### Browsing by Author "Lehtinen, Arttu"

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Item Effects of precipitates and dislocation loops on the yield stress of irradiated iron(2018-12-01) Lehtinen, Arttu; Laurson, Lasse; Granberg, Fredric; Nordlund, Kai; Alava, Mikko J.; Department of Applied Physics; Complex Systems and Materials; University of HelsinkiPlastic 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 1/2(111)dislocation loops on the yield stress of iron, using large-scale three-dimensional discrete dislocation dynamics simulations. Precipitates are included in the simulations using our recent multi-scale implementation [A. Lehtinen et al., Phys. Rev. E 93 (2016) 013309], where the strengths and pinning mechanisms of the precipitates are determined from molecular dynamics simulations. In the simulations we observe dislocations overcoming precipitates with an atypical Orowan mechanism which results from pencil-glide of screw segments in iron. Even if the interaction mechanisms with dislocations are quite different, our results suggest that in relative terms, precipitates and loops of similar sizes contribute equally to the yield stress in multi-slip conditions.Item Excitation Spectra in Crystal Plasticity(2017-12-27) Ovaska, Markus; Lehtinen, Arttu; Alava, Mikko J.; Laurson, Lasse; Zapperi, Stefano; Department of Applied Physics; Complex Systems and MaterialsPlastically deforming crystals exhibit scale-free fluctuations that are similar to those observed in driven disordered elastic systems close to depinning, but the nature of the yielding critical point is still debated. Here, we study the marginal stability of ensembles of dislocations and compute their excitation spectrum in two and three dimensions. Our results show the presence of a singularity in the distribution of excitation stresses, i.e., the stress needed to make a localized region unstable, that is remarkably similar to the one measured in amorphous plasticity and spin glasses. These results allow us to understand recent observations of extended criticality in bursty crystal plasticity and explain how they originate from the presence of a pseudogap in the excitation spectrum.Item Glassy features of crystal plasticity(2016-08-02) Lehtinen, Arttu; Costantini, Giulio; Alava, Mikko J.; Zapperi, Stefano; Laurson, Lasse; Department of Applied Physics; University of MilanoCrystal plasticity occurs by deformation bursts due to the avalanchelike motion of dislocations. Here we perform extensive numerical simulations of a three-dimensional dislocation dynamics model under quasistatic stress-controlled loading. Our results show that avalanches are power-law distributed and display peculiar stress and sample size dependence: The average avalanche size grows exponentially with the applied stress, and the amount of slip increases with the system size. These results suggest that intermittent deformation processes in crystalline materials exhibit an extended critical-like phase in analogy to glassy systems instead of originating from a nonequilibrium phase transition critical point.Item Mesoscale simulations of dislocation-obstacle interactions and dislocation avalanches(Aalto University, 2017) Lehtinen, Arttu; Laurson, Lasse, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, FinlandPlastic deformation in crystals is generated by the motion of line-like defects called dislocations, whose dynamics has thus the key role in determining the mechanical properties of crystalline materials like metals. In addition to line dislocations, metals can contain other defects that serve as obstacles for dislocation motion, leading to strengthening of the metal. The modelling of the interactions between these defects and dislocations is useful when developing new metal alloys. The interactions between dislocations themselves are based on the long-range anisotropic stress-fields that they generate by distorting the surrounding lattice. These long-range interactions and the motion constraints imposed by the lattice lead to complex dynamical behaviour. It has been observed in micropillar experiments of pure crystals that collective motion of dislocations happens in avalanches. The size and duration distributions of these avalanches follow power law scaling. It has been suggested that the exact form of these power laws could be explained with the idea that plastic yielding is a non-equilibrium phase transition. However, results from 2D discrete dislocation dynamics simulations indicate that in the case of pure crystals the collective dynamics of dislocations have glassy features similar to those found in jammed systems. In this thesis we use numerical simulations to study the properties of dislocations in different metals. In publication I we develop a multiscale framework for dislocation-precipitate interactions in body-centered cubic (BCC) iron. In this framework molecular dynamics simulations are used to provide physically justifiable input parameters for 3D discrete dislocation dynamics (DDD). The multiscale model is used in publication II to study the yielding of irradiated BCC iron at elevated temperatures. Irradiated iron contains line dislocations, precipitates and self-interstitial dislocation loops that impede dislocation motion. We show that precipitates and dislocation loops contribute equally to the yield stress when present at equal densities. In publication III we use extensive 3D DDD simulations to study dislocation avalanches in pure aluminium. Analysis of the avalanche statistics indicate that plastic deformation in face-centered cubic (FCC) crystals exhibit extended critical-like phase in analogy to glassy systems, instead of originating from a non-equilibrium phase transition at a critical stress.Item Multiscale modeling of dislocation-precipitate interactions in Fe: From molecular dynamics to discrete dislocations(2016-01-21) Lehtinen, Arttu; Granberg, Fredric; Laurson, Lasse; Nordlund, Kai; Alava, Mikko; Department of Applied Physics; Complex Systems and Materials; University of HelsinkiThe 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 12(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.Item Plastic yielding and deformation bursts in the presence of disorder from coherent precipitates(American Physical Society, 2020-08-13) Salmenjoki, Henri; Lehtinen, Arttu; Laurson, Lasse; Alava, Mikko J.; Department of Applied Physics; Complex Systems and Materials; Tampere UniversityAlloying metals with other elements is often done to improve the material strength or hardness. A key microscopic mechanism is precipitation hardening, where precipitates impede dislocation motion, but the role of such obstacles in determining the nature of collective dislocation dynamics remains to be understood. Here, three-dimensional discrete dislocation dynamics simulations of fcc single crystals are performed with fully coherent spherical precipitates from zero precipitate density up to p(p) = 10(21) m(-3) and at various dislocation-precipitate interaction strengths. When the dislocation-precipitate interactions do not play a major role, the yielding is qualitatively the same as for pure crystals, i.e., dominated by "dislocation jamming," resulting in glassy dislocation dynamics exhibiting critical features at any stress value. We demonstrate that increasing the precipitate density and/or the dislocation-precipitate interaction strength creates a true yield or dislocation assembly depinning transition, with a critical yield stress. This is clearly visible in the statistics of dislocation avalanches observed when quasistatically ramping up the external stress, and it is also manifested in the response of the system to constant applied stresses. The scaling of the yielding with precipitates is discussed in terms of the relation.Item Transient shear banding in time-dependent fluids(2013-02-22) Illa, Xavier; Puisto, Antti; Lehtinen, Arttu; Mohtaschemi, Mikael; Alava, Mikko J.; University of Barcelona; Department of Applied PhysicsWe study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered.Item Transient shear banding in time-dependent fluids(American Physical Society (APS), 2013) Illa, Xavier; Puisto, Antti; Lehtinen, Arttu; Mohtaschemi, Mikael; Alava, Mikko J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered.