102 research outputs found
Dispersion relations formalism for virtual Compton scattering off the proton
No full textWe describe a dispersion relation formalism to the virtual Compton scattering reaction off the proton as a new tool to analyze VCS experiments above pion threshold, where one observes increasing effects of the generalized polarizabilities
Photonuclear physics with low intensity photon beams
No full textSIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
Dislocation networks in precipitation hardened aluminium alloys during plastic deformation: The effects of dislocations on the anelastic behaviour and the evolution of dislocation networks in an AA7075 aluminium alloy.
No full textThe effects of precipitates in the microstructure of an AA7075 aluminium alloy on: the dislocation behaviour, dislocation structure and dislocation structure evolution during plastic deformation were investigated using the physical yield criterion model by van Liempt and Sietsma. The model was in fact applied for the first time to aluminium and an AA7075 aluminium alloy. By constructing an extended Kocks-Mecking plot from the measured tensile and interrupted tensile data the dislocation density, average dislocation segment length and physical yield stress were determined.The aim of the present study was to get a better understanding of the role of precipitates in the evolution of the dislocation structure during plastic deformation. The recovery of anelastic strain during loading and subsequent unloading after plastic deformation was investigated as well. The yield criterion was used to study the anelastic loading, while a constitutive unloading model by Torkabadi et al. was used to study the unloading behaviour. The models were combined to define the fraction of unrecoverable anelastic strain. The anelastic strain is related to springback. Therefore a better understanding of the anelastic strain could be useful for making better predictions of springback after metal forming. The physical yield criterion was further extended by incorporating a continuous uniform dislocation segment length distribution. Insight on the dislocation segment length distribution, and the evolution thereof during plastic deformation, could help to better understand the mechanical behaviour of metals.The evolution of the dislocation structure during plastic deformation is impeded by the presence of precipitates in the microstructure. Therefore, the physical interpretation of α in the Taylor equation which quantifies the dislocation structure, proposed by Arachebelata et al., was modified to incorporate the effects of precipitates in the microstructure. The dislocation structure parameter α should remain constant. However, the average dislocation segment length obtained from the extended Kocks-Mecking plot does not decrease sufficiently to accommodate a constant α. The introduction of the length between precipitates, which is independent from work hardening, into the Taylor equation ensures that α does remain constant, whilst in addition it provides an estimate of the distance between the precipitates. Anelastic strain, caused by reversible glide of dislocations in the pre-yield regime, is introduced into or recovered from the metal during loading and unloading respectively. The anelastic unloading strain determined from the interrupted tensile tests was found to be smaller than what was expected according to the model. Three possible causes were identified: not all Frank-Read sources are at their critical state when unloading is initiated, dislocation loops propagating through the crystal undo portions of the anelastic unloading strain and the retracting dislocations remain stuck behind obstacles whilst retracting under the reducing applied stress. The constitutive model was found to be not suitable for studying the dislocation behaviour during unloading because the change of dislocation segment length with plastic deformation is not accounted for. The development of a physical unloading model is therefore recommended. The distribution of the dislocation segment lengths could explain the non-zero value of the work hardening rate Θ at the abrupt change of slope between the pre and post-yield regime in the extended Kocks-Mecking plot. The value of Θ at this point could be an indication of the distribution width. The continuous uniform distribution is a rather unrealistic description of the dislocation segment length distribution. Therefore, other distribution types are proposed for the further development of the dislocation segment length distribution model of the physical yield criterion.<br/
Unravelling the mechanical behaviour of advanced multiphase steels isothermally obtained below M<sub>s</sub>
No full textThe initial formation of athermal martensite was proven to accelerate the subsequent bainite formation kinetics during isothermal holdings below the martensite-start temperature (Ms). The presence of prior athermal martensite (PAM) within the phase mixture is expected to modify the overall mechanical response of these newly-designed multiphase steels. Differences stem not only from the balance of product phases, but also from the effect of tempering of the PAM with variations in the applied holding time. This work investigates the effect of tempering time on the mechanical behaviour of the PAM and, as consequence, on the overall mechanical response of these microstructures. Results show that, for short holding times (several minutes), PAM yields similar to as-quenched martensite while, for longer holding times, its yielding behaviour becomes comparable to the one exhibited by typical tempered martensite. Furthermore, the use of Kocks-Mecking curves for the analysis of the mechanical performance confirms the bainitic character of the product phase isothermally formed below Ms. Tailoring the bainitic-martensitic microstructure with variations of the holding time below Ms enables to obtain advanced multiphase steels with comparable mechanical properties to those exhibited by conventional bainitic steels, but in shorter processing times due to the acceleration of bainite formation.(OLD) MSE-3Materials Science and Engineerin
Nuclear effects in inclusive pion photoproduction and in nucleon decay experiments
No full textSIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
Photonuclear physics at the Bonn synchrotrons. Present status and future plans at the Bonn synchrotron
No full textSIGLEDEGerman
Photonuclear experiments using large acceptance detectors
No full textSIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
Thermostatistical theory of plastic deformation in metals
No full textThis work aims to describe plastic deformation and microstructure evolution of metals at various scales in terms of dislocation behaviour. The theory is based on statistical thermodynamics, where the entropy is proposed to incorporate the possible paths for dislocation motion. Other than estimating the velocity gradients a dislocation may reach, the number of possible paths (configurations) that are favourable in terms of free energy at a given temperature and strain rate are considered in the entropy. It is demonstrated that the entropy features strongly in plasticity: 1) Its description supplies a physical foundation to the Kocks–Mecking formulation across the scales at a variety of deformation conditions for FCC, BCC and HCP metals, by identifying the activation energy for dislocation annihilation. 2) The transitions from low, medium and high temperature dislocation annihilation mechanisms are physically explained. 3) It aids in describing the conditions for the formation of dislocation cells and their average size, as well as the work hardening behaviour at large strains in FCC and BCC metals. 4) Deformation twinning in HCP, FCC and nano–twinned copper can be described. 5) The transition tempera- tures where different twin modes predominate in HCP metals are predicted. 6) The dynamic recrystallisation behaviour in pure and multicomponent FCC systems can be described; the critical conditions for recrystallisation occurrence are obtained in terms of alloy’s composition and deformation parameters. 7) Solid solution effects in work hardening can be identified. All these results allow to describe various plasticity phenomena in terms of a single parameter: the average dislocation density.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
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