Intrinsic brittleness origination of micro-scale phase-separating metallic glasses: Based on mechanical relaxations and theoretical analysis

Pronounced slow β-relaxation in metallic glasses (MGs) has been proven to be closely related to macroscopic plasticity especially around 0.6–0.8T_g. However, the micro-scale phase-separating (Zr_xLa_1-x)₅₇Co₂₇Al₁₆ (x = 0.2, 0.3, and 0.4) MGs obviously display brittleness around 0.7T_g even though an obvious slow β-relaxation peak is observed based on the dynamic mechanical analysis (DMA). In order to probe their intrinsic brittleness, the slow β-relaxation processes of the (Zr_xLa_1-x)₅₇Co₂₇Al₁₆ (x = 0, 0.2, 0.3, 0.4, and 1) MGs were observed together with their mechanical performances. The results demonstrated that the La₅₇Co₂₇Al₁₆ and Zr₅₇Co₂₇Al₁₆ MGs show obvious ductility around 0.7T_g. Based on DMA results, the La₅₇Co₂₇Al₁₆ MG displays an evident slow β-relaxation peak while the Zr₅₇Co₂₇Al₁₆ MG presents an excess wing. When the phase-separating Zr–La–Co–Al MGs consisting of La-rich and Zr-rich components were heated up to 0.7T_g, only the β-relaxation behavior originating from La-rich components rather than from Zr-rich components is preferentially thermal-activated. In addition, the slow β-relaxation is well described by the Cole-Cole (CC) model, which becomes more obvious with increasing phase-separating degree. Then only partial but not all of potential structural units in whole samples can be activated, leading to the whole intrinsic brittleness of the micro-scale phase-separating MGs. These findings suggests that the large compositional difference between components is harmful to design ductile phase-separating MGs.