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Title:Modeling of plastic deformation due to slip-its implications in characterizing fatigue crack growth thresholds and non-Schmid behavior in transforming alloys
Author(s):Alkan, Sertan
Director of Research:Sehitoglu, Huseyin
Doctoral Committee Chair(s):Sehitoglu, Huseyin
Doctoral Committee Member(s):Lambros, John; Ertekin, Elif; Krogstad, Jessica Anne
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Fatigue threshold
Non-Schmid slip
Abstract:This study will be divided into two main pillars in interrogating the microstructural barriers prevalent at the nanoscale: (i) the effect of twin and grain boundaries along with their contribution on the irreversibility of the crack-tip emitted slip under cyclic fatigue loading, (ii) the atomistic scale lattice resistance against glide motion of dislocations in a perspective of laying out the dislocation core-crystal structure and applied loading interplay in particular embracing the plastic behavior in ordered shape memory alloys. The primary goal of the current work is to provide physical insight for the implications of the slip-mediated plasticity in fatigue crack growth and non-Schmid behavior on both experimental and theoretical grounds. In first part of the current study, the near threshold fatigue crack growth behavior of nanocrystalline Ni-2.89% wt. Co (Ni-Co) alloy with nanotwinned microstructure will be characterized in particular based on the contribution of microstructural variables such as the on-going crack-tip emitted dislocation and twin/grain boundary (i.e. of , and types) interactions, the pre-existing dislocation density and the characteristic dimensions of grain size, twin thickness and spacing. In order to accomplish this task, we architectured the microstructure of nanocrystalline Ni-Co alloy by conducting annealing treatments at various temperatures and promoted grain-twin coarsening as well as varying the pre-existing slip density. Furthermore, we conducted experiments on these engineered microstructures under uniaxial tension and cyclic fatigue loading employing Digital Image Correlation technique at different length scales. The monotonic tension experiments enables to characterize the contribution of microstructural variables on the mechanical response of Ni-Co alloy, such as ductility and strength. On the other hand, the cyclic fatigue crack growth experiments help identify the variation of crack growth behavior and threshold levels of Ni-Co alloy along with the architectured microstructures. The experimental measurements show that nanotwins hierarchically embedded in the microstructure of Ni-Co alloy promotes ductility and fatigue threshold in a profound fashion with decreasing characteristic dimensions. Meanwhile, the primary focus is on nanocrystalline Ni-Co alloy, the current work has been put forward to establish a physical model informed by the multi-scale microstructural parameters which is capable of predicting the fatigue threshold levels of metallic materials devoid of empiricism. To that end, we simulated the interaction of crack-tip emitted slip and the grain/twin boundaries within the framework of Molecular Dynamics and characterized the on-going dislocation reactions as well as the crystalline resistance at the boundary against the glide of dislocations participating in these reactions. Subsequently, the effective threshold stress intensity factor range metric is predicted on theoretical grounds by incorporating physical parameters such as the friction stresses both inside the pristine crystal and at the grain/twin boundaries along with the glide geometry associated with the prevailing dislocation reactions into the continuum scale dislocation motion equations. The modelling efforts for cyclic glide motion of crack-tip emitted dislocations as a function of applied stress factor range , , provided a quantitative basis to determine the microstructure-sensitive crack threshold levels on theoretical grounds. The results indicate that coherent twin boundaries ( type ) impart superior fatigue properties to Ni-Co alloy compared to the less-coincident grain boundaries of and types. As a distinguishing finding of the present study, the increasing frequency of the grain and twin boundaries-linked with the grain size and twin spacing & thickness- are determined to promote the fatigue threshold levels in Ni-Co alloy. In the second part of the study, the mechanisms governing the slip-mediated plasticity of the ordered shape memory alloys, particularly Fe3Al and NiTi, are focused on both experimental grounds (via Digital Image Correlation technique) and employing theoretical atomistic scale dislocation core simulations. The non-Schmid character of the plastic response profoundly governs on the functional performance of this class of alloys imparting tension-compression asymmetries and anisotropic glide resistance as a function of crystal orientation. To accomplish this task, the dislocation core structures are calculated employing Molecular Statics-Dynamics simulations and subsequently the interaction mechanisms of the non-planar dislocation core structure with applied stress tensor components are identified considering the corresponding crystal symmetries involved. The dislocation core shape that is governed by the atomistic scale disregistry distributions under applied loading is demonstrated to play a decisive role on the anisotropic glide resistance which results in deviations from the Schmid law. The theoretical predictions for the anisotropic glide resistance are demonstrated to be in well agreement with the high magnification experiments conducted on single crystals of these alloys. The current methodology followed enables us to build a comprehensive understanding for the non-Schmid glide behavior of dislocations in austenitic phase of Fe3Al and NiTi shape memory alloys considering the effects of both glide and non-glide stress components. Furthermore, generalized yield criteria for these materials are established extending atomistic scale core mechanics to the macro-scale crystal plasticity.
Issue Date:2017-12-06
Rights Information:Copyright 2017 Sertan Alkan
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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