Different Thermal Responses In La-Al-Ni/Cu Metallic Glasses

Due to the disordered atomic packing formed by rapid quenching,metallic glasses(MGs)are metastable and will gradually evolve into their equilibrium phases under suitable time and temporal conditions.This process is generally called physical aging,which is of great importance for metastable materials since it indeed determines their properties.Although this issue has been studied for decades,it is still unclear how the atomic structure evolves during aging and how to control it precisely.As the β-relaxation is an intrinsic feature of glasses,the further study of β-relaxation is helpful to understand the structural evolution during physical aging.However,due to the β-relaxation occurring very quickly and often being shielded by high-intensity α-relaxation,its intensity is usually low on the relaxation spectrum,resulting in some difficulity in study of the β-relaxation of MGs individully.More recently,it has been reported that La-Al-Ni glasses have pronounced β-relaxation behavior,which becomes unpronounced when Ni atoms are replaced by Cu atoms.Such alloys provide us excellent model systems to study the β-relaxation.Moreover,it was demonstrated that the atomic dynamics in glasses is sensitive to the annihilation of excess free volume.As such,in this paper,we plan to investigate how Ni and Cu contents affect the β-relaxation behavior,and also what effect a 4-day pre-annealing at the temperature of 0.9Tg has on the relaxations in both alloy systems.We design two alloy series to see how the variation of element content contributes to the β-relaxation.One is La85-xAl15Nix(x = 10 to 35 at.%),and another is La70Al30-xNix(x = 5 to 20 at.%)· We also select four La85-xAl15Cux(x = 10 to 35 at.%)in comparison with Ni-containing glasses.By combining high energy synchrotron x-ray diffraction(XRD)and x-ray absorption fine structure(XAFS)techniques,together with ab initio molecular dynamics(AIMD)simulation,we carefully compare the atomic structure differences between four selected La50Al15Ni35,La50Al15Cu35,La70Al15Ni15 and La70Al15Cu15 MGs,and propose several structural factors that could affect the β-relaxation.The main conclusions of this study are as follows:(1)The La-Al-Ni glasses tend to have a significant β-relaxation,which becomes more pronounced and shifts to high temperatures when increasing Ni content from 10 to 35 at.%in La85-xAl15Nix or Al content from 10 to 25 at.%in La70Al30-xNix glasses.Moreover,the sub-Tg pre-annealing can stabilize the β-relaxation and isolate it from the a-relaxation.(2)In contrast,the β-relaxation is unpronounced in La-Al-Cu glasses and gradually disappears with excess free volume annihilation in annealed samples.(3)The β-relaxation is mainly the free volume control in La-Al-Cu glasses.The substitution of Ni for Cu atoms can loosen the local structure around Ni,showing large vibration displacement under thermal activation,thus likely leading to the pronounced(3-relaxation behavior in La-Al-Ni glasses.