Atomistic simulation of precipitation hardening in α-iron: influence of precipitate shape and chemical composition (original) (raw)
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Hardening due to copper precipitates in α-iron studied by atomic-scale modelling
Journal of Nuclear Materials, 2004
We present results of a large-scale atomic-level study of dislocation-precipitate interaction. We have considered a 1 2 AE1 1 1ae edge dislocation gliding in a-iron containing coherent copper precipitates of size from 0.7 to 6 nm over a temperature range from 0 to 450 K. The results demonstrate that some features are qualitatively consistent with earlier theoretical conclusions, e.g. the critical resolved shear stress (CRSS) is proportional to L À1 and lnðDÞ, where L and D are precipitate spacing and diameter. Other features, which are intrinsic to the atomic-level nature of the dislocationprecipitate interaction, include strong dependence of the CRSS on temperature, dislocation climb and precipitate phase transformation.
Hardening due to copper precipitates in alpha-iron studied by atomic-scale modelling
J Nucl Mater, 2004
We present results of a large-scale atomic-level study of dislocation-precipitate interaction. We have considered a 1 2 AE1 1 1ae edge dislocation gliding in a-iron containing coherent copper precipitates of size from 0.7 to 6 nm over a temperature range from 0 to 450 K. The results demonstrate that some features are qualitatively consistent with earlier theoretical conclusions, e.g. the critical resolved shear stress (CRSS) is proportional to L À1 and lnðDÞ, where L and D are precipitate spacing and diameter. Other features, which are intrinsic to the atomic-level nature of the dislocationprecipitate interaction, include strong dependence of the CRSS on temperature, dislocation climb and precipitate phase transformation.
Thermodynamic and mechanical properties of copper precipitates inα-iron from atomistic simulations
Physical Review B, 2013
Precipitate hardening is commonly used in materials science to control strength by acting on the number density, size distribution, and shape of solute precipitates in the hardened matrix. The Fe-Cu system has attracted much attention over the last several decades due to its technological importance as a model alloy for Cu steels. In spite of these efforts several aspects of its phase diagram remain unexplained. Here we use atomistic simulations to characterize the polymorphic phase diagram of Cu precipitates in body-centered cubic (BCC) Fe and establish a consistent link between their thermodynamic and mechanical properties in terms of thermal stability, shape, and strength. The size at which Cu precipitates transform from BCC to a close-packed 9R structure is found to be strongly temperature dependent, ranging from approximately 4 nm in diameter (∼2700 atoms) at 200 K to about 8 nm (∼22 800 atoms) at 700 K. These numbers are in very good agreement with the interpretation of experimental data given Monzen et al. [Philos. Mag. A 80, 711 (2000)]. The strong temperature dependence originates from the entropic stabilization of BCC Cu, which is mechanically unstable as a bulk phase. While at high temperatures the transition exhibits first-order characteristics, the hysteresis, and thus the nucleation barrier, vanish at temperatures below approximately 300 K. This behavior is explained in terms of the mutual cancellation of the energy differences between core and shell (wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The proposed mechanism is not specific for the Fe-Cu system but could generally be observed in immiscible systems, whenever the minority component is unstable in the lattice structure of the host matrix. Finally, we also study the interaction of precipitates with screw dislocations as a function of both structure and orientation. The results provide a coherent picture of precipitate strength that unifies previous calculations and experimental observations.
Atomic scale modelling of edge dislocation movement in the alpha-Fe-Cu system
Modelling and Simulation in Materials Science and Engineering, 2000
The aim of the present work is to investigate by molecular dynamics (MD) calculations the interaction between a moving edge dislocation in an α-Fe crystal and a copper precipitate. In the absence of external stresses, two edge dislocations with the same slip plane and opposite Burgers vectors within a perfect α-Fe crystal lattice are investigated. In agreement with Frank's rule, the movement of the dislocations under mutual attraction is found and attention is focused on the interaction between one of the dislocations and the Cu precipitate. The critical resolved shear stress of the Fe was calculated and the influence of different sizes of Cu precipitates on the dislocation mobility was studied. The pinning of the dislocation line at the Cu inclusion as derived from the atomistic modelling agrees with previously published continuum theoretical behaviour of pinned dislocations. Therefore, nanosimulation as a way to model precipitation hardening could be established as a useful scientific tool. § On leave from
Revealing the atomistic nature of dislocation-precipitate interactions in Al-Cu alloys
Journal of Alloys and Compounds, 2019
Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocationprecipitate interactions scenarios for both θ' and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ' Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as unshearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts.
