The effect of aluminium on twinning in binary alpha-titanium

The deformation mechanisms of binary Ti–Al model alloys (0–13.1 at.% Aluminium) have been investigated with respect to the twinning activity using in-situ loading in combination with neutron diffraction as well as detailed post mortem electron backscatter diffraction analysis. A consistent starting grain size and texture was generated for all alloys promoting tensile twinning during compression testing. Long-wavelength neutron diffraction and selected area diffraction transmission electron microscopy analysis were carried out to detect evidence of Aluminium ordering and Ti3Al formation.It was found that raising the Aluminium content in Titanium does first slightly enhance twinning, with {10View the MathML source2} tensile twinning being by far the dominant type, while the critical residual intergranular strains for twin initiation decreases. This suggests that either the lowering of stacking fault energy by Aluminium or its solute solution strengthening effect are important factors. At around 7 at.% Aluminium a turning point in twinning activity was noticed and a further increase in Aluminium did result in a dramatic loss of twinning activity particularly when the material had been exposed to an additional low temperature age. The dramatic decrease of twinning activity is strongly correlated with increasing evidence of short range ordering and also early signs of Ti3Al-formation in case of the highest Aluminium content. In addition, electron backscatter diffraction analysis revealed that the formation of Aluminium ordered zones do severely hinder growth of twin boundaries

STRUCTURE OF SMALL ANGLE TILT BOUNDARIES AND THEIR BEHAVIOR UNDER THE APPLIED SHEAR STRESSES IN fcc METALS

A molecular statics study has been performed on symmetrical tilt boundaries in fcc metals using pair potentials fitted to Cu and Ni. Boundaries are found to consist of a pair of walls of Shockley partials in both metals ; distance between walls is much smaller than that between partials of an isolated extended dislocation. The separation between partial dislocation walls decreases with the increasing tilt angle and boundaries eventually come to have different structures from those observed in lower angle boundaries when the tilt angle exceeds a critical value depending on the potentials used. The critical shear strain for motion of the boundaries increases with the increasing tilt angle which is in good qualitative agreement with experimental observations so far reported

MULTIPLICITY OF GRAIN BOUNDARY STRUCTURES : VACANCIES IN BOUNDARIES AND TRANSFORMATIONS OF THE BOUNDARY STRUCTURE

Un nombre de structures non-équivalentes ont été trouvées dans les études atomistiques des joints de grains périodiques. Ces structures peuvent se transformer mutuellement en ôtant, ou en ajoutant des lamelles d'atomes qui sont parallèles aux joints [19]. Cette multiplicité résultante des structures est très extensive pour des joints de grains généraux avec de longues périodes. On montre ici que les configurations de lacune d'énergie plus basse dans les joints de grain correspondent à la présence locale des unités de structures alternatives, alors que l'absorption ou l'émission des lacunes à un joint peuvent être regardées comme des transformations structurelles locales. Alors, quand la température monte et la concentration équilibre des lacunes monte, on augmente le désordre correspondant à la présence de diverses unités alternatives dans le joint. Ceci peut arriver ou graduellement, ou par une transformation du type ordre-désordre. On montre aussi que l'accumulation de lacunes ou d'impuretés dans le joint peut provoquer des transformations de glissement entre des structures de joint alternatives, conduisant en même temps à la migration des joints. Nous abordons ici l'importance de ces divers genres de transformations structurales pour les phénomènes qui se produisent dans les joints de grain.A number of non-equivalent structures have been found in the atomistic studies of periodic grain boundaries. These can transform into each other by removal or insertion of layers of atoms parallel to the boundary [19]. The ensuing multiplicity of structures is very extensive for long period, general, boundaries. We show here that low energy configurations of vacancies in grain boundaries correspond to the local presence of units of alternative structures so that absorption or emission of vacancies at a boundary can be regarded as local structural transformations. Hence, as the temperature rises and the equilibrium concentration of vacancies increases the disorder corresponding to occurrence of various alternative units in the boundary increases. This may happen either gradually or through an order-disorder transformation. It is further shown that accumulation of vacancies or impurities in the boundary may induce shear transformations between the alternative boundary structures which leads at the same time to the migration of the boundary. The significance of these different structural transformations for various grain boundary phenomena is discussed