Matrix Damage in Iron
- PDF / 219,275 Bytes
- 6 Pages / 595 x 842 pts (A4) Page_size
- 40 Downloads / 207 Views
MATRIX DAMAGE IN IRON A C NICOL, M L JENKINS AND M A KIRK1 Department of Materials, University of Oxford, Parks Rd., Oxford OX1 3PH, UK 1 Materials Science Division, Argonne National Laboratory, Argonne, 60439 IL, US ABSTRACT We present results of a weak-beam transmission electron microscopy study of “matrix damage” in two nearly-pure irons (designated alloys 1A and 2A) produced by neutron irradiation to a fluence of 0.06 dpa at 280oC. The matrix damage in both materials was found to consist of small (2-6 nm) dislocation loops. About 80 % have Burgers vectors b = a, and the remainder have b = a/2. The loops in alloy 1A have a mean image size dmean = 2.8 ± 0.1 nm and a mean maximum image size dmax = 4.2 ± 0.3 nm, while those in 2A have dmean = 3.4 ± 0.1 nm and dmax = 4.5 ± 0.3 nm. The number densities are about 8.5 x 1021 m-3 in alloy 1A, and 6.6 x 1021 m-3 in 2A. It can be shown that the loops can account for the observed irradiation hardening. At least some loops are stable under thermal annealing to temperatures of at least 430oC. This and other indirect evidence suggests that their nature is interstitial. INTRODUCTION The formation of “matrix damage” has been identified as a primary mechanism of the hardening and embrittlement of nuclear reactor pressure vessels and other near-core ferritic components, and is becoming an increasingly important factor in plant-life extension analysis. Matrix damage arises from the aggregation of irradiation-induced defects to form clusters. At present, evidence for matrix hardening is mostly indirect, coming from its effects on mechanical properties. It is now fairly well established that there is a hardening and accompanying change in uniaxial yield stress )Fy which is proportional to the square root of the irradiation dose. This observation is consistent with a hardening mechanism involving the cutting by glide dislocations of irradiation-produced point-defect clusters. Several different cluster types may be involved, depending on the steel and the irradiation conditions. Suggestions for possible cluster types have included interstitial and vacancy loops; loose aggregates such as vacancy sponges; decorated microvoids; and solute-point defect clusters. There is evidence in Magnox C-Mn steels that matrix hardening is influenced by the levels of free nitrogen, suggesting that interstitial solutes play an important role. However, the exact nature of such clusters has so far eluded direct characterisation. In this paper we employ weak-beam microscopy to study matrix damage in pure iron. Weak-beam microscopy, with suitable modifications, has been shown capable of reliably identifying and sizing dislocation loops as small as 1-2 nm [1,2]. EXPERIMENTAL The materials, designated “Hawthorne” alloys 1A and 2A, were supplied by BNFL Magnox Generation. These alloys are essentially pure iron. The minor element compositions of these alloys are given in table1. In both alloys the concentrations of
R1.3.1
composition (wt %) alloy C Mn P N Cu S Al W 1A 0.013 0.017 0.005 0.029
Data Loading...
