Tensile Deformation of Metallic Glass: Understanding the Effects of Specimen Size, Structural State and Testing Temperature


August 2023

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The disordered or amorphous structure of metallic glasses results in a unique physical and chemical feature, which makes them suitable for a variety of applications, such as precise metal parts, sporting equipment, energy conversion technologies, transformer cores, etc. In particular, micro- and nano-sized devices can benefit from the homogenous structure (down to nanoscale), high strength (1-3 GPa), large elastic strain limit (2-3%), and remarkable thermoplastic formability of metallic glasses. However, the disordered structure is also responsible for structural softening, which results in negligible tensile ductility of metallic glasses at room temperature. The lower tensile ductility is considered as a major drawback for limiting their applications. Due to the structural softening, the plastic strain in metallic glasses is localized in narrow shear bands (~20– 40 nm in thickness), which results in catastrophic failure in tension. Numerous factors such as the elastic constants, testing temperature, strain rate, sample size, and cooling rate affect the development of the shear bands. To comprehend the origin of shear band formation and lack of tensile ductility in metallic glasses, it is crucial to investigate the effects of these factors and propose a comprehensive model. While the effect of many parameters on the deformation of metallic glasses has been understood, the sample size effects have remained controversial. The contradictory findings regarding the size effects of metallic glass are triggered by numerous reasons, such as improper sample geometry, use of high-energy irradiation during sample preparation and testing as well as the absence of statistically reliable data from in-situ testing. To understand the tensile deformation behavior of metallic glass on the nanoscale, this study examines the effect of sample diameter, structural state, and testing temperature on the shear band formation process in a Pt57.5Cu14.7Ni5.3P22.5 (Pt-based) metallic glass. The novel thermoplastic drawing method was developed to manufacture numerous dog-bone-shaped tensile specimens with diameters ranging from 100 μm to 100 nm. A custom-built experimental setup was used to fabricate and test hundreds of nanosized tensile samples from Pt-based metallic glass at different temperatures. The fracture morphologies of the samples after tensile testing show a gradual shift from zero ductility (shear band mediated) to ductile necking (homogenous deformation) with decreasing sample diameter. Our observation indicates that a reduction in the testing temperature has a similar effect on the deformation behavior in all stages as the sample diameter. With a smaller sample size and/or lower temperature processing of metallic glass, the critical diameter of homogenous deformation increases. The relationship between the sample size and testing temperature in tensile fracture of metallic glasses can help in understanding the origin of ductility in metallic glass. These findings are verified and analyzed using the current shear band formation models for bulk specimens in metallic glasses. In addition, a comprehensive model for shear band development in metallic glasses is proposed, which describes the effects of sample size, structural state, and testing temperature on size-dependent changes in tensile deformation.



Engineering, Mechanical