Modified 9Cr-1Mo ferritic-martensitic steel (T91) has been extensively investigated as a structural material for GenIV nuclear reactors and Accelerator Driven Transmutation systems. One attractive characteristic of this steel in these applications is its superior radiation damage tolerance in comparison to typical austenitic stainless steels such as 316L. In some GenIV applications, it also has a significantly higher corrosion resistance. Further improvement of both is necessary if GenIV designs are to become commercially viable. Other work has shown an improvement in radiation damage tolerance via cold rolling or sputtering nanoscale multilayered films. Additionally, corrosion resistance can be improved by homogenizing the microstructure. Further, these changes can improve the strength of the material. However, there has been no fabrication of bulk ultra fine grain ferritic-martensitic steel candidates that might offer these avenues of improvement. This work demonstrates the refinement and homogenization of T91 by Equal Channel Angular Extrusion (ECAE) and heat treatment. Processing temperature and strain level were varied to produce multiple levels of refinement. Materials were characterized by microhardness, tensile testing, x-ray diffraction and transmission electron microscopy. An ultra-fine, highly misoriented and homogeneous microstructure was achieved in the material. Refinement was demonstrated both in ferritic and ferritic-martensitic compositions of the steel. Microhardness increased by as much as 70% and ultimate tensile strength by 80%. More significantly, tensile strength was improved by 40% without decreasing ductility.

There have been numerous efforts in processing metallic alloys into fine-grained materials, so as to exhibit high strain rate superplasticity (HSRSP) and/or low temperature superplasticity (LTSP). The current study is to apply the most simple and feasible one-step extrusion method on the commercial AZ31 magnesium billet to result in low temperature and high strain rate superplasticity (LT&HSRSP). The one-step extrusion was undertaken using a high extrusion ratio at 250-350oC, and the grain size after one-step extrusion became ~1-4 mm. The processed AZ31 plate exhibited high room temperature tensile elongation up to 50%, as well as superior LTSP and/or HSRSP up to 1000%. Meanwhile, the AZ31 alloy was also conducted by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of 461% may be attained at a temperature of 150oC, equivalent to 0.46 Tm where Tm is the absolute melting temperature. This result clearly demonstrates the potential for achieving low temperature superplasticity. A detailed investigation, using x-ray diffraction (XRD), electron back scattering diffraction (EBSD), and transmission electron microscopy / selected area diffraction (TEM/SAD), revealed different textures in the as-extruded and as-ECAP bars. These dominant textures were characteristic of <10 0>//ED in the extruded bars and < 76>//ED in the ECAP condition, where ED is the extrusion direction. The results show that the basal planes tend to lie parallel to the extrusion axis in the extruded bars but there is a rearrangement during ECAP and the basal planes become reasonably aligned with the theoretical shearing plane. As to the extruded plates, the {0002} planes tended to lie on the plane that contains the extrusion axis. At different tensile temperatures, different deformation mechanisms would be dominant. Over the lower loading temperatures within 150-200oC, the true strain rate sensitivity, mt, after extracting the threshold stress is determined to be 0.28, suggesting that power-law dislocation creep but the Qt value is not related to any creep mechanism. It should be partly due to thermal activated dislocation slip mechanism. However, more data need to be tested systematically this part in the future study in order to define the correct deformation mechanism. As to the loading temperatures over 250-300oC, the mt value and the true activation energy for the extruded specimens are calculated to be ~0.4-0.5 and ~90-100 kJ/mol, implying that the major deformation mechanism is grain boundary sliding plus minor solute drag creep, with the rate controlling diffusion step being the magnesium grain boundary diffusion.

Diploma thesis is focused on desing of the screw press, which is used for laboratory testing of materials by ECAP. Thesis included design of individual scructural groups and their destcription, including the creating of 3D model.

