Deformation and Transitions at Grain Boundaries VII: Grain Boundary Decohesion and Fracture
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Shen Dillon, University of California, Irvine; Douglas Spearot, University of Florida; Jian Luo, University of California, San Diego; Jennifer Carter, Case Western Reserve University

Wednesday 2:00 PM
February 26, 2020
Room: 5B
Location: San Diego Convention Ctr

Session Chair: Josh Kacher, Georgia Institute of Technology; Jennifer Carter, Case Western Reserve University


2:00 PM  
Microstructural Predictions of Thermo-mechanical Fracture of Hydrided HCP Alloys: T. Hassan1; I. Mohamed1; Mohammed Zikry1; 1North Carolina State University
    A dislocation-density based multiple slip crystalline plasticity formulation and a new computational fracture approach have been used to investigate and predict thermo-mechanical fracture in hexagonal close packed (h.c.p.) materials with a focus on h.c.p. alloys with hydrides that have different crystalline structures than that of the matrix. This predictive framework has been used to understand and predict the interrelated effects of dislocation-density interactions, generation, and recovery on the competition between intergranular and transgranular crack nucleation and propagation The validated predictions indicate that transgranular fracture is dominated by dislocation-density interactions with hydrides and intergranular fracture is dominated by GB misorientations. The proposed modeling framework can provide guidelines for a fundamental understanding of materials subjected to thermo-mechanical loading conditions, such that failure resistant material systems can be attained.

2:20 PM  
Atomic Scale Modeling of Microstructure Effects on the Nucleation, Growth of Voids During Failure of Nanocrystalline Ta: Shayani Parida1; Jie Chen1; Avinash Dongare1; 1University of Connecticut
    Large scale molecular dynamics (MD) simulations have been carried out to investigate the mechanisms and stresses for void nucleation and growth during spall failure of nanocrystalline Tantalum microstructures at the atomic scales. Uniaxial expansion simulations are carried out to investigate the evolution of dislocation densities and twinning/de-twinning behavior during nucleation and growth of voids. The results suggest that voids nucleate at grain boundaries and triple junctions and growth of voids occurs along the grain boundaries accompanied by extensive deformation twinning at the crack tip. The effects of microstructure are considered for systems with grain sizes ranging from 15 nm to 60 nm that are as-created as well as that have undergone uniaxial compression and release to mimic the process of shock compression and spall failure. The role of initial microstructure in the interplay between dislocation-based plasticity and deformation twinning, and their effect on void nucleation stresses will be presented.

2:40 PM  
Understanding the Evolution of Defects Under Extreme Conditions in BCC Tantalum: Sumit Suresh1; Avinash Dongare1; 1University of Connecticut
    The capability of classical molecular dynamics (MD) simulations to model deformation behavior of tantalum microstructures relies on the accuracy of interatomic potentials to predict the contributions from crystal slip and twinning. In this work, a selective dataset for highly strained configurations of tantalum is generated using density functional theory (DFT) calculations to investigate the phase stability and defect energetics under extreme environments. The dataset includes strained configurations of gamma surface energy curves for different bcc slip systems, metastable phases and twin boundary energies and compared with predictions using several interatomic potentials. A comparative study on the capability to model defect evolution in nanocrystalline tantalum will be shown via MD simulations emulating shock loading conditions. A critical analysis of twinning behavior predicted by various interatomic potentials will be detailed with suggestions to fit these potentials to a more comprehensive dataset including strain and/or pressure dependent defect configurations.

3:00 PM  
Modeling Growth of Voids in Various Grain Neighborhoods Using Crystal Plasticity Theory: Paul Christodoulou1; Toby Francis1; Ricardo Lebensohn2; Tresa Pollock1; Irene Beyerlein1; 1University of California, Santa Barbara; 2Los Alamos National Laboratory
    Void growth is dependent on the deformation of the surrounding material and its microstructure, and the deformation is heterogeneous at grain boundaries. The goal of this work is to understand the role of crystallography and grain boundary/grain morphology on void growth in FCC and BCC metals. To this end, voids in model grain neighborhoods were simulated using a dilatation viscoplastic crystal plasticity model implemented in a fast Fourier transform-based solver (DVP-FFT). This talk will discuss the result of void growth in various neighborhoods: within grains aligned at different orientations with respect to loading, at grain boundaries, triple junctions and quadruple junctions. This method is quasi-static in nature, although we seek to study the dynamic properties of this material by simulating triaxialities found as a result of shock loading.

