The deformation mechanism of an ultrafine grained (UFG) Al-Zn alloy has been studied. In this work, Al-15wt%Zn alloy was processed by equal channel angular extrusion (ECAE) route A at 100oC to achieve UFG structure. The deformation mechanism was studied by performing tensile test with various strain rates. Scanning electron microscopy and transmission electron microscopy were used to investigate the microstructure evolution in Al-15wt%Zn alloy with increasing ECAE passes. The observation indicated that the super saturated Al-Zn alloy would decompose and precipitate Zn particles during ECAE process. Increasing ECAE passes, the aluminum grain size was reduced, but the size of Zn particles was increased. However, the net effect of increasing ECAE passes is softening of this Al-Zn alloy. The tensile properties of the UFG Al-Zn alloy can be summarized as follows. (1)The UFG Al-Zn alloy possesses higher tensile strength and elongation as compared to commercial purity Al (AA1050). (2)The strain rate sensitivity of the UFG Al-Zn alloy increases significantly with increasing number of ECAE pass, which might be related to the refined aluminum grain size. After processed by 4-16 ECAE passes, the activation volume of the UFG Al-Zn alloy falls in the range of 25 b3~40 b3, which remains nearly constant value with increasing tensile strain. It is suggested that the controlling mechanism responsible for the tensile deformation of the UFG Al-Zn alloy might be related to a grain boundary mediated mechanism. (3)With increasing ECAE passes, the total tensile elongation of the UFG Al-Zn alloy increases but the uniform elongation show little change. This indicates that the increase in total elongation is mainly due to the contribution from an enhanced post-uniform elongation (PUE). It is suggested that the enhanced PUE might be related to the increase in strain rate sensitivity, which is resulted from the refinement of grain size. More detailed studies are needed to understand the deformation mechanism.

Ultrafine-grained(UFG) AZ31 Mg alloy was obtained by equal-channel angular extrusion(ECAE) and subsequent annealing at elevated temperatures. The basal texture component for ECAEed material is located on the Z plane of the ECAEed billets. Tensile tests were performed at temperatures between room temperature and 125¢J, and strain rates used ranging from 3*10-5 to 6*10-2 s-1. The experimental results showed that a high tensile yield stress of 394 MPa was obtained at room temperature under a strain rate of 3*10-3 s-1. Strengths of UFG AZ31 specimens were greatly improved due to grain refinement. It was found that strain rate sensitivity of UFG AZ31 alloy increased significantly from 0.024 to 0.321 with increasing temperature. The constant k of Hall-Petch equation, £m=£m0 +kd-1/2, decreased with increasing temperature, and decreasing strain rate. Negative k values were ontained at 75¢J and 100¢J under a strain rate 3*10-5 s-1. When compressed along X, Y and X45Z billet orientations, strain localization within shear bands was found in UFG AZ31 specimens. Shear bands are formed inclined near 45 to the compression axis. The smaller the grain size, the thinner the shear band. Different Hall-Petch constant k were found in specimens deformed along different orientations, which is caused by different deformation mechanisms. The formation of tension twins is the primary deformation mechanism for compressed X and Y samples, and basal slip is responsible for the deformation of X45Z sample. tension twins were found in 0.46 £gm grain size specimens.

