Apparatus for In-situ Defect Analysis (AIDA)

38 Pages·2015·4.17 MB·English

by  Liedke, Maciej Oskar

Defects in Solids
Positron Annihilation Spectroscopy (PAS)
Apparatus for In-situ Defect Analysis (AIDA)
Maciej Oskar Liedke
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Outline
• Motivation
• Crystal structures
• Defect dimensionality
– 0D (ZERO dimension) – point defects
• Vacancies, interstitials, impurities
– 1D – linear defects
• Dislocations (edge, screw, mixed)
– 2D – grain boundaries, surfaces
• Tilt, twist
– 3D – open volume defects
• Pores, voids, cracks
• How to measure defects?
– Positron annihilation spectroscopy (PAS)
• Tools for PAS measurements
– Slow-Positron System of Rossendorf (SPONSOR)
– Apparatus for In-situ Defect Analysis (AIDA)
– Monoenergetic Positron Spectroscopy (MePS)
• Conclusion
Page 2 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Why to investigate defects?
Material failures impose a significant threat to the integrity and the safety
of technical systems. Investigations of the microscopic origin and the
development of defects requires advanced methods.
… early days of material analysis
courtesy A. Wagner
Page 3 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Crystal Structures
• All ionic solids and most covalent solids are crystalline
• All solid metals, under normal circumstances, are crystalline
• The ideal crystal has an infinite 3D repetition of identical units, e.g., atoms
• Crystals have a lattice and a unit cell
• Lattices are different types of regular 3D arrays of points in space
• unit cell is either an atom or 3D geometrical arrangement of atoms, which
is repeated identically at every lattice point
• Most metals (~90%) unit cell structures:
Sources: http://poh101.com/index3/mems.html, http://www.insula.com.au/physics/1250/L8.html
Page 4 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Crystal Structures
• Body Centered Cubic (BCC)
BCC
 Cr, Fe, Mo, K, Na, Ta, V
• Face Centered Cubic (FCC)
 Al, Cu, Au, Ag, Pb, Ni, Pt
• Hexagonal Close Packed (HCP)
 Mg, Co, Zr, Ti, Be
FCC
Unit Cell n CN a/R APF
SC 1 6 2 0.52
BCC 2 8 4√3 0.68
FCC 4 12 2√2 0.74
HCP 6 12 0.74
HCP
n – number of atoms per unit cell
CN – coordination number
APF – atomic packing factor
Sources: http://poh101.com/index3/mems.html, http://www.virginia.edu/bohr/mse209/chapter3.htm
Page 5 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Defects
• 0D (ZERO dimension) – point defects
 Vacancy  Interstitial  Impurity
the absence of an an atom (foreign or regular) substitution of a regular
atom from a site on a non-lattice site lattice atom (from within
normally occupied or outside the crystal)
in the lattice a lattice atom moved to an with an atom that does
off-lattice site not normally occupy that
→ vacancy created site

 typical point defect concentration: 0.1-1%
Sources: http://www.insula.com.au/physics/1250/L9.html
Page 6 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Defects
• 1D – linear defects – dislocation
→ line discontinuity in the regular crystal structure
→ characterized by Burgers vector
 Edge dislocations (metals)

 Burgers vector perpendicular to the dislocation line
 insertion or removal of an extra half plane of atoms
in the crystal structure
 Edge dislocations (ionic and covalent solids)

 extra half planes of unit cells
 Disruption to the crystal structure occurs along the dislocation line
→ the line may step up or down out-of-plane
Sources: http://www.insula.com.au/physics/1250/L9.html, http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/dislocations.html
Page 7 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Defects
• 1D – linear defects – dislocation
→ line discontinuity in the regular crystal structure
 Screw dislocations

 Burgers vector parallel to the dislocation line
 changes the character of the atom planes that form
single surface, which “spirals” from one end of the
crystal to the other
 misalignment shifts a block of ions gradually
downwards or upwards

 Mixed dislocations = Edge + Screw
Sources: http://www.insula.com.au/physics/1250/L9.html, http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/dislocations.html
Page 8 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Defects
• 2D – grain boundaries
 Grain boundaries  Tilt boundaries  Twin boundaries
planar defect that separates between two slightly crystals on either side of a
regions of different misaligned grains plane are mirror images of
crystalline orientation → an array of edge each other
(i.e. grains) within a dislocations → no disorder at the twin
polycrystalline solid boundary
Sources: http://www.insula.com.au/physics/1250/L9.html
Page 9 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de
Defects
• 3D – open volume defects
• Pores, voids
• The absence of a number of atoms to form internal surfaces in the crystal
• A typical volume defect is porosity, often introduced in the solid during
processing. Snow → highly porous ice
• Can act as a crack nucleation site as well as allow moisture to penetrate the
material
• micro-cracks
• Internal broken bonds create new surfaces
• Form at grain boundaries and other regions of disorder
Sources: http://www.insula.com.au/physics/1250/L9.html
Page 10 Member of the Helmholtz Association
Dr. Maciej Oskar Liedke I FWKK I www.hzdr.de