The mechanical properties and fracture characteristics of the high-entropy alloy Al0.5CoCrCuFeNi are studied in different structural states (cast and after two heat treatments) at temperatures of 0.5–300 K with quasistatic deformation by uniaxial compression and distension. Mechanical resonance spectroscopy is used to measure the temperature variations of the Young modulus in the different structural states. It is found that heat treatment of the samples leads to an increase (by roughly 25%) in the Young modulus, the nominal yield point τ0.2, and the deforming stress. The form of the deformation curves is analyzed. The temperature interval for the transition from smooth to discontinuous plastic flow is determined. For the cast state the differences in τ0.2 under tension and compression are determined, an anomalous temperature dependence of τ0.2 (for temperatures in the 0.5–4.2 K range) is discovered, and thermal activation analysis of the experimental data yields empirical estimates for the parameters of the interactions of dislocations with local barriers. After heat treatment the samples break up into two parts under compression, as opposed to the cast state, where the samples acquire a “barrel” shape during compression. It is found that fracture of the heat treated samples at temperatures of 300–4.2 K has a viscous character.

In this work, we studied the low-temperature plasticity, elastic and dissipative characteristics, as well as the microstructural evolution of the Fe40Mn40Co10Cr10 high-entropy alloy in a wide temperature range of 300 - 0.5 K. The temperature dependences of yield strength, strain hardening, strength and ductility, as well as acoustic absorption and dynamic Young's modulus are obtained. It was found that the structure of Fe40Mn40Co10Cr10 alloy in the initial state is single-phase with fcc lattice, and in the deformed state at low temperatures it becomes two-phase due to the deformation-induced phase transition. In addition, EBSD analysis of the alloy structure revealed a change in grain morphology and the appearance of twin dislocations after plastic deformation at all investigatedtemperatures. Acoustic studies showed that the transition from the initial to the deformed state changes the character of the temperature dependence of the dynamic Young's modulus from almost linear to exponential, and reduces the absolute values. The Fe40Mn40Co10Cr10 alloy has excellent strength and ductility at a high strain hardening rate, which is explained by the significant contribution of the twinning process. With a decrease in temperature from 300 to 4.2 K, a strong temperature dependence of the yield strength is observed, which indicates the thermal activation of the nature of the plastic deformation of the alloys in this temperature range. In the temperature range of 0.5-4.2 K, an anomaly of the yield strength was observed, namely a decrease in the value of the yield strength with decreasing temperature from 4.2 K to 0.5 K. The anomalous dependence of the yield strength is due to a change in the mechanism of overcoming local barriers from thermoactivated to inertial one, when part of the local obstacles is overcome by dislocations without activation. This leads to a decrease in yield strength with decreasing temperature. At a temperature of 4.2 K and below, the smooth nature of the plastic deformation changes from smooth to serrated. The jumps begin immediately after the yield strength and gradually increase from 40 MPa to ~ 160 MPa. The results obtained are important for practical applications of high-entropy alloys at low temperatures.

The effect of changes in the ultrasonic frequency or in the dislocation structure of the samples on the parameters of the low-temperature α peak of the internal friction and the Young’s modulus defect corresponding to it is investigated experimentally in single crystals of pure iron. A statistical interpretation of the temperature dependence and structural sensitivity of the characteristics of the dynamic relaxation in the neighborhood of the α peak is proposed, based on the assumption of a random scatter of the values of the activation energy of the elementary relaxators. Empirical estimates are obtained for the attempt period and for the mean value and variance of the activation energy of the relaxation process responsible for the α peak of the internal friction in iron.

An integrated approach to the study of the mechanisms of low-temperature dislocation relaxation during low-temperature cyclic deformations of crystalline materials has been developed and tested. The approach is based on the combined use of experimental methods of mechanical spectroscopy in wide frequency-temperature ranges and theoretical methods of statistical and thermoactivation analysis of experimental results. The efficiency of this approach was demonstrated by studying low-temperature relaxation resonances in iron crystals whose dislocation structure was varied by preliminary plastic deformation. In this study, the previous mechanical spectroscopy results for iron in the temperature range of 4 K < T < 150 K at vibrational frequencies of about 1 Hz and 105 Hz were supplemented by a detailed study of the temperature spectra of internal friction and the Young’s modulus of a single-crystal iron plate at intermediate frequencies of about 103 Hz. A two-mode dislocation relaxator model was suggested for interpreting the entire set of experimental results. Its first component is a linear segment of a dislocation line in a first-kind Peierls relief whose relaxation properties are determined by thermal activation of kink pairs. The second component is a chain of geometric kinks capable of thermally activated diffusion movement in a second-kind Peierls relief. Empirical estimates of the energy, force, inertial, and geometric characteristics of both components of such a relaxator have been obtained. This study complements the earlier analysis of the processes of mechanical relaxation in crystals caused by nucleation and movements of kinks on dislocation lines [a review: A. Seeger and C. Wüthrich, Nuovo Cimento B 33, 38 (1976)].