Damage-tolerant Zr-Cu-Al-based bulk metallic glasses with record-breaking fracture toughness
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an Mab) Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA (Received 28 February 2014; accepted 4 June 2014)
Bulk metallic glasses (BMGs) exhibit high yield strength but little tensile ductility. For this class of materials, damage tolerance is a key mechanical design parameter needed for their engineering use. Recently we have discovered a correlation between the local structural characteristics in the glass and the propensity for shear transformations. Based on the dependence of glass structure on alloy composition, zirconium (Zr)-rich Zr–titanium (Ti)–copper (Cu)–aluminum (Al) compositions are predicted to be more prone to spread-out plastic deformation and hence profuse shear banding. This structural perspective has guided us to locate a Zr61Ti2Cu25Al12 (ZT1) BMG that exhibits a record-breaking fracture toughness, on par with the palladium (Pd)-based BMG recently developed at Caltech. At the same time, the new BMG consists of common metals and has robust glass-forming ability. Interestingly, the ZT1 BMG derives its high toughness from its high propensity for crack deflection and local loading-mode change (from mode I to substantially mode II) at the crack tip due to extensive shear band interactions. A crack-resistance curve (R-curve) has been obtained following American Society for Testing and Materials (ASTM) standards, employing both “single-specimen” and “multiple-specimen” techniques as well as fatigue precracked specimens. The combination of high strength and fracture toughness places ZT1 atop all engineering metallic alloys in the strength–toughness Ashby diagram, pushing the envelop accessible to a structural material in terms of its damage tolerance. I. INTRODUCTION
Bulk metallic glasses (BMGs) are currently investigated worldwide, as a new class of promising structural materials. Different from conventional crystalline metallic alloys, the internal structures of BMGs are amorphous, and therefore they do not have well-defined dislocation defects to mediate plastic deformation. As a result, they exhibit extraordinarily high strength, but lack tensile ductility. This brings in an important difference, in terms of the key properties that are needed for engineering design and practical use. For conventional alloys, yield strength is often the design parameter used in materials selection; but for BMGs, fracture toughness is a more tell-tale indicator of mechanical performance, needed in design to guarantee damage tolerance. The fracture toughness assesses a material’s resistance to crack propagation, measured by the energy needed to cause fracture.
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2014.160 J. Mater. Res., Vol. 29, No. 14, Jul 28, 2014
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Simultaneous presence of high fracture toughness and strength imparts high damage tolerance to an alloy. This article discusses a strategy to locate BMGs with high fracture toughness.
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