NEEP 541 Microstructure Fall 2002 Jake Blanchard Outline

NEEP 541 – Microstructure Fall 2002 Jake Blanchard

Outline n Microstructure n Definitions

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

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

Pre-Irradiation n n Vacancy formation energy is lower than interstitial formation energy Equilibrium vacancy concentration greater than interstitial Interstitials are mobile Dislocations not in equilibrium Dislocation density ~1012 /m 2

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

FCC interstitial configuration

BCC interstitial configuration

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

Tetrahedral

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

Di-Interstitials in FCC and BCC

Dumbell - FCC

Dumbell - BCC

Vacancies n n n 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

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

Interstitial Loop

Vacancy loop

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

Loop Growth n n 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

Voids n n n Excess vacancies can form 3 -D clusters Usually some inert gas is needed to stabilize these voids At low temperature (<0. 25 Tm), diffusion is slow so voids don’t form At high temperature (>0. 5 Tm), thermal emission eliminates voids In between, void formation is likely This causes swelling (we’ll come back to this)

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

Destination of Au Interstitials

Dislocation Changes n n n 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

Dislocation Density Evolution

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

Diffusion Coeff. for Ni alloy