In situ resistivity measurements of RAFM base alloys at cryogenic temperatures: The effect of proton irradiation

Abstract

A four-probe technique for measurement of electrical resistance on low-temperature ion-irradiated metallic sheets is described. The design, temperature control system, preparation method of samples and the resistivity measurements are described in detail. The resistivity recovery (RR) curve has been measured on a Fe–5%Cr model alloy irradiated with 5 MeV protons. The procedure to obtain the RR derivative curve is outlined and experimental errors are identified and quantified. Special care has been taken to use a sample with very low impurity content and low dislocation density (1.2 × 10⁸ cm⁻²). Thus, effects in recovery spectrum of the Fe–5%Cr alloy are only due to the presence of Cr and irradiation defects, which will be mainly Frenkel Pairs (FPs) given that the mean energy of the Primary Knock-on Atoms (PKA) is close to 0.35 keV. The results obtained for the Fe–5%Cr under 5 MeV proton irradiation are found to be in overall agreement with previous experimental measurements performed under electron irradiation although some differences appear probably due to the different spatial distribution of the created defects and the higher temperature resolution of annealing steps. The RR spectrum obtained reveals the appearance of the structure of stages I and II and also a partial suppression of the stage III peak with respect to previous results obtained after electron irradiation. The stage III suppression is explained as a superposition of vacancy recombination effects and short-range ordering (SRO) effects which are apparently dependent on the spatial distribution of defects created during irradiation. Moreover, recombination phenomena are observed beyond stage III up to 500 K.

Introduction

Development of nuclear materials capable to withstand hostile environments is one of the principal challenges facing the development of future nuclear reactors. In particular, structural materials of future fusion reactors will be exposed to intense high-energy neutrons, electrons, ions and electromagnetic irradiation as a consequence of fusion reactions in the hot plasma. This radiation will produce a significant amount of defects in the structure of the materials, affecting their physical properties at different scales, and hence, their reliability as structural materials.

In order to identify suitable structural materials for fusion reactors and establish boundary conditions for such materials in operation, it is crucial to fully understand the radiation effects, which can be done using a physically-based modelling approach. This can be achieved undertaking computational simulations to reproduce well-designed experiments carried out under well controlled irradiation conditions and on model alloys. This method allows validating parts of our models and can provide additional information on the atomistic mechanisms responsible for defect evolution. This approach is non-straightforward and strong efforts are still needed to develop the models and to obtain new experimental data.

When it comes to structural materials, it is well known that reduced activation ferritic/martensitic steels (RAFM) are the most promising candidates for fusion and fission applications. Its optimized Cr concentration (typically ∼9%) reduces swelling, radiation-induced ductile–brittle transition temperature (DBTT) [1], [2] and increases protection against corrosion [3]. It is postulated that trapping of interstitials and interstitial clusters due to Cr atoms in Fe alloys might explain the reduced swelling and enhanced nucleation of interstitial loops compared to pure Fe [4]. Nevertheless micro-structural mechanisms responsible for such behaviour and its evolution under irradiation are still not well understood and detailed study of FeCr_x based alloys is still required. The resistivity recovery (RR) measurements can be used as a validation tool to study basic physics of point defects in FeCr_x systems and this has been the case, as this method has been widely used [5], [6], [7], [8] because of its high sensitivity to the presence of defects in metals. RR measurements can isolate the effects of irradiation defects as they are usually performed at cryogenic temperatures. Thus the electron–phonon interaction is suppressed and the measured resistivity values only depend on the ordering of the material microstructure, i.e., the residual resistivity. The cryogenics condition requires performing in situ measurements where residual resistivity values can be obtained before, during and after irradiation and also after temperature cycles. Measuring the radiation-induced resistivity of the microstructure of irradiated Fe and FeCr_x alloys allows correlating defect production with irradiation parameters, in particular dose and energy. In addition, the follow-up of damage during post-irradiation annealing provides valuable information on defect kinetics, their diffusion, recombination and clustering.

As suitable 14 MeV neutron sources are not yet available for material studies, research has been mainly centred on the use of electron irradiations which produce mainly spatially separated FPs. One of the advantages of this type of irradiation is that initial conditions are relatively easy to define in simulation set-up. Thus, using electron irradiation and combining RR experiments [5], positron annihilation (PA) experiments [9] and simulations, which combine Molecular Dynamics (MD), kinetic Monte-Carlo (kMC) and Rate Theory [10], it was possible to shed light on fundamental aspects of defect kinetics in pure Fe. Concerning Fe alloys, it has been shown that the presence of solutes exhibits strong interaction with point defects, which complicates the interpretation of RR stages in terms of defect kinetics. Particularly in bcc Fe–Cr alloys it has been determined that Cr solutes tend to trap self-interstitial atoms (SIA), but, do not interact with vacancies (V) [11]. Thus, the migration mechanisms of very simple defects, such as interstitials, vacancies and their small clusters change with respect to pure Fe. Also, in binary alloys the long range migration of simple defects enhances the rearrangement of solutes, leading to changes in the residual resistivity. Such re-ordering is the so-called short-range ordering (SRO) and can lead to increase or decrease of residual resistivity values as a function of the sign of the Cowley–Warren SRO parameters [12], [13]. All these questions, the limited experimental data available [11], [14], [15], [16], [17], [18] and the current limitations of computing models and codes stresses the need to continue to perform new well controlled experiments to understand the physics of FeCr_x RR spectra.

Up to date most resistivity and positron annihilation experiments for understanding the behaviour of RAFM based alloys have been carried out under electron irradiation [5], [8], [11], [14], [15], [16], [17], [19], [20], [21]. It is worth noting that in this work the irradiation of samples was carried out with 5 MeV protons, which allows to study the behaviour of steel microstructure under a type of irradiation damage different from the one generated by electrons but closer to what neutrons would create. As it is shown in the discussion, this is going to reveal new processes on the RR results.In this work, a pure Fe–5%Cr specimen was irradiated by 5 MeV protons at 50 K. The defects created during irradiation contribute to the so-called radiation induced resistivity (RIR) which is measured at a temperature of 20 K. After irradiation, the sample temperature was raised up by successive temperature steps and then cooled down again to base temperature (T_base = 30 K) in order to perform all the residual resistivity measurements after annealing up to 500 K. The RR curve was obtained with this procedure and its derivative provides the RR spectrum where the stages I, II and III can be identified.

In Section 2 we first describe the studied material and its preparation, the experimental set-up, the irradiation and finally the measurement proceeding. Section 3 is dedicated to the theoretical calculation of the damage produced by proton irradiation, which will set the basis for the discussion of the experimental results. We then present the results obtained and the analysis method in Section 4. Finally, a comparison with previous measurements performed with electron irradiation is presented and discussed.