Deformation and Transitions at Grain Boundaries VII: Grain Boundary Structure: Disconnections, Complexions, Twins
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
Monday 8:00 AM
February 24, 2020
Room: 5B
Location: San Diego Convention Ctr
Session Chair: Yi Guo, Imperial College London; Saryu Fensin, Los Alamos National Laboratory
8:00 AM Invited
Dislocation Density Distribution at Slip Band-grain Boundary Intersections: Yi Guo1; David Collins2; Edmund Tarleton3; Felix Hofmann3; Angus Wilkinson3; Ben Britton1; 1Imperial College London; 2Birmingham University; 3University of Oxford
We use ‘differential aperture micro-Laue diffraction’ (DAXM) to investigate the dislocation density distribution at grain boundary as the slip passage from one grain is blocked by its neighbouring grain. For those slip bands of the same variant blocked by grain boundary, some lead to local dislocation accumulation piling up at the boundary. We also observe a significant new mechanism, where the secondary dislocation seems to form a band structure that correlate with a specific crystallographic plane. We find such dislocation structures can be explicitly explained by the grain boundary ledge (disconnection) theory proposed by Li and Hirth who suggest this mechanism as process of minimising grain boundary energy density. We also include 2D to 3D GND comparisons, which are useful to consider regarding the breadth of 2D based HR-EBSD and related GND measurements within the literature.
8:20 AM
Continuum Dislocation Dynamics at Finite Deformation: Computational Modeling and Preliminary Results: Kyle Starkey1; Anter El-Azab1; Grethe Winther2; 1Purdue University; 2Technical University of Denmark
We present a finite deformation, density-based dislocation dynamics approach for mesoscale deformation of single crystals. A derivation of the dislocation transport equations at finite strain and lattice rotation in both Lagrangian and Eulerian forms is outlined, with a special focus on the kinematic coupling of dislocation density evolution on individual slip systems and on coupling via cross slip and dislocation reactions. A numerical scheme based on a staggered solution of the transport and stress equilibrium problems has been implemented within a finite element framework. We present simulations of several test problems such as low angle and twist boundaries, known geometrically necessary boundaries, which form under special loading modes. We also present preliminary predictions on geometrically necessary boundary formation and compare our results with TEM data. The lattice rotation effects of the forming patterns will be also be highlighted.
8:40 AM
Predictions of Grain Boundary Segregation in bcc Refractory Metals based on Electronic Descriptors: Yong-Jie Hu1; Ge Zhao2; Baiyu Zhang3; Chaoming Yang1; Xiaofeng Qian3; Liang Qi1; 1University of Michigan; 2The Pennsylvania State University ; 3Texas A&M University
We discovered a universal correlation between two electronic descriptors and the solute-defect interaction energies in binary alloys of bcc refractory metals with transition-metal substitutional solutes. One electronic descriptor is the bimodality of the d-orbital local density of states for a matrix atom at the substitutional site, and the other is related to the hybridization strength between the valance sp- and d-bands for the same matrix atom. To test the predictive capabilities of the correlation model, substantial first-principles calculations were performed to calculate the solute segregation energies at complex high-index grain boundary (GB) structures in W and Ta. By comparing with direct first-principles calculations, our model shows excellent performance on the predictions of segregation energies and the overall temperature-dependent GB segregation concentrations for different matrix-solute element pairs. This newly discovered correlation model could be used as a screening tool to study the solute segregation in grain boundaries and other crystalline defects.
9:00 AM
Twinning Nucleation Process at Grain Boundary in BCC Crystals: Scott Mao1; Jiangwei Wang1; 1University of Pittsburgh
Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. In metallic nanostructures, plastic deformation requires higher stresses than those needed in their bulk counterparts, resulting in the “smaller is stronger” phenomenon. Such high stresses are thought to favour twinning over dislocation slip. Deformation twinning has been well documented in face-centred cubic (FCC) nanoscale crystals. However, it remains unexplored in body-centred cubic (BCC) nanoscale crystals, especially twinning nucleation. Here, by using in situ high-resolution transmission electron microscopy, we show that how twinning is nucleated at bi-crystals grain boundary in BCC tungsten. Such deformation twinning and its nucleation is found to be pseudoelastic, manifested through reversible detwinning during unloading. Our work provides direct observations of deformation twinning and nucleation process as well as new insights into the deformation mechanism in BCC nanostructures.
9:20 AM
Unusual Size Effects from Tilted Twin Boundaries in Nano-twinned Metals: Caizhi Zhou1; Sixie Huang1; Irene Beyerlein2; 1Missouri University of Science and Technology; 2University of California at Santa Barbara
Nano-twinned (NT) metals have demonstrated exciting potential to break the strength-ductility trade-off, providing high strength without the loss of ductility. Tilted twin boundaries (TBs) naturally occur in columnar-grained NT metals. Here, using a combination of atomistic simulations and theoretical model, we study the effect of tilted TBs on the twin layer size effect on strength. Our results reveal that NT metals with the ideal TB orientation (i.e., with no tilt) exhibit continuously increasing strength with decreasing twin thickness and no inverse size effect. In contrast, NT metals with tilted TBs exhibit an inverse size effect, and the critical twin thickness, below which strength decreases, increases as the TB-tilt angle increases. Furthermore, the investigation identifies a critical value of TB tilt for which strength becomes independent of twin thickness and is the weakest. Our results provide the insights for the guideline of design of NT metals with desired properties.
