Monotonic and Cyclic Mechanical Behavior of TRIP-assisted High Entropy Alloys

Multi-principal element alloys (MPEAs) have increasingly been shown as candidate materials to cover demands for advanced properties. MPEAs are classified based on the entropy level calculated regarding the chemical composition of the alloy. High-entropy alloys (HEAs) especially are known for their advanced mechanical behaviors. It can be said that various mechanisms are involved during plastic deformation. Transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) are two assistive plasticity mechanisms for improving mechanical responses. The aim of employing various mechanisms is to improve the strain-hardening capability of the introduced microstructure in the HEAs during plastic deformation. Throughout the TRIP mechanism, the base austenite as the soft domain (FCC) transforms to martensite as the hard domain (BCC). Plastic deformation starts with dislocation movements and is carried on by the introduction of dislocation pile-ups in the grains. It has been shown that under the TRIP mechanism, dislocations in the soft domain (FCC) are consumed profoundly by creating the hard domain (BCC) in the microstructure. The present work introduces a new TRIP-assisted HEA composition and investigates the monotonic and cyclic mechanical behaviors at room temperature. Through these studies, a novel dualphase FCC-BCC hetero-structured MPEA via thermo-mechanical processing is tried to improve the mechanical response. Applying monotonic mechanical tests, a meaningful relation between designed microstructures and the TRIP mechanism is proposed. Employing appropriate thermomechanical processes, the 750°C microstructure managed to achieve a yield strength of 1GPa (6 times improvement compared to homogenized microstructure) with proper ductility. It has been shown that the degree of recrystallization played a critical role in the mechanical behavior of this TRIPassisted multi-phase HEA, where intragranular dislocations mediate uniform plastic deformation at higher stress levels. In addition, the modified microstructure under 750°C annealing condition assisted by the TRIP mechanism at 0.2% strain amplitude showed about a quarter million cycles with a stress amplitude of 370 MPa. The introduced heterogeneous TRIP-assisted microstructure resisted up to 6000 cycles at a total strain amplitude of 0.4% with a stress amplitude of 670 MPa. The results illustrate that the extent of TRIP-related deformation mechanisms can be limited by increasing the sigma phase fraction in the multi-phase HEA through the adjustment of thermomechanical processing parameters. The advanced characterization techniques of electron backscatter diffraction (EBSD), X-ray diffraction (XRD), energy dispersive Xray spectroscopy (EDS), and scanning electron microscopy (SEM) confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.