Kinetics of heterogeneous dislocation precipitation of NbC in alpha-iron
Acta Materialia, 2008
We propose Monte Carlo simulations of the precipitation kinetics of NbC on dislocations in a-iron based on an atomistic description of the main mechanisms controlling the kinetic pathway. The algorithm takes into account realistic diffusion properties, with a rapid diffusion of C atoms by interstitial jumps and a slower diffusion of Fe and Nb atoms by vacancy jumps. A simple model of dislocation includes interactions with solute atoms, through local segregation energy and long range elastic fields. The relative importance of local segregation energies and long range elastic stresses on the precipitation characteristics, such as early segregation of carbon atoms on dislocations, transient precipitation of metastable carbides, and homogeneous and heterogeneous NbC precipitation, is discussed.
Materialia, 2023
Copper addition in austenitic stainless steel is known to provide good high-temperature strength through precipitation strengthening. However, the mechanism of such strengthening and its overall contribution to strength in austenitic stainless steels have not been adequately investigated. The present work employs molecular dynamics (MD) simulation to investigate the interaction between an edge dislocation and copper precipitate in austenitic stainless steel. The mechanism controlling the strength was found to be trailing partial detachment for smaller precipitates up to a maximum radius of 3 nm, whereas leading partial detachment controls the strength for larger precipitate sizes, irrespective of the inter-precipitate spacing. Besides, modulus strengthening was identified as the primary strengthening contributor for the coherent Cu precipitate in the present alloy, which was subsequently aligned with the theoretical predictions by the existing Russell-Brown (R-B) model, adopting some modifications. The discrepancy between the simulation results and the original R-B model is attributed to the interaction between the two partial dislocations in presence of the precipitate. The modified R-B model was then used in combination with Thermo-calc and DICTRA predictions to estimate the strength of Cu-added austenitic stainless steel. However, the predictions overestimated the experimental data due to the model's inability to account for the random distribution of precipitates. To address this limitation, a subsequent discrete dislocation dynamics (DDD) simulation was conducted, incorporating a random distribution of Cu precipitates. Finally, the DDD simulation results demonstrated a good match with the experimental results, which can be further extended to other alloy systems as well.
Molecular dynamics simulations of strengthening due to silver precipitates in copper matrix
physica status solidi (b), 2017
Molecular dynamics simulations of edge dislocation interactions with coherent and incoherent silver precipitates in the copper matrix are applied to investigate precipitation strengthening. Simulated shear tests with spherical and octahedral precipitates revealed that dislocations can cut a precipitate or circumvent it by the Orowan mechanism. Precipitates with radii below 3 nm are cut whereas both processes were observed for radii in the range of 3-9 nm. The reason for the occurrence of the Orowan mechanism is that dislocation reactions at the interface can lead to sessile dislocations. Orowan circumvention is more likely for spheres than for octahedra which is due to different dislocation types existing at the matrix/precipitate interfaces. On average, the critical resolved shear stress is found to be slightly higher for Orowan processes. In case of small precipitates, the critical resolved shear stress depends strongly on the coherency, whereas for larger precipitates, it is mainly influenced by dislocation reactions at the interface. In some cases, the formation of a jog was observed which can reduce the critical resolved shear stress whereas it was increased significantly in the cases of pronounced cross-slip without jog formation.
Journal of Nuclear Materials, 2014
A multiscale approach is presented to study the effect of chromium and nickel concentration on the deformation behavior of iron systems. A combination of molecular dynamics (MD) and dislocation dynamics (DD) simulations are employed. In this framework, the critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation mobility is obtained from MD simulations. Since accurate measurements of dislocation velocity are difficult to obtain through experiment, atomistic simulations constitute an adequate alternative tool. Then this information is used by DD to simulate large systems with high dislocation and defect densities. In particular, we study the effect of nickel and chromium concentration on the strength, as well as the effect of dislocation loops concentration on the yield stress of the aforementioned systems. The results reveal, among other, defect free zones, in accordance to experimental observations, and an evolution law for the defect density.