Mg based bulk metallic glasses: Glass transition temperature and elastic properties versus toughness

摘要

In this work, optimal compositions for bulk metallic glasses (BMGs) formation in the ternary Mg-Cu-Nd and Mg-Ni-Nd systems are located at the Mg_(57)Cu_(34)Nd_9 and Mg64Ni 21Nd15, respectively, with the critical diameter of 4 mm for the rods fabricated by copper mold casting. As indicated by notch toughness testing, Mg_(64)Ni_(21)Nd_(15) BMG (KQ = 5.1 MPa√m) manifests higher toughness with respect to the Mg _(57)Cu_(34)Nd_9 (KQ = 3.6 MPa√m), even though both BMGs have similar compressive fracture strength of 870-880 MPa. Such an improvement in toughness for Mg BMGs correlates with the reduction of shear modulus and the enhancement of thermal stability to resist to the structural relaxation at room temperature, which is indicated by the elevated glass transition temperature Tg. Under the Mode I loading condition, morphology in fracture surface of the Mg_(64)Ni_(21)Nd _(15) BMG varies along the crack propagation path. Fractographic evolution of the fracture surface follows the Taylor's meniscus instability criterion. For the Mg-based BMGs, shear modulus scales with the glass transition temperature, and can be expressed as μ = 4.7 + 625Tg/V m[1-4/9(T/Tg)2/3]. Meanwhile, correlation between the calorimetric Tg and elastic properties at Tg can be rationalized with Egami's model.