Effect of solution annealing temperature on microstructural evolution and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys

Recent studies have shown that NbC precipitates in high-entropy alloys (HEAs) are strong obstacles for dislocation motion, which can substantially contribute to the mechanical performance over a wide temperature range. However, there are limited reports discussing the exceptional adjustability of microstructures and mechanical properties using thermomechanical processing (TMP). Therefore, the microstructures and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys (HEAs) tuned via TMP were systematically investigated. Homogenized HEAs with -0.3 at% C and Nb were solution annealed at 1100, 1200 and 1300 degrees C, followed by cold-rolling plus annealing. The microstructures and mechanical properties of the HEAs were systematically characterized using scanning electron microscopy, transmission electron microscopy and uniaxial tensile tests. The results indicated that NbC distribution evolved from eutectic-like primary NbC clusters surrounded by disorderly distributed secondary NbC to dispersive nano-sized secondary NbC as the solution annealing temperature increased above the NbC solvus temperature. Two kinds of microstructures were correspondingly produced: 1) samples solution annealed at 1100 and 1200 degrees C consisted of fine grain lamellae embedded with NbC precipitates and coarse grain lamellae without precipitates, which exhibited a combination of high strength (UTS -740 MPa) and good ductility (elongation -60%); and 2) samples solution annealed at 1300 degrees C obtained a homogeneous fine grain microstructure, which demonstrated excellent synergy of high strength (UTS -760 MPa) and exceptional ductility (elongation -63%). The enhanced strength of samples solution annealed at 1100 and 1200 degrees C originated primarily from the high back stress strengthening of lamellar microstructure, while precipitation strengthening caused by dispersed nanosized NbC precipitates and grain boundary strengthening provided extra strengthening for the sample annealed at 1300 degrees C. This work provides a new strategy for tailoring the microstructure and mechanical properties of NbC-reinforced HEAs via adjusting the solution annealing temperature of TMP.