Highlights

Damage Evolution in Thin Foils of UHP-Fe and FeCr Alloys at High Doses

Scientific Achievement

We have carried out an investigation of damage development in UHP Fe and Fe-8%Cr at higher doses up to 2 x 10¹⁹ ions m^-2 (~13 dpa).  In thin-foil irradiations with 150 keV Fe⁺ ions at temperatures from room temperature (RT) to 500^oC significant complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 x 10¹⁸ ions m^-2, seeming to involve cooperative interaction, alignment and coalescence of smaller loops.  First strings of loops all with the same ˝<111> Burgers vectors formed.  In UHP Fe irradiated at 300^oC the damage then developed into colonies of resolvable interstitial loops with ˝<111> Burgers vectors.  By a dose of 2 x 10¹⁹ ions m^-2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops.  Similar but not identical damage structures developed in UHP Fe irradiated at RT and in Fe-8%Cr irradiated at both RT and 300^oC.  At 500^oC, square-shaped interstitial loops with b = <100> nucleated and grew to large sizes.  Damage structures in FeCr were similar but on a finer scale. Small voids were found at both 300^oC and 500^oC.  Evidently we unveiled a truth that there is a transition from 1/2<111> type predomination into <100> predomination at certain temperature between 450^oC and 500^oC.

Significance

The very complex and beautiful microstructures which develop at high doses were quite unexpected, and could not have been understood without these in situ experiments. Perhaps the most significant result here is the transition in loop types from ˝ <111> at lower temperatures to <100> at higher temperatures.  This transition is the subject of further experiments.  A similar result has now been found in ex situ irradiations, and it may well have important consequences on the use of ferritic alloys at high temperatures in future fusion and Generation IV fission reactors.  Some of this work has been published in Phil. Mag. 88, 2851-2880 (2008).  A second paper is in press in J.Nuclear Materials, and further papers are in preparation.

Performers