Thermomechanically induced phase separation at elevated temperatures in a CoCr0.4NiSi0.3 medium-entropy alloy

The microstructural evolution of an as-cast CoCr0.4NiSi0.3 medium-entropy alloys (MEA) was investigated under quasi-static tensile tests from 200 degrees C to 800 degrees C. The face-centered cubic (FCC) matrix exhibited ordered nano-precipitates induced by phase separation, resulting in varying strengthening mechanisms caused by metastable phase separation at elevated temperatures. The directional vacancy diffusion of solute atoms along specific crystal planes of dynamically recrystallized (DRXed) grains leads to phase separation at elevated temperatures. This occurs through pathways along the crystal planes of {0 (21) over bar} in the DRXed sigma phase at 400 degrees C and the {05 (5) over bar0} in the secondary alpha-M5Si3 phase at 800 degrees C. The diffusion mechanism involves lattice defects at lower temperatures and lattice interdiffusion at elevated temperatures. Additionally, the FCC-structured MEA enhances strength and plasticity through the TRIP effect by decomposing perfect dislocations to form the 9R phase. The interaction mode between the primary precipitated L1(2) phase and dislocations varies with temperature. At 400 degrees C, a/3<112> dislocation pairs cut through the L1(2) phase, while at 800 degrees C, an Orowan mechanism bypasses the L1(2) phase. The favorable mechanical properties of the as-cast MEA at elevated temperatures can be attributed to the temperature-dependent evolution of metastable phase configurations and their interactions with dislocations during thermomechanically tensile testing, providing insights into the evolution of metastable phases in CoCrNi-based MEAs under elevated temperatures.