Spatio-temporal dynamics of jerky flow in high-entropy alloy at extremely low temperature

Abstract

Despite a large body of literature, mechanisms contributing to low temperature jerky flow remain controversial. Here, we report a cross-over from a smooth at room and liquid nitrogen temperatures to serrated plastic flow at 4.2 K in high-entropy CrMnFeCoNi alloy. Several complimentary investigations have been carried out to get a coherent physical picture of low temperature jerky flow in these alloys. Microstructural characterisations at 77 K and 4.2 K show that the number of Lomer-Cottrell (L-C) locks at 4.2 K is much higher than that at 77 K, inducing stronger barriers for dislocation glide at 4.2 K. A stability analysis shows that the jerky flow results from an interaction between dislocation inertial motion with L-C locks. The instability results from a competition between inertial and viscous time scales characterised by a Deborah number. A detailed nonlinear time series analysis of experimental serrated stress signals shows that jerky flow is chaotic characterised by the existence of a finite correlation dimension and a positive Lyapunov exponent. Further, the minimum degree of freedom required for the chaotic dynamics turns out to be four, consistent with four collective modes degrees of freedom used in our model equations. These results highlight the crucial ingredients for jerky flow at liquid helium temperatures.

Lattice distortion and magnetic property of high entropy alloys at low temperatures

“…But the magnetic property of HEAs might have little effect on their deformation behaviors. Although pronounced differences in magnetization were observed for CrFeCoNi HEA with those of Cr-CoNi MEA and CrMnFeCoNi HEA, similar deformation behaviors referred to the observation of serration in stress-strain curve were observed for both CrFeCoNi HEA [68] and CrMnFeCoNi HEA at 4.2 K [30] . Evidently, the three alloys exhibit similar deformation mechanisms at cryogenic temperatures, but their magnetic properties differ tremendously at corresponding temperatures.…”

“…The extensively studied CrCoNi MEA [24][25][26] , CrFeCoNi [ 27 , 28 ] and CrMnFeCoNi HEAs [29][30][31] have been reported breaking out the strength-ductility trade-off at low and cryogenic temperatures. The dominated deformation mechanism would change from dislocation planar-slip to a synergy of dislocation and stacking faults and/or deformation-induced twinning with temperature decreasing [32] .…”

Atomic-level mechanism of spallation microvoid nucleation in medium entropy alloys under shock loading

“…Recently, high/medium-entropy alloys (H/MEAs), in which multiple principal elements in equal or near equal molar ratios are occupied randomly on topologically ordered crystallographic lattices, have emerged as an exciting new class of metallic structural materials [19][20][21][22][23]. This unique atom-packing structure with a high chemical disorder imparts HEAs' many extraordinary mechanical properties, such as an excellent combination of strength and ductility [21,[24][25][26][27][28], superior performance at an extremely low temperature [29], high hydrogen and corrosion resistance [30,31], an exceptional "self-sharpening" capability [7] and shock resistance [32,33]. For example, conspicuous element aggregation provides considerable resistance to dislocation glide, which overcomes the strength and ductility trade-off [34,35].…”