Deformation and Transitions at Interfaces : Grain Boundary Interactions with Dislocation and Twins in Hexagonal Metals
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

Thursday 8:30 AM
March 2, 2017
Room: 23B
Location: San Diego Convention Ctr

8:30 AM  Invited
Influence of Twin-grain Boundary Interactions on Further Twin Growth and Twin Transmission in HCP Metals: Carlos Tome1; M. Arul Kumar1; Irene J Beyerlein1; 1Los Alamos National Lab
    Deformation twins in HCP polycrystals nucleate from grain boundaries (GBs) and propagate to span the entire grain. The shear transformations associated with twins are constrained by the grain boundaries and most twins are terminating at GBs. This constraint creates a stress reversal in the twin region that can be quantified as a backstress, and which tends to suppress the twin growth. At the same time, the local reaction at the GBs creates stress concentrations in the neighboring grain in the vicinity of twin tip. This will lead to twin transmission across GBs. In this work we employ a full-field Fast-Fourier-Transform modeling tool to characterize the role of twin-grain boundary interactions on twin growth and twin transmission. In this process we have developed a correlation between twin growth, grain size and neighboring grain crystal orientation effect. We also make a connection between twin transmission, neighboring grain orientation and material anisotropy.

8:50 AM  
Investigation of Dislocation Activities during Slip Transmission across Alpha/Beta Interface in Ti-alloy Using Microscopic Phase-Field: Pengyang Zhao1; Chen Shen2; Ju Li3; Michael Mills1; Yunzhi Wang1; 1The Ohio State University; 2GE Global Research, US; 3Massachusetts Institute of Technology
    Plastic deformation of alpha/beta Ti-alloy is largely controlled by the interplay between the inter-phase interfaces and dislocation motion, of which the underlying mechanisms still remain unclear. Using the recently developed microscopic phase-field model, we study the dynamic process of dislocation-mediated slip transmission across alpha/beta interfaces. Quantitative simulation is achieved by incorporation of the generalized stacking fault (GSF) energy surfaces obtained from ab initio calculations and matching two GSFs at alpha/beta interface with experimentally revealed Burgers orientation relationship. A continuous dislocation flux driven by external stress (accounting for dislocation sources in real materials) is considered to glide across alpha-beta-alpha sandwich configuration and the simulation result reveals successive pile-up, reaction, transmission, and storage events. In particular, mechanisms proposed previously based on experimental characterizations are examined quantitatively and new mechanisms of slip transmission assisted by Shockley partials are proposed based on the current simulations. A coarse-grained dislocation density based description is also proposed.

9:10 AM  Invited
Slip-induced Twinning in Ti: Maryam Ghazisaeidi1; 1Ohio State University
    Slip induced twinning is often predicted with phenomenological parameters such as favorable Schmid factor in adjacent grains and the slip/twin compatibility parameter known as the Luster-Morris factor. Although, a combination of these factors often predicts twin nucleation correctly, experimental observations show outlier cases occasionally. On the other hand, the process of slip transfer through the grain boundary depends on the atomic structure of the boundary and the possibility of various dislocation reactions. We study dislocation/grain boundary interactions in Ti using molecular statics calculations to reveal the underlying mechanisms of slip induced twin nucleation in Ti. We conduct a systematic study of several grain boundaries with various combinations of Schmid and Luster-Morris factors. We then simulate the interactions of <a> type prismatic slip with these boundaries. Different trends of dislocation/GB interactions are discussed and the results are compared to existing experimental observations.

