Phase-field modeling on shear banding in amorphous alloys with improved ductility

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Phase-field modeling on shear banding in amorphous alloys with improved ductility

 

Author: Zhang, Hongying
Title: Phase-field modeling on shear banding in amorphous alloys with improved ductility
Degree: Ph.D.
Year: 2013
Subject: Metallic glasses.
Amorphous substances.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: 163 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2653101
URI: http://theses.lib.polyu.edu.hk/handle/200/7300
Abstract: Metallic glasses (MGs) have attracted scientific and technological attention due to their excellent physical, mechanical and chemical properties. Although the brittle nature of bulk metallic glasses (BMGs) limits their practical applications, it has been found that some new kinds of amorphous alloys show improved ductility through introducing the pores or secondary phases in their microstructures. The improved ductility is also observed in MGs with decreasing sample sizes. To date, the mechanisms governing the improved ductility in those MG systems are still under debate. Thus for the design of MGs for engineering applications, it is urgent to explore the mechanisms of the improved ductility. In this work, we use the phase-field modeling methods to study the origin of improved ductility in porous BMGs, in-situ formed BMG matrix composites and nano-sized MGs which are typical MG systems showing ductility. Through simulations on the plastic deformation in porous BMGs, we found that the pore with much smoother surface can detour the incident shear bands, resulting in the improved ductility. On the other hand, adiabatic shear banding occurs when the pores are not filled with gas or are in vacuum, avoiding the brittle failure due to the generation and multiplication of new shear bands.
The development of the in-situ formed BMG matrix composites containing dendrite microstructures is another outstanding advancement of the applications of the metallic glasses. In this work, the effects of two factors of the dendrites, i.e., the rotation angle representing the dispersion pattern and the fracture energy of such crystalline phase, on the plastic deformation of BMG composites are discussed. It is observed that the tips of the dendrite play a major role in the bifurcation and detour of the shear bands when the rotation angle is from 0° to 15°. It is found that the dendrite with high fracture energy can achieve high crack resistance and then cause more obvious bifurcation and detour of the shear bands, while the ones with lower fracture energy can cause more fracture area (both in matrix and in crystalline phases) to absorb more strain energy, which could result in improved ductility of the BMG composites. These features observed from simulation are useful in explaining the mechanisms of the improved ductility in the BMG composites and are also helpful in guiding the design of those composites for engineering applications. Besides, the improved ductility and mechanical strength are also observed in the simulation when the size of the sample decreases, especially to the nano-scale (20 nm - 200 nm). Furthermore, some meaningful features and phenomena such as the emergence of necking and ductility during deformation are revealed from the simulated results and are found to be consistent with the experimental results. In this work, we elucidate two factors that lead to the size effect on the mechanical properties of MG nanowires, i.e., the fractions of initial deformation defects in the nanowire interior and on the nanowire surfaces. With the decreasing diameter, it is noted that the initial states of the deformation defects on the surfaces which are difficult to be quantitatively measured by experiments, play an important role in the deformation of MG nanowires. Based on the results of simulations, the Hall-petch like relation between the fracture strengths and the sample diameters are derived, which can well explain the discrepancies among the previous reports in literatures.

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