Influence of stress state and strain path on deformation induced martensitic transformations

The mechanically induced transformation behavior of 12Crsingle bond9Nisingle bond 4Mo (ASTM A 564) austenitic stainless steel is investigated in different stress states. This phenomenon is studied experimentally on a plane-stress biaxial test facility. The facility can load a sheet specimen simultaneously in shear and tension which enables us to investigate the effect of stress state on transformation kinetics. The martensite fraction is monitored via a magnetic sensor while the strain is measured using a camera and a dot-tracking software

Influence of plastic strain on deformation-induced martensitic transformations

The effects of plastic strain on deformation-induced martensitic transformations have been investigated experimentally. Austenitic metastable stainless steel samples were heated to a temperature at which the transformation is suppressed and were plastically strained to different amounts. The resulting pre-strained material was cooled to room temperature and a tensile test was conducted during which transformation was monitored via a magnetic sensor. Results of these tests are discussed concerning the existing theories that describe the mechanically induced martensitic transformation phenomenon

Combined athermal and isothermal martensite to austenite reversion kinetics, experiment and modelling

A novel laser heat treatment setup is presented and used to characterize the reverse transformation of martensite to austenite resulting from highly dynamic laser heat treatments of stainless steel. During laser heat treatments the irradiated spot and its surroundings can experience completely different thermal loads, yet both experience reverse transformation. The experiments are conducted such to reflect these diverse conditions. Next to experiments, a new kinetic model is reported which combines both athermal and isothermal transformation mechanisms to cope with the diversity in conditions in a unified framework. The experimental results show that reverse transformation can proceed extremely fast, yet saturates at intermediate temperatures. Additionally, it is shown that there is good agreement between experiment and model and it is essential to embed both the athermal and isothermal transformation mechanism in the model for achieving this performance. Initial steps towards model validation are performed showing good predictability of a non-isothermal heat treatment with conditions realistic and relevant for industrial laser heat treatments

Microscopic investigation of damage mechanisms and anisotropic evolution of damage in DP600

Weight reduction and fuel consumption play an important role on material selection in automotive industry. In this respect, ferritic-martensitic dual phase steels are gaining popularity thanks to their versatile combination of strength and formability. In this study, we investigate evolution of damage and active damage mechanisms in a commercial DP600 steel. Interrupted tensile tests are conducted in both rolling (RD) and transverse directions (TD). Subsequently, damage mechanisms and void evolution is characterized by cross-sectional SEM micrographs. The results reveal that, in both RD and TD, damage occurs by three different damage mechanisms. Namely, void formation due to inclusions, cracking of martensite islands and decohesion between ferrite and martensite. From these damage mechanisms, void formation due to large inclusions occur in the early stages of deformation, whereas the other two are both active throughout the complete stretching. The most commonly observed damage mechanism was martensite cracks and seem to be the primary reason of failure. In addition, void evolution studies clearly show that damaged area as well as number of voids increase more rapidly in RD than TD. Furthermore, in both directions, damage concentrates at the mid plane of the specimens, leading to an inhomogeneous distribution of voids in the thickness direction

Response of 2D and 3D crystal plasticity models subjected to plane strain condition

The plane strain assumption is generally applied in crystal plasticity finite element (CPFE) simulations in a 2D space to characterize the macroscopic material response considering microstructural features. However, the reliability and accuracy of 2D approximations need to be addressed. In this paper, crystal plasticity finite element simulations of 2D and 3D RVEs are performed with local and averaged plane strain assumptions in Abaqus/Standard. Plane strain postulation is implemented via plane strain elements in 2D and zero average thickness strain in 3D. Irregularly shaped RVEs are generated using the open-source software library Voro++. A conforming mesh is rendered to assign periodic boundary conditions on geometrically periodic RVEs. Periodic boundary condition (PBC) is applied using a prescribed macroscopic deformation gradient tensor. A rate-independent finite strain crystal plasticity model is employed as the user-defined material behavior in finite element simulations. A discrepancy is observed between macroscopic flow curves of 2D and 3D RVEs. The comparison was made for three cases of latent hardening in the crystal plasticity model. In all cases, 3D flow curves exceed 2D results. The results indicate that the deviation is caused by out-of-plane slip activation in 3D simulations, which proves to be an additional hardening source