In the present study, the powder metallurgy was evaluated as a technique to produce high performance Al &amp / #8211 / 8Fe &amp / #8211 / 1.7V &amp / #8211 / 7.9Si (wt%) alloys for elevated temperature applications and the role of powder particle size range and extrusion ratio in the microstructural and mechanical properties of the extruded alloys was investigated. For this purpose, an air atomization method was employed to produce powders of the high temperature alloy and after that the produced powders were sieved and cold compacted. The compacted billets were subsequently hot extruded at 450 &amp / #8211 / 480 &deg / C. Five selected ranges of powders which were different in particle size (&amp / #8722 / 2000+212 &amp / #956 / m, &amp / #8722 / 212+150 &amp / #956 / m, &amp / #8722 / 150+106 &amp / #956 / m, &amp / #8722 / 106+90 &amp / #956 / m, and &amp / #8722 / 90 &amp / #956 / m) and three different extrusion ratios (144:1, 81:1, and 26:1) were used in this study. In the first part of the thesis, microstructure and thermal stability of as &amp / #8211 / air atomized powders were described. &amp / #945 / &amp / #8211 / Al matrix and &amp / #945 / &amp / #8211 / Al13(Fe, V)3Si phases were characterized in all rapidly solidified powders by XRD. The fraction of the intermetallic phases was reduced as the powder particle size increased. DTA analysis revealed an exothermic reaction at 581 &deg / C in all alloy powders of different size fractions. In the second part of the study, the effect of powder particle size and extrusion ratio on microstructural and mechanical properties (at different temperatures) of the extruded alloys was investigated. The results showed that decrease in powder particle size and increase in extrusion ratio refined the microstructure and improved the mechanical properties. It was revealed that the effect of powder size was more evident than that of extrusion ratio. Remarkable increases in mechanical properties (e.g. 60.7% increase in ultimate tensile strength at 250 &deg / C) were observed as a result of rapid solidification process (atomization) and hot extrusion.

<p>This work presents a study and implementation of the simulation of residual stresses in castings. The objects of study are a cast iron truck Hub part (provided by the company Volvo 3P) and an optimized version of the Hub resulting from the application of a topology optimization process. The models are solved through an uncoupled thermo-mechanical solidification analysis, performed both in the FE commercial software Abaqus and the FD commercial software Magmasoft and the results are compared. First, a thermal analysis is carried out where the casting is cooled down from a super-heated temperature to room temperature. The thermal history obtained, is then used as an external force to calculate the residual stresses by means of a quasi-static mechanical analysis, using a J2-plasticity model. The simulation procedures are explained through a simplified model of the Hub and then applied to the geometries of interest. A results comparison between the original Hub and its optimized version is also presented. The theoretical base is given in this work as well as detailed implementation procedures. The results shows that the part subjected to the topology optimization process develop less residual stresses than its original version.</p>

Inconel 738 (IN 738), like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes. The cause of this cracking, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion along one of the solid-liquid interfaces due to on-cooling tensile stresses. To address the problem of liquation cracking in weldments, recent developments in welding research have resulted in supposedly exclusive solid-state friction joining processes, such as linear friction welding (LFW), for joining crack susceptible structural alloys. The objective of this work was therefore to investigate the weldability of the difficult-to-weld IN 738 superalloy by LFW and to analyze the resulting microstructural changes in the alloy due to the welding process. LFW was performed on Linear Friction Welding Process Development System (PDS) at the Aerospace Manufacturing Technology Centre of the Institute for Aerospace Research, National Research Council (NRC) of Canada. In order to study and decouple the effect of non-equilibrium thermal cycle and imposed compressive stress during the joining, physical simulation of the LFW process was performed by using Gleeble 1500-D Thermo-Mechanical Simulation System at the University of Manitoba. Detailed microstructural study of welded and Gleeble-simulated materials was carried out. Correlation between the simulated microstructure and that of the weldments was obtained, in that, a significant grain boundary liquation was observed in both the simulated specimens and actual weldments due to non-equilibrium reaction of second phase particles, including the strengthening gamma prime phase. These results show that in contrast to the general assumption of LFW being an exclusively solid-state joining process, intergranular liquation, caused by non-equilibrium phase reaction(s), occurred during the process. However, despite a significant occurrence of liquation in the alloy, no HAZ cracking was observed. Nevertheless, the result showed that crack-free welding by linear friction welding is not due to preclusion of grain boundary liquation as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crack-formation effect of the imposed strain and to a concept observed and reported for the first time in this work, which is strain-induced rapid solidification. Furthermore, microstructural evolution during joining cannot be understood without considering the concept of non-equilibrium liquation reaction and strain-induced rapid solidification of the metastable liquid, which are carefully elucidated in this thesis.

