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Title: The structural characteristics of twin boundaries in body centred cubic metals
Author: Bristowe, Paul David
ISNI:       0000 0001 3480 4902
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1975
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The structure and energy of {112} twin boundaries and twinning dislocations in a-iron, molybdenum and tungsten have been studied using a real space discrete lattice model. This method of calculation, otherwise known as the computer simulation or atomistic technique, involves the iterative minimisation of the potential energy of the atomic configuration. It has revealed that two distinct twin boundary structures can be stable in those body centred cubic metals. These are the conventional twin boundary defined by a reflection orientation relation and a twin in which the boundary consists of a layer of cells which project on the {110} plane as isosceles triangles. In the case of iron the two boundaries, when fully relaxed, are found to have almost identical energies but for molybdenum and tungsten the reflection boundary is preferred. The twin boundary energies for these two metals are three or four times as large as those for iron. The results suggest that in some bcc metals both types of boundary may arise in which case each perfect twinning dislocation is likely to be dissociated into two partials each with Burgers vector 1/12 . The core structures of bcc edge twinning dislocations with Burgers vectors 1/12 and 1/6 have been examined with the aid of differential displacement mapping procedures. In molybdenum and tungsten the perfect twinning dislocation of Burgers vector 1/6 [111] was found to be stable and well localised. However, in iron the perfect twinning dislocation always dissociated into two separate partials each of Burgers vector 1/12 [111] so that ribbons of isosceles and reflection twin were obtained. The separation of these partials was found to be large and the steps diffuse corresponding to negligible lattice resistance to nucleation and growth of a new layer. In addition the interaction of vacancies and divacancies with twin boundaries and twinning dislocations has been investigated. These calculations have located the regions of maximum binding energy so that the ability of the boundaries and dislocations to act as vacancy sources or sinks can be evaluated. Finally the results are discussed in relation to the theory of coincidence site lattices. Also the overall reliability of the computer simulation method for investigating the structure of crystalline interfaces is assessed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available