Radiation-induced grain boundary segregation in dilute alloys
thesisposted on 27.09.2012 by Shenhua Song
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Modelling of irradiation-induced segregation or thermal non-equilibrium segregation needs data on the impurity-point defect binding energy. These values are generally unavailable. In this work, an initial approach to determining impurityinterstitial binding energies in metals is established with some success on the basis of strain field arguments and the earlier work is slightly modified for more accurate calculations of oversized impurity-vacancy binding energies. The method is applied to predictions of various impurity-point defect binding energies in several transition metal matrices. With the aid of the predictions, some experimental results on radiation-induced segregation are reasonably satisfactorily interpreted. A radiation-induced grain boundary segregation (RIS) model is established for dilute alloys based on the complex mechanism and combined with McLean's equilibrium segregation model. In the model, radiation-enhanced solute diffusion is taken into consideration. Theoretical predictions are made for segregation of phosphorus in the neutron-irradiated a-Fe matrix. There exists a segregation transition temperature below which combined radiation-induced non-equilibrium and radiation-enhanced equilibrium segregation is dominant, and above which thermal equilibrium segregation is dominant; peaks in the temperature dependence of segregation shift to lower temperatures with decreasing neutron dose rate and/or increasing neutron dose; the combined radiation-induced non-equilibrium and radiation-enhanced equilibrium peak segregation temperature and the thermal equilibrium peak segregation temperature are about 150 and 550°C, respectively, for phosphorus grain boundary segregation in the a-Fe matrix at neutron dose rate = 10-6 dpa/s and neutron dose = I dpa . Grain boundary segregation of solutes in the neutron-irradiated and unirradiated (thermally aged) 2.25Cr1Mo steels doped with P and Sn is examined by means of field emission gun scanning transmission electron microscopy (FEGSTEM) which has very high spatial resolution (- 1 nm). The material is irradiated to a dose of 0.042 dpa at a dose rate of 1.05 x 10-8 dpa/s in a swimming pool-type light-water research reactor in the Paul Scherrer Institute (PSI) of Switzerland. Grain boundary microanalysis is performed in the Nuclear Electric Berkeley Technology Centre of the UK. Comparison of the experimental and predicted results shows that the predictions are generally consistent with the observations.
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