On the fracture mechanisms of beta metastable TRIP-TWIP Ti alloys

A constant growing use of titanium alloys results from their excellent properties including good corrosion resistance, specific strength and biocompatibility. However, their extensive use is partly hindered by a lack of ductility, strain hardening, and fracture toughness. Recently, several β-metastable titanium alloys were designed to simultaneously activate both transformation-induced plasticity and twinning-induced plasticity effects, resulting in significant improvement of both their strain hardening capacity and resistance to plastic localization. However, their fracture properties have not been investigated yet. The present thesis aims at uncovering the fracture process of these alloys. An ultra-large fracture resistance resulting from a high resistance to damage nucleation is observed in several β-metastable Ti alloys. An unexpected fracture phenomenology involving adiabatic shear banding occurs under quasi-static loading, which is strongly dependent on the geometry of the tensile specimen. Their excellent ductility is also kept after short ageing treatments. A significant temperature rise up to the melting temperature of the alloys is estimated using the fusible coating method and is associated with a very large crack propagation speed, measured by high speed imaging. The evolution of the microstructure during quasi-static straining and after fracture is characterized by transmission electron microscopy. A pre-necking localization of the deformation is also detected by digital image correlation analysis.(FSA - Sciences de l'ingénieur) -- UCL, 202