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As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex ...chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A-atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO.
•An improved version of the FIRE algorithm is now implemented in LAMMPS.•The widely used FIRE algorithm requires a symplectic time integration scheme.•Default parameters are recommended to ensure ...optimal performance.•Its application is shown and discussed using multiple examples from materials science.
In atomistic simulations, pseudo-dynamical relaxation schemes often exhibit better performance and accuracy in finding local minima than line-search-based descent algorithms like steepest descent or conjugate gradient. Here, an improved version of the fast inertial relaxation engine (fire ) and its implementation within the open-source atomistic simulation code lammps is presented. It is shown that the correct choice of time integration scheme and minimization parameters is crucial for the performance of fire.
The identification of defects in crystal structures is crucial for the analysis of atomistic simulations. Many methods to characterize defects that are based on the classification of local atomic ...arrangement are available for simple crystalline structures. However, there is currently no method to identify both, the crystal structures and internal defects of topologically close-packed (TCP) phases such as Laves phases. We propose a new method, Laves phase crystal analysis (LaCA), to characterize the atomic arrangement in Laves crystals by interweaving existing structural analysis algorithms. The new method can identify the polytypes C14 and C15 of Laves phases, typical crystallographic defects in these phases, and common deformation mechanisms such as synchroshear and non-basal dislocations. Defects in the C36 Laves phase are detectable through deviations from the periodic arrangement of the C14 and C15 structures that make up this phase. LaCA is robust and extendable to other TCP phases.
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The mechanical behaviour of MgAl alloys can be largely improved by the formation of an intermetallic Laves phase skeleton, in particular the creep strength. Recent nanomechanical studies revealed ...plasticity by dislocation glide in the (Mg,Al)2Ca Laves phase, even at room temperature. As strengthening skeleton, this phase remains, however, brittle at low temperature. In this work, we present experimental evidence of slip transfer from the Mg matrix to the (Mg,Al)2Ca skeleton at room temperature and explore associated mechanisms by means of atomistic simulations. We identify two possible mechanisms for transferring Mg basal slip into Laves phases depending on the crystallographic orientation: a direct and an indirect slip transfer triggered by full and partial dislocations, respectively. Our experimental and numerical observations also highlight the importance of interfacial sliding that can prevent the transfer of the plasticity from one phase to the other.
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•Co-deformation of Mg-Laves phase composites shows cracking, slip deformation and interfacial sliding.•The active co-deformation mechanism depends on the interfacial orientation.•Slip transfer into the Laves phase occurs on prismatic and basal planes.•Interfacial sliding is favoured by matrix dislocations absorbed at the interface.
Grain boundary (GB) strengthening of metallic materials faces limitations as grain sizes are reduced to the nanoscale, primarily due to the transition from the strengthening to the softening effects. ...Understanding the intrinsic mechanisms behind such pronounced transitions is essential for optimizing the mechanical properties of nanocrystalline (NC) alloys. In this work, the critical grain size responsible for this transition is determined for NC CoCrNi medium-entropy alloys using molecular dynamics simulation, and the underlying mechanisms concerning varying temperatures and strain rates are systematically investigated for the samples with the critical grain size. The transition from Hall-Petch (HP) strengthening to Inverse Hall-Petch (IHP) softening is established by decreasing the average grain sizes from 21 nm to 3 nm. The critical grain size for the current alloys is estimated to be 12 nm, and the underlying deformation mechanisms are illustrated as follows: In the IHP regime, GB-mediated deformation becomes dominant, leading to softening. Conversely, in the HP regime, dislocation slip dominates the deformation mode, contributing to strengthening. It is found that an increase in temperature from 77 K to 1100 K leads to a decrease in the average flow stress and Young's modulus. Meanwhile, the deformation mechanisms change from dislocation slip (77K–700K) to GB-mediated deformation (700 K–1100 K). Furthermore, the underlying mechanisms correlating to the critical strain rate are uncovered. It can be attributed to the shift in deformation mechanisms from dislocation slip at relatively low strain rate regimes (5 × 107 s−1-2 × 109 s−1) to dislocation drag at relatively high strain rate regimes (2 × 109 s−1-5 × 109 s−1). Our atomistic insights into the tensile response of NC CoCrNi may provide a crucial basis for designing superior properties to meet the growing demands of advanced engineering fields.
