strongly controlled by air temperature during the early and late season at the forest, while ET at the shrub site was more sensitive to warmer temperatures in the mid-growing season. At the shrub sites, ET was energy limited with no observed soil moisture limitation on T. While 2H and 18O of volume weighted precipitation became more depleted with elevation, the opposite was true in xylem water, where 2H and 18O became more enriched with elevation. Plant water uptake was more reflective of snow water at the forest site than both shrub sites, particularly early in the year and during dry periods. Near-surface bulk soil water had more negative lc-excess at the forest throughout the season and with depth, highlighting increased contributions from soil evaporation. This study combined direct measurements of sap flux, ET, and critical zone isotopes to provide new details on multi-year plant-soil-water dynamics, critical zone water cycling, and species-specific plant water uptake patterns in seasonally frozen soils, which have not previously been reported in cold regions. Our results suggest that advances in treeline will increase overall ET and lower interannual variability; however, the large growing season water deficit and stable water isotope signature at the forest indicates strong reliance on soil moisture from late fall and snowmelt recharge and the potential for plant water stress. Differences between the shrub species were apparent in the sap flux and stable isotope measurements, highlighting the need to further evaluate species specific responses and feedbacks when predicting hydrological fluxes across subarctic ecosystems. Overall, our results suggest that predicted changes in vegetation type and structure in northern regions will have a considerable impact on water partitioning and will vary in a complex way in response to changing precipitation timing, phase and magnitude. / Thesis / Candidate in Philosophy

Les interactions entre les eaux souterraines et les eaux de surface sont complexes et jouent un rôle prépondérant dans le fonctionnement des hydrosystèmes, tant en termes quantitatifs que qualitatifs. Ces deux compartiments interagissent à travers différentes interfaces emboitées depuis l’échelle locale, notamment la zone d’interface rivière-nappe, appelée zone hyporhéique (ZH) jusqu’à l’échelle régionale. La ZH se définit comme un buffer entre les eaux souterraines et les eaux de surface. Les variations des flux d’eau et de chaleur au niveau de cette zone modulent les processus biogéochimiques. Le but de cette thèse est d’améliorer la compréhension du régime thermique et hydrologique de la ZH et de sa variation spatiale. Cette problématique est abordée par l’utilisation de stations de mesures haute fréquence des échanges nappe-rivière (MOLONARI) sur le bassin des Avenelles (sous bassin de la Seine (46 km2)) et de la modélisation. Le suivi haute fréquence a permis de réaliser une analyse des données expérimentales présentant le régime hydro-climatique du bassin ainsi que l’évolution spatiale et temporelle du fonctionnement des différents compartiments de l’hydrosystème. Un cas synthétique a été réalisé afin de caractériser les différents facteurs contrôlant le régime thermique de l’interface nappe rivière et la variation de stocks d’énergie au sein de la ZH. La démarche de modélisation suivie pour le cas synthétique fournit un cadre d’analyse des données des cinq stations MOLONARI. Les données acquises sur ces stations ont été utilisées afin de déterminer les propriétés hydrodynamiques et thermiques de la ZH ainsi que des couches géologiques sous-jacentes par inversion à l’aide du modèle METIS couplé à un script de balayage de l’espace des paramètres afin de quantifier les flux d’eau et de chaleur à l’interface nappe-rivière le long d'un corridor hydraulique. Les résultats du modèle mettent en lumière la variabilité spatio-temporelle des échanges de chaleur au niveau des cinq stations MOLONARI. La quantification des flux de chaleur a fourni un nouvel éclairage des interactions entre les eaux de surface et les eaux souterraines à l’interface nappe-rivière du bassin des Avenelles. / The river-aquifer interactions are complex and play a preponderant role in hydrosystems functioning, in both qualitative and quantitative terms. These two compartments interact through various nested interfaces from the local scale in particular the river aquifer interface called the Hyporheic Zone (HZ) to the regional scale. The HZ acts as a buffer between the stream and the groundwater. The water and heat fluxes variation at this zone modulates the biogeochemical processes.The aim of this thesis, is to improve the understanding of the thermal and hydrological regime of the HZ and its spatial variation. This problem is approached by the use of high frequency measurement stations of river-aquifer exchanges (LOMOS) in the Avenelles basin (sub-basin of the Seine basin (46 km2)) and by modeling. The high frequency monitoring allowed to realize an analysis of the experimental data characterizing the hydroclimatic regime of the Avenelles basin as well as the spatial and temporal evolution of the hydrosystem various compartments functioning.A synthetic case was realized to characterize the main factors controlling the thermal regime at the river aquifer interface and the energy storage variation within the ZH. The modeling approach followed by the synthetic case provides an analysis framework of the five LOMOS data.The acquired LOMOS data were used to determine the hydrogeological and thermal properties of the HZ and of the underlying aquifers by inversion by means of a 2D finite element thermo-hydrogeological model (METIS) coupled with a parameters screening script to quantify water and heat fluxes through the stream – aquifer interface over along the stream network.The model results highlight the spatiotemporal variability of the heat exchanges at the five LOMOS. The quantification of heat fluxes provided a new lighting of the stream-aquifer interactions of the Avenelles basin.