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The present work aims to reveal the microstructural evolution and post-processing mechanical behavior of difficult-to-work alloys upon severe plastic deformation. Severe plastic deformation is applied using equal channel angular extrusion (ECAE) where billets are pressed through a 90o corner die achieving simple shear deformation. Three different materials are studied in this research, namely Ti-6Al-4V, Ti-6Al-4V reinforced with 10% TiC and AISI 316L stainless steel. Microstructure and mechanical properties of successfully extruded billets were reported using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tension and compression experiments and microhardness measurements. The effects of extrusion conditions (temperature and processing route) on the microstructure and mechanical properties are investigated. The underlying mechanisms responsible for observed mechanical behaviors are explored. It is seen that ECAE shear deformation leads to refinement in α plates and elimination of prior β boundaries in Ti-6Al-4V. Decreasing extrusion temperature and increasing number of passes decreases α plate size and grain size. Refined α grain size leads to a significant increase in tensile and compressive flow stresses at room temperature. Texture produced by ECAE has a pronounced effect on mechanical properties. Specifically it leads to tension/compression asymmetry in flow strengths and strain hardening coefficients may be described by the activation of differing slip systems under tension and compression loading. ECAE of Ti-6Al-4V+10%TiC samples also improved mechanical properties due to α plate size refinement. Nevertheless, further extrusion passes should be carried out for tailoring reinforcement size and distribution providing optimum strength and ductility. ECAE deformation of AISI 316L stainless steel at high homologous temperatures (0.55 to 0.60 Tm) results in deformation twinning as an effective deformation mechanism which is attributed to the effect of the high stress levels on the partial dislocation separation. Deformation twinning gives rise to high stress levels during post-processing room temperature tension and compression experiments by providing additional barriers to dislocation motion and decreasing the mean free path of dislocations. The highest tensile flow stress observed in the sample processed at 700 oC following one pass route A was on the order of 1200 MPa which is very high for 316L stainless steel. The ultimate goal of this study is to produce stabilized end microstructures with improved mechanical properties and demonstrate the applicability of ECAE on difficult-to-work alloys.

NiTiHf alloys have attracted considerable attention as potential high temperature Shape Memory Alloy (SMA) but the instability in transformation temperatures and significant irrecoverable strain during thermal cycling under constant stress remains a major concern. The main reason for irrecoverable strain and change in transformation temperatures as a function of thermal cycling can be attributed to dislocation formation due to relatively large volume change during transformation from austenite to martensite. The formation of dislocations decreases the elastic stored energy, and during back transformation a reduced amount of strain is recovered. All these observations can be attributed to relatively soft lattice that cannot accommodate volume change by other means. We have used Equal Channel Angular Extrusion (ECAE), hot rolling and marforming to strengthen the 49.8Ni-42.2Ti-8Hf (in at. %) material and to introduce desired texture to overcome these problems in NiTiHf alloys. ECAE offers the advantage of preserving billet cross-section and the application of various routes, which give us the possibility to introduce various texture components and grain morphologies. ECAE was performed using a die of 90º tool angle and was performed at high temperatures from 500ºC up to 650ºC. All extrusions went well at these temperatures. Minor surface cracks were observed only in the material extruded at 500 °C, possibly due to the non-isothermal nature of the extrusion. It is believed that these surface cracks can be eliminated during isothermal extrusion at this temperature. This result of improved formability of NiTiHf alloy using ECAE is significant because an earlier review of the formability of NiTiHf using 50% rolling reduction concluded that the minimum temperature for rolling NiTi12%Hf alloy without cracks is 700°C. The strain level imposed during one 90° ECAE pass is equivalent to 69% rolling reduction. Subsequent to ECAE processing, a reduction in irrecoverable strain from 0.6% to 0.21% and an increase in transformation strain from 1.25% to 2.18% were observed at a load of 100 MPa as compared to the homogenized material. The present results show that the ECAE process permits the strengthening of the material by work hardening, grain size reduction, homogeneous distribution of fine precipitates, and the introduction of texture in the material. These four factors contribute in the increase of stability of the material. In this thesis I will be discussing the improvement of mechanical behavior and stability of the material achieved after various passes of ECAE.

