Mechanical behaviour of Twinning Induced Plasticity (TWIP) steels

TWinning Induced Plasticity (TWIP) steels are single phase austenitic alloys that successfully combine the properties of high strength and ductility. Thus, TWIP steels are an ideal candidate material for applications where the absorption of energy is required, such as armour or automotive crash safety systems. The TWIP effect arises due to the formation of thin lenticular deformation twins during straining. These twins act as strong barriers to dislocation movement. This results in a dynamic grain refinement process, leading to an increasing work hardening capacity and superior ductility. This thesis presents work carried out to develop our understanding of the mechanical properties and the micromechanics of twinning in a TWIP steel during deformation. In-situ X-ray synchrotron diffraction loading experiments were conducted to investigate the evolution of deformation texture, lattice strain and peak width during deformation at quasi- static strain rates. The lattice strain evolution indicated that twinning occurs very early during deformation and remarkably initiates before the macroscopic yield point. In addition, the in-situ experimental observations were modelled successfully using an elasto-plastic self consistent (EPSC) model. The armour capabilities of the material was investigated using Hopkinson pressure bar and blast testing. The characteristics of twinning were found to be dependent on the strain rate. Fewer active twin systems were observed at high strain rates (i.e. >1000 s-1) while the twins were relatively thicker compared to those observed at lower strain rates. TWIP steels have not obtained widespread use particularly in the automotive sector due to their relatively low yield stress compared to alternative advanced high strength steels. Cold rolling and annealing was performed on the as-received TWIP steel to explore alloy strengthening using a grain refinement mechanism. The influence of initial grain size on twinning was investigated and a critical twin stress of 50 MPa at the single crystal limit was determined.Open Acces