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Microstructure and micro-texture of titanium alloys

 

The two phases, α and β, have anisotropic properties. A global texture (at the scale of one cm3, or of the piece) or a local texture (at the scale of a grain) can lead to behavioural anisotropy, or even to local damage. These textures may be created by a strain texture and recrystallization, such as phase transformation. Studies were thus undertaken pertaining to the understanding of the simultaneous formation of the microstructure and the micro-texture associated with the β -> α + β phase transformation. This research dealt primarily with the phase alpha texture at the issue of the precipitation at the β/β grain boundaries, namely the αGB and αWGB morphologies. Furthermore, the study of the variant formed at the grain boundaries was extended to analyze the possible existence of a rule for selection of the variant. This work was carried out in collaboration with L. Germain and N. Gey from LEM3 (Metz) and is supported by the CNRS, the Région Lorraine and the LabEx DAMAS.

The statistical study by EBSD of the transformation products formed in conditions of isothermic transformation showed a strong sensitivity of the number of alpha variants present in a beta grain to the transformation conditions. The number increases when the transformation temperature decreases. Each αWGB colony shows a single alpha orientation inherited from a single αGB precursor. An increase in the number of colonies weakens the micro-texture, considered at the scale of a β grain. The origin of this is a more rapid formation of αGB precursors, as was shown by an in-depth study of the first stages of precipitation.


The consequences on the texture, considered at the scale of a statistical number of β grains, have been analyzed [1]. The phenomena of variant selection must be taken into account, according to which the crystallographic orientation of αGB grains depends on crystallographic parameters of β/β grain boundaries. Thus, for 200 grain boundaries analyzed, in more than 50% of cases and irrespective of the temperature of the analysis, alpha variants formed at the grain boundaries respect the following selection rule: {100} β1 // {0001}α GB // {110} β2.

<font size="1"><i>EBSD cartography (figures of inversed poles) at a single scale showing the alpha micro-texture at the end of the transformation at: a) 750°C; b) 800°C; c) 830°C. The beta phase is represented in black and the boundaries of beta grains by dotted white lines. The figures indicate the number of alpha orientations in each beta grain.</font></i>

These first results showed that the alpha texture obtained at the end of the transformation depends on the transformation conditions, kinetics and morphological evolution, and not only on the texture of the parent phase, as had been generally thought.

 

Research is underway to examine other selection criteria, as well as other transformation conditions: cooling, prior β plastic distortion, influence of the alloy's chemical composition.

 

[1] Salib, M. Etude microstructurale et cristallographique de la précipitation de la phase alpha aux joints de grains beta dans un alliage de titane. soutenue le 2 mars 2015 Université de Lorraine, 2015