Brown University School of Engineering

“Fracture in amorphous alloys: in search of a length scale”

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Monday, April 02, 2012 4:00pm - 5:00pm

BROWN UNIVERSITY Joint Materials/Solid Mechanics Seminar Series “Fracture in amorphous alloys: in search of a length scale” U. Ramamurty Department of Materials Engineering Indian Institute of Science, Bangalore-560012, India Abstract: A material physics-based condition, at which fracture will initiate, is essential for reliable design of components and structures. Fracture criteria for brittle materials like ceramics are stress based whereas those in ductile materials like metals and alloys are strain based. But, both need the identification of a critical length scale, l*, which is related to some microstructural length scale, to be prescribed a priori. Bulk metallic glasses (BMGs) are a new class of structural materials, some of which exhibit high crack initiation toughness due to shear band mediated plastic flow at the crack-tip. However, they do not have any crack growth resistance due to the lack of a microstructure which would otherwise hinder the unmitigated crack growth. Thus, at crack initiation, the fracture behavior of ductile BMGs transitions from that of ductile alloys to that of ceramics. A suitable fracture criterion is needed to identify l* at which this transition takes place. Mixed-mode fracture experiments coupled with detailed finite element simulations are conducted identify the fracture criterion in a nominally ductile BMG. These results show that fracture in amorphous alloys is controlled by the attainment of a critical strain and that a stable crack grows inside a shear band at the notch root before attaining criticality at l*»60 m. The Argon and Salama model, which is based on meniscus instability phenomenon at the notch root, has been modified to rationalize the physics behind this length scale. This model suggests that the mean ridge heights on fractured surfaces were found be correlated to the toughness of the BMG. In contrast, the fracture mechanism in brittle metallic glasses is elusive. Some interesting morphologies observed on fracture surfaces will be presented. Monday, April 2, 2012 4:00-5:00 pm B&H Room 190