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Dive into the research topics where C. Gammer is active.

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Featured researches published by C. Gammer.


Philosophical Magazine | 2010

Electron microscopy of severely deformed L12 intermetallics

D. Geist; C. Gammer; Clemens Mangler; C. Rentenberger; H. P. Karnthaler

Severe plastic deformation (SPD) can be used to make bulk, nanostructured materials. Three L12 long-range ordered (LRO) intermetallic compounds were studied by TEM methods. The superlattice glide dislocations can dissociate according to two schemes: antiphase boundary (APB) coupled unit dislocations or superlattice intrinsic stacking fault (SISF) coupled super Shockley partials; both of them are analysed by weak-beam TEM methods. The nanostructures resulting from SPD carried out by high pressure torsion (HPT) are strongly affected by the different dissociation schemes of the dislocations. APB-dissociated superlattice dislocations and especially the APB tubes they form lead to the destruction of the LRO by HPT deformation as observed in Cu3Au and Ni3Al, whereas in Zr3Al heavily deformed (∼100,000% shear strain) at low temperatures the order is not destroyed since the deformation occurs by SISF-dissociated dislocations. In addition to the effects on the LRO the different dissociation schemes of the dislocations have a strong impact on the refinement and destruction of the crystalline structure by SPD. They seem to be decisive for the dynamic recovery considered as the limiting factor for the final grain sizes and the possibility of reaching amorphisation. Finally, the correlation between the reduction of the LRO and the structural refinement occurring during SPD is different in the three different alloys: In Cu3Au, the LRO is already strongly reduced before the structural refinement reaches saturation, in Ni3Al both are occurring simultaneously, whereas in Zr3Al, the formation of the nanograins does not seem to be connected with disordering.


Radiation Effects and Defects in Solids | 2012

Radiation effects in bulk nanocrystalline FeAl alloy

A. Kilmametov; Adam G. Balogh; M. Ghafari; C. Gammer; Clemens Mangler; C. Rentenberger; R. Valiev; H. Hahn

Bulk-ordered nanocrystalline FeAl intermetallic compound with a grain size of 35 nm was prepared using severe plastic deformation. Nanocrystalline and coarse-grained counterparts with a grain size of 160 nm were subjected to 1.5 MeV Ar+ ion irradiation at room temperature. Enhanced irradiation resistance of nanocrystalline FeAl has clearly been identified by means of grazing-incidence X-ray diffraction and Mössbauer spectroscopy. At the identical damage dose, the nanocrystalline FeAl retains long-range ordering in the B2-superlattice structure, while the coarse-grained state becomes already substantially disordered. The present experimental studies verify that fully dense ordered intermetallic alloys are promising candidate materials for radiation environments.


Microscopy and Microanalysis | 2016

Study of Structure of Li- and Mn-rich Transition Metal Oxides Using 4D-STEM

Alpesh K. Shukla; Colin Ophus; C. Gammer; Quentin M. Ramasse

The structure of Liand Mn-rich transition metal oxides (Li1+xM1-xO2, where M is usually a combination of transition metals such as Mn, Co and Ni, called LMRTMO henceforth) has been debated extensively for the past several years. It has been recently shown, by imaging entire primary particles at atomic resolution at multiple zone axes using high angle annular dark field (HAADF-) scanning transmission electron microscopy (STEM), that the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure [1] as shown in Figure 1 (a). Using HAADF-STEM, it was demonstrated that the particles did not contain two phases (trigonal and monoclinic) in the bulk as described earlier [2], since the size of the primary particles was small enough show that the monoclinic phase was the only phase present in entire particles. However, larger sized particles are often preferred for the cathode materials in order to obtain better volumetric energy density. For these applications, imaging entire particles at atomic resolution is very difficult and in some cases impossible owing to regions of higher thickness. In this paper we demonstrate the use of 4D-STEM [3] using large fields of view on a commercial cathode material to confirm that the bulk of the primary particles is made up of a single phase and consists of domains corresponding to three variants of monoclinic phase.


Advanced Materials | 2015

Functionalizing Aluminum Oxide by Ag Dendrite Deposition at the Anode during Simultaneous Electrochemical Oxidation of Al

Lidija D. Rafailović; C. Gammer; C. Rentenberger; T. Trišović; Christoph Kleber; Hans Peter Karnthaler

A novel synthesis strategy is presented for depositing metallic Ag at the anode during simultaneous electrochemical oxidation of Al. This unexpected result is achieved based on galvanic coupling. Metallic dendritic nanostructures well-anchored in a high surface area supporting matrix are envisioned to open up a new avenue of applications.


