Deformation behaviour of bulk metallic glasses under complex stress states

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Deformation behaviour of bulk metallic glasses under complex stress states


Author: Chen, Shun Hua
Title: Deformation behaviour of bulk metallic glasses under complex stress states
Degree: Ph.D.
Year: 2015
Subject: Metallic glasses -- Mechanical properties.
Bulk solids.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xxv, 217 leaves : illustrations ; 30 cm
Language: English
InnoPac Record:
Abstract: Due to the non-ordered atomic arrangements, bulk metallic glasses (BMGs) exhibit unique properties, such as high corrosion resistance, excellent mechanical performances and attractive processing potential. One of the key issues of BMGs is the room-temperature brittleness, especially under tensile loading. Over the past three decades, much effort has been devoted to improve the global room-temperature plasticity of BMGs, and great achievements have been made by optimizing compositions, fabricating composite micro/nano-structures and introducing geometric confinements. However, these studies mainly focus on the deformation behaviour of BMGs under uniformly distributed stress states. In practical applications of structural materials, they are seldom subject to uniform stress distributions (either under compression or tension), but are mostly under complex stress states. In this work, the prime aim is to shed light on the deformation behaviour of BMGs under complex stress states and give insights into the deformation mechanisms of macroscopic BMG structures. The effect of stress gradients on the deformation behaviour of BMGs has initially been examined using three-point bending tests. The results have shown that both compressive and tensile stress gradients are able to improve the plastic deformation behaviour of BMGs, which have further been validated by examining and comparing the fracture surface features under different stress states. Subsequently, by designing Z-shaped specimens with stress gradients, we have achieved some tensile axial elongations in monolithic BMGs. It is demonstrated that the stress concentration areas of the BMG specimens facilitate the formation of shear bands and the presence of "soft" and {212040}hard regions due to the stress gradient confines the propagation. Thereafter, by tailoring a complex stress distribution on curved BMG specimens, for the first time, tunable large axial elongations have been achieved in these BMG structural elements under tensile loading, resulting from the controllable plastic deformation in localized regions and the straightening of the curved segments. With the designed complex stress distribution, the deformation process of the curved part in the BMG structural elements has been found to consist of three stages, i.e., the shear band initiation stage (stage I), the shear band multiplication stage (stage II) and the final rapid fracture stage (stage III). With uniformly distributed stresses, the conventional tensile specimens fracture (stage III) catastrophically after stage I. However, in the curved specimen, the complex stress distributions result in not only more stable shearing processes in stage I, but also the evolution of stage II before stage III, which causes the formation of a large number of shear bands. These findings suggest that although the tensile BMG specimens exhibit brittle behaviour under a uniform stress state, large plastic deformation in localized regions could be achieved by controlling the complex stress distributions. The controllable geometries provide a feasible route for further understanding the deformation mechanisms of BMGs under more complex stress states, and for designing macroscopic BMG structures in potentially structural applications.
To uncover the deformation mechanisms of BMG structures, compression tests on five BMG structural elements have been carried out. The findings reveal three different types of deformation modes in the designed BMG struts, which can be regarded as three fundamental deformation modes in macroscopic BMG structures. Thereafter, a series of cellular BMGs with remarkable energy absorption capacity have been designed and examined under compressive testing. The greatly enhanced energy absorption capacity is due to the large plastic bending of the struts, the blocking of the crack tips and the severe plastic deformation at the nodes. Analytical analysis capturing the effect of the strut thickness and the number of unit cells on the deformation behaviour of the cellular BMGs has been conducted and discussed. The findings have demonstrated that the designed macroscopic cellular structures are more efficient in dissipating mechanical energy than the microscopic cellular structures. The achievement of high energy absorption capacity in cellular BMGs provides a valuable guidance on designing cellular BMGs for energy absorption applications.

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