| Summary: | Bulk metallic glasses (BMGs) are a new class of structural materials and exhibit unique combinations of mechanical properties. As a result, their mechanical behavior has been an active area of scientific pursuit in the recent past and considerable emphasis has been paid to understand plastic deformation in them. It is now well accepted that shear transformation zones (STZs), aided by free volume, are the fundamental carriers of plasticity. At a microscopic level, deformation at low temperatures and high stresses tends to localize into shear bands. Most BMGs posses high fracture toughness despite high yield strengths and poor global ductility. However, the micro-mechanisms of fracture and fatigue in this new class of materials are not fully understood yet. The overall objective of this study is to provide insights into the fracture and fatigue response of amorphous alloys, which is important both from scientific and technological perspectives. The key questions we seek to answer through this study are the following. Do amorphous alloys undergo a ductile-brittle transition (DBT), and if so what are the reasons for it? What are the parameters that influence fatigue crack initiation in amorphous alloys and whether fatigue life can be improved by surface treatments? A related question is whether the BMGs are susceptible to deformation-induced crystallization (DIC). A Zr-based BMG, Zr41.2Ti13.75Cu12.5Ni10Be22.5 was utilized to conduct this study. By comparing the fracture and fatigue behaviors in the as-cast and annealed states {annealing was carried out below the glass transition temperature (Tg) because of established embrittlement effects}, we seek to provide answers for the questions posed above.
We begin by examining the influence of temperature on the toughness of BMGs. Impact toughness measurements show that the annealed samples, which are brittle at room temperature, recover the lost toughness beyond a critical temperature (TDB) and exhibit a sharp DBT. However, the hardness remains unaffected across the TDB. Fractography reveals nano-scale patterning and cleavage fracture in the brittle state, while the formation of thick vein-patterns and shear fracture are characteristics of the ductile state of the annealed samples. We explore various micro-mechanistic possibilities for explaining the features of this transition, including a critical Poisson’s ratio-toughness correlation.
Next, to understand the origins of fatigue crack initiation, we study the un-notched fatigue response of as-cast and sub-Tg annealed Zr-based BMG specimens. Because of embrittlement and nano-crystallization at the crack initiation region, the annealed specimens exhibit a lower fatigue life than the as-cast specimens. Shot-peening of the as-cast specimens did not exhibit significant improvement in their fatigue performance because of competing effects between the compressive residual stress field (CRSF) and deformation-induced softening. To further investigate surface and repeated loading effects, the tribological response of the as-cast Zr-based BMG was compared with specimens annealed above and below the Tg. A good correlation between the hardness (increasing as a function of the annealing temperature) and wear rate was obtained. The formation and peeling of the oxide layer formed during testing was the primary wear mechanism in all the specimens.
Lastly, crystallization was observed within the deformed region of the as-cast Zr-based BMG repeatedly scratched with a sharp diamond indenter. But, transmission electron microscopy (TEM) does not reveal any evidence of crystallization within the indents formed within an electron transparent film formed by laser deposition of the as-cast Zr-based BMG. Absence of crystallization in deformed regions obtained by designing critical experiments, which avoid artifacts generated during sample preparation, suggests that the occasional observation of DIC might be an exception rather than the rule in BMGs.
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