[Séminaire] Cahit Can Aydiner ‘Caractérisation du comportement des polycristaux métalliques par mesures de champs très haute résolution’

Résumé :

On spatial, temporal and energetic aspects of mechanical twinning in polycrystals

Mechanical twinning is an eccentric strain accommodation mechanism employed by low-symmetry metals and in particular hexagonal metals (e.g., Magnesium and Titanium). With a temporally abrupt nucleation (as a zone of mirror-orientation inside the grain), the twin front first advances toward the opposite grain boundary and then accommodates further strain by lateral expansion. Twinning only takes place in one direction of a load path, i.e., it is unipolar. Upon reverse load, a twinned zone reverts back (detwinning) but not as abruptly with the absence of the nucleation step. Magnesium alloys, aside from the interest in their engineering use as light metals, have also been the model systems for twinning since typical forming processes generate intense crystallographic textures in wrought Magnesium products. Combined with a low activation stress for twinning, this leads to mass twin activity only in one sense of the load and very little (anomalous) on the opposite load path where hard slip systems factor in. As will be detailed with area-scanning in situ microscopic DIC data (the main technique of our group at Boğaziçi University), the mass twinning regimes show twin (and thus strain) patterning whose form is heavily linked with texture. The c-axes-aligned rolling texture shows an extremely sharp autocatalytic activity, with the massive twinning band showing nucleation, advance (across the sample) and expansion stages analogous to the activity of an individual twin inside a grain. Our complementary measurements for the energetics of this massive twinning regime with thermography (most recent experiments at Ecole Centrale) and band speed analysis by high-speed DIC (experiments at METU, Ankara) consider this unique case. The talk will thus bring together multiple experimental approaches (the discussion also bringing high energy XRD findings) to try to shed light on the coordinated twin proliferation in polycrystals.