ИСТИНА

The temper embrittlement of steels remains an actual problem, and becomes particularly acute for the safety of nuclear reactors after prolonged operation. AES analysis of the grain boundaries (GB) in the test samples of reactor steels is an efficient method to control the content of impurity and kinetics of segregation. Quantitative description of the segregation kinetics in GB can be obtained if: (1) local AES analysis providing reliable data about the composition of the surfaces cleaved along GB, (2) changes of the chemical composition in GB may be measured during sufficiently long operation period, and (3) there is a physical model of segregation process that takes into account the influence of chemical and structural factors on the transport of impurities to the GB. The use of standard AES techniques for the cleaved surfaces of GB gives results that are generally distorted by the presence in analysis zone Me23C6 type precipitates with the average size ~100 nm and the surface density of particles ~4·1012 m-2. AES results can be corrected, if the average composition of carbide precipitates is known: for WWER-1000 vessel steel we obtained ~(Cr0.9Mo0.1)23C6 by TEM and SEM techniques. The effect of transgranular cleavage, existing on the GB surface, can be also estimated using morphological features of such sites and their proportion in the zone of analysis. Using SEM methods we have obtained that for WWER-1000 vessel steel a cleaved area does not exceed 15%, and the value of AES data correction is comparable with the measurement accuracy. We compared several physical models describing the segregation kinetics, in particular a simple [1-2] and more complex one, which takes into account the grain structure of the steel [3]. Applying the AES corrected data does not reveal the advantage of a particular model for t > 20 years. However, application of the model [3] gives definitely more confidence for t < 10 years results, whereas the kinetic dependencies, suggested in [1] and [2], constrains the dependence of the model [3] from above and below. Accuracy of the impurity segregation kinetics prediction at long times (t > 10 years) essentially depends on the estimation of the impurity equilibrium in the GB. Thermodynamic model was used for evaluation of the GB energy and elements concentrations in GB depending on temperature, pressure and composition in the grain volume. As a result, the value of 4.1 at.% was obtained for phosphorus equilibrium concentration in the GB, what is close to theexperimental value of 3.7 at.% for WWER-1000 vessel steel 195000 hours at 315 °C [3]. [1] D. McLean, Grain boundaries in metals (Clarendon Press, Oxford, 1957). [2] B.S. Bokshtein, A.N. Khodan, O.O. Zabusov, D.A. Maltsev, B.A. Gurovich. Phys. Met. Metallogr. 115 (2014) 146. [3] M.V. Sorokin, Z.V. Lavrukhina, A.N. Khodan, D.A. Malzev, B.S. Bokstein, A.O. Rodin, A.I. Ryazanov, B.A. Gurovich. Materials Letters 158 (2015) 151–154