Fundamentals of Radiation Damage- Lecture 5 Flashcards
What is radiation induced segregation a consequence of?
Radiation enhanced diffusion
What is the increase in diffusion or enhancement of atom mobility in an irradiated metal due to?
Enhanced concentration of the defects.
Creation of new defect species.
Require a diffusion pathway and displaced atoms can provide/improve this.
Why do concentration/strain gradients form (in relation to radiation induced segregation)?
Due to diffusing defects and alloying elements
What can changes in grain boundary composition result in?
Microstructure changes like precipitation, dislocation loop structure, void structure.
Lead to intergranular corrosion and stress corrosion cracking.
How do the concentrations of Cr, Ni, Si and P vary around GBs in irradiated stainless steel?
All constant in bulk of grains.
At GBs: Cr drops sharply to a minimum, Ni, Si and P increase sharply to maxima.
How does radiation induced segregation progress in metals?
In alloys elements will be of different sizes. Radiation damage can form Frenkel pairs of vacancies and interstitials. To reduce the strain energy stored in the lattice undersize solute substitutional atoms will preferentially exchange with solvent atoms in interstitial positions, oversize solute atoms will tend to remain on or return to substitutional sites. Vacancies preferentially exchange with oversize solute atoms. GBs are sinks for defects, both vacancies and interstitials. As vacancies diffuse towards GBs, oversize solute atoms diffuse in the opposite direction. Interstitial undersize solute atoms diffuse towards GBs. Result in enrichment of undersize and depletion of oversize solute atoms near GBs.
What affects whether an element enriched or depletes at GBs?
Relative strength of atom association with respective defect fluxes. Basically is it large or small so will diffuse by vacancy or interstitial-mediated mechanism respectively.
Problem of radiation induced segregation for stainless steels
Leads to changes in susceptibility to intergranular corrosion and SCC. Cr provides passivity to stainless steels by forming very thin protective Cr2O3 film. Cr depletes at GBs in irradiated stainless steels. Loss of Cr at GBs causes increased susceptibility to intergranular corrosion and SCC.
Other effects of radiation induced segregation and example in Ni-Al solid solution
Can lead to local solute concentration that exceeds solubility limit and results in precipitation.
Can stabilisé a second phase that would otherwise not be present in thermodynamic equilibrium and vice versa.
E.g in Ni-12.8at%Al solid solution alloy irradiated with 5MeV Ni+ ions to 10^16cm^-2 results in formation of a Ni3Al phase which is separate and distinct from the parent phase and embrittling.
