4.2 Materials Modelling

4.2.1.Determination of (proton) radiation damage on Fe and Fe-Cr alloys by electrical resistivity measurements

Background and Objectives: Within the Fusion Materials Topical Group there is considerable effort in understanding and modelling radiation effects in Fe and Fe-Cr alloys, the main ingredients of EUROFER low-activation structural steels. Experimental validation of the developed modelling tools is of primary importance for the reliable prediction of radiation damage at fusion relevant dose levels. The general objective of this activity is to provide experimental data on the radiation response of Fe-Cr alloys, to assist the modelling activities and test theoretical predictions. In NCSR “D” a low temperature ion irradiation facility has been developed during the last years, offering in-situ electrical resistivity measurements and fast annealing capabilities. In 2011 a series of Fe-Cr alloys have been irradiated with high energy protons and their resistivity recovery has been studied.
Work performed in 2011 (Co-operation: within the Fusion Materials Topical Group):
(i) A series pure Fe-Cr model alloys with chromium concentrations of 5.8, 10.8 and 15.0 at. % were irradiated with 5 MeV protons from the TANDEM accelerator. The samples were prepared in the form of thin foils (thickness ~ 50 µm) so that the high energy ions penetrate fully through the thickness of the specimens and a homogeneous production of radiation damage is achieved. The irradiation was performed at cryogenic temperature (50K) and the induced radiation damage was recorded in-situ by means of electrical resistivity measurements. During post-irradiation annealing the electrical resistivity recovers gradually towards its pre-irradiation value. The recovery proceeds in so-called “stages”, i.e., temperature intervals with particularly high recovery rate. It has been found that the position and amplitude of the stages depends on the chromium concentration. A striking feature in the recovery curves of the 10.8 and 15 at. % alloys is that the resistivity after irradiation and annealing becomes lower than the initial pre-irradiation value. This is attributed to Cr ordering effects. The experimental results are detailed in Annex 37. Furthermore, a full analysis of the experimental dataset in the light of recent theoretical results is underway. The analysis could not be completed to date due to the lack of irradiation and recovery data on pure Fe, under the same conditions, which will serve as a reference. Such experiments have been recently performed, thus the analysis of the whole dataset will be completed shortly.
The above results were presented in the meetings of the Fusion Materials Topical Group organised by EFDA.

4.2.2.Determination of phase stability and agglomeration phenomena in Fe and Fe-Cr model alloys

Background and objectives: The general objective of this activity is to provide experimental data for the testing of the different theoretical predictions and to further assist in the development of the modelling activities. Within 2011 the aim was to a) to complete the determination of the magnetic properties of EFDA FeCr model alloys continuing the work that started in earlier years and compare the results with the theoretical ones as predicted by the CCFE group (UKAEA) and b) to measure the thermal properties of FeCr alloys as a function of temperature in the temperature range 2 to 300 K.

Work performed in 2011 Co-operation: Within Materials and Modelling group (TG-M), TEKES (University of Helsinki)
(i) Small-angle neutron scattering (SANS) measurements were performed at JCNS at FRM-II reactor in Garching on thermally aged FeCr alloys in order to investigate agglomeration phenomena and compare with existing data from modelling theories. Problems in the measurement of transmission of the samples in order to deduce absolute cross sections made further analysis of the data not possible. Also the shut-down of the reactor during most of 2011 did not allow revision of the transmission measurements. A proposal for beam time application was applied and new measurements are expected to be performed in 2012. To support the activities referring to thermal ageing effects in FeCr alloys the dissolution of already existing Cr precipitates in FeCr alloys has been investigated. The means of investigation was molecular dynamics computer simulation, using the parallel code PARCAS and a two-band embedded-atom method potential. A number of configurations of FeCr alloys containing Cr precipitates of various sizes embedded in matrices of either pure Fe or with a 15% random Cr distribution was set up and their behaviour after thermal aging at temperatures ranging between 600-2000 K was examined. The temperature range was selected so that it would include the (a+a׳)-a transition in the standard FeCr phase diagram. The investigation is still underway to include longer MD timescale and lower temperatures. The initial results are presented in Annex 38.

(ii) The set-up of the electrical resistivity measurements was finalised with respect to the electrical contacts which had to be of pressure type in order to avoid any weld of the leads on the sample. In addition, the specific heat of Fe and FeCr alloys was measured as a function of temperature in the temperature range 2 to 300 K and the results are compared with different theoretical models. The data were analysed by using a linear and cubic term, arising from conduction electrons and phonon excitations, respectively. From these terms, the density of states and the Debye temperature are determined. The detailed results are presented in Annex 39. The magnetic properties of Fe and FeCr alloys, supplied by EFDA, were compared to the modelling predictions of the CCFE group using the magnetic cluster expansion (MCE) model. In Annex 40 the results are presented.
The above results were presented in the meetings of the Materials Topical Group (MATREMEV) organised by EFDA.

4.2.4.Effect of iron ion irradiation on the structural and magnetic properties of thin Fe layers

Background and objectives: For Fusion energy applications FeCr steels are the candidate structural materials. Understanding the neutron irradiation effects on Fe and FeCr steels is of paramount importance for the development of radiation resistance materials. The energy of the incident fusion neutrons is 14 MeV. However, as the neutrons penetrate within the material they lose energy and thus the Primary Knock-on Atoms (PKA) of Fe have a range of energies with a maximum energy of around 1 MeV. Thus, the main part of the damage arising from neutron or other ions irradiation is due to energetic Fe atoms which create the radiation damage cascades. This phenomenon can be studied by employing Fe ion irradiation of Fe or FeCr alloys. Iron irradiation also has the advantage that this phenomenon can be studied by selecting the specific incident energy in contrast to neutron irradiation in which the primary knock-ons have a range of energies. Since the penetration of energetic Fe ions is of the order of few nm to µm depending on the incident Fe energy, thin films need to be investigated.

Work performed in 2011 (Co-operation: Within Materials and Modelling group (TG-M), CEA- JANNUS):
(i) Evaluation of the PKA energy spectrum in iron irradiated by fast neutrons simulating PKAs distribution was calculated using codes MCNP5 and SPECTER. MCNP was used to calculate neutron fluence and energy spectra for beams of fast neutrons incident on an iron specimen. SPECTER was used to evaluate the PKA spectra. The energy spectrum of the Fe primary knock-ons is used as input for selecting the proper Fe ion energy in order to simulate neutron damage effects in iron using ion beams. The results are presented in Annex 41.

(ii) Simulations were performed concerning the irradiation of iron ions on iron thin films using the SRIM code. These results were used in order to select appropriate thickness of the iron films to be fabricated for the investigation of the irradiation/implantation effects of iron ions which simulate the neutron damage in FeCr alloys. Fe thin films of various thicknesses were fabricated and were structurally and magnetically characterised. The results are presented in Annex 42. Due to technical problems of the JANNUS accelerator the ion irradiations scheduled in November 2011 were rescheduled for March 2012.
The above results were presented in the meetings of the Fusion Materials Topical Group organised by EFDA.