Heavy-ion irradiations of Fe and Fe-Cr model alloys Part 1: Damage evolution in thin-foils at lower doses

The evolution of radiation damage in Fe and Fe-Cr alloys under heavy-ion irradiation was investigated using transmission electron microscopy. Thin foils were irradiated with 100 or 150 keV Fe+ and Xe+ ions at room temperature (RT) and 300 degrees C. Dynamic observations followed the evolution of damage and the early stages in damage development are reported. Small (2-5 nm) dislocation loops first appeared at doses between 10(16) and 10(17) ions m(-2) in all materials. Loop number densities depended strongly on the foil orientation in pure Fe but not in Fe-Cr alloys. Number densities did not depend strongly on Cr content. For a given material, defect yields were higher for Xe+ ions than for Fe+ ions, and were higher at RT than at 300 degrees C. Loops with both < 100 > and 1/2 < 111 > Burgers vectors were identified. The proportion of < 100 > loops was larger, especially in pure Fe. Dynamic observations showed that: the contrast of some new loops developed over intervals as long as 0.2 s; hopping of 1/2 < 111 > loops was induced by the ion and electron beams and was pronounced in ultra-pure iron; and many loops were lost during and after ion irradiation by glide to the foil surface. The number of loops retained was strongly dependent on the foil orientation in Fe, but less so in Fe-Cr alloys. This is due to lower loop mobility in Fe-Cr alloys, probably due to pinning by Cr atoms. Reduced loop loss probably explains the higher loop number densities in Fe-Cr alloys compared with pure Fe.