Influence of Secondary Phase on Elastic and Acoustic Characteristics of Magnesium Alloys of the Mg–Zn–Y–Gd System

Abstract

The microstructural, phase, acoustic, and elastic properties of nine cast magnesium alloys with an LPSO structure (X phase) were studied within a concentration range of Y, Gd, Zn, and Zr that are promising for practical applications, considering their subsequent thermal dispersion strengthening (Y ≤ 7.6, Zn ≤ 2.78, Gd ≤ 4.9, and Zr ≤ 0.68 wt %). A comparison of the experimental data revealed that the propagation rate of longitudinal and transverse waves in the alloys decreases, while the attenuation coefficient increases proportionally to the total weight percentage of alloying elements forming the X phase. Furthermore, the elastic and acoustic properties correlate more significantly with the total weight percentage of alloying elements in the Mg alloy rather than with the atomic parameters of the phase-forming alloying elements (Y/Zn) commonly used in metallurgy. It was shown that the variation in wave propagation rate in Mg alloys with the X phase predominantly correlates with an increase in their density. At a low content of the secondary phase, wave attenuation is determined by grain size, while in the presence of a secondary phase in the form of conglomerates at grain boundaries, it is influenced by the ratio of grain size, secondary phase size, and wavelength. It was found that the parameter of the ratio of secondary phase size to wavelength, introduced by analogy with the conventional acoustic parameter of the ratio of grain size to wavelength, weakly correlates with the acoustic properties of alloys with the X phase. Additionally, there is a discrepancy between the dependences of the attenuation coefficient on grain size and wave frequency. This discrepancy may be due to the unaccounted for influence of the X phase in the form of banded insertions into α-Mg grains, as well as the method of calculating the secondary phase size, which requires further investigation.