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Article History

Received: 17 October 2023

Revised: 6 May 2024

Accepted: 9 May 2024

First Online: 3 June 2024

Conflict of interest

: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

: The SHPB is used as a loading device, with the samples placed between the incident bar and the transmitted bar. By processing the collected electrical signal, the force between the bar and the sample, the relative displacement between the two bars, the variation curve of the sample reduction and the force can be obtained. The bullet fired by the air gun strikes the incident rod to obtain the incident wave. The sample deforms at a high speed under the action of incident pulse loading, and the reflected wave is generated in the incident rod. The transmitted wave is produced in the transmission rod. And the strain gauge is used to receive and record the strain signals of the incident, reflected, and transmitted waves.The hat-shaped sample was designed by Meyer and Hartmann (Ref ). The schematic diagram of a hat-shaped sample is shown in Fig. . The shadow area represents the shear zone, which is subjected to the shear force to form a shear band in it. The shear stress, shear strain, strain rate, true stress, and true strain can be calculated as follows (Ref , ):where E is the elastic modulus of the bar; A is the area of the bar; τ, ,, σT, and εT are the shear stress, shear strain rate, shear strain, true stress, and true strain, respectively. εi(t) and εt(t) represent voltages from the incident bar and the transmitted bar, respectively. C0 is the elastic-wave speed in the SHPB. h is the height of the shear zone; s represents the width of the shear band; di and de are the top and bottom diameters of the hat-shaped specimen, respectively.

: Meyers et al. (Ref ) and Nesterenko et al. (Ref ) proposed the mechanism of sub-crystalline rotational dynamic recrystallization (RDR) when studying the microstructure evolution of an adiabatic shear band in Ta. The RDR mechanism (Ref , , ) can be used to explain microstructure evolution at high strain rates. The main process is described as follows: firstly, randomly-distributed dislocations generate in the crystal, and elongated dislocation cells form with the increase in the dislocation density, and then elongated sub-crystals appear. In order to meet the needs of continuous deformation, the elongated sub-crystals are broken, and finally, the sub-crystals rotate to form the structure with recrystallization characteristics.According to the mechanism of RDR, the rotation of sub-grain boundaries is about 30 °. The time required can be calculated as follows (Ref ):where L1 and δ are the average sizes of the sub-grain diameter and grain boundary thickness. η1 and D0 are the grain boundary energy and constants related to grain boundary diffusion, respectively. Qb represents the activation energy of grain boundary diffusion, Qb = (0.4 − 0.6) Q; Q is the activation energy of grain growth, taking Qb = 0.4 Q in this work; k is the Boltzmann constant, k = 1.38 × 10−23 J/K, where R represents the gas constant, with a value of 8.314 J/(K…mol); θ is the orientation difference of the sub-grain, which ranges from 0° to 30°. Due to the lack of relevant data on NiCrFe medium-entropy alloys, the activation energy of NiCrFeCoMn high-entropy alloy was adopted. The relevant parameters of formulas and are shown in Table .Substituting the above parameters into formulas and can obtain the kinetic results of recrystallized grains generated by sub-grain boundary rotation within the shear band of NiCrFe medium-entropy alloy.