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

In this work, optimal compositions for bulk metallicglasses (BMGs) formation in the ternary Mg-Cu-Nd and Mg-Ni-Nd systems are located at the Mg(57)Cu(34)Nd(9) and Mg(64)Ni(21)Nd(15), 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 (K(Q) = 5.1 MPa root m) manifests higher toughness with respect to the Mg(57)Cu(34)Nd(9) (K(Q) = 3.6 MPa root 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 T(g). Under the Model 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 mu = 4.7 + 625T(g)/V(m)[1-4/9(T/Tg)(2/3)]. Meanwhile, correlation between the calorimetric T(g) and elastic properties at T(g) can be rationalized with Egami's model. (C) 2011 Elsevier B.V. All rights reserved.