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NEEP 541 – Phase Transformation due to Radiation

NEEP 541 – Phase Transformation due to Radiation. Fall 2003 Jake Blanchard. Outline. Phase Transformations Examples Definitions Read Wollenberger (page 63+). Introduction. Many materials are used in a multi-phase microstructure For example, consider a Ni-based alloy

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NEEP 541 – Phase Transformation due to Radiation

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  1. NEEP 541 – Phase Transformation due to Radiation Fall 2003 Jake Blanchard

  2. Outline • Phase Transformations • Examples • Definitions • Read Wollenberger (page 63+)

  3. Introduction • Many materials are used in a multi-phase microstructure • For example, consider a Ni-based alloy • Strength comes from distribution of fine Ni-Al-Ti precipitates • Precipitates are spherical (D=20 nm) • Under high-T irradiation, the precipitates redistribute around point defects

  4. Another Example • If there is some Cu in -Fe • At 700 C, Cu is soluble in the iron • Ageing at 500 C causes Cu to precipitate out (D=3 nm) • Irradiating the material with electrons to 0.001 dpa will cause the Cu to precipitate out near loops, causing hardening

  5. Phase Transformation • Irradiation can cause phase transformations at temperatures for which we would expect phase stability • We’ll consider diffusion controlled transformations • Result is either decomposition of solid solution or dissolution of one or more phases into solid solution • Mechanisms are nucleation and growth or spinodal decomposition

  6. Spinodal Decomposition • Composition fluctuations grow in parent phase until amplitudes reach composition of new phase • Relatively rare in irradiation induced transformations

  7. Nucleation and Growth • This is more common with irradiation-induced changes • Small phase nucleates and then grows by diffusion

  8. Transport • The transformations require atom transport • Irradiation must promote this transport • Transport is ballistic or via radiation enhanced diffusion

  9. Ballistic Transport • Driven by cascades • Disorders long range structure • Induces mixing

  10. Radiation Enhanced Diffusion • Solute diffusivities increase under radiation due to increased vacancy concentration

  11. Radiation Induced Segregation (RIS) • Inverse Kirkendall Effect • Point defects, created by radiation, tend to gravitate towards sinks (typically large defects) • Atoms will diffuse in the same direction as interstitials and in the opposite direction of vacancies • If vacancy diffusion is controlling and A diffuses faster than B, then A will be depleted near the defect

  12. Example • Consider surface of Ni sample as sink • Irradiation with Ni ions leads to changes in concentration • Al, Ti, and Mo are depleted near surface • Si is enriched • Evidence is that diffusion of Si is interstitial controlled, while the rest are vacancy controlled

  13. Steel • In steel, Si is enriched near surface

  14. Temperature Effects • Inverse Kirkendall only operates in intermediate temperatures • At low temperature, defect concentrations build and tend to annihilate rather than diffuse to sinks • At high temperatures, thermal diffusion dominates and equilibrium atom concentrations are reached

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