Lorsqu'un champ magnétique est appliqué au cours de la solidification directionnelle, une convection dans la phase liquide peut être induite par l'effet thermoélectrique. En effet la présence d'un gradient de température le long du front de solidification peut provoquer la circulation du courant thermoélectrique, qui interagit avec le champ magnétique appliqué pour créer un écoulement (convection thermo électromagnétique-TEMC). Les conditions de transport de soluté et de l'énergie sont affectées par cette convection, donc il y a influence sur l'espacement des dendrites et la macro-ségrégation des composants de l'alliage. Dans ce travail, l'influence du champ magnétique sur la solidification directionnelle d'alliages métalliques est étudiée. Des travaux expérimentaux de la solidification directionnelle de Sn-Pb et Sn-Bi alliages sont réalisés. La solidification directionnelle dans la configuration Bridgman est effectuée avec ou sans champ magnétique appliqué. L'influence, sur la solidification, du champ magnétique et d'un courant électrique (AC et DC) appliqués est étudiée. Les mouvements du liquide provoquent de fortes macro-ségrégations ainsi qu'un modification des espacements interdendritiques. Les résultats expérimentaux sont interprétés à la lumière d'une modélisation heuristique. Le cas d'un champ magnétique tournant a été aussi étudié. Ainsi, la valeur de la rotation du champ est choisie pour ralentir assez brassage électromagnétique sans pour autant supprimer les effets de TEMC. À faible vitesse de tirage et faible vitesse de rotation faible champ une macro-ségrégation en forme de spirale a pu être obtenue.

Ce travail traite de la thermohydraulique d'un bain de melt couplée à la physicochimie pour ladescription du comportement de mélanges de matériaux (non-eutectiques).On décrit le transitoire d'établissement de température dans un liquide avec dégagement de puissancevolumique en présence de solidification sur une paroi refroidie. Le modèle développé à cet effet estvalidé par rapport aux résultats des essais LIVE réalisés à KIT. Dans les conditions de ces essais onmontre que la température d'interface suit la température liquidus (correspondant à la composition dubain liquide) pendant le transitoire d'établissement de la température dans le bain et des croûtessolides.Par ailleurs, on propose un modèle d'interaction entre un liquide non-eutectique (soumis à dissipationvolumique de puissance) et une paroi fusible dont la température de fusion est inférieure à latempérature liquidus du bain. Les prédictions du modèle sont comparées aux résultats des essaisARTEMIS 2D. On en déduit une nouvelle formulation de la température d'interface (inférieure àliquidus température) entre le liquide et la couche pâteuse en paroi.

Inconel 738 (IN 738), like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes. The cause of this cracking, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion along one of the solid-liquid interfaces due to on-cooling tensile stresses. To address the problem of liquation cracking in weldments, recent developments in welding research have resulted in supposedly exclusive solid-state friction joining processes, such as linear friction welding (LFW), for joining crack susceptible structural alloys. The objective of this work was therefore to investigate the weldability of the difficult-to-weld IN 738 superalloy by LFW and to analyze the resulting microstructural changes in the alloy due to the welding process. LFW was performed on Linear Friction Welding Process Development System (PDS) at the Aerospace Manufacturing Technology Centre of the Institute for Aerospace Research, National Research Council (NRC) of Canada. In order to study and decouple the effect of non-equilibrium thermal cycle and imposed compressive stress during the joining, physical simulation of the LFW process was performed by using Gleeble 1500-D Thermo-Mechanical Simulation System at the University of Manitoba. Detailed microstructural study of welded and Gleeble-simulated materials was carried out. Correlation between the simulated microstructure and that of the weldments was obtained, in that, a significant grain boundary liquation was observed in both the simulated specimens and actual weldments due to non-equilibrium reaction of second phase particles, including the strengthening gamma prime phase. These results show that in contrast to the general assumption of LFW being an exclusively solid-state joining process, intergranular liquation, caused by non-equilibrium phase reaction(s), occurred during the process. However, despite a significant occurrence of liquation in the alloy, no HAZ cracking was observed. Nevertheless, the result showed that crack-free welding by linear friction welding is not due to preclusion of grain boundary liquation as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crack-formation effect of the imposed strain and to a concept observed and reported for the first time in this work, which is strain-induced rapid solidification. Furthermore, microstructural evolution during joining cannot be understood without considering the concept of non-equilibrium liquation reaction and strain-induced rapid solidification of the metastable liquid, which are carefully elucidated in this thesis.