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In this work, molecular dynamics simulations are conducted to investigate the shock responses and corresponding deformation mechanisms in single crystalline (SC) and nanocrystalline ...(NC) microstructure of the medium entropy alloy (MEA) CoCrNi. The effects of lattice distortion (LD) and chemical short-range order (CSRO) on the shock wave propagation, defect evolution, and the cavitation process are explored to distinguish the unique shock properties of MEA. The results reveal an anomalous anisotropy in the Hugoniot elastic limit different from that seen in pure FCC metals since LD reduces the barrier for Shockley partial (SP) formation but increases the resistance for SP propagation. With sufficient dislocations nucleated in the first shock compression stage, LD aids in the formation of nanotwins by slowing down dislocation propagation in the following release and tension stages. However, because a higher degree of CSRO increases the average intrinsic stacking fault energy above that of the random material, more stacking faults annihilate in the release stage, reducing the chances for nanotwinning. We show that voids prefer to nucleate at Ni segregation sites (with high CSRO) due to the large hydrostatic tensile strain created by the lattice mismatch between the neighboring Ni and CoCr regions, and moreover, the nucleation event favors the grain boundary during spallation in NCs.
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We perform molecular dynamics simulations to investigate shock-induced amorphization in CoCrNi, a medium entropy alloy (MEA) and its mean-field variant without lattice distortion. We ...show that a critical velocity exists above which amorphization occurs. At a low shock velocity of 800 m/s, dislocation slip and twins dominate and amorphization does not happen, but as the shock velocity increases, the deformation mechanism transitions from slip and twinning to solid-state amorphization. Under ultra-high shock velocities, extensive amorphization occurs, following the precursor of shock wave, eliminating anisotropy in spall strength. Compared to the mean-field model, lattice distortion in the MEA causes substantially more amorphization, resulting in a lower spall strength, since voids nucleate and grow preferentially in the amorphous regions.
Under high strain-rate loading, prominent increases in pressure usually triggers phase transition (PT), but the concomitant temperature rise may also cause melting. Quasi-isentropic (QI) compression ...provides a strategy to explore solid-state phase transition by reducing the temperature rise while retaining high pressure. Using large-scale molecular dynamics simulations, we investigate PTs in single crystal CoCrNi medium entropy alloys (MEAs) under QI compression. With the applied strain rates ranging from 108 s−1 to 1011 s−1, the strain-rate dependence and anisotropy of yield stress and solid-state PT path are revealed by comparing the mechanical responses along three compressed crystallographic orientations (100, 11¯0, and 111). Positive strain-rate sensitiveness is found in the yield stress along the 11¯0 and 111 directions, while insensitiveness along the 100 direction. Various PTs occur alongside massive plastic deformation in the post-yield regime. As the strain rate rises, face-centered-cubic (FCC) to body-centered-cubic (BCC) PT overrides the stacking fault-induced hexagonal-close-packed (HCP) phase formation and dominates the plasticity for the 100 loading. By contrast, crystalline PTs give way to amorphization for 11¯0 and 111 loading at high strain rates. Chemical short-range order hinders dislocation slip and promotes dislocation interactions, which further facilitate early formation of the BCC phase, suggesting a potential strategy to tailor polymorphism in MEAs.
•Phase transition (PT) in CoCrNi medium entropy alloy is studied by MD simulations. medium entropy alloy under quasi-isentropic compression.•The effects of loading orientation and strain rate on PT path are investigated.•The role of CSRO in incipient plasticity and BCC phase expansion is revealed.•The competition between crystalline PTs and amorphization is discussed.
Synchro-Shockley dislocations, as zonal dislocation, are the major carrier of plasticity in Laves phases at high temperatures. The motion of synchro-Shockley dislocations is composed of localized ...transition events, such as kink-pair nucleation and propagation, which possess small activation volumes, presumably leading to sensitive temperature and strain rate dependence on the Peierls stress. However, the thermally activated nature of synchro-Shockley dislocation motion is not fully understood so far. In this study, the transition mechanisms of the motion of synchro-Shockley dislocations at different shear and normal strain levels are studied. The transition processes of dislocation motion can be divided into shear-sensitive and -insensitive events. The external shear strain lowers the energy barriers of shear-sensitive events. Thermal assistance is indispensable in activating shear-insensitive events, implying that the motion of synchro-Shockley dislocations is prohibited at low temperatures.
In magnesium alloys with multiple substitutional elements, solute segregation at grain boundaries (GBs) has a strong impact on many important material characteristics, such as GB energy and mobility, ...and therefore, texture. Although it is well established that GB segregation is inhomogeneous, the variation of GB solute composition for random boundaries is still not understood. In the current study, atomic-scale experimental and simulation techniques were used to investigate the compositional inhomogeneity of six different GBs. Three-dimensional atom probe tomography results revealed that GB solute concentration of Nd in Mg varies between 2 and 5 at.%. This variation was not only seen for different GB orientations but also within the GB plane. Correlated atomistic simulations suggest that the inhomogeneous segregation behavior observed experimentally stems from local atomic rearrangements within the GBs and introduce the notion of potential excess free volume in the context of improving the prediction of per-site segregation energies.
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