9:40 AM Break
10:00 AM
Dislocation Interactions and the Formation of Low-angle Grain Boundary: Pranay Chakraborty1; Tengfei Ma1; Abigail Hunter2; Yinan Cui3; Lei Cao1; 1University of Nevada; 2Los Alamos National Laboratory; 3Tsinghua University
The simplest low-angle grain boundaries (GB) is the tilt GB that involves an array of pure edge dislocations. However, the dynamic process of the formation of low-angle GB or tilt GB is difficult to capture. In this work, we use a first-principles informed phase-field dislocation dynamics simulations to capture one possible formation mechanism of the tilt GB. We found that the morphology of the dislocations changes dramatically with interplannar distance between them. When the interplanar distance is large, dislocations form oval shape due to the lower self-energy of screw dislocations. When the interplanar distance is small, the initial dislocations gradually form an array of edge dislocations and thus a tilt GB. We found that the underlying mechanism is the strong elastic interaction between the edge components of the dislocation loops, which is verified by the analytical solution of linear elasticity.
10:20 AM
The Evolution of Interfacial States in Nanocrystalline Al-Ni-Ce and their Influence on Mechanical Behavior and Thermal Stability: Glenn Balbus1; Yolita Eggeler1; Jungho Shin1; Fulin Wang1; Verena Maier-Kiener2; Daniel Kiener2; Daniel Gianola1; 1University of California, Santa Barbara; 2Montanuniversität Leoben
Nanocrystalline metals can host many desirable properties; however, the high density of grain boundaries manifests as instabilities such as catastrophic shear localization and rapid coarsening, limiting the use of nanocrystalline metals. One strategy for improving thermal stability is through careful alloying with elements that exhibit a high tendency for grain boundary segregation, but the effects of grain boundary chemistry and energy on properties are non-trivial. Here, we report the effects of local chemistry and structure on the thermal and mechanical behavior of nanocrystalline Al-Ni-Ce characterized through indentation, ultrafast calorimetry, and in situ electron microscopy. Nanocrystalline Al-Ni-Ce alloys possess many desirable characteristics - a small grain size (< 10 nm), variety of atomic sizes, and strong propensity for elemental segregation. Our results suggest that these alloys have exceptional thermal stability (0.7 Tm) and mechanical properties (5.5 GPa Hardness), and tunable shear localization behavior due to the reconfiguration of grain boundaries.
10:40 AM
Effect of Silica and Rare Earth Oxides on the Grain Boundary Chemistries in Hot-pressed Boron suboxide (B6O) and the Observance of Nanolayer Films: Kristopher Behler1; Christopher Marvel2; Jerry LaSalvia3; J. Dunn-Synowczynski3; S.D. Walck1; Martin Harmer2; 1CCDC Army Research Laboratory, SURVICE Engineering; 2Lehigh University; 3CCDC Army Research Laboratory
Boron suboxide was hot-pressed using rare earth oxides and/or silica sintering aids. Small amounts of silica (1 vol.%) have shown improvements in the densification and hardness, while larger amounts have shown differing microstructures as observed by abnormal grain growth (AGG). The addition of 5 vol.% silica exhibits AGG at elevated temperatures suggesting a change in the grain boundary complexion. The addition of Y and La resulted in suppression of AGG, whereas, Yb promoted AGG. Observed differences in excess Si at the grain boundaries were measured using AC-STEM/EDS. While ordered monolayers of Y, La and Yb were observed, the Si appeared to be disordered. The addition of Eu, exhibited at least three chemically distinct grain boundary chemistries with varying amounts of Si:Eu ranging from Si-rich submonolayers to monolayers of Eu to ~1nm thick nanolayer films. Experimental results, DFT results and procedures will be presented.
11:00 AM
Thick Amorphous Complexions Enabled by Compositional and Thermal Manipulation: Charlette Grigorian1; Timothy Rupert1; 1University of California, Irvine
The behavior of nanostructured materials is determined by the structure and chemistry of interfacial complexions. In particular, amorphous complexions have been found to significantly improve the thermal stability, mechanical properties, and radiation tolerance of nanocrystalline alloys. In this work, we describe efforts to increase the thickness and stability of amorphous complexions by modifying alloy composition and the cooling rate after firing. Thick amorphous complexion formation is demonstrated in nanocrystalline Cu-Zr-Hf alloys, showing that increasing the chemical complexity of the grain boundary has a positive effect. Notably, these alloys have exceptional thermal stability, retaining a fine nanostructure even after two weeks of annealing at > 95% of the melting temperature. The cooling rate was found to be particularly important for sustaining thick amorphous films in nanocrystalline alloys, allowing us to begin to develop an understanding of the important kinetic factors which must be considered during processing.