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Dislocation Cross-Slip in Face-Centered Cubic Solid Solution Alloys

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Dislocation Cross-Slip in Face-Centered Cubic Solid Solution Alloys Dislocation Cross-Slip in Face-Centered Cubic Solid Solution Alloys THÈSE NO 8383 (2018) PRÉSENTÉE LE 12 FÉVRIER 2018 À LA FACULTÉ DES SCIENCES ET TECHNIQUES DE L'INGÉNIEUR LABORATOIRE DE MODÉLISATION MÉCANIQUE MULTI-ÉCHELLE PROGRAMME DOCTORAL EN MÉCANIQUE ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Wolfram Georg NÖHRING acceptée sur proposition du jury: Prof. J. Botsis, président du jury Prof. W. Curtin, directeur de thèse Prof. D. Rodney, rapporteur Prof. K. Albe, rapporteur Dr P. Derlet, rapporteur Suisse 2018 Whence, in order to discover ores, I think it better to abandon the way of bestial and fearless men and to choose the way of using the signs that are exhibited to us through the benignity of Nature, founded on truth and approved by all experts because of their experience, which, as is evident, does not consist of words or promises of incomprehensible and vain things. — Vannoccio Biringuccio, De la pirotechnia (1540) Acknowledgements This work was carried out from 2014 to the time of writing at the École polytechnique fédérale de Lausanne (EPFL), in the Laboratory for Multiscale Mechanics Modeling (LAMMM). Support was provided by the European Research Council through the Advanced Grant “Predictive Computational Metallurgy”, ERC Grant agreement No. 339081 – PreCoMet, and by EPFL through the use of the facilities of its Scientific IT and Application Support Center. Before introducing the subject matter, I would like to address a few words of thanks. First and foremost, I would like to thank my thesis advisor, Prof. William A. Curtin, for his support. He always encouraged me to focus on the important parts of physical problems and to keep the larger context in mind. His guidance was very helpful for someone who tends to get bogged down in details. Moreover, I am thankful for his kind and patient leadership style, and for the opportunity to visit several interesting conferences and workshops abroad. I also thank the members of the jury for agreeing to participate in my PhD exam. Furthermore, I would like to thank the current and former members of LAMMM: Fabio Pavia, Kris Baker, Mike Francis, Phil Moseley, Aitor Luque, Céline Varvenne, Wu Zhaoxuan, Varun Rajan, Francesco Maresca, Binglun Yin, Nikolaos Bouklas, Till Junge, Satish Rao, Ben Szajewski, Mostafa Khosrownejad, Ali Tehranchi, Max Hodapp, Predrag Andric, Shankha Nag, Rasool Ahmad, Yi Hu, Eleanor Mak, Vladimir Dorodnitsyn and Florian Maurin; as well as a number of visiting scholars: Ronald Miller, Derek Warner, Ryo Kobayashi, Maryam Ghazisaeidi, Jun Song and Daniel Mulvihill. I enjoyed the friendly atmosphere in the lab and the lively discussions. Moreover, I owe a debt of gratitude to the secretary of LAMMM, Géraldine Palaj, for her assistance with numerous administrative issues. I fondly recall the time when Céline, Kris, Aitor and others would supply LAMMM with cake almost every week. I am also grateful to Kris for making me aware of the String method, and to Céline and Aitor for their work on the average atom method. Both methods became indispensable for my work on cross-slip. Furthermore, I am indebted to Aitor for his kind support. I also want to thank Wu Zhaoxuan for many interesting discussions about, and assistance with, atomistic modeling, simulation of dislocations, and technical problems. I thank Satish, who has studied cross-slip extensively himself, for his advice and interesting discussions. Ben introduced me to hiking in Switzerland, which I enjoy a lot. Mike was the “master of ceremony” of many lively discussions. I thank Mostafa, who had entered the PhD program before me, for helpful advice regarding my next steps. Moreover, I thank Ali for his advice and our tremendous discussions. Never forget: you wont find the global minimum. I thank Max, Predrag and Aitor for our many meetings. I wonder how many lemonade bottles we emptied over the course of three years. I fondly recall several hikes and many inspiring conversations with Varun and Max, who share my sense of humor. In this spirit, I wish Max much success with the further development of dislocation jokes. Finally, I thank Francesco, Till and Nikolaos for their sage advice. A special thanks goes to Till for translating the abstract i Acknowledgements of this thesis into French. I also thank my friends from the “Stammtisch”, in particular Simon Schütz, with whom I departed on many recreational expeditions into the alps. Without him as planner and cartographer, I would not have went on some hikes and been lost on others. Moreover, I want to thank my friends from Germany for their support. I cannot name them all here, however, I want to thank Simon for our Skype chats, Felix