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 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

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 dpa will cause the Cu to precipitate out near loops, causing hardening

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

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

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

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

Ballistic Transport Driven by cascades Disorders long range structure Induces mixing

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

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

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

Steel In steel, Si is enriched near surface

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