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Plasticity in body centered cubic (BCC) crystals is shown to be controlled by slow screw dislocation motion, owing to the thermally-activated process of kink pair nucleation and migration. Through three dimensional discrete dislocation dynamics simulations, this work unravels the mystery of how such slow screw dislocation behavior contributes to extremely rapid strain bursts in submicron BCC tungsten (W) pillars, which is typical of BCC metals. It is found that strain bursts are dominated by the motion of non-screw dislocations at low strain rate, but are more influenced by screw dislocations at high strain rate. The total, and partial strain burst magnitude due to screw dislocations alone, are found to exhibit rate dependence following a power law statistics with exponent of 0.65. Similar power law statistics are also obeyed for the standard deviation of the corresponding plastic strain rate. The role of screw dislocations is attributed to the changing nature of dislocation source operation at different strain rates. The corresponding spatial distribution of plastic deformation is also discussed based on the uniqueness of the simulation method in reproducing the distribution of slipped area and plastic strain with very high spatial resolution.more » « less
Twin boundary (TB) strengthening in nanotwinned metals experiences a breakdown below a critical spacing at which softening takes over. Here, we survey a range of nanotwinned materials that possess different stacking fault energies (SFEs) and understand the TB strengthening limit using atomistic simulations. Distinct from Cu and Al, the nanotwinned, ultralow SFE materials (Co, NiCoCr, and NiCoCrFeMn) intriguingly exhibit a continuous strengthening down to a twin thickness of 0.63 nm. Examining dislocation slip mode and deformation microstructure, we find the hard dislocation modes persist even when reducing the twin boundary spacing to a nanometer regime. Meanwhile, the soft dislocation mode, which causes detwinning in Cu and Al, results in phase transformation and lamellar structure formation in Co, NiCoCr, and NiCoCrFeMn. This study, providing an enhanced understanding of dislocation mechanism in nanotwinned materials, demonstrates the potential for controlling mechanical behavior and ultimate strength with broadly tunable composition and SFE, especially in multi-principal element alloys.
Miller, Victoria M. ; Maier, Petra ; Jordon, J. Brian ; Neelameggham, Neel R. (Ed.)Tensile samples of an Mg alloy ZK10 sheet were tested at a range of temperatures and strain rates designed to rather evenly probe a range of Zener Hollomon parameter values, from ln(Z) ≈ 15 (10–4 s−1 and 623 K) up to ln(Z) ≈ 50 (10–3 s−1 and 300 K). In contrast with more commonly examined Mg alloy AZ31B sheet material, ZK10 sheet material shows modest strain anisotropy (r-value) at low temperatures for both 45° (r45 ≈ 1.2) and TD (rTD ≈ 1.4) sample orientations, despite showing evidence of significant prismatic slip of dislocations, which often leads to high r-values at low temperatures. These low r-values become even lower (r45 ≈ 0.84 and rTD ≈ 0.89) at high temperatures. These behaviors are hypothesized to occur due to a distinct initial texture and deformation mechanism activity, which includes a modest level of tensile twinning andmore » « less
slip at both room and elevated temperature. A version of the viscoplastic self-consistent (VPSC) code, which accounts for the kinematics of dislocation climb, is used to simulate the behavior of a textured Mg alloy ZK10 sheet reveals that both the glide of pyramidal dislocations and the climb of basal < a > dislocations are required to describe the behavior at elevated temperatures.
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p) of 3.2, activation energy ( ) 620 ± 145 kJ mol−1(570 ± 145 kJ mol−1when accounting for the role of temperature on water content), activation volume ( E G ) ~5 V G ×10−6 m3mol−1, and rate constant ( k0) 1.8 ×103 m ps−1. Our is within uncertainty of that predicted for dislocation creep of wet olivine ( E G E*= 480 ± 40 kJ mol−1). Grain size in strain rate‐stepping samples adjusted to the olivine piezometer within 1.3–7.9% strain. The active grain boundary migration processes during deformation and dynamic recrystallization affect the kinetics of postdeformation grain growth, as grain boundary migration driven by strain energy density ( ρGBM) may delay the onset of grain growth driven by interfacial energy (γGBM). We compared our postdeformation grain growth rates with data from previously published hydrostatic annealing experiments on synthetic olivine. At geologic timescales, the growth rates are much slower than predicted by the existing wet olivine grain growth law.