Thermodynamic investigation of systems related to TWIP steels

Sammanfattning: The world is facing serious challenges regarding environmental issues. Carbon dioxide levels increase every day. In an attempt to decrease carbon emissions the automotive industry searches for lighter, stronger materials. TWinning Induced Plasticity (TWIP) steels show an impressive combination of strength and ductility. The possibility of adding high amounts of aluminum that decreases the density while maintaining the mechanical properties makes this type of steel very interesting for use in automotive applications. The only thing keeping the TWIP steels from being used in the automotive industry is that the Yield Strength (YS) is too low. The TWIP steels usually have a YS around 400 MPa. For them to be useful in automotive applications YS around 600-700 MPa is necessary. One of the most promising ways of improving the YS is by precipitation hardening. This work has been performed within a European Research Fund for Coal and Steel (RFCS) project called Precipitation in High Manganese steels (PrecHiMn). As the name of the project suggests, the goal of this project has been to study the precipitation in TWIP steels. The precipitation hardening is achieved through the addition of carbide and carbonitride formers such as Nb, Ti and V. In order to build advanced models to simulate precipitation it is important to have a good understanding of the thermodynamics of systems related to TWIP steels.The goal of this work has been to study the thermodynamic properties of systems related to TWIP steels, more specifically the system forming the matrix phases of TWIP steels. Therefore the Al-C-Fe-Mn system has been studied as well as the Al-Ti-V system. Complete thermodynamic descriptions that reproduce the experimental data well have been produced including descriptions of order-disorder transformations.