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In this work, elastic properties of Mg-based bulk metallic glasses (BMGs) with different chemical compositions were investigated. By compositional tuning in the quaternary Mg–Cu–Ag–Y alloys, the Poisson’s ratio m of 0.332 is achieved at Mg56Cu21Ag14Y9 BMG, in excess of the previously suggested critical value (m 5 0.31–0.32) for the brittle-to-tough transition in metallic glasses. With the properties of the constituent elements, the predicted values of the bulk modulus B and shear modulus l of Mg-based BMGs are 8% and 10% greater than the measured value, respectively. Notch toughness KQ of the ten investigated Mg-based BMGs varies between 3.6 and 8.2 MPaOm. Intrinsic brittleness of Mg glass is associated with its tiny plastic zone size (in micrometer scale) and weak resistance to crack propagation. The toughness variations are lack of significant correlation with the m or l. Among the investigated alloys, the Mg59.5Cu22.9Ag6.6Gd11 BMG manifests a good combination of improved toughness and high glass-forming ability.

I. INTRODUCTION

In comparison with conventional polycrystalline alloys, bulk metallic glasses (BMGs) that were developed over the past decades manifest many attractive properties, such as high fracture strength, high elastic limit (
2%), excellent corrosion resistance, and near-net-shape processability. However, their applications as structural materials are restricted because of their catastrophic failure under tensile loading and lower fracture toughness (small plastic zone size).1–3 Recently, it is revealed that fracture toughness of BMGs has a significant compositional dependency, depending on the host constituent elements and even on the micro-alloying.4–9 Among the BMG family, Mg-based BMGs exhibit significant brittleness with respect to the Zr-, Ti-, and Cubased glassy alloys, even though their glass-forming ability (GFA) is no longer a bottle-neck to fabricate the centimeter-scale bulk materials.10,11 However, very few studies considering the compositional dependency on fracture toughness of Mg-based BMGs were carried out so far. As shown by limited data, notch toughness of Mg65Cu25Tb10 BMG is quite lower (
2 MPaOm),4,12 comparable to the silicate glasses. Thus, it remains an open question whether such low toughness can be improved through the alloy screening and composition tuning. Recently, it is suggested that the toughness/plasticity of metallic glasses (MGs) has a “one-to-one” correlation a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.5 J. Mater. Res., Vol. 26, No. 7, Apr 14, 2011

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with their elastic properties,4,7,13,14 such as the Poisson ratio m, equivalently as a ratio of the shear modulus l to bulk modulus B owing to a relation of l/B 5 (3/2)(1  2m)/ (1+m), and even the single shear modulus. It is understood from several different views, including (i) the fracture processes ahead of a crack tip are mainly controlled by two competing factors: plastic flow and dilatation caused by the ope