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Title:Quantifying the interface deformation mechanisms in nanostructured metals with atomic simulations and continuum field representations
Author(s):Li, Ruizhi
Director of Research:Chew, Huck Beng
Doctoral Committee Chair(s):Chew, Huck Beng
Doctoral Committee Member(s):Lambros, John; Geubelle, Philippe H; Sehitoglu, Huseyin
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Nanostructured metals
Interface
Molecular dynamics simulation
Grain boundary tractions
Abstract:Nanostructured metals possess ultra-high mechanical strength. A well-established consensus is that the deformation behaviors of these materials are governed by the extremely high density of interfaces. Previous studies have shown that, at nanoscale, interfaces act not only as obstacles of dislocations but also as main sources for dislocation emission. However, due to the lack of proper descriptors for interface structures, it is difficult to quantify the interface-dislocation interaction mechanisms in the nanostructured metals. Targeting to resolve this problem, my research is divided into two steps. In the first step, the interface-dominant plastic deformation mechanisms of semi-coherent Cu–Ag and Cu-Al nanolayered metals subjected to out-of-plane tension are investigated by molecular dynamics simulations. The results show that the initially planar Cu–Ag nanolayers abruptly become wavy at a critical tensile strain. High stress concentrations subsequently develop at the summits and valleys of the wavy Cu–Ag interlayer interfaces, from which micro-twinning partials are emitted. On the other hand, stacking fault tetrahedra (SFTs) are observed initiating from the Cu–Al semi-coherent interface. The closed SFTs within the Cu interlayers and open-ended SFTs within the Al interlayers envelop Cu-Al interface and result in considerable strain hardening of the Cu–Al nanolayers. These contrasting observations in Cu-Ag and Cu-Al nanolayers are associated with the shear resistance along the respective interfaces and they both introduce new length scale effect to determine the macro strength of the nanolayered metals. While postdictive MD simulations can be used to characterize the mechanics of the interfaces, in the second step of my research, the notion of continuum-equivalent traction fields is introduced as local quantitative interface descriptors to predict the deformation behaviors directly from the atomic interface structure. These descriptors are applied to symmetrical-tilt 〈110〉 Ni grain boundaries and successfully predict the critical stress for dislocation emissions. Additionally, the traction signatures along the grain boundaries are used to explain the tension-compression asymmetry of grain boundaries to nucleate dislocations. Traction signatures descriptors can potentially be used to establish the relationship between the atomic structure of grain boundary and its propensity to impede, absorb, or transmit dislocations. These descriptors are also expected to broadly apply to more complex interface structures, such as heterogeneous interfaces.
Issue Date:2017-02-17
Type:Thesis
URI:http://hdl.handle.net/2142/97655
Rights Information:Copyright 2017 Ruizhi Li
Date Available in IDEALS:2017-08-10
Date Deposited:2017-05


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