List of Journal Publications
2009
5.
Kumar, R., Nicola, L., der Giessen, E. Van
Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study Journal Article
In: MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, vol. 527, no. 1-2, pp. 7–15, 2009.
Abstract | BibTeX | Tags: Discrete dislocations, Grain boundary, size effects, Thin films | Links:
@article{Kumar2009,
title = {Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study},
author = {R. Kumar and L. Nicola and E. Van der Giessen},
doi = {10.1016/j.msea.2009.08.072},
year = {2009},
date = {2009-01-01},
journal = {MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING},
volume = {527},
number = {1-2},
pages = {7–15},
abstract = {Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.},
keywords = {Discrete dislocations, Grain boundary, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.
2006
4.
Nicola, L., Xiang, Y., Vlassak, J. J., der Giessen, E. Van, Needleman, A.
Plastic deformation of freestanding thin films: Experiments and modeling Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 54, no. 10, pp. 2089–2110, 2006.
Abstract | BibTeX | Tags: Bauschinger effect, Computer simulation, Dislocations, size effects, Thin films | Links:
@article{Nicola2006,
title = {Plastic deformation of freestanding thin films: Experiments and modeling},
author = {L. Nicola and Y. Xiang and J. J. Vlassak and E. Van der Giessen and A. Needleman},
doi = {10.1016/j.jmps.2006.04.005},
year = {2006},
date = {2006-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {54},
number = {10},
pages = {2089–2110},
abstract = {Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress-strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank-Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.},
keywords = {Bauschinger effect, Computer simulation, Dislocations, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress-strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank-Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.
2005
3.
Nicola, L., der Giessen, E. Van, Needleman, A.
Two hardening mechanisms in single crystal thin films studied by discrete dislocation plasticity Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 85, no. 14, pp. 1507–1518, 2005.
Abstract | BibTeX | Tags: discrete dislocation plasticity, hardening, Thin films | Links:
@article{Nicola2005b,
title = {Two hardening mechanisms in single crystal thin films studied by discrete dislocation plasticity},
author = {L. Nicola and E. Van der Giessen and A. Needleman},
doi = {10.1080/14786430500036611},
year = {2005},
date = {2005-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {85},
number = {14},
pages = {1507–1518},
abstract = {Two-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate are used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling is analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density is found close to the impenetrable film-substrate interface. There is a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state are independent of film thickness. In such films the stress outside the boundary layer is on average very low, so that the film-thickness-independent boundary layer is responsible for the size effect. A larger size effect is found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreases while the stress in the film increases. The size dependence is only described by a Hall–Petch type relation for films thicker than the threshold value.},
keywords = {discrete dislocation plasticity, hardening, Thin films},
pubstate = {published},
tppubtype = {article}
}
Two-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate are used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling is analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density is found close to the impenetrable film-substrate interface. There is a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state are independent of film thickness. In such films the stress outside the boundary layer is on average very low, so that the film-thickness-independent boundary layer is responsible for the size effect. A larger size effect is found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreases while the stress in the film increases. The size dependence is only described by a Hall–Petch type relation for films thicker than the threshold value.
2004
2.
Nicola, L., der Giessen, Erik Van, Needleman, Alan
Relaxation of thermal stress by dislocation motion in passivated metal interconnects Journal Article
In: JOURNAL OF MATERIALS RESEARCH, vol. 19, no. 4, pp. 1216–1226, 2004.
Abstract | BibTeX | Tags: discrete dislocation plasticity, metal interconnects, Thin films | Links:
@article{Nicola2004,
title = {Relaxation of thermal stress by dislocation motion in passivated metal interconnects},
author = {L. Nicola and Erik Van der Giessen and Alan Needleman},
doi = {10.1557/JMR.2004.0158},
year = {2004},
date = {2004-01-01},
journal = {JOURNAL OF MATERIALS RESEARCH},
volume = {19},
number = {4},
pages = {1216–1226},
publisher = {Materials Research Society},
abstract = {The development and relaxation of stress in metal interconnects strained by their surroundings (substrate and passivation layers) is predicted by a discrete dislocation analysis. The model is based on a two-dimensional plane strain formulation, with deformation fully constrained in the line direction. Plastic deformation occurs by glide of edge dislocations on three slip systems in the single-crystal line. The substrate and passivation layers are treated as elastic materials and therefore impenetrable for the dislocations. Results of the simulations show the dependence of the stress evolution and of the effectiveness of plastic relaxation on the geometry of the line. The dependence of stress development on line aspect ratio, line size, slip plane orientation, pitch length, and passivation layer thickness are explored.},
keywords = {discrete dislocation plasticity, metal interconnects, Thin films},
pubstate = {published},
tppubtype = {article}
}
The development and relaxation of stress in metal interconnects strained by their surroundings (substrate and passivation layers) is predicted by a discrete dislocation analysis. The model is based on a two-dimensional plane strain formulation, with deformation fully constrained in the line direction. Plastic deformation occurs by glide of edge dislocations on three slip systems in the single-crystal line. The substrate and passivation layers are treated as elastic materials and therefore impenetrable for the dislocations. Results of the simulations show the dependence of the stress evolution and of the effectiveness of plastic relaxation on the geometry of the line. The dependence of stress development on line aspect ratio, line size, slip plane orientation, pitch length, and passivation layer thickness are explored.
2003
1.
Nicola, L., der, Giessen Van, Needleman, E.
Discrete dislocation analysis of size effects in thin films Journal Article
In: JOURNAL OF APPLIED PHYSICS, vol. 93, no. 10, pp. 5920–5928, 2003.
Abstract | BibTeX | Tags: discrete dislocation plasticity, size effects, Thin films | Links:
@article{Nicola2003,
title = {Discrete dislocation analysis of size effects in thin films},
author = {L. Nicola and Giessen Van der and E. Needleman},
doi = {10.1063/1.1566471},
year = {2003},
date = {2003-01-01},
journal = {JOURNAL OF APPLIED PHYSICS},
volume = {93},
number = {10},
pages = {5920–5928},
abstract = {A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.},
keywords = {discrete dislocation plasticity, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.
