Impurity effects in the microscopic elastic properties of polycrystalline Mg-Zn-Y alloys with a synchronized long-period stacking ordered phase

Inelastic X-ray scattering (IXS) experiments were performed on polycrystalline Mg97Zn1Y2 and Mg85Zn6Y9 alloys with synchronized long-period stacking ordered (LPSO) phase of about 24% and 100%, respectively, together with the reference pure Mg at room temperature for investigating the impurity effects in the microscopic elastic properties of the LPSO alloys. Inelastic neutron scattering (INS) measurements were also carried out on a polycrystalline Mg97Zn1Y2 alloy. The IXS measurements were carried out for the momentum transfers Q between 1.6 and 15.9 nm−1 including the whole first- and partially second Brillouin zones, and for the energy transfers ω below 40 meV. Peaks arising from longitudinal acoustic (LA) modes were clearly observed in the IXS spectra of both the LPSO alloys and pure Mg, while transverse acoustic (TA) modes can mainly be detected in the second Brillouin zone. The dispersion relations in pure polycrystalline Mg are in good agreement with those of previous INS data on single crystal Mg. Broader inelastic signals and larger quasielastic peaks are characteristic for the LPSO alloys due to the phonon scattering by the Zn/Y impurities. Only the TA excitation energies increase by adding the impurities, which indicate a harder stiffness of the bond angles relating to the existence of the L12-type clusters in the LPSO alloys. Besides, new impurity-derived dispersion-less excitation modes are observed at about 10 meV by adding the Zn/Y impurities. By comparing with the INS data on the polycrystalline LPSO alloy, the contributions of the impurities to these excitation modes are discussed using the differences in the scattering cross-sections between neutrons and X-rays.