9:30 AM  
{1012} Twin Faceting on Non-tilt Interfaces: Christopher Barrett1; Haitham El Kadiri1; 1Mississippi State University
    Asymmetric tilt faceting has been demonstrated as a major mechanism effecting {1012} twin growth and enabling it to become the predominant twin mode in hexagonal close-packed metals. Non-tilt faceting, however, is still poorly understood. Low-energy twist facets have been recently observed using electron back-scatter diffraction and atomistic simulations. Here we develop interfacial defect theory for mixed and twist faceting using trichromatic patterns and demonstrate their application for {1012} twins. Atomistic simulations of twin nucleation validate the importance of these facets. Using mixed and pure twist facets, a new mechanism of twin nucleation in single crystal magnesium is presented. The twin embryo develops off of a high energy {1011} stacking fault, reducing the stacking fault energy by reshuffling atoms into a new orientation. Once the embryo reaches a critical thickness, traditional {1012} boundaries are formed via disclination dipole nucleation and dominate further growth.

9:50 AM  Invited
Intergranular and Transgranular Fracture Modes in H.C.P. Alloys: Ismail Mohamed1; S. Ziaei1; 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 intergranular and transgranular fracture in hexagonal cubic packed (h.c.p.) materials with a focus on h.c.p. alloys subjected to larges changes in strains, strain-rates, and temperatures. This validated 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 predictions indicate that iodine pits that have diffused into the GB dominate intergranular fracture and that trangranular fracture is dominated by interrelated dislocation-density interactions and threshold fracture stresses along trangranular cleavage planes.

10:10 AM Break

10:30 AM  Invited
Dislocation/Boundary Interaction in Titanium: Molecular Dynamics Study: Mohammad Shahriar Hooshmand1; Maryam Ghazisaeidi1; 1The Ohio State University
    Dislocation/grain boundary interactions play an important role in mechanical properties of materials such as strength, ductility and fracture toughness. We implemented molecular statics approach to study the interactions between < a > screw, < c > edge on (10-10) prism plane and < c+a > mixed dislocation on pyramidal-I plane with two different Symmetrical tilt grain boundaries (STGBs)- namely (-1011) and (-1013) in Ti. Depending on the slip system, atomic structure of STGB and stress/strain states, different dislocation reactions were identified. In some cases, we observed (10-12) twin nucleation due to slip transfer at the boundary. Other than twin nucleation, slip transmission through the boundary, dissociation of the incoming dislocation into the Shockley partials and twin dislocation formation on the were boundary observed. These results shed light on the underlying mechanisms governing the deformation of Ti.

10:50 AM  Invited
Imaging and Analyzing Slip in Three Dimensions: Rulin Chen1; Jonathan Lind2; Reeju Pokharel3; David Menasche1; Anthony Rollett1; Robert Suter1; 1Carnegie Mellon University; 2Lawrence Livermore National Laboratory; 3Los Alamos National Laboratory
    Precise, non-destructive measurements of lattice orientation in three dimensions are used to image rotations associated with slip events during plastic deformation of polycrystals. These rotations form low angle interfaces within what were previously virtually perfect crystalline grains. Near-field High Energy Diffraction Microscopy has been used to perform such measurements on materials including hexagonal zirconium and cubic copper. In hexagonal materials, it is straightforward to pick out intra-granular rotations corresponding specific slip systems and to characterize the variations in rotation axis and angle corresponding to individual events. Furthermore, the degree to which these rotations occur on expected crystallographic planes and the variations in the above quantities within these planes can be characterized. This talk will contrast behavior observed in hexagonal materials with characterizations in copper. The results of this analysis can be used to inform models of polycrystal plasticity.

11:10 AM  Invited
Early Stages of Microstructure and Texture Evolution during Beta Annealing of Ti-6Al-4V: Adam Pilchak1; Gordon Sargent2; Lee Semiatin1; 1Air Force Research Laboratory; 2UES, Inc.
    The early stages of microstructure evolution during annealing of Ti-6Al-4V in the beta phase field were established. For this purpose, a series of short-time heat treatments was performed using sheet samples that had a noticeable degree of alpha-phase microtexture in the as-received condition. Reconstruction of the beta-grain structure from electron-backscatter-diffraction measurements of the room-temperature alpha-phase texture revealed that microstructure evolution at short times was controlled not by general grain growth, but rather by nucleation-and-growth events analogous to discontinuous recrystallization. The nuclei comprised a small subset of beta grains that were highly misoriented relative to those comprising the principal texture component of the beta matrix. From a quantitative standpoint, the transformation kinetics were characterized by an Avrami exponent of approximately unity, thus suggestive of metadynamic recrystallization. The recrystallization process led to the weakening and eventual elimination of the initial beta texture through the growth of the population of highly misoriented subgrains.

