EXPERIMENTAL-METHOD TO DETERMINE THE ROLE OF HELIUM IN NEUTRON-INDUCED MICROSTRUCTURAL EVOLUTION

A method is presented which allows the determination of the role of helium on microstructural evolution in complex alloys and which avoids many of the problems associated with other simulation experiments. It involves a direct comparison of the materials' response to a primary difference in fission and fusion environments, namely the rate of helium generation. This is accomplished by irradiating specimens in a fission reactor and conducting microstructural analyses which concentrate on alloy matrix regions adjacent to precipitates rich in boron or nitrogen. Such precipitates generate well-defined atmospheres of helium (and other transmuted, atoms) due to (n,α) reactions during irradiation. Microstructural comparisons are made between the regions bearing implanted helium and those just beyond the helium range. The method takes advantage of virtually no variation in alloy composition and pre-irradiation microstructure over short distances, and can be used to study any material of interest if the appropriate precipitates can be developed. A variety of helium/dpa ratios can be obtained in one specimen by using precipitates of various sizes. Procedures are outlined for calculation of background and injected helium levels as well as displacement doses generated by neutrons and alpha particles. An example of the analysis method is shown for an experimental austenitic stainless steel containing boride particles and irradiated to 3 and 7 × 1022 n/cm2(E >0.1 MeV). © 1979.