Interaction of solutes with irradiation-induced defects of electron-irradiated dilute iron alloys
Introduction
Radiation-induced segregation causes a change in the microstructure of alloys during irradiation. There is a close correlation between the radiation-induced segregation and the size-factor of solutes. Enrichment of solutes at defect sinks takes place below stage III because the mixed-dumbbells migrate, thereby transporting solutes. For this reason, to study the interaction of solutes with self-interstitials is essential to know the various properties of materials subject to irradiation.
From the studies on the interaction of solutes with self-interstitials in FCC metals, it has been generally accepted that there are two kinds of solute traps for freely migrating self-interstitials corresponding to the size-factor of solutes in the metals 1, 2, 3, 4. Oversized solutes, remaining at their lattice sites, trap self-interstitials. In contrast to this, undersized and some slightly oversized solutes combine with self-interstitials to form mixed-dumbbells, displacing themselves from their lattice sites 1, 2, 3.
For BCC metals experimental work has been concentrated on Fe alloys, using electrical resistivity and magnetic after-effect measurements 5, 6, 7, 8, 9, 10. Electrical resistivity recovery measurements were performed in some detail by Maury et al. on several kinds of Fe dilute alloys electron-irradiated at around 20 K 5, 6, 7, 8. They obtained the result that the recovery in such alloys as Fe–Cr [6]and Fe–Mn [8](both solutes are slightly oversized) is enhanced compared to that in Fe between ID and IE. This demonstrates a marked difference from FCC metals.
With respect to the resistivity measurements by Maury et al., there still remain two experimental problems to be improved. One is the contamination of specimens by Si resulting from the quartz tube used for heat treatment in a hydrogen atmosphere. The other is the uncertainty of resistivity measurements stemming from ferromagnetic contribution of residual resistivity in the absence of a magnetic field. These problems become serious under the condition that the measurements must be carried out on dilute alloys containing only a small amount of Frenkel pairs (20–30 ppm).
Maury et al. attributed a distinct recovery stage at around 180 K in Fe–Si to the migration and annihilation of mixed-dumbbells formed between ID and IE [5]. This result hence raises the question whether or not any undersized solute stabilizes mixed-dumbbells thermally up to temperatures above stage I. This induced the authors to examine the resistivity recovery of Fe–P and Fe–Be, both P and Be being significantly less undersized than Si, and to study the dependence of solute size-factor on the recovery temperature, that is, the thermal stability of mixed-dumbbells.
Section snippets
Experimental
The high-purity Fe (RRR=3000–3500) was obtained by zone refining ATOMIRON (99.999%, Showa Denko). The purities of solute elements were as follows: molybdenum (99.999%, JM) and chromium (MARZ grade, MRC) with subsequent zone-refining, silicon (99.9999%, Oki Electric Industry), Phosphorus (99.9999%, Kojundo Chemical Laboratory), and beryllium (99%, NGK Insulators). The five kinds of Fe dilute alloys (Fe–Mo, Fe–Cr, Fe–Si, Fe–P and Fe–Be) were fabricated by zone-leveling. All zone-refining and
Results and discussion
Fig. 1 shows the measured isochronal resistivity recovery spectra between 77 and 146 K of Fe and Fe–Mo alloys irradiated simultaneously. The irradiation-induced resistivity in Fe, Δρ0=0.712 nΩ m corresponds to a Frenkel pair concentration of 24 ppm [15]. From this figure, it is obvious that Mo solutes annihilate the stage IE(≈135 K) and the recovery decreases with increasing solute concentration between 120 and 140 K. Similar recovery characteristics have been ordinarily observed in FCC alloys
Conclusions
The conclusions obtained from the recovery spectra of Fe dilute alloys electron irradiated are as follows:
- 1.
The oversized Mo solutes trap migrating self-interstitials and release them above stage IE.
- 2.
The undersized Si solutes trap migrating self-interstitials and the resulting mixed-dumbbells migrate and annihilate above stage IE.
- 3.
The mixed-dumbbell in Fe–Cr (Cr; slightly oversized) migrates below IE.
- 4.
The mixed-dumbbells in Fe–P and Fe–Be are thermally unstable and migrate below IE in spite of the
Acknowledgements
The authors would like to express their cordial thanks to Dr Aono of the Hitachi Research Laboratory and Dr Makii of the Research Laboratory, Kobe Steel, Ltd. for providing of the samples of Fe–Mo and Fe–Cr alloys.
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