NEEP 541 – Microstructure

1. NEEP 541 – Microstructure Fall 2002 Jake Blanchard

2. Outline • Microstructure • Definitions

3. Defects • Planar Defects • Grain boundaries • Interphase boundaries • Twin boundaries • Domain boundaries • Stacking defaults • Volume Defects • Cavities • Precipitates • cracks • Point Defects • Vacancies • Interstitials • Impurities • Clusters • Line Defects • Dislocations • Strings of pt defects

4. Pre-Irradiation • Point defects, dislocations, grain boundaries are key • These act as sinks and traps for moving defects • Typically in thermodynamic equilibrium Formation energy Defect concentration

5. Pre-Irradiation • Vacancy formation energy is lower than interstitial formation energy • Equilibrium vacancy concentration greater than interstitial • Interstitials are more mobile • Dislocations not in equilibrium • Dislocation density ~1012 /m2

6. Damage Structure • Displacements create v’s and i’s • These move by diffusion • Get recombination • …and clustering • Di-vacancies, di-interstitials • Voids • Stacking fault tetrahedra

7. FCC interstitial configuration

8. BCC interstitial configuration

9. Octahedral http://www.techfak.uni-kiel.de/matwis/amat/def_en/makeindex.html

10. Tetrahedral

11. Lattice Effects • Interstitials deform surrounding lattice • They “share” spot with lattice atom • Called a dumb-bell configuration

12. Di-Interstitials in FCC and BCC

13. Dumbell - FCC

14. Dumbell - BCC

15. Vacancies • Produce weaker lattice deformation • Form planar and 3-D clusters • Lattice collapses around planar clusters (dislocation loops) • 3-D clusters are voids or stacking fault tetrahedra (SFT) • Impurities (inert gases) stabilize voids

16. Dislocation loops • Lattice collapses around platelet of vacancies or interstitials • Frank loops form • Loops grow • Loops convert to perfect loop • Loops rotate

17. Interstitial Loop

18. Vacancy loop

19. Loop Growth • Loops grow by collecting point defects as they diffuse • Typical growth equation is: Point defect flux to loop Thermal emission

20. Loop Growth • Z represents preferential attractive interaction between dislocation loops and moving interstitials over that with vacancies • That is, dislocation loops are more likely to absorb interstitials than vacancies • This tends to leave an excess of vacancies in the lattice • Hence, interstitial loops tend to grow and vacancy loops tend to shrink

21. Voids • Excess vacancies can form 3-D clusters • Usually some inert gas is needed to stabilize these voids • At low temperature (<0.25Tm), diffusion is slow so voids don’t form • At high temperature (>0.5Tm), thermal emission eliminates voids • In between, void formation is likely • This causes swelling (we’ll come back to this)

22. Microstructure Development • 3 changes occur as damage takes place • Changes in dislocation structure • Void formation and growth • Changes in chemical state (segregation, precipitation)

23. Destination of Au Interstitials

24. Dislocation Changes • Dislocations change shape and size • Length increases by loop growth and interactions with other dislocations • Length decreases due to annihilation • In annealed materials, length increases • In cold-worked materials, length decreases

25. Dislocation Density Evolution

26. Microchemical Changes • Cascades can destroy clusters and dissolve precipitates (effective diffusion) • Increased point defect densities enhance diffusion

27. Diffusion Coeff. for Ni alloy