Room-Temperature Plasticity in Amorphous and Amorphous-Nanocrystalline Alloys

非晶及非晶納米晶合金的室溫塑性

Student thesis: Doctoral Thesis

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Award date2 Oct 2019

Abstract

Plasticity in amorphous and amorphous nanocrystalline alloys, including nano-grained amorphous-amorphous and amorphous-crystalline alloys, recently attracted tremendous research interest because of their great potential in various structural and functional applications. For monolithic amorphous alloys, it has been an enduring and heated debate whether the yield strength of amorphous alloys is size dependent or size independent. Through extensive in-situ and ex-situ micro- and nano-compression studies, we demonstrate that the average yield strengths of amorphous alloys are size dependent as above a critical length scale while becomes size independent as below. Interestingly, such a transition is stochastic in compression and featured with a strong fluctuation in the apparent yield strengths when the micropillar size is near the critical length scale. A stochastic shear-band initiation model is then developed, which yields a universal scaling relation onto which the data across a wide range of pillar sizes and a variety of metallic glasses can be collapsed. 

In addition, we also studied the plasticity in nano-grained amorphous alloys (or named nanoglass). Through detailed experimental and theoretical analyses, we uncovered the mechanism of plasticity initiation in the Ni-P nano-grained amorphous alloys which contains nano-sized hard amorphous inclusions and soft amorphous inter-granular regions. In this dual nano-phase amorphous structure, plasticity is mainly initiated through the elongation and coalescence of the amorphous nano-grains, resulting in a shear band much wider than in its monolithic counterpart. Consequently, the nano-grained amorphous alloys turns out to be softer at the microscopic scale. At the fundamental level, our finding provides a universal mechanism which explains the unusual strength weakening observed in a variety of dual nano-phase amorphous systems. 

Furthermore, we successfully fabricated a series of Fe-based amorphous nanocrystalline alloys through the controlled nanocrystallization in Fe-based amorphous ribbons, which resulted in rather thin amorphous inter-granular films with an average thickness reducing from 10 nm to less than 1 nm as the volume fraction (Vf) of the nanocrystals increased from 16% to 95%. Based on extensive microcompression experiments, we identified four types of distinctive deformation behaviors in our amorphous nanocrystalline alloys, including (I) multiple shear banding for Vf < 16%, (II) single shear banding for 25%< Vf < 67% , (III) shear banding induced cavitation and cracking for 72%< Vf < 86% and (IV) distributed plasticity for Vf > 90%. Through transmission electron microscopy and strain rate sensitivity analyses, we reveal that plasticity in our Fe-based amorphous-nanocrystalline alloys is mainly dominated by the soft amorphous inter-granular films rather than the hard nanocrystals and, therefore, the unusual phenomenon of multiple brittle-to-ductile transitions, as manifested by the four deformation types, could be well explained by the micromechanics and thermodynamics that govern the residual stress and shear-induced cavitation in an amorphous-nanocrystalline alloy. From the perspective of applications, our current findings provide quantitative insights into the design of strong yet ductile amorphous nanocrystalline alloys which could have important structural and functional applications.