The Fracture Toughness And Crack Propagation Of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ Metallic Glass

Strength and fracture toughness?damage tolerance?are the two primary attributes of a material,in terms of its mechanical properties.Metallic glasses?MGs?are known to have extraordinary strength,thus,their applications are mainly limited by fracture toughness.In some special cases,materials have to suffer from extreme temperature conditions.Therefore,it is of great importance to investigate the effects of temperature on the fracture toughness of MGs.In the present study,a representative,well-studied Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅?at%?MG is chosen as the model material,we systematically investigate the effects of cryogenic temperature,high temperature and sub-Tg isothermal annealing on the fracture toughness of MG.In addition,to further elucidate the fractographic evolution and its correlations with fracture toughness,the fracture surfaces are carefully examined based on scanning electron microscope?SEM?images and three-dimensional?3D?fractographies.A significant drop in the fracture toughness is found at cryogenic temperature.The decreased shear bands density and propagation distance suggest a reduced plastic zone size at cryogenic temperature,which causes the drop in fracture toughness.Decreasing the temperature to the cryogenic level leads the decreasing Poisson's ratio,?,and a drop in the shear transformation zone?STZ?volumes,suggesting that shear flow are frustrated at cryogenic temperature.With crack propagation,the crack speed increases to a dynamic behavior,which causes the elastic softening behavior and the development of shear bands to weaken.As a result,the fracture surface roughness decreases along the crack propagation direction until a stable value is reached.Within the temperature range of 373 K and 578 K,no significant changes in the fracture toughness are observed.MG specimens with high toughness values tend to have larger dimple sizes and fracture process zone.In addition,to further elucidate the fractographic evolution and its correlations with fracture toughness,the fracture surfaces are carefully examined based on three-dimensional?3D?fractographies.Sub-T_g isothermal annealing can induce severe embrittlement in MGs.No visible shear bands are found ahead of the notch tip for annealed samples,suggesting the absence of large plastic zone.After isothermal annealing,the fractographic features and fractographic evolutions change dramatically,reflecting the difference in energy accumulation and dissipation.By using digital image correlation?DIC?method,direct and precise measurements of the strain concentration ahead of notch tip are performed.No significant elastic strain field evolution difference is observed.