Equal-channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) to produce ultra-fine grain (UFG) material, and its principle and microstructural developments. The majority of papers on SPD materials have been devoted to the face centered cubic (FCC) structure materials such as Al, Cu and Ni. The UFG of high alloy ECAP processing has been difficult up to now, but we were successful in this study. Fe-20%Cr steel with extremely low C and N has different slip behavior from the FCC. The mechanical properties and corrosion resistance were investigated in term microstructural evolution during ECAP processing. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Aufgrund ihrer herausragenden Eigenschaften haben ultrafeinkörnige Werkstoffe, die aus konventionellen normalkörnigen Halbzeugen über eine extrem große Kaltverformung hergestellt wurden, in den letzten zwei Jahrzehnten zunehmend an Bedeutung erlangt. In der vorliegenden Arbeit wird das Fließspannungsverhalten eines Reinaluminiumwerkstoffes (EN AW-1070 – Al99,7) und einer ausscheidungshärtbaren Aluminiumlegierung (EN AW-6060 – AlMgSi) mit Korngrößen von bis zu 660 nm und 310 nm in einem weiten Bereich von Dehnungen und Dehnraten analysiert und mit den zzt. existierenden Modellvorstellungen zu den mikrostrukturellen Abläufen in Verbindung gebracht. Um die Voraussetzung zur Herstellung von ultrafeinkörnigen Werkstoffen zu schaffen, wurden mehrere Werkzeugprototypen für die ECAP-Umformung im Labormaßstab entwickelt und erprobt. Die Untersuchungen zum Fließspannungsverhalten erfolgten anhand von Zug- und Druckversuchen über insgesamt sieben Dekaden der Dehnrate bis in den Bereich der hochdynamischen Belastung von 10^3 s^-1. Die Tests zeigen, dass das Fließspannungsverhalten ultrafeinkörniger Aluminiumwerkstoffe vollständig mithilfe der thermisch aktivierbaren Mechanismen erklärbar ist, wobei Ausscheidungen eine wichtige Rolle spielen. / Because of their exceptional properties ultrafine-grained materials, processed from conventional polycrystalline materials by severe plastic deformation, have gained increasing scientific and industrial interest during the last two decades. Based on the concept of work-hardening for f.c.c. metals the commercially pure aluminium AA1070 (Al99,7 – soft annealed) and the aluminium alloy AA6060 (AlMgSi – peak aged) were investigated. ECAP was used to introduce very high strains and an ultrafine-grained microstructure with grain sizes down to 660 nm and 310 nm. Subsequently compression and tensile tests were performed in a wide range of strain rates over seven decades up to the range of impact loading of 10^3 s^-1. The results indicate that strain path and the corresponding dislocation structure is important for the post-ECAP yielding and the following hardening response. Furthermore the precipitates of the AA6060 clearly constrain the interactions of dislocations in work-hardening stage III – causing lower strain rate sensitivity. If compared to the AA1070 they avoid hardening in stage V where an additional rate and temperature depending effect contributes – caused by the interaction of deformation induced vacancies and dislocations. The results indicate that the strain-hardening behavior can be described by thermal activated mechanisms.

Made available in DSpace on 2017-07-21T20:42:32Z (GMT). No. of bitstreams: 1 Fernanda Aparecida Farias.pdf: 6839448 bytes, checksum: d86819dd1de8464678930b61469c884f (MD5) Previous issue date: 2009-12-11 / A duplex stainless steel with austenitic–ferritic structure, type S32205/S31803 was submitted to equal channel angular pressing, whose is based on very severe plastic deformation, at relatively low temperature, with no change in the dimensions along transversal section of the material. The processed material was heat treated at 1000° and 1200°C during 7.5, 15, 30 and 60 minutes, in order to evaluate the recovery aspects and recrystallization steps of the severely deformed duplex steel. The die was manufacturing from SAE 1045 steel (cemented, quenched and annealed) and heat treated and the plungers from tool steel. The extrusions were conducted at room temperature, using a universal type mechanical test machine, according three different pressing speeds: 1.5, 6.0 and 12.0 mm/min. The processing stress increase continuously during extrusion by friction forces between sample and die walls and very high pressing stress level was reached. The results show the increase of stress according to increase of speed processing. For 1,5 mm/min processing speed, the second pass could be done with maximum reached stress close to 3GPa. The friction forces promoted a very heterogeneous deformations process along the samples sections with increase of hardness from 257HV (start material) to 575HV (highest deformed point). This effect was observed by grains