F. Granberg

Multiscale modeling of dislocation-precipitate interactions in Fe: From molecular dynamics to discrete dislocations

The stress-driven motion of dislocations in crystalline solids, and thus the ensuing plastic deformation process, is greatly influenced by the presence or absence of various pointlike defects such as precipitates or solute atoms. These defects act as obstacles for dislocation motion and hence affect the mechanical properties of the material. Here we combine molecular dynamics studies with three-dimensional discrete dislocation dynamics simulations in order to model the interaction between different kinds of precipitates and a 1/2〈111〉{110} edge dislocation in BCC iron. We have implemented immobile spherical precipitates into the ParaDis discrete dislocation dynamics code, with the dislocations interacting with the precipitates via a Gaussian potential, generating a normal force acting on the dislocation segments. The parameters used in the discrete dislocation dynamics simulations for the precipitate potential, the dislocation mobility, shear modulus, and dislocation core energy are obtained from molecular dynamics simulations. We compare the critical stresses needed to unpin the dislocation from the precipitate in molecular dynamics and discrete dislocation dynamics simulations in order to fit the two methods together and discuss the variety of the relevant pinning and depinning mechanisms.

Improving atomic displacement and replacement calculations with physically realistic damage models

Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett−Robinson−Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-dpa by providing more physically realistic descriptions of primary defect creation in materials and may become additional standard measures for radiation damage quantification.The Norgett−Robinson−Torrens displacements per atom model is the benchmark to assess radiation damage in metals but has well-known limitations. Here, the authors use molecular dynamics to introduce material-specific modifications to describe radiation damage more realistically.

Tensile testing of Fe and FeCr nanowires using molecular dynamics simulations

Using molecular dynamics, we have studied the behaviour of cylindrical [001]-oriented Fe and FeCr nanowires under uniaxial tensile strain with both an embedded atom method (EAM) and a Tersoff-like bond order potential. The mechanical properties were analysed and the deformation mechanism was studied and compared between the potentials. The effects of chromium content and size of the wire were studied. Both potentials show elongation by deformation twinning in the 〈111〉/{211} system resulting in a significantly stiffer and stronger [110]-axial nanowire. The pure iron nanowires are elastically softer than bulk iron and an addition of chromium has both a softening and weakening effect. The bond order potential shows a strong dependence on chromium concentration, while the dependence is considerably weaker for the EAM potential.

Investigation of the thermal stability of Cu nanowires using atomistic simulations

We present a method for determining the melting point of copper nanowires based on classical molecular dynamics simulations and use it to investigate the dependence of the melting point on wire diameter. The melting point is determined as the temperature at which there is a significant change in the fraction of liquid atoms in the wire, according to atomic bond angle analysis. The results for the wires with diameters in the range 1.5 nm to 20 nm show that the melting point is inversely proportional to the diameter while the cross-sectional shape of the wire does not have a significant impact. Comparison of results obtained using different potentials show that while the absolute values of the melting points may differ substantially, the melting point depression is similar for all potentials. The obtained results are consistent with predictions based on the semi-empirical liquid drop model.

Radiation damage buildup by athermal defect reactions in nickel and concentrated nickel alloys

ABSTRACT We develop a new method using binary collision approximation simulating the Rutherford backscattering spectrometry in channeling conditions (RBS/C) from molecular dynamics atom coordinates of irradiated cells. The approach allows comparing experimental and simulated RBS/C signals as a function of depth without fitting parameters. The simulated RBS/C spectra of irradiated Ni and concentrated solid solution alloys (CSAs, NiFe and NiCoCr) show a good agreement with the experimental results. The good agreement indicates the damage evolution under damage overlap conditions in Ni and CSAs at room temperature is dominated by defect recombination and migration induced by irradiation rather than activated thermally. GRAPHICAL ABSTRACT IMPACT STATEMENT A new method simulating the Rutherford backscattering Spectrometry in channeling conditions (RBS/C) was proposed. The RBS/C simulations reveal that the radiation damage buildup in Ni, NiFe and NiCoCr was dominated by athermal defect reactions.

Cascade debris overlap mechanism of 〈100〉 dislocation loop formation in Fe and FeCr

Two types of dislocation loops are observed in irradiated α-Fe, the 1/2〈111〉 loop and the 〈100〉 loop. Atomistic simulations consistently predict that only the energetically more favourable 1/2〈111〉 loops are formed directly in cascades, leaving the formation mechanism of 〈100〉 loops an unsolved question. We show how 〈100〉 loops can be formed when cascades overlap with random pre-existing primary radiation damage in Fe and FeCr. This indicates that there are no specific constraints involved in the formation of 〈100〉 loops, and can explain their common occurrence.

Local segregation versus irradiation effects in high-entropy alloys: Steady-state conditions in a driven system

We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme, a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipitation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal that the high-entropy alloy—independent of the initial configuration—installs a certain fraction of short-range order even under particle irradiation. Moreover, the results provide evidence that defect accumulation is reduced in the high-entropy alloy. This is because the reduced mobility of point defects leads to a steady state of defect creation and annihilation. The lattice defects generated by irradiation are ...

Interaction of Dislocations with Carbides in BCC Fe Studied by Molecular Dynamics

Abstract Iron carbide (Fe3C), also known as cementite, is present in many steels and has also been seen as nanosized precipitates in steels. We examine the interaction of edge dislocations with nanosized cementite precipitates in Fe by molecular dynamics. The simulations are carried out with a Tersoff-like bond order interatomic potential by Henriksson et al. for Fe-C-Cr systems. Comparing the results obtained with this potential for a defect free Fe system with results from previously used potentials, we find that the potential by Henriksson et al. gives significantly higher values for the critical stress, at least at low temperatures. The explanation was found to be the difference in the core structure of the edge dislocation. The results show that edge dislocations can unpin from cementite precipitates of sizes 1 nm and 2 nm even at a temperature of 1 K, although the stresses needed for this are high. On the other hand, a 4 nm precipitate is not sheared by edge dislocations at low temperatures (≤100 K) on our simulation timescale.

Effects of precipitates and dislocation loops on the yield stress of irradiated iron

Plastic deformation of crystalline materials is governed by the features of stress-driven motion of dislocations. In the case of irradiated steels subject to applied stresses, small dislocation loops as well as precipitates are known to interfere with the dislocation motion, leading to an increased yield stress as compared to pure crystals. We study the combined effect of precipitates and interstitial glissile