Le transport de contaminants, l’altération des roches ainsi que les grands cycles biogéochimiques sont contrôlés par les temps de séjour de l’eau. Ces temps de séjour représentent le temps de transit durant lequel l’eau « voyage » dans un bassin versant. Contraindre ces temps de transit est donc un enjeu essentiel pour quantifier l’impact de l’homme sur la qualité de l’eau en rivières et dans les aquifères et pour évaluer la résilience des écosystèmes aquatiques continentaux. Cependant, les rivières comme les nappes phréatiques sont constituées d’un mélange d’eau de différents âges (une distribution des temps de transit ou des temps de résidence) qui varie avec le temps, en fonction des aléas météorologiques et climatiques, rendant difficile leur caractérisation ainsi que leur prédiction. Dans cette thèse, nous inférons ces temps de résidence à l’aide de traceurs géochimiques et de modèles guidés par les données. Nous montrons comment cette connaissance permet de quantifier l’altération des roches cristallines. Nous développons ensuite un cadre original de modélisation à base physique, capable de représenter la variabilité saisonnière et interannuelle des débits et des temps de transit mesurés en rivière. Nous montrons comment le processus de battements de nappes et son interaction avec les couches perméables du sol mène à la génération d’un ruissellement qui explique les fluctuations saisonnières de qualité de l’eau en rivières, traduites par des mesures de silice dissoute. Enfin, nous esquissons un cadre général de représentation de la réactivité à l’échelle du versant capable de rendre compte des processus biogéochimiques. En effet, représenter la dégradation des éléments réactifs (oxygène, nitrates, carbone) permettra d’évaluer les mesures de réduction d’intrants agricoles, de prédire l’évolution long terme de ces solutés en rivières, et donc leur potentiel d’eutrophisation ainsi que d’évaluer des mesures pour réconcilier agriculture et environnement. Cette réactivité apparaît comme le dernier maillon manquant pour comprendre, mesurer et prédire, les impacts anthropiques sur la zone critique. / Groundwater travel time controls contaminant transport, weathering processes and biogeochemical cycles. Groundwater travel time is a fundamental descriptor characterizing the transit time of water inside the catchment, from precipitation events to the streams. Quantifying these transit times is pivotal to predict the impact of anthropogenic pressure and assess freshwater ecosystems resilience. However, streamwater and groundwater are a mixture of water of different ages (the transit time and the residence time distribution), which vary according to climatic forcings. This makes difficult its characterization and prediction. Here we infer residence times with geochemical tracers and data-driven models. We show how this can be constrained by silicate weathering at the catchment scale. We then develop a novel process-based framework, which can model discharge and transit time seasonal and interannual variabilities. We identify water table fluctuations, its interaction with permeable soil layers and the resulting subsurface stormflow generation as a key process for seasonal water quality variations described by dissolved silica measurements. Finally, we draw a reactivity framework to represent biogeochemical processes. Indeed, evaluating reactive solute degradation is needed to assess the efficiency of reducing fertilizer loads, to predict the long term evolution of in stream solute concentrations and the eutrophication potential of freshwater bodies. Modeling the reactivity at the catchment scale is the missing link to understand, quantify and predict the effect of anthropogenic pressure on the critical zone.

