Visco-elasticity and mechanical relaxation behavior in La-based metallic glasses

Dynamic mechanical relaxation process is one of the important issues to understand mechanical/physical properties of metallic glasses. In the current research, dynamic mechanical relaxation behavior of La₆₂Al₁₄(Cu_5/6Ag_1/6)₂₀(Ni_1/2Co_1/2)₄ and (La_0.7Ce_0.3)₆₅Al₁₀Co₂₅ bulk metallic glasses were investigated by mechanical spectroscopy. Unlike the typical Zr- or Cu-based metallic glasses, (La_0.7Ce_0.3)₆₅Al₁₀Co₂₅ metallic glass shows an evident β relaxation process. On the other hand, La₆₂Al₁₄(Cu_5/6Ag_1/6)₂₀(Ni_1/2Co_1/2)₄ metallic glass presents a “shoulder” relaxation process. Physical aging below the glass transition temperature T_g induces a decrease of the concentration of “defects”, which corresponds to a reduction of the intensity of the β relaxation. During aging process, the amplitude of β relaxation (△E") decreases, while the relaxation time (τ_β) increases. In organic glasses, it was reported that the changes of In(τ_β) and In(△E_max) are linearly correlated with In(τ_β)=b-a In(△E"_max), this correlation has also been found in metallic glass. This behavior is discussed in the framework of the asymmetric double well potential (ADWP) model, and U and △ are used to characterize ADWP. It is suggested that during the aging process, the U/△ ratio remains basically unchanged, which is close to the coefficient describing the linear correlation between In(τ_β) and In(△E_max) (U/△~a).