Mechanical Properties and Thermal Stability of Nanocrystalline High-entropy Alloys

High entropy alloys (HEAs) have been the subject of numerous investigations during past 20 years. Various alloy systems have been explored to identify HEA systems with improved property combinations, leading to an extraordinary growth of this field. Equiatomic single face-centered cubic (FCC) structured CoCrFeMnNi alloy, also known as Cantor alloy, has attracted increased attention in the past decades largely because of its excellent mechanical properties. The most remarkable feature of this alloy is the superior combination of ductility and strength at cryogenic temperatures in comparison with that at room temperature, especially in a fine-grained state. Generally, CoCrFeMnNi alloy exhibits a dramatic ductility and strong work hardening performance at cryogenic temperature but lacks comparable strength. To improve the strength, CoCrFeMnNi alloys with reduced Cr content and with the addition of different contents of carbon as interstitial impurity were synthesized for following study. High-pressure torsion (HPT) process, as the most effective severe plastic deformation (SPD) method, was performed to obtain nanocrystalline HEAs. Meanwhile, the evolution of microstructure and hardness was investigated during HPT process. Subsequently, the mechanical properties were analyzed in specimen with saturated microstructure. Based on electron microscopy characterization, the microstructure and composition fluctuation were investigated on the nanocrystalline HEAs. The results indicated that carbon interstitial alloying significantly promoted the grain refinement, dislocation density increase, improvement of yield strength and the carbon segregation at the grain boundaries. Herein, the mechanism of the grain fragmentation, deformation behavior and strengthening and fracture mechanisms were discussed in the following chapters. The C segregation behavior during HPT at room and cryogenic temperature were studied in detail. The results of this work could be a good reference for the production of high strength HEAs using SPD methods. Post deformation annealing has been used in SPD processed alloys to gain comprehensive performance avoiding the brittle fracture. Hence, the exploration of thermal stability of the C alloyed nanocrystalline HEAs is essential to promote the improvement of the mechanical properties. Using electron microscopy, the elemental segregation, nucleation of precipitates, decomposition of matrix phase decomposition and grain growth were illustrated after annealing at different temperature. The results suggested that the single FCC phase nanocrystalline HEA is thermally stable up to 400 °C. Significant co-segregation of alloy constituent elements and precipitation occur from 500 to 600 °C. New phases such as CoFe B2 phase, NiMn FCC phase and M7C3 carbides formed during the annealing at the medium temperature interval from 500 to 600 °C. The development of the precipitation process and the effect of precipitates on the mechanical properties are unveiled in following chapters. Consequently, the results in the present work optimized the current inference on the thermal stability of nanocrystalline HEAs and proposed a theoretical model for the precipitation process