DEUTERIUM AND TRITIUM APPLICATIONS TO THE HYDROGEN LOCAL CONCENTRATION MEASUREMENT IN METALS RELATED TO EMBRITTLEMENT PHENOMENA

The knowledge of the critical hydrogen concentration (1) necessary for a crack to be initiated and to propagate remains of major concern for the modelling of hydrogen embrittlement and stress corrosion cracking processes. Secondary ion Mass Spectroscopy (SIMS) and tritium autorad iography on samples charged with deuterium or tritium can be interesti ng tools for the study of the trapping of hydrogen on microstructural defects (2-4), the measurement of concentration profiles to determine local concentration and diffusion kinetics (5-8) or the observation of preferential paths for diffusion (9). The aim of this paper is to sho w some applications of the SIMS and tritium autoradiography techniques for quantitative measurements of the hydrogen concentration in metals and alloys. A short overview is presented on recent work devoted to: -hydrogen diffusion and trapping in nickel base superalloys; -hydrogen concentration measurements at the crack tip of DCB 4120 low alloy ste el samples; -tritium distribution in welded 316L stainless steel plate s. The experimental part is focused on the procedure of the tritium au toradiography technique and on the quantification of autoradiographs ( film procedure) by optical microdensitometry. The experimental conditi ons of deuterium profiling for the measurement of local deuterium conc entrations by SIMS is also briefly described. The microstructure of ni ckel base superalloys (tables I and II) is a very important parameter for the optimization of their resistance to hydrogen embrittlement. Th e changes in the deuterium concentration profiles as a function of mic rostructure and the autoradiographs of the tritium distribution show a very good concordance and lead to the following conclusions: -a decre ase in the penetration distance of deuterium when the size of the gamm a' phase decreases (fig. 3); -a larger solubility of hydrogen in the g amma' (gamma/gamma' eutectic and gamma' precipitates) phase when compa red to the gamma phase (fig. 4, 5); -hydrogen frapping at the gamma' p recipitates limits the hydrogen penetration.The preferential embrittle ment of the gamma' phase leading to a decrease of the mechanical prope rties of the nickel base superalloy in the presence of hydrogen (fig. 2, 6) may be attributed to the large hydrogen amount absorbed by the g amma' phase and hydrogen trapping at the gamma/gamma' interfaces. Trit ium autoradiography and deuterium profiling were performed on bolt-loa ded DCB 4120 specimens in order to measure the hydrogen concentration in the plastic zone at the crack tip under stress corrosion conditions . The results show: - a significant hydrogen penetration in the sample s corroding freely in 3.5% NaCl (pH 6) (fig. 7). This illustrates the high sensitivity of the tritium autoradiography technique for the meas urement of very small residual tritium concentrations; - a larger trit ium concentration in deformed samples (fig. 7); - a small hydrogen enr ichment at the crack tip with an apparent segregation ratio ranging be tween 1.5 and 2 (fig. 8, 9). The increased tritium concentration can b e attributed to trapping on microstructural defects in the plastic zon e.Although the detection sensitivity of the tritium autoradiography te chnique is very high, the effective hydrogen concentration at the crac k tip is certainly undervaluated in these bce materials where hydrogen desorption at room temperature is large. Tritiated 316L stainless ste el pieces were used to analyse the tritium distribution before and aft er welding (1.6 to 2.2 kW power, 2.5 m/min) with a 3 kW CO2 laser devi ce. When compared to the unwelded samples (fig. 10), a strong tritium enrichment is observed in the fusion lines and over several hundreds o f micrometers around the melted zone (fig. 12, 13) whereas the maximum tritium concentration (6 at. ppm) is found in the heat affected zone. In the case of materials where the hydrogen motion is low, the microd ensitometric analysis (fig. 11) of autoradiographs representative of t he tritium distribution leads to reliable quantitative data.