Strain rate effect and temperature effect of CoCrNi-based medium-entropy alloy with interstitial C doping

To further explore the influence of interstitial C atom on the strain rate effect and temperature effect of CoCrNi-based medium-entropy alloy, the compression mechanical behavior, microstructure evolution and deformation mechanism of CoCrNiSi0.3C0.048 medium-entropy alloy were systematically studied at a wide temperature and strain rate range. The investigated alloy is composed of face-centered cubic (FCC) matrix and three-level precipitate microstructure, i.e. the primary Cr23C6 carbides (2−10 μm), the secondary SiC precipitates (200−500 nm), and the tertiary SiC precipitates (～50 nm). The results show that the serrated flow phenomenon is observed on the true stress-strain curve of the alloy at 400 ℃, and the amplitude of the serrations decreases gradually with the increase of strain and ultimately vanishes. In addition, the abnormal stress peak (the 3rd-type strain aging phenomenon) appears on the curve of the quasi-static flow stress with temperature, but at high strain rate, the abnormal stress peak disappears. Through the analysis of the characterization of the deformed microstructure, it is speculated that the main reason for the phenomenon of 3rd-type strain aging under quasi-static conditions may be the existence of interstitial C atoms. During the process of continuous plastic deformation and development, a series of mixed structures similar to heterogeneous structures are generated, which are composed of dense dislocation cells, micro bands, stack faults, dislocation clusters and deformation twins. These mixed structures intensify the interaction between interstitial atoms and moving dislocation, and then pin the dislocation, which results in dynamic strain aging phenomenon occurs. The reason why the 3rd-type strain aging does not appear under dynamic conditions may be that the solute atoms move slower than the dislocation. The dislocation cannot be pinned in time. In addition, the precipitation of a large number of nanoscale SiC precipitates weakens the "pinning" effect of interstitial atoms under dynamic loading.