In the course of this PhD thesis metastable Cu50Zr50-xTix (0≤ x ≤ 10) and (Cu0.5Zr0.5)100-xAlx (5 ≤ x ≤ 8) alloys were prepared and characterised in terms of phase formation, thermal behaviour, crystallisation kinetics and most importantly in terms of mechanical properties. The addition of Al clearly enhances the glass-forming ability although it does not affect the phase formation. This means that the Cu-Zr-Al system follows the characteristics of the binary Cu-Zr phase diagram, at least for Al additions up to 8 at.%. Conversely, the presence of at least 6 at.% Ti changes the crystallisation sequence of Cu50Zr50-xTix metallic glasses and a metastable C15 CuZrTi Laves phase (Fd-3m) precipitates prior to the equilibrium phases, Cu10Zr7 and CuZr2. A structurally related phase, i.e. the “big cube” phase (Cu4(Zr,Ti)2O, Fd-3m), crystallises in a first step when a significant amount of oxygen, on the order of several thousands of mass-ppm (parts per million), is added. Both phases, the C15 Laves as well as the big cube phase, contain pronounced icosahedral coordination and their formation might be related to an icosahedral-like short-range order of the as-cast glass. However, when the metallic glasses obey the phase formation as established in the binary Cu-Zr phase diagram, the short-range order seems to more closely resemble the coordination of the high-temperature equilibrium phase, B2 CuZr. During the tensile deformation of (Cu0.5Zr0.5)100-xAlx bulk metallic glasses where B2 CuZr nanocrystals precipitate polymorphically in the bulk and some of them undergo twinning, which is due to the shape memory effect inherent in B2 CuZr. Qualitatively, this unique deformation process can be understood in the framework of the potential energy landscape (PEL) model. The shear stress, applied by mechanically loading the material, softens the shear modulus, thus biasing structural rearrangements towards the more stable, crystalline state. One major prerequisite in this process is believed to be a B2-like short-range order of the glass in the as-cast state, which could account for the polymorphic precipitation of the B2 nanocrystals at a comparatively small amount of shear. Diffraction experiments using high-energy X-rays suggest that there might be a correlation between the B2 phase and the glass structure on a length-scale less than 4 Å. Additional corroboration for this finding comes from the fact that the interatomic distances of a Cu50Zr47.5Ti2.5 metallic glass are reduced by cold-rolling. Instead of experiencing shear-induced dilation, the atoms become more closely packed, indicating that the metallic glass is driven towards the more densely packed state associated with the more stable, crystalline state. It is noteworthy, that two Cu-Zr intermetallic compounds were identified to be plastically deformable. Cubic B2 CuZr undergoes a deformation-induced martensitic phase transformation to monoclinic B19’and B33 structures, resulting in transformation-induced plasticity (TRIP effect). On the other hand, tetragonal CuZr2 can also be deformed in compression up to a strain of 15%, yet, exhibiting a dislocation-borne deformation mechanism. The shear-induced nanocrystallisation and twinning seem to be competitive phenomena regarding shear band generation and propagation, which is why very few shear offsets, due to shear banding, can be observed at the surface of the bulk metallic glasses tested in quasistatic tension. The average distance between the crystalline precipitates is on the order of the typical shear band thickness (10 - 50 nm) meaning that an efficient interaction between nanocrystals and shear bands becomes feasible. Macroscopically, these microscopic processes reflect as an appreciable plastic strain combined with work hardening. When the same CuZr-based BMGs are tested in tension at room temperature and at high strain rate (10-2 s-1) there seems to be a “strain rate sensitivity”, which could be related to a crossover of the experimental time-scale and the time-scale of the intrinsic deformation processes (nanocrystallisation, twinning, shear band generation and propagation). However, further work is required to investigate the reasons for the varying slope in the elastic regime. As B2 CuZr is the phase, that competes with vitrification, it precipitates in a glassy matrix if the cooling rate is not sufficient to freeze the structure of the liquid completely. The pronounced work hardening and the plasticity of the B2 phase, which are a result of the deformation-induced martensitic transformation, leave their footprints in the stress-strain curves of these bulk metallic glass matrix composites. The behaviour of the yield strength as a function of the crystalline volume fraction can be captured by the rule of mixtures at low crystalline volume fractions and by the load bearing model at high crystalline volume fractions. In between both of these regions there is a transition caused by percolation (impingement) of the B2 crystals. Furthermore, the fracture strain can be modelled as a function of the crystalline volume fraction by a three-microstructural-element body and the results imply that the interface between B2 crystals and glassy matrix determines the plastic strain of the composites. The combination of shape memory crystals and a glassy matrix leads to a material with a markedly high yield strength and an enhanced plastic strain. In the CuZr-based metastable alloys investigated, there is an intimate relationship between the microstructure and the mechanical properties. The insights gained here should prove useful regarding the optimisation of the mechanical properties of bulk metallic glasses and bulk metallic glass composites.