for many exhilarating messages, and Benni and Kathrin for visiting me thrice. Furthermore, I thank Johannes Joel Möller for proofreading the manuscript and basically for his friendship since we were “little” students. Last but not least, I want to thank my family. Without their support this work would not have been possible. Lausanne, December 5, 2017 W. G. N. ii Abstract The mechanical strength of metals depends on their resistance against various microscopic deformation processes. In ductile metals, the most important process is shearing of the crystal lattice by dislocations. One of the fundamental aspects of dislocation motion is cross-slip of screw dislocations, the process by which they change their glide plane. In Face-Centered Cubic (FCC) metals, cross-slip is supposed to play a role in dislocation structuring, work hardening, recovery, fatigue, etc. Most prior studies on cross-slip in FCC metals focused on pure metals. There have been few studies of solute effects on cross-slip, which are important for engineering alloys. Here, the effects of substitutional solutes are studied using atomistic simulations and statistical modeling. In the first part of the thesis, the mechanism and energy of cross-slip of short (40 Burgers vectors long) dislocations in Ni-Al, Al-Mg and Cu-Ni alloys are determined using atomistic calculations. These calculations are carried out with real random alloys and with “average” alloys, where the real atom types are replaced by a single average type. By comparison, it is shown that cross-slip is controlled by fluctuations in the solute concentration, i.e. the activation energy for cross-slip is a distributed variable with a large variance around the mean value. The latter changes only little with concentration. Most importantly, activation energies that are significantly lower than the mean value can be observed in random alloys. A linear correlation between the activation energy and the energy difference between the state of the dislocation before and after cross-slip is observed. An analytical, parameter-free model of this energy difference is developed, which takes random changes in solute-dislocation and solute-solute binding energies into account. Thus, it is possible to predict the distribution of activation energies for nucleation of cross-slip. In the second part, cross-slip of long (102–103 Burgers vectors) dislocations is studied using a random walk model. Cross-slip is seen as a discrete process, where one Burgers vector long subsegments of the dislocation cross-slip one after another. Associated with each step is a random energy due to random changes in solute binding energies, as well as a deterministic energy change due to constriction formation and stress effects. The random walk model allows the calculation of the activation energy distribution for arbitrary dislocation lengths and stresses. Cross-slip of long dislocations is unlikely at zero stress, due to increasing frequency of high activation energies with increasing length. However, an external stress eliminates these high barriers. Cross-slip then becomes a weakest-link problem. Like in the case of short dislocations, activation energies that are significantly lower than average-alloy estimates can be observed in real random alloys. Keywords: screw dislocation, cross-slip, face-centered cubic crystals, solid solution, atomistic simulation, transition path calculation, random walk, modeling iii Zusammenfassung Die Festigkeit der Metalle ist abhängig von ihrem Widerstand gegen verschiedene mikroskopische Verformungsmechanismen. Der wichtigste Mechanismus in duktilen Metallen ist die Abscherung des Kristallgitters durch Versetzungen. Ein wichtiger Aspekt der Versetzungsbewegung ist die Quergleitung von Schraubenversetzungen, also die Änderung ihrer Gleitebene. Im Falle der kubisch-flächenzentrierten (kfz-) Metalle nimmt man an, dass die Quergleitung bei der Bildung von Versetzungsstrukturen, der Kaltverfestigung, der Ermüdung, etc. eine Rolle spielt. Bei früheren Untersuchungen von Quergleitung in kfz-Metallen standen zumeist die reinen Metalle im Mittelpunkt. Es liegen nur wenige Studien zum Einfluss von Fremdatomen auf die Quergleitung vor. Dieser spielt in technischen Legierungen eine wichtige Rolle. In der vorliegenden Arbeit wird der Einfluss von substitutionellen Fremdatomen mittels atomistischer Simulationen und statistischer Modellierung untersucht. Im ersten Teil der Arbeit werden der Mechanismus und die Energie der Quergleitung kurzer (40 Burgersvektoren langer) Versetzungen in Ni-Al, Al-Mg und Cu-Ni mit atomistischen Simulationen bestimmt. Diese Berechnungen werden mit echten und “gemittelten” Mischkristallen durchgeführt. Bei letzteren werden die verschiedenen echten Atomtypen durch einen einzigen durchschnittlichen Typ ersetzt. Im Vergleich zeigt sich, dass die Quergleitung
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