Helium 3 precipitation in AISI 316L stainless steel induced by radioactive decay of tritium: Microstructural study of he
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INTRODUCTION
THE development of the thermonuclear technology has given rise to a renewed interest in the study of the behavior of helium in metals. A great amount of work is still required for the understanding of the role of helium on the mechanical properties of structural materials for fusion technology, especially austenitic stainless steels. [1"21Indeed, tritium present as a combustible will be absorbed by numerous components during the life of fusion reactors and can generate helium 3 by radioactive decay according to the reaction following: ~H ~ 23He + _off + 5,7 keV where 5,7 keV stands for the mean energy value of the /3- radiation. Helium solubility in metals is known to be very small (less than 10 -1~ appm in nickel at 1700 K (10 MPa) [3~) and it precipitates as bubbles very easily. The introduction of helium in metals in significant amounts requires specific techniques such as helium generation by nuclear reactions, ionic implantation, or radioactive decay of tritium. Depending on the method, the generated helium will be either 3He or 4He; no information concerning any isotopic effect between 3He and 4He is available. The helium bubbles are preferentially found in grain boundaries and at dislocations, t4,5'rj However, contradictory observations have been reported concerning the precipitation of helium bubbles on inclusions or precipitates. [7,81 The gas bubble's distribution in a metal, as well as A.M. BRASS and J. CHENE, Senior Researchers, are with the Laboratory of Structural Metallurgy, CNRS, University of Pads-South, Orsay, France. A. CHANFREAU, formerly with CNRS, is Postdoctor, Department PuA, CEA-DAM, Bmyrres Le Ch~tel, France. Manuscript submitted July 8, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
the growth mechanism of the bubbles, is strikingly dependent on several experimental factors: The mode of helium introduction:tl,gj Vacancies associated with lattice defects or created by irradiation or implantation reactions, together with hightemperature working conditions, facilitate bubble nucleation.[10-13] Helium concentration: The dependence of gas bubble distribution and growth with the helium concentration in the metal remains controversial. An increase in the bubble density and/or the mean bubble radius with helium concentration is frequently observed, t6'I4ASI The precipitation and growth of helium bubbles in grain boundaries was also shown to be dependent on the helium concentration, t14~As a consequence, increased helium concentration in the metal leads to a decrease in the material's ductility[16"171 at high temperature 'and often to intergranular cracking, tr] The influence of helium concentration on the yield strength and ultimate tensile strength is more controversial. ]7,17-19] The influence of the testing conditions on the mechanical properties of helium-containing materials is tightly correlated with the possibility of helium bubble precipitation and growth during testing. Furthermore, helium 3 precipitation as bubbles may be responsible for ductility losses of material
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