Highlights

Loop Formation and Motion in Fe and Fe-Cr alloys

Scientific Achievement

The development of radiation damage under heavy-ion irradiation in thin foils of pure Fe of different purities and a range of FeCr alloys has been investigated in in situ experiments using the Argonne IVEM-Tandem Facility. Specimens of Fe and FeCr alloys with Cr contents 5, 8 and 11wt.% were irradiated with 100 or150 keV Fe⁺ and Xe⁺ heavy ions at room temperature (RT) and 300^oC.  Dynamic observations followed the evolution of damage.  We report here on early stages in damage development, up to a dose of 2 x 10¹⁸ ions m^-2 (~ 1dpa). Small (2-5 nm) dislocation loops first appeared at doses between about 10¹⁶ and 10¹⁷ ions m^-2 in all materials.  The threshold dose for visible damage in Fe-Cr alloys was considerably lower that in pure Fe irradiated under the same conditions and loop number densities at a given dose were higher.  The highest loop number densities were found in the Fe11%Cr alloy.  For a given material, defect yields were higher for Xe⁺ ions than for Fe⁺ ions, and were higher at RT than at 300^oC.  Loops with both <100> and ˝ <111> Burgers vectors were identified. The proportion of <100> loops was larger, especially in pure Fe where they predominated.  Dynamic observations showed: (1) that the contrast of new loops developed over time intervals as long as 0.2s and on average was longer for loops produced by Xe⁺ ions than Fe⁺ ions; (2) loop motion (“hopping”) of ˝ <111> loops was induced by the ion and electron beams and was particularly pronounced in ultra-pure (UHP) iron; and (3) that many loops were lost during and after ion irradiation, probably by glide to the foil surface.  The number of loops retained in the foil was strongly dependent on the foil orientation in Fe, but less so in FeCr alloys.  This is due to a lower loop mobility in FeCr alloys and probably explains the lower number densities of loops in thin foils of Fe.

Significance

The appearance of loops over such a long timescale seems to be quite new, and has excited considerable interest in the modeling community.  The high mobility of loops and the phenomenon of “hopping”, and the effect of Cr additions on reducing hopping, have important implications in understanding the development of radiation damage in this class of materials.  This work has been published in Phil. Mag. 88, 2851-2880 (2008).

Performers

Z. Yao, M. L. Jenkins (U. Oxford); M. Hernández-Mayoral (CIEMAT); M. A. Kirk (Argonne-MSD)