The transformation strain associated with displacive phase transformations can be utilised to improve mechanical properties of structural components in steels. The advantages of the transformation plasticity can be fully utilised by allowing the transformation to occur under the influence of external stress or strain. In this thesis, mathematical models have been formulated to calculate the transformation strain and texture during martensitic and bainitic transformations. The models are able to deal with a variety of complexities including various starting austenite textures and different states of externally applied stress. A variant selection model has been proposed based on Patel and Cohen’s theory and the effect of variant selection on the transformation strain and texture has been discussed in detail. A new theory has been proposed to calculate the extent of variant selection. An attempt has been made to separate the effects of stress and strain on transformation plasticity and variant selection. It has been shown that Patel and Cohen’s plastic strain theory is more suitable than the elastic infinitesimal strain deformation model to calculate the interaction energies between crystallographic variants and external load. Using theoretical knowledge and with the help of a neural network model, new alloys have been prepared to be used as the welding filler metals to reduce the residual stress and to achieve higher toughness. Neutron diffraction studies have revealed that newly developed filler metals do indeed reduce the residual stress. Synchrotron X-ray data have been utilised to determine the texture of austenite and martensite as transformation occurs under load. A mathematical model has been developed to predict the Debye diffraction patterns observed experimentally
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