Compared to more common used austenitic stainless steels, ferritic stainless steels contain very low amounts of the expensive alloying element Ni. In addition, they have good corrosion properties, but are sometimes suffering from poor weldability and bad mechanical properties. This is mainly due to the presence of large grains after casting and large grain growth during heat treatment or welding. Processes for reducing the grain size (grain refining) of metal alloys are widely known and proven before to be suitable for many alloys. A successful grain refining process can increase the strength of an alloy without decreasing the ductility. This can be achieved by different methods, such as rolling or cooling. In this work, the focus has been on studying the aspect from a metallurgist point of view, to analyse the possibilities to create small particles in the liquid stage to enhance the solidification. The focus has been on oxide and nitride formation for nucleation of smaller grains during solidification. This study was made by forming particles, develop the analysis methods and thereafter to study the effect of particles on the structure of a ferritic stainless steel. The particles were formed by additions of Ti, Ce and Zr in to a liquid Fe-20mass% Cr alloy containing different amounts of oxygen and nitrogen. The electrolytic extraction technique was used to achieve a good understanding of the reaction processes and the particles size, number, composition and morphology. The grain sizes and the particles were then studied in as-cast samples as well as in specimens heat treated for 60 minutes at 1200, 1300 and 1400°C in a Confocal Scanning Laser Microscope (CSLM). Also, based on Scanning Electron Microscope (SEM) determinations and Thermo-Calc calculations, the precipitated particles were divided as primary and secondary particles. Thereafter, the grain refining potential was studied for each of these types. In this work, particles engineering by using small particles have been proven suitable for the Fe-20mass% Cr ferritic stainless steel alloys. Although the work has been based on small-scale samples and experiments, the results show good tendencies with respect to grain refining as well as a clear relationship between the particles in the steels and the microstructures. It was found that Ti-Ce additions in Fe-20mass% Cr alloys resulted in complex oxides including Ti, Ce and Cr. These oxides were observed to have high agglomeration tendencies and therefore to form larger particles or clusters. The grain refining potential on the solidification structure was insignificant, despite a relatively large amount of particles. However, Ti-Zr additions in Fe- 20mass% Cr alloys on the other hand resulted in primary precipitated particles such as ZrO2 and ZrO2+ZrN. Furthermore, ZrN nucleated ferrite during solidification, which lead to a clearly observed grain refining effect. This effect was also increased with an increased number of particles. The amount of particles (primary and secondary) was also found to increase with an increased amount of nitrogen. Secondary particles (mostly TiN) were precipitated near grain boundaries, which lead to a location based pinning effect of the grain growth during heat treatment at 1200 °C. This pinning effect was increased by an increased nitrogen content and thereby an increased number of particles. Finally, the pinning effect was clearly reduced during heat treatment at 1400 °C. / QC20100524

Made available in DSpace on 2014-10-09T12:41:34Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:02Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Sedimenten i området Kolkajen-Ropsten, Stockholm, innehåller höga halter av tjäroljor till följd avtidigare produktion i gasverket på området. Stockholms stad planerar att utföra byggnation i detnuvarande vattenområdet, vilket ökar risken för spridning av föroreningar. För att skydda arbetandeunder planerad byggnation och framtida boende måste föroreningssituationen åtgärdas. Tjäroljornainnehåller höga halter av polyaromatiska kolväten (PAH) och det är föroreningen av PAH-16 som styråtgärdsbehovet för området. En metod som använts med god framgång i flera projekt i Norden ochandra delar av världen är stabilisering och solidifiering, s/s, av sediment. Metoden går ut på att blandain utvalda bindemedel i sedimenten som då härdar. Därigenom minskar föroreningens mobilitet och degeotekniska egenskaperna förbättras. Om kravet på skjuvhållfasthet (&gt;140 KPa), låg permeabilitet (10-8-10-9 m/s) samt minskad