From an experimental perspective, carefully and reproducibly controlling the state of a glass and realizing these states in mechanical test samples has been very challenging, and has hampered accurate and reproducible fracture toughness measurements 2. From a theoretical perspective, currently there exists no general framework to quantify the disordered atomic structures of glasses and no complete understanding of the relation between these structures and glassy dynamics, most notably, irreversible plastic deformation that occurs in response to external driving forces. Glasses are non-equilibrium materials featuring disordered atomic structures whose properties are processing and history dependent 6, 7. Progress in understanding, predicting, and controlling the fracture toughness of MGs has been, on the whole, limited by the lack of: first, a theoretical understanding of the intrinsically disordered, non-equilibrium glassy states of matter second, techniques to generate well-reproduced, well-defined glassy states and third, accurate and reproducible fracture toughness samples and measurements. A major impediment, however, for their widespread usage as structural materials is not their strength, but rather their often low and highly variable fracture toughness 3. MGs constitute a relatively new and broad class of amorphous materials with a combination of plastic-like processability and exceptional strength and elasticity-superior to their crystalline counterparts-holding great promise for wide-ranging structural and functional applications 3, 4, 5, 6. Understanding the fracture toughness of glassy materials, which lack long-range crystalline order and are characterized by intrinsically disordered non-equilibrium structures, is a pressing problem in general and particularly for metallic glasses (MGs) 2. It is of enormous practical importance, as it is a major limiting factor in the structural integrity of a broad range of natural and engineering systems, and of great fundamental importance, as it challenges our understanding of the strongly non-linear and dissipative response of materials under the extreme conditions prevailing near defects 1. The fracture toughness quantifies a material’s ability to resist catastrophic failure in the presence of a crack. 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. Consequently, a similar toughening transition as a function of strain rate is predicted and demonstrated experimentally. The observed transition is interpreted to result from a competition between the T f-dependent plastic relaxation rate and an applied strain rate. The ordinary temperature, which has been previously associated with a ductile-to-brittle transition, is shown to play a secondary role. We show that the notch fracture toughness of metallic glasses exhibits an abrupt toughening transition as a function of a well-controlled fictive temperature ( T f), which characterizes the average glass structure. ![]() 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. The fracture toughness of glassy materials remains poorly understood.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |