Mesoscale simulations of dislocation-obstacle interactions and dislocation avalanches

Arttu Lehtinen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Plastic 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.
Translated title of the contributionDislokaatiovyöryjen ja erkaumalujittumisen mallintaminen mesoskaalan simulaatiomenetelmillä
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Alava, Mikko, Supervising Professor
  • Laurson, Lasse, Thesis Advisor
Publisher
Print ISBNs978-952-60-7272-2
Electronic ISBNs978-952-60-7271-5
Publication statusPublished - 2017
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • dislocation avalanches
  • plastic deformation
  • precipitate hardening
  • irradiation damage

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