L’altération des roches silicatées constitue le dénominateur commun d’une multitude de problématiques environnementales et sociétales. Du fait de la difficulté d’extrapoler au milieu naturel les cinétiques de dissolution des minéraux mesurées in vitro, cette thèse propose de réviser en profondeur l’approche actuelle de la réactivité minérale du laboratoire au terrain. Ce travail démontre que l’évolution intrinsèque des propriétés texturales et structurales de l’interface réactive au cours de la dissolution induit des variations de vitesse qui ne peuvent être expliquées dans le cadre des théories thermocinétiques classiques. Nous proposons une nouvelle méthode permettant de sonder la réactivité biogéochimique des minéraux sur le terrain et de révéler les interactions réciproques entre le minéral et le monde microbien au sein de la minéralosphère. Nous démontrons la pertinence des phénomènes de passivation pour l’altération de surface et l’incapacité des microorganismes à les surmonter. / Chemical weathering of silicate minerals is central to numerous environmental and societal challenges. This study addresses the long-standing question of the inconsistency between field and laboratory estimates of dissolution kinetics, by revisiting current approaches of mineral reactivity. It is demonstrated that evolution of feldspar reaction rates are inaccurately describedby current kinetics rate laws, due to textural and structural changes occurring at the fluid-mineral interface over the course of the dissolution process. A novel method is developed to enable probing biogeochemical weathering rates in the field. Bacterial and fungal metagenomic data reveal that subtle reciprocal relationships are established between microorganisms and mineral substrates within the mineralosphere. This thesis emphasizes the impact of passivation phenomena on dissolution rates, under field-relevant reacting conditions and the incapacity of microorganisms to overcome the passivation barrier.

Made available in DSpace on 2016-12-08T17:19:17Z (GMT). No. of bitstreams: 1 0 Capa-Sumario.pdf: 106772 bytes, checksum: 6233f32589849e604817fec8f6305e39 (MD5) Previous issue date: 2008-02-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work examines the influence of austenitizing parameters on the microstructure of ductile cast iron austempered from the critical zone, called in this work as NAZC Nodular Austemperado a partir da Zona Crítica (ductile iron austempered from the critical zone). Initially a differential thermal analysis was performed to determinate the critical temperatures range, according to this analysis the critical zone is placed between 750 e 850o C. Alloyed ductile iron specimens were austenitized within the critical zone and water quenched to evaluate the amount of phases present in the final microstructure, proeutectoid ferrite and martensite. This preliminary study was conducted in order to quantify the amount of austenite (martensite at room temperature) which would become ausferrite later in the study of austempering. The treatment of austempering results in a dual microstructure of ausferrite and proeutectoid ferrite. The microstructural and quantitative analysis allowed the evaluation of the amount of phases formed, as well the evolution of microstructural transformation. This study was the basis to determine the parameters of austenitizing to austempering. Austempering treatments were performed under several conditions of austenitizing, until a microetruture related to a high mechanical strength and ductility were obtained. It s possible control the microetruture and consequently the properties from the control of the austenitizing temperature and time and austempering conditions. The microstructural analyses of austempered specimens showed that the austenitizing temperature within the critical zone of 790 °C and times of 3 and 4 hours led to a final microstructure formed by approximately 49 and 64% of ausferrite. An austempered material in these conditions of austenitizing should present an interesting set of mechanical properties, a mechanical strength next to the pearlitic grades associated with a considerable elongation, near the ferritic grades. The austempered materials from the critical zone of this study also present a good distribution between the phases proeutectoid ferrite and ausferrite, even near eutectic cell boundary and around graphite nodules, what must produce good properties of fatigue. / Este trabalho avalia a influência dos parâmetros de austenitização sobre a microestrutura do ferro fundido nodular austemperado a partir da zona crítica, denominado neste trabalho como NAZC - Nodular Austemperado a partir da Zona Crítica. Inicialmente foi realizada uma análise térmica diferencial para determinação da faixa de temperaturas crítica, conforme essa análise a