Dislocation-based serrated plastic flow of high entropy alloys at cryogenic temperatures

Abstract

Serrated plastic deformation at temperatures close to 0 K has been previously reported in some metals and alloys, and is associated with two possible origins: (i) thermomechanical instability or (ii) mechanical instability. While some recent results indicate that serrations are a mechanical dislocation-based phenomenon, a comprehensive model does not exist. CoCrFeMnNi, an expectedly ideal candidate, exhibits severe serrated plastic deformation with large stress drops in excess of 100 MPa. Furthermore, it also shows cryogenic serrated plastic deformation at a higher temperature (35 K) than previously reported for any other alloy. The exacerbated nature of serrated plastic deformation in CoCrFeMnNi led to the following inferences: (i) temperature and dislocation density are indisputable controlling parameters for cryogenic serrated plastic deformation and they cannot supersede each another; (ii) a phenomenological model is elucidated based on the increasing difficulty for cross-slip with decreasing temperature, leading to sudden massive dislocation proliferation event;(iii) the model establishes a gradual transition from completely non-serrated to completely serrated deformation, mediated by cross-slip, as opposed to the conventional model which proposed a discrete transition; (iv) solute dislocation interaction and associated Stacking Fault Energy (SFE) during deformation plays a key role in controlling dislocation constriction and cross-slip and correspondingly serrated plastic deformation; (v) the need/direct influence of deformation twinning, transformation induced plasticity and especially thermomechanical factors on serrated plastic deformation is invalidated. Some of these points were further clarified through comparisons with CoCrNi and CoNi, also presented in the present article.

Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin

“…3) for the CrCoNi alloy tested at 20 K indicate extensive deformation-induced twinning in the highly deformed grains (under high stress triaxiality) within the plastic zone in the vicinity of the crack tip. Similar deformation mechanisms have been reported for CrCoNi-based alloys at these low temperatures in uniaxial tensile tests where the degree of triaxiality is far lower (18)(19)(20)(21)(22)25). We then made sections from these regions into TEM foils using a focused ion beam (FIB) lift-out method, finishing with a 5-kV Ga + polish, for HRTEM and four-dimensional scanning transmission electron microscopy (4D-STEM).…”

“…Impact tests on the CrCoNi alloy have reported high Charpy V-notch energies of close to 400 J at 77 K, which were reduced by ~10% at 4.2 K (18). However, it remains unclear how samples that contain a sharp crack would perform at temperatures below 77 K, where anomalies in the temperature dependence of strength and ductility have been reported (19)(20)(21)(22). Furthermore, full resistance-curve measurements that define both the crack-initiation and crack-growth fracture toughness have not been performed on medium-or high-entropy alloys at low temperatures approaching that of liquid helium.…”

Combining Machine Learning and Molecular Dynamics to Predict Mechanical Properties and Microstructural Evolution of FeNiCrCoCu High-Entropy Alloys

“…The visualization of the local strain rate field reveals different microscopic characteristics between smooth flow and abrupt flow. Tirunilai et al [ 23 ] found that the key parameters for low-temperature jagged plastic deformation are temperature and dislocation density. The strengthening mechanism of superimposed faults in jagged plastic deformation was revealed.…”

Micro-mechanical deformation behavior of CoCrFeMnNi high-entropy alloy

“…CoCrFeMnNi has been reported to exhibit serrated flow at certain cryogenic and elevated temperatures [8,10,[16][17][18][19]. The high-temperature phenomenon is usually interpreted in terms of dynamic strain aging (DSA), explicitly as the pinning and depinning of dislocations by solute atoms [12,18,20].…”