Low temperature peak of internal friction in high entropy Al0.5CoCrCuFeNi alloy

Abstract

The dynamic elasticity and internal friction of the highly entropic Al0.5CoCrCuFeNi alloy were studied in a wide low temperature range of 4.2–300 K. In the experiments, the bending vibrations of thin plates at a frequency of 530 Hz were studied in two structural states: samples prepared after crystallizing a cast alloy (I); the same samples after prolonged high-temperature annealing (II). The temperature dependences of internal friction Q−1(T) and dynamic Young’s modulus E(T) were obtained and analyzed for these structural states. In state I, both dependences are monotonic functions without any specific features: the internal friction decreases with a decrease in temperature from QI−1(T=300K)≃10−3 to QI−1(T=4.2K)≃10−4, while Young’s modulus increases from EI(T = 300 K); 180 GPa to EI(T = 4.2 K); 190 GPa. Upon the transition to state II, the QII−1(T) plot acquires a peak with a height of about 10−4 and a center at Tp; 228 K, without significant changes in internal friction at other temperatures. Young modulus EII(Т) significantly increases (by 20% on average) at all temperatures, and near Tp; 228 K a blurred step with a height of ΔEII ≈ 0.3 GPa ≈ 10−4 EII(T = 4.2 K) appears on the EII(T) plot. Statistical and thermoactivation analysis of the anomaly of the alloy’s viscoelastic properties near Tp; 228 K by the previously developed algorithm [V. D. Natsik, Yu. A. Semerenko, FNT 42, 185 (2016)] led to the conclusion that it is caused by the resonance interaction of the plate’s elastic vibrations with the dynamic processes in the system of dislocation relaxators, i.e., the thermally activated unpinning of the dislocation segments from the local defects [M. Koiwa and R. R. Hasiguti, Acta Met. 13, 1219 (1965)]. High temperature annealing of the cast alloy is accompanied by significant changes in the morphology and dislocation structure of the samples, which results in a significant increase in their elasticity (Young’s modulus), and the appearance of an internal friction peak at moderately low temperatures. The properties of this peak are similar to those of Hasiguti peaks or β-peaks in simple metals with FCC and BCC lattices.