NiAl ist eine intermetallische Verbindung mit kubischer B2-Struktur. Unterhalb der Spröd-duktil-Übergangstemperatur, die bei Normaldruck bei etwa 300°C liegt, können nach dem Von-Mises-Kriterium nicht ausreichend viele unabhängige Gleitsysteme zur homogenen plastischen Verformung aktiviert werden, da die Bruchspannung unterhalb der kritischen Schubspannung zur Aktivierung des sekundären Gleitsystems liegt. Um NiAl trotzdem massiv plastisch verformen zu können, muss die Verformung bei hohem Druck erfolgen. Dies ist beispielsweise möglich bei der Verformung mittels Gleichkanal- Winkelpressen (ECAP) oder Hochdrucktorsion (HPT). Hierbei werden sehr hohe Verformungsgrade erreicht. In dieser Arbeit wird nun die Gefüge- und Texturentwicklung bei Verformung mittels HPT bei Temperaturen zwischen Raumtemperatur und 500°C in Abhängigkeit von Temperatur und hydrostatischem Druck sowie Verformungsgrad untersucht und mit den entsprechenden Größen bei der Verformung mit ECAP verglichen. Es zeigt sich, dass bei der HPT-Verformung die für kubisch-raumzentrierte Metalle typische Schertextur auftritt; zudem findet sich ab einer Verformungstemperatur von 100°C eine geneigte Würfellage, die durch dynamische Rekristallisation entstanden ist und deren Intensität mit der Verformungstemperatur ansteigt. Auch bei den ECAP-Proben findet man die typische Schertextur, eine Würfeltextur tritt nicht auf. Bei beiden Prozessen kommt es zur Kornfeinung, die beeinflusst wird von Verformungstemperatur, Druck und Verformungsgrad. Eine verbesserte Duktilität kann im Zugversuch nicht festgestellt werden, während eine gesteigerte Festigkeit durch Mikro- härtemessungen bestätigt wird. Bei genügend hoher Verformung findet man auch nach der Verformung von Einkristallen eine typische Schertextur. / NiAl is an intermetallic compound with a cubic B2 structure. Below the brittle-to-ductile transition temperature at about 300°C under ambient pressure the number of independent slip systems that have to be activated for a homogeneous plastic deformation is not sufficiently high according to the von Mises criterion. This is because the fracture stress is smaller than the critical yield stress for the activation of the secondary slip system. Despite this, severe plastic deformation is possible under a high pressure. It can be realized by deforming by equal channel angular pressing (ECAP) or high pressure torsion (HPT). Thereby a very high degree of deformation is reached. In this work, texture and microstructure evolution after deformation by HPT at different tem- peratures between room temperature and 500°C and at different hydrostatic pressures and degrees of deformation are analysed and compared to texture and microstructure after ECAP. After deformation by HPT, a texture typical for sheared body-centred metals is found; besides, for a deformation temperature of 100°C and higher an oblique cube component appears. It develops by dynamic recrystallization and grows with increasing deformation temperature. The shear texture also arises in the ECAP deformed samples, but no oblique cube texture is found. Both processes lead to grain refinement, which is influenced by deformation temperature, pressure, and degree of deformation. Tensile testing doesn’t show an enhanced ductility, but an improved strength is confirmed by microhardness measurements. If the degree of deformation is high enough, a shear texture can even be found after deforming single crystals.

No description available.

Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos visco-plastic self consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [14]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an onto mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [11] and [36]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths.

