Contrast factors of irradiation-induced dislocation loops in hexagonal materials

Irradiation-induced defects, such as dislocation loops, significantly affect the mechanical properties of structural alloys, altering slip and influencing creep and growth. As a consequence, the quantitative characterization of irradiation-influenced defect structures as a function of dose, thermal treatments and/or cold work is essential for models which predict changes in mechanical properties due to the accumulation of irradiation defects. Whole pattern diffraction line profile analysis (DLPA) is a modern tool for microstructure characterization based on first-principles physical models, well established for dislocation density measurements in plastically deformed materials. However, the DLPA procedures that have been tailored for deformed materials account for the strain anisotropy of hexagonal crystals with theoretical contrast factors calculated specifically for dislocation types generated by plasticity which, if directly applied to irradiated materials, will inherently introduce inaccuracies. In an effort to specifically address dislocation structures consisting of irradiation defects, a method was developed to calculate theoretical contrast factors for any general elliptically shaped dislocation loop. The values of the contrast factors are calculated and compiled in tables for six common elliptical < a >-type and < c + a >-type loops for ten hexagonal crystals, in order to provide a database for future DLPA work on irradiated materials. The use of the dislocation loop specific contrast factors is demonstrated on neutron-irradiated Zr-2.5Nb.