11:30 AM  
In-situ Probe of Twinning Dynamics at a Tensile Twin Tip in Mg: Lin Jiang1; M. Arul Kumar2; Irene Beyerlein2; Dalong Zhang1; Xin Wang1; Subhash Mahajan3; Enrique Lavernia1; Julie Schoenung1; 1University of California Irvine; 2Los Alamos National Laboratory; 3University of California-Davis
    Twinning dynamics in metals, such as twin nucleation and propagation is of great interest. In the present study, we developed an in-situ STEM technique for introducing local deformation around a tensile twin (TTW) tip, and thus capturing the twinning dynamics around the TTW tip in Mg. We also utilized full field Fast Fourier Transform model to simulate the local stress variations introduced by the TTW. For the first time, we directly captured the nucleation of a (0-111) compression twin (CTW) around a propagating (0-112) TTW tip during the stressing. Also, we have directly observed the dissociation of this CTW (about 100 nm) into a twin nucleus with only 16-20 atom layers during the unloading. Numerical simulation revealed that the local stress induced by the TTW led to the nucleation and detwinning of the CTW and the (0-111) [01-1-2] CTW was a preferred twin variant to nucleate around the TTW tip.

11:50 AM  Invited
Mesoscale Response of Titanium Alloy Tensile Samples Measured through High Energy X-ray Experiments: Joel Bernier1; Paul Shade2; Todd Turner2; Darren Pagan1; David Menasche3; Robert Suter3; Peter Kenesei4; Jun-Sang Park4; Jonathan Almer4; 1Lawrence Livermore National Laboratory; 2Air Force Research Laboratory; 3Carnegie Mellon University; 4Argonne National Laboratory
    High energy synchrotron x-ray characterization methods have been utilized to map the 3D microstructure and evolving intergranular elastic strain state of a single phase polycrystalline titanium alloy tensile specimen. The stress state of individual grains is found to deviate significantly from the macroscopic applied boundary conditions due to the influence of the local grain neighborhood. The experimental data was used to calibrate and benchmark a crystal plasticity finite element model. The combined experimental and simulation results indicate that the residual stress state in the material prior to starting the mechanical test also had an influence on the micromechanical state of the sample during loading. Finally, we explore the use of a line focused x-ray beam to measure grain cross-section averaged elastic strains/stresses, resulting in sub-grain resolution in the direction orthogonal to the plane of the x-ray beam (along the sample tensile axis).

12:10 PM  Invited
Heterogeneous Deformation in Polycrystalline Mg-Y by In Situ 3D-XRD: Leyun Wang1; Zhonghe Huang1; Xiaoqin Zeng1; Sangbong Yi2; Erica Lilleodden2; Peter Kenesei3; Jun-Sang Park3; 1Shanghai Jiao Tong University; 2Helmholtz-Zentrum Geesthacht; 3Argonne National Laboratory
    Small amount of Y solutes significantly improves the ductility of Mg alloys. This phenomenon is often attributed to non-basal slip modes becoming more active with Y solutes. On the other hand, quantitative evaluation of how Y alters the relative critical resolved shear stress (CRSS) for different slip modes is still missing. In this work, we tackle this problem by using a three dimensional X-ray diffraction (3D-XRD) technique available at the Advanced Photon Source. Polycrystalline Mg-Y tensile specimens were incrementally deformed with 3D-XRD characterization to track the evolution of individual grains, including their crystal orientation, position, and stress state. Activation of slip systems is inferred from crystal orientation change in each grain. CRSS for basal slip, prismatic slip, and pyramidal c+a slip are estimated from the analysis. For comparison, same experiment was performed for pure Mg to understand the role of Y solutes for ductility improvement.