zona crítica se situa entre 750 e 850o C. Amostras de ferro nodular com adição de elementos de liga foram austenitizadas dentro da zona crítica e temperadas em água para avaliação da quantidade de fases presentes na microestrutura final, ferrita pró-eutetóide e martensita. Esse estudo preliminar foi realizado a fim de quantificar a quantidade de austenita (martensita na temperatura ambiente), a qual se transformaria em ausferrita no estudo posterior de austêmpera. O tratamento de austêmpera resulta em uma microestrutura bifásica composta por ausferrita e ferrita pró-eutetóide. As análises microestruturais e quantitativa permitiram avaliar a quantidade de fases formadas, bem como a evolução da transformação microestrutural. Esse estudo foi a base para a determinação dos parâmetros de austenitização para austêmpera. Foram realizados tratamentos de austêmpera em diversas condições de austenitização, até que fosse obtida uma microestrutura relacionada à elevada resistência mecânica e ductilidade. É possível controlar a microestrutura e consequentemente as propriedades a partir do controle da temperatura e tempo de austenitização e das condições de austêmpera. As análises microestruturais de amostras austemperadas mostraram que a temperatura de austenitização dentro da zona crítica de 790o C e tempos de 3 e 4 horas levaram a uma microestrutura final formada por aproximadamente 49 e 64% de ausferrita. Um material austemperado nessas condições de austenitização deve apresentar um interessante conjunto de propriedades mecânicas, ou seja, uma resistência mecânica próxima a de classes perlíticas associada a um considerável alongamento, próximo ao de classes ferríticas. Os materiais austemperados a partir da zona crítica deste estudo também apresentam uma boa distribuição entre as fases ferrita pró-eutetóide e ausferrita, mesmo próxima aos contornos de célula e nódulos de grafita, o que deve conferir boas propriedades de fadiga.

Made available in DSpace on 2016-12-08T17:19:36Z (GMT). No. of bitstreams: 1 0 Capa - 1 Introducao.pdf: 288577 bytes, checksum: 440b6dcfb8697dcbdfbfecd21ba397c1 (MD5) Previous issue date: 2010-07-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work is part of a research project at UDESC, which aims to understand and compare the process of fracture of ADI with dual matrix structure, such as sites of plastic deformation concentration and crack nucleation, and paths of crack propagation. The fracture mechanisms of two types of duals ADI with different ausferrite volume fraction were compared to those of conventional ADI. Intercritical austenitising temperatures chosen were 790°C and 820°C, and resulted in dual ADI with 17% and 85% of ausferrite respectively. The full ADI, which was austenitised at 900°C, was used to serve as reference material. The three materials were austempered at 360°C for 2h. Results showed that in dual ADI with predominantly ferrite, the nucleation of cracks occurred along the interface graphite/matrix. Increasing the ausferrite volume fraction, the nucleation of cracks tended to occur along the inclusions and intercellular boundaries. Moreover, in dual ADI, the crack propagation and concentration of plastic deformation occurred preferentially in the ferrite. With respect to the fracture´s analysis, all materials showed that the quantities of fracture ductile/fragile were equivalent. The fractures had an aspect predominantly ductile, being formed mostly by dimples with some regions of quasi-cleavage. / Este trabalho faz parte de uma linha de pesquisa executada na UDESC, que visa entender e comparar o processo de fratura dos ADI duais, tais como, locais de concentração de deformação plástica e de nucleação de trincas, e caminhos de propagação de trinca. Os mecanismos de fratura de dois tipos de ADI duais com diferente fração volumétrica de ausferrita foram comparados com o ADI pleno. As temperaturas de austenitização intercrítica escolhidas foram 790°C e 820°C, e resultaram em ADI duais com 17 e 85% de ausferrita respectivamente. O ADI pleno, no qual foi austenitizado a 900°C, foi utilizado para servir de material de referência. Os três materiais foram austemperados a 360°C por 2h. Verificou-se que para o ADI dual com predominância de ferrita, a nucleação da trinca ocorreu junto à interface grafita/matriz. Conforme aumentou a fração volumétrica de ausferrita na matriz, a nucleação da trinca tendeu a ocorrer junto à inclusão intercelular e em contornos de célula. Além disso, nos ADI duais, a propagação da trinca e a concentração de deformação plástica ocorreu preferencialmente na ferrita. Vale ressaltar que os três materiais apresentaram quantidades de fratura dúctil/frágil equivalentes. As fraturas tinham uma característica predominantemente dúctil, sendo formadas na maioria por alvéolos com algumas regiões de quase-clivagem.