Growing economical and ecological requirements have driven the automotive industry towards lowering fuel consumption by reducing vehicle weight and in the meantime maintaining safety standards. This pushes the development of advanced light weight alloys. As a promising candidate for automotive applications, the twinning induced plasticity (TWIP) steels have received tremendous attention due to their superior mechanical properties, which combines high ultimate tensile strength and ductility simultaneously. However, their plasticity mechanism and recovery behavior are still not completely understood. In the first part of this investigation, a modified physical-based model is developed based on the recent observations of strong influence of grain orientation and Bauschinger effect. The model describes well the serrated plastic flow of Fe-22Mn-0.6C TWIP steel under quasi-static tensile loading at ambient temperature. The discrete twinning is considered responsible for the serration phenomenon in Fe-22Mn-0.6C TWIP steel. The model also provides a new approach to estimate the volume fraction of deformation twins. In the second part of this investigation, the recovery of pre-strained Fe-22Mn-0.6C TWIP steel is studied. A model describes the evolution of yield stress during annealing is proposed, which indicates that the recovery treatment does not remove deformation twins induced by cold rolling but decreases dislocation density in matrix and removes the dislocations piled up against twin and grain boundaries. The recovery activation energy rec Q is derived from experimental results. The value Qrec = 1.73 eV = 167 kJ/mol implies that the diffusion from dislocations cores governs the recovery of investigated TWIP steel. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

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A set of codes with 3D periodic boundary conditions has been developed to model dislocations in semiconductors. Several schemes have been used to investigate the atomic structure of dislocations; classical potentials incorporated in a Molecular Dynamics framework, a tightbinding k-space scheme and ab initio pseudopotential codes developed at Cambridge and Edinburgh. An error has been detected in previous work that modelled dislocations using periodic boundary conditions. It is demonstrated, for the 90° and 30° Shockley partials, that a mismatch at the periodic boundaries leads to erroneous atomic and particularly electronic structures. A new approach is proposed which through its geometry obviates this problem. The Stillinger Weber potential has been found to predict a completely new type of reconstruction for the 90° partial. Recent work by other authors confirms this and predicts significantly different results to earlier work. A thorough investigation has been made into the bonding processes involved in the core of the 90° partial. This study has involved reproducing much of the earlier work to understand why there has been such poor agreement between various authors. The reconstruction of the 90° partial is found to involve a symmetry lowering displacement intimately connected to its electronic structure. The band-gap is predicted to be clear of states, except for the possibility of shallow states at both band edges, which contradicts the findings of the most recent work on this partial by other authors. The interaction of phosphorous with the 90° partial has been studied using the tightbinding model. The Hamiltonian has been parameterised by comparing the predictions to an earlier ab initio cluster method study. Good qualitative agreement with the ab initio work is obtained, including the prediction of a strong dislocation locking effect by phosphorous. Preliminary studies on the unreconstructed 30° partial show that phosphorous is strongly bound to the three-fold coordinated sites resulting in no states in the indirect band-gap. The modelling of interstitial copper at the core of the 90° partial has been initiated. The ab initio codes have been used and new silicon and copper pseudopotentials tested. The first attempt to model copper located interstitially in the core was not successful and the reasons for this have been identified. However, it is evident from this investigation that the neutral copper strongly repels and does not form bonds with the surrounding silicon atoms. A review is given of two techniques that have been developed to obtain the thermally averaged structure and concentration of vacancies at dislocations, together with a preliminary investigation on the Frank partial.

Thesis (Ph. D.)--Ohio State University, 2004. / Title from first page of PDF file. Document formatted into pages; contains xv, 148 p.; also includes graphics Includes bibliographical references (p. 143-148).

The effects of a polycrystalline surface layer on the critical resolved shear stress, yield stress and the work hardening rate of tin single crystals has been investigated. The change in these parameters was found to be independent of the thickness of the layer. The results have been interpreted in terms of the inhibition of surface sources and the higher stress level required to initiate interior sources. An exploratory investigation of the flow mechanism in tin was undertaken. Tin single crystals oriented for slip on the (110) [001] slip system were deformed in the temperature range from room temperature to -80°C, with the lower limit of temperature being sensitively dependent on the strain-rate. Twinning was observed to be the mode of deformation below this lower limit. Strain-rate change tests at temperatures from -27°C to R.T. were performed in order to assess the activation energy and activation volume. On the basis of these values, the flow mechanism was postulated to be the non-conservative motion of jogs in screw dislocations. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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