RSC Advances | 2016

Surface enhanced Raman scattering of dendritic Ag nanostructures grown with anodic aluminium oxide

Lidija D. Rafailović; C. Gammer; J. Srajer; T. Trišović; J. Rahel; Hans Peter Karnthaler

We present the application of newly developed Ag nanodendrites (Ag-ND) grown together with anodic aluminium oxide for surface-enhanced Raman scattering (SERS). The Ag-ND yield very pronounced SERS using a self-assembled monolayer (SAM). This is confirmed by simulations showing hot spots in the electromagnetic field at the surfaces of the Ag-ND. SERS measurements reusing Ag-ND demonstrate its long-term stability even after one year.


Microscopy and Microanalysis | 2011

Three-Dimensional Analysis by Electron Diffraction Methods of Nanocrystalline Materials

C. Gammer; Clemens Mangler; H. P. Karnthaler; C. Rentenberger

To analyze nanocrystalline structures quantitatively in 3D, a novel method is presented based on electron diffraction. It allows determination of the average size and morphology of the coherently scattering domains (CSD) in a straightforward way without the need to prepare multiple sections. The method is applicable to all kinds of bulk nanocrystalline materials. As an example, the average size of the CSD in nanocrystalline FeAl made by severe plastic deformation is determined in 3D. Assuming ellipsoidal CSD, it is deduced that the CSD have a width of 19 ± 2 nm, a length of 18 ± 1 nm, and a height of 10 ± 1 nm.


Microscopy and Microanalysis | 2016

Automated Analysis of Large Datasets Acquired with STEM Diffraction Mapping

C. Gammer

In scanning nanobeam electron diffraction a sample region is scanned by a converged electron probe and for each probe position an electron diffraction pattern is recorded along with the regular ADF-signal. Thus, each pixel of the resulting STEM map is a full diffraction pattern for the corresponding beam position. Acquisition of a map by recording a diffraction pattern and shifting the electron probe is limited to a small number of probe positions, due to the time needed to record a diffraction pattern. Therefore, the electron detector is used in continuous recording mode and synced with the scan coils. This technique is only limited by the speed of the CCD camera, yielding up to 20 f/s. The acquisition is fully automated and large maps can be recorded in around 30 minutes. Recent developments in fast electron detectors enable recording datasets in a significantly shorter time. A Gatan K2 IS direct electron detector operating at a frame rate of 400 f/s was used to record diffraction maps with 256x256 probe positions in less than 3 minutes. The resulting dataset consists of more than 65,500 diffraction patterns, with around 2000x2000pixels each [1].


Microscopy and Microanalysis | 2016

Nanoscale Strain Mapping During in situ Deformation of Annealed Al-Mg Alloys

Thomas C. Pekin; Jim Ciston; C. Gammer; Andrew M. Minor

Al-Mg alloys represent an attractive option for weight-sensitive applications such as exterior car paneling, but serrated flow during plastic deformation limits their applicability (due to both poor formabilty and strain rate sensitivity for impact qualifications). Although serrated flow is a commonly experienced phenomenon during plastic deformation of these alloys [1], the fundamental dislocation processes responsible remain poorly understood. Recent advances in local strain mapping using nanobeam electron diffraction (NBED) have demonstrated the ability to observe single defects and the strain fields around them [2]. By observing dislocations, their strain fields, their movement under stress, as well as their interactions with each other and precipitates, we aim to provide insight into the fundamental mechanisms of serrated flow.


Microscopy and Microanalysis | 2016

Local strain measurements during in situ TEM deformation with nanobeam electron diffraction

Andrew M. Minor; C. Gammer; Yu Deng; Colin Ophus; Peter Ercius; Jim Ciston

A.M. Minor, C. Gammer,Y. Deng, C. Ophus, P. Ercius, J. Ciston 1. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2. Department of Materials Science & Engineering, University of California, Berkeley, CA 94720 3. Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, Austria 4. Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, P.R. China


Microscopy and Microanalysis | 2016

Multimodal Acquisition of Properties and Structure with Transmission Electron Reciprocal-space (MAPSTER) Microscopy

Jim Ciston; Colin Ophus; Peter Ercius; Hao Yang; Roberto dos Reis; Christopher T. Nelson; Shang-Lin Hsu; C. Gammer; Burak V. Ӧzdöl; Yu Deng; Andrew M. Minor

1. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA 2. Department of Materials Science and Engineering, University of California, Berkeley, USA 3. Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, Austria 4. Western Digital Media, Fremont, USA 5. Laboratory of Solid State Microstructure, Nanjing University, Nanjing, P.R. China

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T. Trišović

Serbian Academy of Sciences and Arts

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D. Geist

University of Vienna

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Bernhard Gollas

Graz University of Technology

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