Deformation and Transitions at Interfaces : Fracture and Decohesion
Sponsored by: TMS Functional Materials Division (formerly EMPMD), TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Thin Films and Interfaces Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Rozaliya Barabash, OakRidge National Lab; Shen Dillon, Universe of Illinois; Jian Luo, University of California, San Diego; Doug Spearot, University of Florida

Tuesday 2:00 PM
February 28, 2017
Room: 23B
Location: San Diego Convention Ctr

Session Chair: David Jones, Los Alamos National Laboratory

2:00 PM  Invited
Investigations on the Origin of Crack Initiation and Propagation Susceptibility of Prior Austenite Grain Boundaries in DP and Martensitic Steels: Fady Archie1; Stefan Zaefferer1; 1Max-Planck-Institut für Eisenforschung GmbH
    Prior austenite grain boundaries (PAGbs) in martensite are highly favored locations for initiation and propagation of cracks. A comprehensive statistical analysis of crystallography and crack susceptibility illustrates the underlying mechanisms of damage initiation at PAGbs under monotonic and cyclic deformation modes. The study is based on coupled high resolution SEM-based diffraction techniques, namely, electron channeling contrast imaging, and electron backscatter diffraction. Results from local micro-cantilever bending experiments have been employed to study the influence of martensite variant selection on crack initiation at the prior austenite boundaries. The low fracture strength of PAGbs is analyzed with respect to two contributing factors, namely, solute element segregation, and localized residual elastic stresses. The first is studied by atom probe tomography, the latter by full field elastic strain analysis using focused ion beam milling and digital image correlation. Possible tracks for grain boundary engineering of DP-steels are revealed through varying heat treatment parameters.

2:20 PM  
Hydrogen Embrittlement and Grain Boundary Fracture in Nickel: A Perspective from Atomistic Simulations: Douglas Spearot1; Remi Dingreville2; Doruk Aksoy1; 1University of Florida; 2Sandia National Laboratories
    The most challenging issue regarding hydrogen embrittlement is associated with the convolutions and interactions between mechanisms that cause hydrogen-induced intergranular fracture. In this presentation, a statistical approach combined with molecular dynamics simulations is used to study the influence of hydrogen on intergranular decohesion. In order to isolate the role of hydrogen effects on the natural strength of the grain boundary, bicrystal systems are carefully selected based on their mechanical attributes and compatibility (elastic impedance, Schmid factors) across the grain boundary interface. The influence of hydrogen segregation on the grain boundary strength and work of separation will be discussed in terms of both the grain boundary structure and mechanical characteristics for sets of grain boundaries.

2:40 PM  
Segregation of Lead and Hydrogen Isotopes to Grain Boundaries in Nickel and Their Effect on Fracture: Richard Karnesky1; Samantha Lawrence1; Khalid Hattar1; Stephen Foiles1; Brian Somerday2; 1Sandia National Laboratories; 2Southwest Research Institute
    Atom-probe tomographic measurements and atomistic calculations demonstrate the segregation of deuterium (D) and of lead (Pb) to grain boundaries in nickel (Ni), lowering grain boundary energy. The degree of segregation of each element depends strongly on the structure of the grain boundary; coherent Σ3 boundaries show essentially no segregation and higher Σ boundaries have enrichments of over ten from the bulk concentrations, for example. The combined segregation of Pb and D to grain boundaries in Ni-201 co-embrittles the alloy and changes the predominant mode of fracture from what is observed in material that lacks either or both of the segregants.

3:00 PM  
Mesoscale Modeling of the Influence of Microstructural Gradients on Fracture: Gustavo Castelluccio1; Hojun Lim1; John Emery1; Corbett Battaile1; 1Sandia National Laboratories
    Traditional fracture mechanics theories infer crack tip driving force (local field) by surveying macroscopic physical quantities away from the crack (far field). In the case of inhomogeneous or anisotropic materials, fracture mechanics singularity approaches are not fully theoretically sound and their application relies on extrapolating methodologies with ad-hoc modifications. This work employed mesoscale-sensitive finite element simulations to investigate the impact of grain size and texture on the crack tip behavior. A dislocation crystal plasticity model was used to convey grain size effects by computing the constraint on dislocation structures. We assessed the impact of microstructural variability of single phase metals considering various displacement-based measurements on the crack tip driving forces, and quantified the variability in opening and sliding modes. The results show that grain size and texture affect the resisting fracture toughness and can induce a Mode II deformation. Furthermore, driving force measurements present different sensitivity to microstructural effects.