The purpose of this study was to investigate how design changes in Cochlear Nucleus cochlear implants (CIs) (CI24M, CI24R, CI24RE and CI422) affected electrode impedance and ECAP measures, and to determine if these design changes affected post-lingually deafened adults and children with congenital hearing loss in a similar way. Results of this study showed that electrode impedance was inversely related to the area of the electrode contacts in the array: lowest for the full-banded CI24M CI and highest for adults who used the CI422 device which has the smallest electrode contacts of all four devices. The noise floor of the NRT system likely plays a significant role in the finding that CI users with older devices (the CI24M, and CI24R CIs) had higher ECAP thresholds than individuals with the CI24RE electrode array. The position of the electrode array in the cochlea was also found to have a significant effect on ECAP measures. CI users with modiolar hugging (the CI24R and CI24RE CIs) electrode arrays were found to have lower ECAP thresholds than CI users whose electrode arrays were seated more laterally in the cochlear duct (e.g. the CI24M and CI422 implants). The position of the electrode contacts relative to the modiolus of the cochlea was found to be related to slope of the ECAP growth functions. The lowest slopes were found in CI24RE users. It also had a significant impact on the width of the channel interaction function. Electrode arrays seated further from the modiolus have significantly more channel interaction than electrode arrays that hug the modiolus of the cochlea. Differences between results recorded from post-lingually deafened adults and children with congenital hearing loss were minimal. The difference only reflected on the ECAP slopes. Slopes in children with congenital hearing loss were significantly steeper than those recorded from adults. This may indicate that children with congenital hearing loss may have better neural survival than adults with acquired hearing loss. In conclusion, the results of the current study show evidence of the effects of variations in design and function of the implanted components of the Nucleus CI. Perhaps the most significant finding from the current data set is that electrode arrays located closer to the modiolus of the cochlea have lower thresholds and exhibit less channel interaction than electrode arrays that are positioned more laterally. An argument could be made that lower stimulation levels and less channel interaction may result in better outcomes and/or longer battery life. For CI candidates who do not have significant residual acoustic hearing, the CI24RE implant might be a better choice than the more recently introduced CI422 electrode array.

Title: Complex investigation of fine-grained polycrystals of Cu and CuZr alloy processed by equal channel angular pressing and high pressure torsion Author: RNDr.Ondřej Srba Department: Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University Prague Supervisor: Doc. RNDr. Miloš Janeček, CSc. Abstract: In the thesis the microstructure development, mechanical, elastic and corrosion properties of deformed specimens of pure Cu and binary alloy CuZr processed by equal channel angular pressing (ECAP) are investigated. Several properties of pure Cu processed by ECAP are compared with properties of the same material processed by high pressure torsion (HPT). The microstructure development is characterized in detail by several experimental techniques (light and electron microscopy, electron back scatter diffraction, positron annihilation spectroscopy, etc.). The microstructure development in specimens processed by ECAP is characterized by the continuous fragmentation of the initial coarse grain structure and the formation of new grains having the sizes in the submicrocrystalline range (of 460 nm and 260 nm in Cu and CuZr alloy, respectively). During the deformation by ECAP the fraction of high-angle grain boundaries, the dislocation density and the concentration of vacancies are...