morphology and orientations changes after processing and along the sample dimensions. In the as heat treated samples surfaces two distinct structures type formation was verified with small surfaces positioned with well defined angles near of 90° and 109° between itself. The region with 90° between surfaces show a lot of cubic form structure probably as a consequence of dislocation pile up annihilation on the surface of body-centered-cubic phase (ferrite). In this crystalline structure, the slip system occurs mainly in the {110} planes which have 90º between then. In another side, the slip planes of face-centered-cubic structure slip systems (planes {111}) make 109º angles between then and consequently, dislocation annihilation on the surface may result in structures with this angles on the austenite regions. Sample surface observations detected “roof like” morphology with a well defined near 109º structures which suggest the dislocation annihilation on austenite. Another structure detected on the sample surface was the cold induced martensite. The preliminary evaluation of recrystallization of some processed and treated samples, at 1000°C for 7,5, 15 and 30 minutes identified located points of recrystallization in the grain contours. With increase of the time of heat treatment from 7,5 to 15 minutes, increase of recrystallization nuclei was detected, whereas after treatment of 30 minutes verifies increase of the size of crystals formed without expressive increase of the number of nucleis. / Este trabalho tem por objetivo processar um aço inoxidável dúplex de estrutura austenítica-ferrítica, tipo UNS S32205/S31803, através da extrusão em canal angular (ECA) e caracterizar o aço processado. O princípio da ECA está baseado na aplicação de uma deformação plástica severa, a uma temperatura relativamente baixa, sem nenhuma mudança nas dimensões da seção transversal do material. A matriz utilizada na extrusão foi construída a partir de um aço 1045, cementado e temperado, e os punções de aço ferramenta. A extrusão foi realizada a temperatura ambiente usando uma máquina de ensaios mecânicos, em três velocidades diferentes de aplicação da pressão de extrusão: 1,5, 6,0 e 12 mm/min. A tensão necessária para realizar a extrusão aumentou continuamente ao longo do processamento, atingindo valores próximos 3GPa. Isto é devido, principalmente, à força de atrito entre o corpo de prova e as paredes o canal da matriz. Os resultados também mostram que o aumento da tensão atingida durante o processamento está relacionado à velocidade de processamento e número de passes. A força de atrito promoveu grande heterogeneidade de deformação ao longo das seções das amostras com aumento de dureza de 257HV (material recebido) para 575HV(ponto que sofreu maior deformação). Este efeito foi observado pela morfologia e mudança de orientação dos grãos ao longo das dimensões da amostra processadas. Após o processamento, algumas amostras do material foram termicamente tratadas a 1000° e 1200°C por períodos 7,5, 15, 30 e 60 minutos. Na superfície das amostras processadas por 2 passes de ECA, a temperatura ambiente, com velocidade de 1,5 mm/min. e posterior tratamento térmico a 1200°C por 60 min, foi verificado formação de dois tipos de estruturas distintas com pequenas superfícies posicionadas com ângulos de 90° e ângulos próximos a 109° bem definidos entre si. A região de 90° entre as superfícies mostra uma estrutura predominante de formas cúbicas, provavelmente em conseqüências da aniquilação de empilhamento de discordância na superfície cúbica de corpo centrado da fase ferrita. Na estrutura cristalina, o sistema de deslizamento ocorre principalmente nos planos {110}, os quais têm ângulos de 90° entre si. Por outro lado, os planos de deslizamento da estrutura cúbica de face centrada, {111}, formam ângulos próximos a 109° entre eles e consequentemente, a aniquilação de discordância na superfície resulta em estruturas na região da austenita. A observação da superfície das amostras detectou uma morfologia semelhante a um telhado (“roof like”) com estruturas com ângulos próximos a 109°, os quais sugerem a aniquilação de discordância na austenita. Outra estrutura detectada nas superfícies das amostras processado por ECA com 1 passe a velocidade de 1,5mm/min. e posterior tratamento térmico a 1200°C durante 60 minutos, foi a martensita induzida por deformação a frio. A avaliação preliminar da recristalização de algumas amostras processadas por ECA com 1 passe a velocidade de 1,5mm/min., e posterior tratamento térmico a 1000°C por 7,5, 15 e 30 minutos identificou pontos localizados de recristalização nos contornos de grão. Com o aumento do tempo de tratamento térmico de 7,5 para 15 minutos, aumentou o número de núcleos de recristalização, ao passo que após tratamento por 30 minutos foi verificado aumento no tamanho dos cristais formados sem expressivo aumento o número de núcleos