3:20 PM  Invited
Understanding Fracture Initiation Under Bending Conditions in AA6451 Using a Multiscale and Multimodal Electron Microscopy Approach: Josh Kacher1; Yung Suk Jeremy Yoo1; Sazol Das2; 1Georgia Institute of Technology; 2Novelis
    AA6xxx is a class of heat-treatable Al alloys that achieves a high strength-to-weight ratio by its heterogeneous microstructure, which includes intermetallic particles on the order of microns, sub-micron scale precipitates and dispersoids, and heterogeneities inherent to polycrystalline materials. Understanding how this complex microstructure dictates bending performance of these alloys, including fracture initiation processes, is critical for their expanded use in applications such as the automotive industry. In this study, different compositions and heat treatments of AA6451 samples were tested under bending and investigated post mortem. Crack formation was correlated at the mesoscale to orientation information using electron backscatter diffraction to gain a statistical understanding of microstructural influences on crack initiation. This statistical understanding was linked to specific deformation modes by fabricating lamellae from select locations for transmission electron microscopy and transmission Kikuchi diffraction analysis. Discussion will focus on the role of microstructure on dictating fracture initiation in AA6451.

3:40 PM Break

4:00 PM  Invited
Fatigue-crack Healing in Pure Nanocrystalline Pt Enabled by Boundary Evolution: Christopher Barr1; Ta Duong2; Daniel Bufford1; Nathan Heckman1; Michael Demkowicz2; Khalid Hattar1; Brad Boyce1; 1Sandia National Laboratories; 2Texas A&M University
    In most structural materials, fatigue cracks are assumed to only advance or arrest, with no possibility of retreat. However, by observing high-cycle fatigue of nanocrystalline Pt with in-situ transmission electron microscopy, we observed unexpected nanoscale crack healing. Under tension-tension fatigue loading, a fatigue crack was observed to propagate through a field of nanocrystalline grains, deflecting along expected cleavage planes and arresting in proximity to a triple junction. Over the course of 100K cycles, the crack tip subsequently healed about one grain diameter and eventually propagated in a different direction. Such behavior may be explained by the grain boundary evolution. Molecular dynamics analysis of the crystallographic configuration suggests that a near-Σ3 boundary migrated ahead of the crack tip, resulting in contact between crack flanks and cold welding. There are several implications for the observed crack healing behavior, including a possible explanation for enhanced crack growth resistance under mixed mode loading conditions.

4:20 PM  
In-situ Investigation of Intergranular Crack Initiation in Hydrogen Embrittled Inconel 725: Mengying Liu1; Lai Jiang1; Emmeline Sheu1; Michael Demkowicz1; 1Texas A&M University
    We perform an experimental investigation of the mechanisms of hydrogen-assisted crack initiation at grain boundaries in initially flaw-free samples of polycrystalline Inconel 725 (UNS725). We design specialized tensile specimens, introduce hydrogen into them using electrochemical charging, and perform in situ tensile tests in a scanning electron microscope (SEM). Consistent with previous studies, cracks are found to initiate along grain boundaries, including coherent twin boundaries. To elucidate the effect of plasticity on the crack initiation process, we use digital image correlation (DIC) to characterize surface plastic strain distributions. Potential mechanisms for hydrogen-assisted crack initiation will be discussed.

4:40 PM  
Atomistic Modeling of Effects of Alloy Element and Impurity Segregation on Grain Boundary Embrittlement in BCC Fe: Axel Seoane1; Ziqi Xiao1; Xian-Ming Bai1; 1Virginia Polytechnic Institute and State University
    Fe-based ferritic alloys are promising structural materials for next-generation fast reactors. Radiation can cause segregation of alloying elements or impurities to local sinks such as grain boundaries (GBs). The local chemistry change due to radiation may affect the cohesive strength of GBs and lead to GB embrittlement or strengthening. In this work, molecular dynamics and density functional theory calculations are conducted to study the embrittlement potency of different solute elements at GBs in bcc Fe such as P, Si, O etc. Four different GBs are studied: Σ3(111), Σ3(112), Σ5(210), Σ5(310). Molecular Dynamics simulations show that a correlation exists between the substitution energy of the solute and grain boundary embrittlement potency for all grain boundaries. The correlation is further tested using density functional theory calculations. The interactions between solute elements and Fe at GBs such as their bonding characteristics are studied to explore the possible origin of GB embrittlement.