NiAl ist eine intermetallische Verbindung mit kubischer B2-Struktur. Unterhalb der Spröd-duktil-Übergangstemperatur, die bei Normaldruck bei etwa 300°C liegt, können nach dem Von-Mises-Kriterium nicht ausreichend viele unabhängige Gleitsysteme zur homogenen plastischen Verformung aktiviert werden, da die Bruchspannung unterhalb der kritischen Schubspannung zur Aktivierung des sekundären Gleitsystems liegt. Um NiAl trotzdem massiv plastisch verformen zu können, muss die Verformung bei hohem Druck erfolgen. Dies ist beispielsweise möglich bei der Verformung mittels Gleichkanal- Winkelpressen (ECAP) oder Hochdrucktorsion (HPT). Hierbei werden sehr hohe Verformungsgrade erreicht. In dieser Arbeit wird nun die Gefüge- und Texturentwicklung bei Verformung mittels HPT bei Temperaturen zwischen Raumtemperatur und 500°C in Abhängigkeit von Temperatur und hydrostatischem Druck sowie Verformungsgrad untersucht und mit den entsprechenden Größen bei der Verformung mit ECAP verglichen. Es zeigt sich, dass bei der HPT-Verformung die für kubisch-raumzentrierte Metalle typische Schertextur auftritt; zudem findet sich ab einer Verformungstemperatur von 100°C eine geneigte Würfellage, die durch dynamische Rekristallisation entstanden ist und deren Intensität mit der Verformungstemperatur ansteigt. Auch bei den ECAP-Proben findet man die typische Schertextur, eine Würfeltextur tritt nicht auf. Bei beiden Prozessen kommt es zur Kornfeinung, die beeinflusst wird von Verformungstemperatur, Druck und Verformungsgrad. Eine verbesserte Duktilität kann im Zugversuch nicht festgestellt werden, während eine gesteigerte Festigkeit durch Mikro- härtemessungen bestätigt wird. Bei genügend hoher Verformung findet man auch nach der Verformung von Einkristallen eine typische Schertextur. / NiAl is an intermetallic compound with a cubic B2 structure. Below the brittle-to-ductile transition temperature at about 300°C under ambient pressure the number of independent slip systems that have to be activated for a homogeneous plastic deformation is not sufficiently high according to the von Mises criterion. This is because the fracture stress is smaller than the critical yield stress for the activation of the secondary slip system. Despite this, severe plastic deformation is possible under a high pressure. It can be realized by deforming by equal channel angular pressing (ECAP) or high pressure torsion (HPT). Thereby a very high degree of deformation is reached. In this work, texture and microstructure evolution after deformation by HPT at different tem- peratures between room temperature and 500°C and at different hydrostatic pressures and degrees of deformation are analysed and compared to texture and microstructure after ECAP. After deformation by HPT, a texture typical for sheared body-centred metals is found; besides, for a deformation temperature of 100°C and higher an oblique cube component appears. It develops by dynamic recrystallization and grows with increasing deformation temperature. The shear texture also arises in the ECAP deformed samples, but no oblique cube texture is found. Both processes lead to grain refinement, which is influenced by deformation temperature, pressure, and degree of deformation. Tensile testing doesn’t show an enhanced ductility, but an improved strength is confirmed by microhardness measurements. If the degree of deformation is high enough, a shear texture can even be found after deforming single crystals.

Les composites à matrice Titane avec des renforts nanométriques présente des améliorations intéressantes quant aux propriétés mécaniques, sans affecter la ductilitédu matériau. Ce travail de thèse se concentre sur l’élaboration et la caractérisation dematériaux composites de Titane obtenus par deux différents procédés de métallurgie despoudres : La densification par déformation plastique sévère utilisant l’ECAP (Equal ChannelAngular Pressing) et les procédés de frittage par hydrogénation/déshydrogénation (HDH).L’ECAP est un procédé de densification rapide utilisant la déformation plastique desmatériaux, se faisant à relativement basse température. Les procédés HDH utilisent ladéshydrogénation du titane comme un levier sur les mécanismes de frittage. Les différentsnano-renforts utilisés dans ces travaux sont les particules sphériques de TiC et les aiguillesde TiB. Cette étude montre l’influence de la nature et de la forme des renforts, ainsi que duprocédé de métallurgie des poudres utilisé, sur les propriétés et la microstructure final desmatériaux denses. / Titanium based composites using nano-sized reinforcements are goodcandidates for the improvement in mechanical properties without affecting ductility. Thisstudy is dedicated to fabrication and characterisation of Ti-based composites using twodifferent powder metallurgy processes: Densification using severe plastic deformation viaEqual Channel Angular Pressing (ECAP) and Hydrogenation/Dehydrogenation (HDH)sintering processes (pressureless sintering and hot pressing).ECAP is a fast process basedon a severe plastic deformation of material at relatively low temperature. HDH processes usethe dehydrogenation of Ti as a leverage of the sintering. The different nanosizedreinforcements used in this study are the TiC spherical particles and the whisker shaped TiB.This study shows the influence of either the reinforcement nature and type, and the powdermetallurgy processes used, on the final microstructure and properties of the dense materials.