Mechanical glass transition revealed by the fracture toughness of metallic glasses

Jittisa Ketkaew,Corey S O’Hern,Jan Schroers,Gabriela Pereira,Ze Liu,Takeshi Egami,Wen Chen,Meng Fan,Eran Bouchbinder,Rui Yamada,Mark D Shattuck,Udo D Schwarz,Wojciech Dmowski,Amit Datye,Hui Wang

doi:10.1038/s41467-018-05682-8

Abstract

The fracture toughness of glassy materials remains poorly understood. In large part, this is due to the disordered, intrinsically non-equilibrium nature of the glass structure, which challenges its theoretical description and experimental determination. We show that the notch fracture toughness of metallic glasses exhibits an abrupt toughening transition as a function of a well-controlled fictive temperature (Tf), which characterizes the average glass structure. The ordinary temperature, which has been previously associated with a ductile-to-brittle transition, is shown to play a secondary role. The observed transition is interpreted to result from a competition between the Tf-dependent plastic relaxation rate and an applied strain rate. Consequently, a similar toughening transition as a function of strain rate is predicted and demonstrated experimentally. The observed mechanical toughening transition bears strong similarities to the ordinary glass transition and explains the previously reported large scatter in fracture toughness data and ductile-to-brittle transitions.

Highlights

The fracture toughness of glassy materials remains poorly understood
The selected geometry and particular notch radius of the samples were kept constant for all of the experiments reported in this work
The glass was rapidly quenched to room temperature at a rate exceeding (Tf−Tg)/τSR in order to minimize structural relaxation during the quench

Summary

Introduction

The fracture toughness of glassy materials remains poorly understood In large part, this is due to the disordered, intrinsically non-equilibrium nature of the glass structure, which challenges its theoretical description and experimental determination. We show that the notch fracture toughness of MGs exhibits an abrupt toughening transition as a function of the fictive temperature (Tf), whereas the ordinary temperature plays a secondary role. This observed mechanical toughening transition, which we theoritically explain, bears strong similarities to the ordinary glass transition and explains the previously reported large scatter in fracture toughness data and ductile-to-brittle transitions. The presented results open the way for a broader usage of tougher and well-reproducible MGs as structural materials

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Conclusion