It is well known that under plastic deformation, dislocations are not only created but also move through the crystal, and their mobility is impeded by their interaction with the crystal structure. At high stress and temperature, this `drag' is dominated by phonon wind, i.e. phonons scattering off dislocations. Employing the semi-isotropic approach discussed in detail in Ref. [arXiv:1804.01586], we discuss here the approximate functional dependence of dislocation drag $B$ on dislocation velocity in various regimes between a few percent of transverse sound speed $c_t$ and $c_t$ (where $c_t$ is the effective average transverse sound speed of the polycrystal). In doing so, we find an effective functional form for dislocation drag $B(v)$ for different slip systems and dislocation characters at fixed (room) temperature and low pressure.
In this paper we discuss the effect of a non-constant dislocation drag coefficient on the very high strain rate regime within an analytic model describing mobile-immobile dislocation intersections applicable to polycrystals. Based on previous work on dislocation drag, we estimate its temperature and pressure dependence and its effects on stress-strain rate relations. In the high temperature regime, we show that drag can remain the dominating effect even down to intermediate strain rates. We also discuss the consequences of having a limiting dislocation velocity, a feature which is typically pr…
Plastic deformation is mediated by the creation and movement of dislocations, and at high stress the latter is dominated by dislocation drag from phonon wind. By simulating a 1-D shock impact problem we analyze the importance of accurately modeling dislocation drag and dislocation density evolution in the high stress regime. Dislocation drag is modeled according to a first-principles derivation as a function of stress and dislocation character, and its temperature and density dependence are approximated to the extent currently known. Much less is known about dislocation density evolution, lead…
Although continuum theory has been widely used to describe the long-range elastic behavior of dislocations, it is limited in its ability to describe mechanical behaviors that occur near dislocation cores. This limit of the continuum theory mainly stems from the discrete nature of the core region, which induces a drag force on the dislocation core during glide. Depending on external conditions, different drag mechanisms are activated that govern the dynamics of dislocations in their own way. This is revealed by the resultant speed of the dislocation. In this work, we develop a theoretical frame…
The mobility of dislocations is an important factor in understanding material strength. Dislocations experience a drag due to their interaction with the crystal structure, the dominating contribution at high stress and temperature being the scattering off phonons due to phonon wind. Yet, the velocity dependence of this effect has eluded a good theoretical understanding. In a previous paper, dislocation drag from phonon wind as a function of velocity was computed from first principles in the isotropic limit, in part for simplicity, but also arguing that macroscopically, a polycrystalline metal …
The anharmonic interaction and scattering of phonons by a moving dislocation, the photon wind, imparts a drag force $v B(v, T, ρ)$ on the dislocation. In early studies the drag coefficient $B$ was computed and experimentally determined only for dislocation velocities $v$ much less than transverse sound speed, $c_t$. In this paper we derive analytic expressions for the velocity dependence of $B$ up to $c_t$ in terms of the third-order continuum elastic constants of an isotropic crystal, in the continuum Debye approximation, valid for dislocation velocities approaching the sound speed. In so doi…
This paper verifies two laws for plastic flows of face-centered cubic crystals that deform at constant strain rates and fixed ambient temperatures. The first law relates steady-state flow stress to ambient temperature and strain rate. The second law requires an increase of configurational entropy towards a maximum reached in the steady state. The large scale least squares analysis is provided which allows the physics-based parameters of thermodynamic dislocation theory to be identified in accordance with these laws.
Compared to cubic metals, whose primary slip mode includes twelve equivalent systems, the lower crystalline symmetry of hexagonal close-packed metals results in a reduced number of equivalent primary slips and anisotropy in plasticity, leading to brittleness at the ambient temperature. At higher temperatures, the ductility of hexagonal close-packed metals improves owing to the activation of secondary c+a pyramidal slip systems. Thus understanding the fundamental properties of corresponding dislocations is essential for the improvement of ductility at the ambient temperature. Here, we present t…
The mechanical behavior in compression of [111] LiF micropillars with diameters in the range 0.5 $μ$m to 2.0 $μ$m was analyzed by means of discrete dislocation dynamics at ambient and elevated temperature. The dislocation velocity was obtained from the Peach-Koehler force acting on the dislocation segments from a thermally-activated model that accounted for the influence of temperature on the lattice resistance. A size effect of the type "smaller is stronger" was predicted by the simulations, which was in quantitative agreement with previous experimental results by the authors \cite{SWC14}. Th…
Atomic-Scale Insights into Solute Drag Effects on Grain Boundary Motion in Mg-Al and Mg-Ca Alloys
College of Materials Science and Engineering, State Key Laboratory of Cemented Carbide, Hunan University（湖南大学材料科学与工程学院）；McGill University（麦吉尔大学）；Institute of Solid State Physics, Chinese Academy of Sciences（中国科学院固体物理研究所）；School of Physics and Electronics, Hunan University（湖南大学物理与电子学院）；Hunan University（湖南大学）
Under high pressure, Zr undergoes a transformation from its ambient equilibrium hexagonal close packed $α$ phase to a simple hexagonal $ω$ phase. Subsequent unloading to ambient conditions does not see a full reversal to the $α$ phase, but rather a retainment of significant $ω$. Previously, the thermal stability of the $ω$ phase was investigated via in-situ synchrotron X-ray diffraction analysis of the isothermal annealing of Zr samples shocked to 8 and 10.5 GPa at temperatures 443, 463, 483, and 503 K. The phase volume fractions were tracked quantitatively and the dislocation densities were t…
