Free volume evolution dominated by glass forming ability determining mechanical performance in Zr x Ti65-x Be27.5Cu7.5 metallic glasses

The shear deformation and fracture behaviors of Zr_xTi_65-xBe_27.5Cu_7.5 metallic glasses (MGs) were systematically investigated in the range x = 20, 25, 30, 35, 40, 45, 50 at% under multi-axial loading mode by small punch test (SPT). Results show that enhanced plasticity can be obtained in MGs with composition ranging from 30 to 45 Zr at%, which is attributed to the large free volume dependent on their better glass forming ability (GFA). It is reasonably concluded that the total free volume consists of two parts: intrinsic free volume ($V_0)$ and excess free volume ($V_G$). Intrinsic free volume, involved in an ideal amorphous structure, is defined as the free volume of MGs undergoing fully structural relaxation, which depends on the atomic size of each element. On the basis of atomic dense packing model, intrinsic free volume of the studied MGs is basically same due to a little difference in the atomic size between Zr and Ti. Excess free volume is crucially dominated by the GFA of MGs. In the case of little difference in atomic size of Zr and Ti, the MGs with large GFA possess a much looser atomic stacking structure, leading to the larger content of excess free volume. The large content of free volume not only favor the better atomic mobility, but also facilitate the nucleation of multiple shear bands, which can dramatically improve the plasticity of MGs. Thus, the studied MGs with Zr content ranging from 30 to 45 at% possess the larger content of total free volume than that with 20, 25 and 50 Zr at% due to their better GFA, depicting the enhanced plasticity induced by the initiation and interaction of multiple shear bands. In addition, the MGs with large content of free volume exhibits the lower driving force of shear bands nucleation, showing a soft state.