Phase Stability, Micromechanical Behavior, and Low-Temperature Tensile Properties in Ferrous Medium Entropy Alloys

DoctorWith recent advances in the aerospace, marine shipbuilding, and natural gas industries, the demand for metallic alloys having desirable strength and ductility in cryogenic environments has been increased. High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (FCC) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. The underlying strengthening mechanism of these materials is associated with deformation twinning. In the HEA research areas, on the other hand, little research has been conducted on the utilization of metastability-engineering at cryogenic temperatures, and the FCC to body-centered cubic (BCC) martensitic transformation in HEAs. Firstly, in the present thesis, novel ferrous MEAs (FMEAs) exhibiting sequential operation of deformation-induced phase transformation from parent FCC to newly formed BCC phases at low temperatures was developed, and low-temperature tensile properties were evaluated. Detailed deformation responses in relation with microstructural evolution were investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy, combined with in-situ neutron diffraction (ND) analysis. The micro-process analysis qualitatively revealed the correlation between microstructure evolution and the mechanical responses at low temperatures. Secondly, phase stress evolution of the FCC and deformation-induced BCC martensite phases was measured in FMEAs. This was done during tensile deformation at −137 °C using in-situ ND measurements for the quantitative interpretation of the role of martensitic transformation on the improved lowtemperature tensile properties. Thirdly, an integrated experimental-numerical analysis on FMEA was conducted to understand the micromechanical response of the constituent phases. The deformation-induced microstructure evolution related to the phase transformation mechanism and strain partitioning behavior was analyzed using ex-situ EBSD. The mechanical responses related to the stress partitioning between constituent phases and deformation-induced transformation rate were determined using in-situ ND in combination with the nanoindentation analysis. The three-dimensional microstructure volume elements based crystal plasticity models were built based on the experimental results, and the simulations were conducted to quantitatively investigate the stress-strain partitioning behavior