A canonical quantization procedure is applied to the interaction of elastic waves --phonons-- with infinitely long dislocations that can oscillate about an equilibrium, straight line, configuration. The interaction is implemented through the well-known Peach-Koehler force. For small dislocation excursions away from the equilibrium position, the quantum theory can be solved to all orders in the coupling constant. We study in detail the quantum excitations of the dislocation line, and its interactions with phonons. The consequences for the drag on a dislocation caused by the phonon wind are poin…
The investigation of the deformation behaviour of intermetallic phases is mostly limited to high temperatures due to their low ductility at ambient temperature. Therefore, within this study, nanoindentation experiments on the Al$_{2}$Cu phase were performed from ambient temperature up to 300°C in conjunction with TEM investigations of the deformed material. It was found that the Al$_{2}$Cu phase starts to soften from temperatures above 150°C. The low temperature deformation behaviour was dominated by serrated yielding and transitioned towards a smooth deformation behaviour above 150°C. No slip…
In this work, molecular dynamics (MD) simulations were used to investigate elementary dislocation properties in a Co-free high entropy (HEA) model alloy ($Cr_{15}Fe_{46}Mn_{17}Ni_{22}$ at. %) in comparison with a model alloy representative of Austenitic Stainless Steel (ASS) ($Cr_{20}Fe_{70}Ni_{10}$ at. %). Recently developed embedded-atom method (EAM) potentials were used to describe the atomic interactions in the alloys. Molecular Statics (MS) calculations were used to study the dislocation properties in terms of local stacking fault energy (SFE), dissociation distance while MD was used to i…
We derive a continuum-level plasticity model for polycrystalline materials in the high energy density regime, based on a single dislocation density and single mobility mechanism, with an evolution model for the dislocation density. The model is formulated explicitly in terms of quantities connected closely with equation of state (EOS) theory, in particular the shear modulus and Einstein temperature, which reduces the number of unconstrained parameters while increasing the range of applicability. The least constrained component is the Peierls barrier $E_P$, which is however accessible by atomis…
The universality class of the avalanche behavior in plastically deforming crystalline and amorphous systems has been commonly discussed, despite the fact that the microscopic defect character in each of these systems is different. In contrast to amorphous systems, crystalline flow stress increases dramatically at high strains and/or loading rates. We perform simulations of a two-dimensional discrete dislocation dynamics model that minimally captures the phenomenology of nanocrystalline deformation. In the context of this model, we demonstrate that a classic rate-dependence of dislocation plast…
This work presents recent the progress in the development of the Concurrent Atomistic-Continuum (CAC) method for coarse-grained space- and time-resolved atomistic simulations of phonon transport. Application examples, including heat pulses propagating across grain boundaries and phase interfaces, as well as the interactions between phonons and moving dislocations, are provided to demonstrate the capabilities of CAC. The simulation results provide visual evidence and reveal the underlying physics of a variety of phenomena including: phonon focusing, wave interference, dislocation drag, interfac…
Refractory high entropy alloys (R-HEAs) are having properties and uses as high strength and high hardness materials for ambient and high temperature, aerospace and nuclear radiation tolerance applications, orthopedic applications etc. The mechanical properties like yield strength and ductility of TaNbHfZr R-HEA depend on the local nanostructure and chemical ordering. In this study we have computationally obtained various properties of the TaNbHfZr alloy like the role of configurational entropy in the thermodynamic property, rate of evolution of nanostructure morphology in thermally annealed sy…
The Ni-base superalloy 718 is the most widely used material for turbomachinery in the aerospace industry and land-based turbines. Although the relationship between processing and the resulting properties is well known, an understanding of the specific deformation mechanisms activated across its application temperature range is required to create more mechanistically accurate property models. Direct atomic-scale imaging observations with high angle annular dark-field scanning transmission electron microscopy, complemented by phase-field modeling informed by generalized stacking fault surface ca…
When a thin stream of aqueous sodium alginate is extruded into a reacting calcium chloride bath, it polymerizes into a soft elastic tube that spontaneously forms helical coils due to the ambient fluid drag. We quantify the onset of this drag-induced instability and its nonlinear evolution using experiments, and explain the results using a combination of scaling, theory and simulations. By co-extruding a second (internal) liquid within the aqueous sodium alginate jet and varying the rates of co-extrusion of the two liquids, as well as the diameter of the jet, we show that we can tune the local …
A simple extension of the thermodynamic dislocation theory to non-uniform plastic deformations is proposed for an analysis of high-temperature torsion of aluminum bars. Employing a small set of physics-based parameters, which we expect to be approximately independent of strain rate and temperature, we are able to fit experimental torque-twist curves for five different twist rates and at one fixed ambient temperature. We find that thermal softening due to temperature rise is significant at high twist rates.
This work demonstrates the development of a strong and ductile medium entropy alloy by employing conventional alloying and thermomechanical processing to induce partial recrystallization (PR) and precipitation strengthening in the microstructure. The combined usage of electron microscopy and atom probe tomography reveals the sequence of microstructural evolution during the process. First, the cold working of homogenized alloy resulted in a highly deformed microstructure. On annealing at 700°C, B2 ordered precipitates heterogeneously nucleate on the highly misoriented sites. These B2 promotes p…
