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Department Metallkunde und Werkstoffprüfung

Hochauflösende Werkstoffanalytik

Dr. Francisca Mendez Martin

Materials Development
For the development of new materials or the improvement of the already existing, special attention has to be turned on the dependence of the physical, chemical and mechanical properties with the material´s microstructure, from the production route until service, from atomic to macro levels. One of the main research fields in this working group is to study higher-performance materials with Atom Probe Tomography (APT).

Atom probe tomography
Two different Atom Probe microscopes are available in our group: on one side, the 3DAP (3-Dimensional Atom Probe) system from Oxford nanoscience, and on the other side the LEAP 3000 X HR (Local Electrode Atom Probe) system from Cameca. The physical principles of these two microscopes are the same: the atoms from a highly sharp tip (less than 100 nm in diameter and around 10° of shank angle) are field evaporated triggered by a high-voltage pulse or thermal pulse (via laser). The atoms are field evaporated layer by layer from the material to a position sensitive detector. The detector allows determining simultaneously the time of flight of the ions by measuring the time between a laser flash or voltage pulse and the arrival on the detector, obtaining the mass over charge ratio. A reconstruction of the position of the atoms and the mass to charge ratio can be done afterwards.

Some applications of APT are:     

  • chemical composition characterization of different precipitates/phases in nanometer/atomic scale      
  • investigation of interfaces    
  • calculation of volume and phase fractions      
  • measurements of lattice parameters     
  • morphology of precipitates
  • studies on diffusion coefficients between phases

Sample preparation and Dual Focused Ion Beam
Moreover, specimen preparation techniques are of special importance if necessary to reveal the microstructure of the investigated material. In our working group a dual focused ion beam (FIB), Versa 3D from FEI, is available for specimen preparation as well with five different detectors for material characterization:

  • ICD: In-Column Detector.   
  • ETD: Everhardt Thornley Detector.      
  • CBS: Concentric Backscatter Detector     
  • EDS: Energy Dispersive X-ray Spectroscopy, Octane Silicon Drift Detector (SDD) Series from EDAX company.      
  • EBSD: Electron Backscatter Diffraction, Hikari camera and a forward backscatter detector, from EDAX company.

The dual FIB microscope is a microscope with two different sources: electron source and ion source. The combination of these two sources allows electron imaging during the ion column prepared a site specific specimen via cutting or doing deposition. The FIB is mainly used for site specific APT specimen preparation, but also applied for TEM (transmission electron microscopy) specimen preparation, cut and view of cross-sections, crystallography characterization and chemical composition mappings.

Two phases Hard metal specimen for APT analysis. The first step was a lift out extraction from the bulk material, followed by a welding and cutting process on pre-tips, the final step was the annular milling to define the final shape of the tip.

 

 

3D Atom Probe reconstruction of a Nickel-base superalloy 718. y‘ and y‘‘ precipitates within an 60at.% Ni-Isosurface are defined for the analysis of size distribution and volume fraction.