Made available in DSpace on 2017-07-21T20:42:44Z (GMT). No. of bitstreams: 1 Marcos Pontes.pdf: 4006153 bytes, checksum: 42b95748e61f5bc3de1b41ca1fbd649b (MD5) Previous issue date: 2012-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The processing by equal channel angular extrusion of a duplex stainless steel UNS32205/S31803, with ferritic-austenitic structures, was studied, processing until four passes each sample. The reason of this processing was to promote a grain refinement in the material, to achieve ultrafined grain material by severe plastic deformation. Different number of passes were used in this processing to evaluate how the number of extrusions in the same sample influences the deformation imposed to the material. After processing, some different heat treatments were done in the samples to evaluate the recrystallization process in the processed material. In this study, it was possible to verify the increasing level of deformation in the material after every new processing in the sample, this was verified by the changes in grain orientation and by microstructure evaluation. It was possible to reduce the stress level to values that can avoid the breaking of punctures during processing, this allowed to process the material with a greater number of passes in a material with a high resistance like the duplex stainless steels. Strain bands were found in the structure of the processed material, this indicates high levels of deformation. / Foi realizado o estudo do processamento de um aço inoxidável dúplex SAF2205,com microestrutura ferrítica-austenítica, por extrusão em canal angular por até quatro passes. O objetivo deste processamento foi a obtenção de um refino de grãos no material, atingindo uma estrutura de grãos ultrafinos, a partir da aplicação de deformações plásticas severas. Foram realizados diferentes modos de processamento, variando o número de passes pela matriz e depois foi avaliado como este parâmetro influencia na deformação imposta ao material. Após o processamento por ECA (Extrusão por Canal Angular), foram realizados diferentes tratamentos térmicos para avaliar a cinética de recristalização do material. Foi possível verificar o nível crescente de deformação do material após cada passe, com o acompanhamento da orientação dos grãos e avaliações microestruturais. Foi possível reduzir os níveis de tensões o suficiente para evitar a quebra de punções durante o processamento, permitindo um número maior de passes para um material com a resistência do aço inoxidável dúplex. Foram verificadas bandas de deformação no material processado, indicando níveis elevados de deformação.

Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess several processing challenges due to their anisotropic structural response, the wide variety of deformation textures they exhibit, and limited ductility at room temperature. The aim of this work is to investigate, both experimentally and theoretically, the effect os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number of phases on the flow stress anisotropy and tension compression asymmetry, and the mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered cubic (fcc) phase. Mg and its alloys have high specific strength that can potentially meet the high demand for light weight structural materials and low fuelconsumption in transportation. Zn-Al alloys, on the other hand, can be potential substitutes for several ferrous and non-ferrous materials because of their good mechanical and tribological properties. Both alloys have been successfully processed using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic textures for revealing the relationship between microstructural parameters, crystallographic texture and resulting flow stress anisotropy at room temperature. For AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC) crystal plasticity model. The flow stress anisotropy and tension-compression (T/C) asymmetry of the as received and processed samples at room temperature were measured and predicted using the same VPSC model coupled with a dislocation-based hardening scheme. The governing mechanisms behind these phenomena are revealed as functions of grains size and crystallographic texture. It was found that the variation in flow stress anisotropy and T/C asymmetry among samples can be explained based on the texture that is generated after each processing path. Therefore, it is possible to control the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by controlling the processing route and number of passes, and the selection of processing conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard phase size, morphology, and distribution were found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples.