Multiscale modeling of fracture and deformation in interface controlled materials [Elektronische Ressource] / vorgelegt von Nils C, Brödling

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• Publié par: universitat_stuttgart
• Publié le: 01 janvier 2007
• Nombre de lectures: 33
• Langue: English
• Poids de l'ouvrage: 12 Mo

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Max-Planck-Institut für Metallforschung
Stuttgart

Multiscale Modeling of Fracture and Deformation in
Interface Controlled Materials

Nils Brödling
Dissertation
an der
Universität Stuttgart

Bericht Nr. 217
September 2007 Multiscale Modeling
of Fracture and Deformation
in Interface Controlled Materials
Von der Fakult˜at Chemie der Universitat˜ Stuttgart
zur Erlangung der Wurde eines Doktors der˜
Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung
Vorgelegt von
˜Dipl.-Ing. Nils C. Brodling
aus Detmold
Hauptberichter: Prof. Dr. phil. E. Arzt
Mitberichter: Prof. Dr. rer. nat. A. Hartmaier
Mitprufer: Prof. Dr. rer. nat. F. Aldinger˜
Tag der Einreichung: 23.07.2007
Tag der mundlichen Prufung: 06.09.2007˜ ˜
Max-Planck-Institut fur˜ Metallforschung, Stuttgart
Juli 2007To my parentsAcknowledgements
This work has been conducted at the Max Planck Institute for Metals Research in
Stuttgart, and partially at the chair for General Material Properties, University of
Erlangen.
First I would like to express my deepest gratitude to Prof. Dr. Huajian Gao, who
accepted me as his doctoral student and gave me the opportunity to work on some
of the most challenging problems in materials science. I am deeply indebted for his
guidance and support. I have beneflted much from the extremely stimulating wor-
king culture in our department and from our joint discussions with various visiting
colleagues.
IamdeeplyindebtedtoProf.Dr.AlexanderHartmaierforhispersistentsupport,his
guidanceandencouragementthatheprovidedduringmytimeattheMaxPlanckIn-
stituteforMetalsResearch,unchangedafterrespondingtohiscalltotheUniversity
of Erlangen, and later during my stay at the chair for General Material Properties,
University of Erlangen. Not forgetting the numerous lessons on discrete dislocation
and atomistic modeling and for the many inspiring discussions that helped me to-
ward my goal of completing my PhD research, thank you.
To my thesis committee members, especially Prof. Dr. Eduard Arzt for his sup-
port and for agreeing on assuming the Hauptbericht for my dissertation, and also
Prof. Dr. Fritz Aldinger for agreeing on assuming the Prufungsvorsitz, I am indeb-˜
ted.
For his great support and giving me the opportunity to work at MIT, thank you
Dr. Markus Buehler. The unique working climate in your modeling group and your
enthusiasm were very stimulating and beneflcial to my research. I enjoyed the fre-
quent discussions on atomistic modeling with you and your co-workers.
I would also like to express my gratitude to Dr. Chun Lu and Dr. Liu Zishun for in-
viting me to IHPC where I learnt about other computational methods, particularly,
the level set dislocation dynamics method.
The environment at MPI has been nothing but conducive and for that there are a
few to thank. Particulary, I would like to acknowledge Franz-Werner Gergen from
the EDV department for his continuous help, and our secretary Silvia Casanova
for her great support and continuous care especially in the last year. Furthermore,
for some of my calculations I used the MD code \IMD\ developed by the groupiv
of Prof. Dr. Trebin at the University of Stuttgart (Institut fur Theoretische und˜
Angewandte Physik). I gratefully acknowledge the support from Dr. Franz G˜ahler
in order to make the necessary modiflcations to the code. Some of my large-scale
computationswerecarriedoutattheveryrecentlyinstalled\Woodcrest\ Compute-
Cluster in Erlangen, and I like to commend Dr. Georg Hager from the Center of
Excellence in High Performance Computing particular for his excellent continuous
support especially in the test phase beginning of this year.
I would like to extend my thanks to all the colleagues and guests at the Max Planck
Institute for Metals Research and at the chair for General Material Properties, Uni-
versity of Erlangen, for their help and for creating a pleasant working atmosphere.
I gratefully acknowledge flnancial support from the German Academic Exchange
Service (DAAD-Kurzzeitstipendium fur Doktoranden) and funding from the Inter-˜
national Graduate-Scholarship program of the Agency for Science, Technology and
Research, Singapore.
Myfamilyandfriends,thankyoufortheencouragementandsupport,specialthanks
to Shuyi, my parents Marianne and Wilhelm, and my sister Nicole.Abstract
Many nanostructured metals are characterized by scale dependent mechanical pro-
perties and by size efiects due to geometrical conflnement. Dislocation activities,
interface mediated plasticity, and macroscopic yielding are quite difierent from tho-
se in unconstrained metals. The role of interfaces for the material properties and
for the governing deformation mechanisms remains unclear despite the large efiorts
made in experimental and theoretical investigations. Here we approach the efiect of
geometrical conflnement on the atomic and on the mesoscopic scale. We elucidate
size efiects on failure mechanisms and on scale dependent plasticity of nanostructu-
red dual phase composite materials with the aid of computer simulations.
Cleavage failure of dual phase layered materials is simulated with a mesoscopic mo-
del to clarify the scaling behavior of the materials fracture toughness. The model
accounts for the conflnement efiect that a layer geometry imposes on the collective
dislocation behaviour near a moving crack tip. The critical layer thickness at which
the bulk fracture toughness of the elastic-plastic material is reached as well as the
bulk fracture toughness itself increase with the cohesive strength of the interface,
but become smaller for higher yield strengths. The main conclusion drawn in this
work is that fracture toughness as a function of layer thickness saturates gradually
if dislocation activity is dispersed, dilute and not compact around the crack tip. It
increases abruptly with the thickness when dislocation activity right at the crack
tip is possible and a compact, shielding dislocation array forms near the crack tip.
Furthermore this work provides preliminary understanding of the governing me-
chanisms that control the limiting length scale for the strengthening of bioinspired
metallic nanocomposits. Large-scale molecular dynamics simulations are performed
to investigate the plastic deformation behavior of a bioinspired metallic nanocom-
posite which consists of hard nanosized Ni platelets embedded in a soft Al matrix.
The simulation results are analyzed with respect to the prevailing deformation me-
chanisms quantifying the contribution of dislocation-based plasticity and interface-
mediated interfacial slip as a function of the nanostructural scaling. The results of
the simulations show that interfacial sliding contributes signiflcantly to the plastic
deformation despite a strong bonding across the interface. Critical for the strength
of the nanocomposite is the geometric conflnement of dislocation processes in the
plastic phase. The conflnement efiect strongly depends on the length scale and the
morphology of the metallic nanostructure. The main conclusion drawn for this ma-
terial is that below a critical length scale, the softening caused by interfacial sliding
prevails, giving rise to a maximum strength at the optimum size.