utlakning uppfylls kan de stabiliserade massorna utgöra grunden för planeradbyggnation.För att utvärdera möjligheterna av denna åtgärdsmetod analyserades en sats av prover från området föratt tillhandahålla information om platsspecifika parametrar. Analysen visade att sedimenten hade högvattenkvot och organiskt innehåll, dominerades av fina fraktioner samt en föroreningsgrad av PAH-16på 13000 mg/kg TS. Utifrån dessa parametrar och med stöd från en litteraturstudie valdes fyra receptpå bindemedel ut, med olika mängd bindemedel. De bindemedel som använts är: Bascement frånSkövde som är ett portland-flygaskecement, Monofill som består av 20 % cement och 80 % granuleratmasugnsslagg samt aktivt kol. De olika recepten blandades med sediment från området och lämnadesatt härda under 28 dygn. Efter härdningen utvärderades de stabiliserade massorna med avseende påminskad utlakning av PAH jämfört med obehandlat sediment, skjuvhållfasthet och permeabilitet.Analyser av de fyra blandningarna visade att kravet på skjuvhållfasthet och permeabilitet uppfylldesför samtliga tillsatser av bindemedel och överlag innebar större mängder tillsatt bindemedel högreskjuvhållfasthet och lägre permeabilitet. Resultaten från laktester indikerade att en större tillsats avaktivt kol minskade utlakningen av PAH.Tillsatsen av bindemedel som bedöms vara bäst lämpad för stabilisering av sedimenten från områdetvar en total mängd av cement och Monofill på 250 kg/m3 (30 % cement och 70 % Monofill) samt entillsats av aktivt kol på 5 % av TS.Inblandning i sedimenten enligt receptet ovan resulterade i en skjuvhållfasthet på 413 KPa samt enönskat låg permeabilitet. Utlakningen av PAH-16 reducerades med ~45 % jämfört med obehandlatsediment. / The sediments in the area Kolkajen-Ropsten, Stockholm, have shown high levels of tar oils due to thegasworks in the area. The city of Stockholm is planning construction in the area which increases therisk of contaminants spreading to the surrounding area. In order to protect workers during the plannedconstruction and future inhabitants, remediation has to be carried out. The tar oils have highconcentrations of polyaromatic hydrocarbons (PAH) and it is pollution of PAH-16 that dictates theneed for remediation.A remediation method that has been used in similar projects, with good results, is stabilization andsolidification (s/s) of the sediments. This is done by mixing binder materials with the sediments whichharden and it results in lowered mobility of the contaminants and improved geotechnical properties ofthe soil. If the shear strength is high enough (&gt;140 KPa), the permeability is low enough (10-8-10-9m/s) and the leaching is reduced sufficiently, the stabilized sediments can be used as a foundation.To evaluate the possibility of this remediation method, a batch of sediment samples from the area wasanalyzed to obtain information about site specific parameters. The analysis showed that the sedimentshad high water content and were rich in organic matter. They were dominated by small particlefractions and had a PAH-16 concentration of 13000 mg/kg DM. With regard to these site specificparameters and a literature study, four different recipes with different amounts of binders wereselected and tested in a laboratory study. The used binders were: cement from Skövde which is aportland-fly ash cement, Monofill composed of 20 % cement and 80 % granulated furnace slag, andaddition of activated carbon.The sediments were mixed with binders according to the recipes and left to harden for 28 days.Following the 28 days of hardening, the stabilized mixtures were evaluated with regard to: thereduction of leaching of PAH in comparison to the untreated sediments, shear strength, andpermeability.Analyses of the four mixtures showed that the desirable values of both shear strength and permeabilitywere achieved for all mixtures, and that a higher concentration of binders resulted in a higher shearstrength and lower permeability. The results from the leaching tests indicated that a higherconcentration of activated carbon resulted in a lowered leaching of PAH.The mixture of binders that was deemed to be most suitable for stabilizing the sediments from the areawas a total amount of cement and Monofill at 250 kg/m3 (30 % cement and 70 % Monofill) with anaddition of activated carbon at 5 % of DM (dry matter).Addition of this binder mixture to the sediments resulted in a shear strength of 413 KPa and a desiredlow permeability. The reduced leaching of PAH-16 was ~45 % compared to untreated sediments.