He bubble growth in a martensitic steel for fusion reactors

Abstract

Small‐angle neutron scattering (SANS) has been used to study thegrowth of He bubbles in a martensitic steel for fusion reactors. The samples had been homogeneously implanted at 523 K with 400 appm He, then subjected to thermal annealing at temperatures up to 1248 K. Suitable reference samples, submitted solely to identical thermal treatments, were also measured. The He bubble volume distribution functions obtained show that a dense population of bubbles 10 Å or less in size is accompanied by a secondary distribution of bubbles one order of magnitude larger, as expected after post‐implantation annealing. These results are discussed in terms of background subtraction and available Q‐range.

Helium bubble evolution in F82H-mod – correlation between SANS and TEM

“…The corresponding He bubbles average radii and volume fractions are reported in Table 1. At 525°C the difference in SANS cross-section between implanted and reference sample is smaller than the experimental errors, as already found in the previous experiment [5,6]: that has been tentatively attributed to the slight differences in the thermal history of the two samples and to the occurrence of a dense, fine precipitation of the M 2 C phase around this temperature [15]. The fact that no significant difference is observed by TEM between the as-implanted sample and the one tempered at 525°C after implantation further supports this interpretation.…”

“…It is however noted that the SANS resolution too strongly decreases below 10 A, as is also shown by the correspondingly large error bands, mostly because of the background arising from the matrix; consequently, quantitative information on the true shape of the distributions can hardly be obtained for the smaller radii region. For this same reason, for R < 10 A errors as high as 40-50% are estimated [5,6] on the He-bubble volume fraction and average radii, which are summarized in Table 1. More specifically, no quantitative conclusion can be based on the very small, mathematically averaged radii determined at 825 and 975°C.…”

“…A sample-to-detector distance of 8.00 m with a wavelength k of 6 A was used. Combined with the previous SANS measurements carried out on these same samples [5,6] this gave a Q interval (the modulus of the scattering vector being Q ¼ 4p sin h=k, where 2h is the full scattering angle) ranging from 0.007 to 0.25 A À1 , which corresponds to particle sizes ranging from 10 to 150 A approximately. The size distributions were determined by indirect transformation of the SANS cross-section:…”

“…This procedure minimises the magnetic volume and facilitates the TEM adjustment, enabling the observation of He bubbles with a minimum size (apparent diameter) of approximately 10 A. Reference is made to [8] and to previous work [5,6] for a general presentation of SANS and of its application to the study of martensitic steels. The SANS measurements were carried out by means of the D22 instrument at the High Flux Reactor of the Institut Laue Langevin, Grenoble.…”

“…Transmission electron microscopy (TEM) investigations enable direct observation of Hebubbles and deliver reliable data concerning local bubbles morphology and spatial distribution, but their resolution limit is approximately 10 A for He-bubbles in magnetic Fe-alloys. Small-angle neutron scattering (SANS) is therefore an appropriate technique to investigate this phenomenon, as also shown by previous studies [5,6], and to complement the microstructural information obtained by TEM, which is the subject of this paper.…”