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MPIE-Colloquium: Atomic Resolution Observations of Step Structure and Dynamics in Grain Boundaries

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MPIE-Colloquium: Atomic Resolution Observations of Step Structure and Dynamics in Grain Boundaries

The development of aberration correction for electron microscopy has greatly increased our ability to characterize materials at the atomic scale. The technological advances that have extended the resolution limit to 0.5Å have also made it possible to record images with better signal-to-noise and at faster rates. In this work, atomic resolution images of moving steps in grain boundaries in gold bicrystals were obtained from extended HR(S)TEM time series by averaging between structural events. The resulting precision of atomic displacements and the small volume of material involved in the observed structures allowed a direct atom-by-atom comparison with MD simulations, revealing details of the transition beyond the temporal resolution of experimental techniques. Specifically, simulations uncovered a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. This analysis is part of a broader study of grain boundary behavior during deformation and capillary shrinkage of island grains. Dynamic observations show that the rate of shrinkage is non-parabolic, and the mechanism is controlled by step motion. Finally, some current developments in electron microscopy will be outlined with a view toward future research on interfaces in materials. [more]

MPIE-Colloquium: Structural Defects and Local Interfacial Chemistry of Complex Oxide Heterointerfaces

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MPIE-Colloquium: Structural Defects and Local Interfacial Chemistry of Complex Oxide Heterointerfaces

Transition metal oxide superlattices have been widely investigated during recent years as they are one of the largest material groups where physical and chemical properties such as ferroelectricity, magnetism, ionic and electronic conductivity are closely coupled to structural parameters. Cation sub¬sti¬tution in complex oxides is an effective way to develop the functionalities through carrier doping, band engineering, or application of chemical pressure. For example, the coupling between charge and spin degrees of freedom across the interfaces and the local charge carrier concentration profiles have profound influences on the occurrence of superconductivity in low dimensional systems. Super-conductivity arises when a parent insulator compound is doped beyond some critical con-centration. Furthermore, the magnetic behaviour and conductivity of complex oxide superlattices can be tuned by controlling the layer thickness and by selecting appropriate intervening layer materials. Various methods for growing controlled superlattice structures exist, a favourite has been pulsed laser deposition (PLD), but molecular beam epitaxy (MBE) is now also popular because of the controlled deposition rate and the flexibility allowed by the use of individual element sources. In theory, this allows composition control to the level of individual atomic layers. The PLD process requires higher temperatures and pressures than MBE. It also involves significantly higher energies for the impinging particles, which has potential implications for the interface roughness. In this presentation, I will discuss mapping of the local structure and interfacial chemistry of various complex oxide hetero-interfaces through advanced scanning transmission electron microscopy (STEM) in combination with energy-dispersive x-ray (EDX) analysis and electron energy-loss spectroscopy (EELS).1 EELS allows for local probing of chemical composition and bonding, as well as electronic and magnetic structure, making the combination of STEM and EELS ideal for discovery of structure-property correlations at the atomic scale.2,3 References 1 F. Baiutti et al., Nature Comm. (2015), DOI: 10.1038/ncomms9586, in press. A.V. Boris et al., Science 332 (2011) 937-940. E. Detemple et al., Appl. Phys. Lett. 99 (2011) 211903. E. Detemple et al., J. Appl. Phys. 112 (2012) 013509. A. Frano et al., Adv. Mater. 26 (2014) 258-262. F. Wrobel et al., submitted (2015). K. Song et al., APL Materials 2 (2014) 032104. D. Zhou et al., APL Materials 2 (2014) 127301. D. Zhou et al., Adv. Mater. Interfaces 2 (2015) 1500377. D. Zhou et al., Ultra¬micro¬sco¬py 160 (2016) 110–117. 2 PAvA gratefully acknowledges the intense collaboration with the following people without their contributions this work wouldn’t have been possible: F. Baiutti, E. Benckiser, C. Bernhard, A.V. Boris, M. Castro-Colin, G. Cristiani, E. Detemple, K. Du, A. Frano, E. Gilardi, G. Gregori, H.-U. Habermeier, V. Hinkov, B. Keimer, M. Kelsch, F.F. Krause, G. Logvenov, Y. Lu, J. Maier, V.K. Malik, A.F. Mark, Y. Matiks, M. Morenzoni, K. Müller-Caspary, E. Okunishi, P. Popovich, T. Prokscha, Q.M. Ramasse, M. Reehuis, A. Rosenauer, Z. Salman, H. Schmid, W. Sigle, K. Song, V. Srot, A. Suter, Y. Wang, P. Wochner, F. Wrobel, M. Wu, D. Zhou. 3 The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007/2013] under grant agreement no 312483 (ESTEEM2). [more]

Role of orientation and grain interactions on the deformation of Ti64

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Role of orientation and grain interactions on the deformation of Ti64

  • Date: May 23, 2016
  • Time: 13:30 - 14:00
  • Speaker: Prof Prita Pant
  • Department of Metallurgical Engineering and Materials Science, IIT-Bombay, Mumbai, India Speaker Bio: Prof Prita Pant graduated with a Ph.D from Cornell University in 2004 and is currently an Associate Professor at the Indian Institute of Technology Bombay. Her research interests focus on correlating deformation response of metals and alloys with microstructure. Her group has been using both bulk and micro-deformation techniques along with microscopy to investigate the evolution in microstructure with deformation and quantify the effect of dominant microstructural features on mechanical behavior. They also carry out both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations to investigate deformation behavior.
  • Location: Max-Planck-Institut für Eisenforschung GmbH
  • Room: Room 1034 Hall 9
  • Host: Prof. Gerhard Dehm / Dr. Nagamani Jaya Balila
  • Contact: stein@mpie.de

Titanium and its alloys are extensively used for aerospace and biomedical applications due to their high specific strength – even at elevated temperature, and excellent corrosion resistance. Due to its hexagonal crystal structure, alpha titanium is highly anisotropic. Hence it is essential to understand the role of crystallographic orientation and texture in deformation. In this poster, we will present results from bulk deformation and nanoindentation experiments, where we have investigated the role of orientation and of grain interaction on deformation. We show that both orientation and near neighbor interactions play an equally important role in the deformation of polycrystals. We further show that boundaries between hard and soft grains are significantly harder than boundaries between soft grains due to greater incompatibility in deformation between hard and soft grains. In the last five minutes of my talk, she will present some results from her ongoing Dislocation Dynamics simulation studies of deformation of the ferrite phase during wire drawing of pearlitic steel.. [more]

 
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