3:20 PM  Invited
Multi-probe, Multi-scale Analysis of Plasticity and Crack Blunting at Lath Martensitic Boundaries: Cem Tasan1; 1MIT
    Lath martensite plays the most critical microstructural role in commercial steels used in strength requiring applications. A deeper understanding of the governing factors of its plastic behavior, especially in presence of pre-cracks, is not yet fully available. Employing in-situ scanning electron microscopy deformation experiments with microstructure, surface topography and strain mapping (i.e. using electron backscatter diffraction (EBSD), secondary electron imaging, and microscopic-digital image correlation (μ-DIC), respectively) in samples with and without focused ion beam milled cracks, we address this problem. Results of the experiments and accompanying crystal plasticity simulations unravel the underlying factors in the heterogeneous plastic accommodation in martensitic steels, and especially the key roles of local differences in crystallography, lath size and orientation, defect and boundary densities.

3:40 PM Break

4:00 PM  Invited
The Nature of Grain Boundaries and Their Response to Shock Compression and Release in Tantalum: Marc Meyers1; Eric Hahn1; Saryu Fensin2; Tim Germann2; 1UCSD; 2LANL
    Molecular dynamics is used to evaluate the energies of grain boundaries and their dependence on misorientation. The role of grain boundaries in the generation and propagation of defects (dislocations and twins) is evaluated in both shock compression and release. This enables a quantitative understanding of the effect of polycrystallinity on shock compression and spalling.

4:20 PM  
The Influence of Second-phase Distribution on Dynamic Damage and Spall Strength: David Jones1; Saryu Fensin1; Daniel Martinez1; Carl Trujillo1; George Gray1; Ellen Cerreta1; 1Los Alamos National Laboratory
    We present a series of experiments studying the influence of the distribution of a second phase in a material on its response to dynamic tensile loading, or spall fracture. Investigation with high purity metals has shown that the void nucleation, growth and coalescence process can preferentially initiate at microstructural features such as grain boundaries, inclusions and vacancies. Similarly, a second phase can introduce nucleation sites and affect the spall resistance. Here, we have loaded two samples of Cu-1%Pb in plate impact experiments; with the lead either distributed in long bands or homogenously though the copper. An increased amount of damage was observed in the homogenized sample, due to the greater number of Cu-Pb interfaces present.

4:40 PM  
The Role of Interfaces in Nucleation of Dynamic Damage in FCC and BCC Materials: Saryu Fensin1; Eric Hahn2; Tim Germann1; Ellen Cerreta1; George Gray1; 1Los Alamos National Laboratory; 2University of California, San Diego
    For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. For high purity metals, it has been observed that voids tend to heterogeneously nucleate at grain boundaries and all grain boundaries are not equally susceptible to void nucleation. Several factors can affect the failure stress of a grain boundary, such as structure, energy and excess volume, in addition to its interactions with dislocations. Flyer plate simulations were carried out for four boundary types with different energies and excess volumes in both materials. These boundaries were chosen as model systems to represent various boundaries observed in “real” materials. In this work, we will compare the mechanisms behind void nucleation in FCC (Cu) and BCC (Ta) materials by using molecular-dynamics simulations. We will also explore the influence of grain boundary energy, excess volume and plasticity at the boundary on the failure stress of a grain boundary.

5:00 PM  
Void Nucleation and Growth at Grain Boundaries in Flat and Surface Perturbed Copper Bicrystals: Elizabeth Fortin1; Matthew Catlett1; Jenna Lynch1; Eric Loomis2; Pedro Peralta1; 1Arizona State University; 2Los Alamos National Laboratory
    Analysis of damage and deformation due to spall around grain boundaries (GBs) can provide a basis for connecting micro- to macroscale behavior. The addition of surface perturbations to bicrystal samples can provide insight on how strain localization occurs next to GBs during shock loading and how rippled release waves interact with boundaries to produce spall. Surface perturbed bicrystals were shocked using laser ablation at the Trident laser at Los Alamos National Laboratory, while flat bicrystals were shocked using flyer-plates via gas gun. Samples were cross-sectioned to perform characterization of damage using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM) to gather information on the characteristics of the GB, with emphasis on how rippled surfaces and material anisotropy affect strain localization and spallation as compared to flat specimens. Initial results show damage localized at the GB and surface perturbations led to heterogeneity of spall damage distribution in the grain bulks.

5:20 PM  Invited
Development of Long-range Crystallographic Correlations in Microstructures: Mukul Kumar1; Jonathan Lind1; David Bober1; 1Lawrence Livermore National Laboratory
    A positive correlation between the grain boundary character distribution and enhancements in properties is now well-established. What is frequently ignored, however, is the role of the correlated grain boundary network or the long-range orientation neighborhood, with most studies focusing on the characteristics of the individual boundaries. In this context, the development of twin-related domains (TRD) that comprise a supra-grain network in the microstructure has been little studied, though one can readily recognize their importance for intergranular processes, say crack propagation. In this presentation, we will attempt to address this issue: first, we will show examples where TRDs matter, and second, follow, statistically, the evolution of the complex morphology of TRDs during recrystallization and grain growth processes. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and supported by US DOE OBES, Materials Science and Engineering Division.