Simultaneously enhancing strength and toughness of Zr-based bulk metallic glasses via minor Hf addition

Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x = 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) bulk metallic glasses (BMGs) were successfully prepared by copper mold casting. The effects of minor Hf addition on the mechanical properties and fracture mechanisms were investigated. The quasi-static compressive mechanical properties of Zr65.5-xHfxNb3Cu13Ni11Al7.5 alloys were significantly improved by Hf addition. The optimum compressive fracture strength (σf) and fracture strain (εf) are 1516 MPa and 2.6%, respectively, for the Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy. The impact toughness of the Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy (Ak = 127 kJ/m2) is nearly four times than that of the Zr65.5Nb3Cu13Ni11Al7.5 alloy (Ak = 33 kJ/m2). The increase of the fracture surface area and the ratio of vein-like patterns regions caused by Hf addition play an important role in fracture energy dissipation under dynamic loading. The dependence of mechanical properties on the Hf content is closely related to the variation of free volume in the current alloys. The relationship between the Hf addition and the free volume change is discussed in detail. These results may provide new insights into simultaneously enhancing the plasticity and toughness of BMGs with minor alloying.