Hardening effects of He irradiation on Fe-Cu alloy

Abstract

To study the effects of ion irradiation on hardening and microstructural evolution of solute clusters in Fe-based alloys, Fe-Cu alloy was irradiated with 2.0 MeV He ions to a fluence of 1.0 × 10¹⁷ ions cm⁻² at 563 K. Nanoindentation and atom probe tomography (APT) were used to characterize the hardness and solute distributions, respectively. Correlation between the mechanical property and microstructure was also discussed. Nanoindentation results showed that the irradiation-induced hardening quickly increased as irradiation depth/damage increased at first and then slowly reached saturation at shallower depth. APT results also showed that well-defined Cu clusters were precipitated after irradiation and grown to a clear saturation at about 0.05 dpa, but the precipitation was suppressed slightly at higher damage/damage rate due to cascade mixing. The good agreement between calculated strength and measured strength demonstrates that the Cu culsters are the dominant feature for hardening.

Introduction

The reactor pressure vessel (RPV) is exposed to harsh conditions with high temperature, high pressure and neutron irradiation under its service [1], [2]. During neutron irradiation, matrix atoms would displace from original location when knocked on by the high energy neutrons, which will cause the point defects concentration in matrix breaking the thermal equilibrium. These defects may evolve into other defects, such as dislocation loops and voids. The solute clusters will also be induced or promoted due to the high diffusion capacity of supersaturated vacancies. All of those would lead to hardening and embrittlement of RPV steels, which limit the lifetime of nuclear power plants. The early pressurized water reactor has run for more than 50 years beyond its designed lifetime. However, the microstructural evolution of Cu clusters and their correlations to mechanical properties are partly understood.

Extensive researches show that Cu atoms has a lower solubility at the operated temperature of about 563 K, which is responsible for the formation of Cu clusters and the embrittlement of RPV steels [3], [4], [5], [6]. Most studies have focused on the evolution of Cu clusters and matrix defects during thermally-age or neutron irradiation [3]. No obvious difference is found besides the coarsening of Cu clusters in thermal ageing, which does not occur in neutron-irradiated RPV steels [4]. Furthermore, the hardening and the square root of volume fraction of clusters have a good liner relationship during thermally-age and neutron irradiation [3]. Bergner and Gillemot [5] studied the effects of Cu-rich clusters, dislocation loops, and nanovoids on the irradiation-induced hardening of Cu-bearing low-Ni RPV steels. They found that Cu-rich clusters still mainly contributed to irradiation hardening due to the higher number density though Cu-rich clusters possessed the lowest obstacle strengths compared with nanovoids and loops. Meslin [6] found that the atom number of Cu clusters increased with damage increasing in neutron-irradiated Fe-0.08 wt% Cu alloy. The size of clusters can reach up to 7 nm in diameter after neutron-irradiated of Fe-0.8 at.% Cu alloy, while the average size and number density can still keep constant even in high dose Fe irradiation, according to the study of Shipeng Shu [7]. Ion irradiation with accelerator implanting is always used to simulate the environment of neutron irradiation due to the high cost, long time and high radioactivity of actual neutron-irradiated materials [5], [6], [7]. Liu [8] investigated the radiation hardening in ion-irrradiated Fe based alloys by nanoindentation and found that proton irradiation led to more hardening than Fe-ions irradiation. They believed that protons could produce damages in small clusters while neutron, Fe-ions mostly produced damages in large clusters, however limited experiment is carried out to further confirm this opinion. So the microstructural evolution of the atom clusters especially in small size under ion irradiation need to be further studied.

Meanwhile, the limited irradiation range of ions makes it difficult to measure the mechanical properties of irradiated thin layers. Nanoindentation [6], [9] has been widely used to characterize the hardness of a thin damage layer produced by ion irradiation, and three effects including damage grade effects (DGE), indentation size effects (ISE), and implantation and surface effects [10] should be considered. Ions implantation experiments show that damage area deepens with the particles energy increasing, or particles mass decreasing. Therefore, high-energy light particles irradiation, such as proton and He ions, may produce a larger damage area economically. In addition, the (n, α) transmutation reaction produced helium under neutron irradiation would make nuclear materials deteriorate. So in this study, He ions irradiation was used to investigate the effects of ion irradiation on the microstructural evolution of Cu clusters in Fe-Cu alloy. Nanoindentation and APT were used to characterize the hardness and solute clusters in the irradiated alloy. Then correlation between the mechanical property and microstructure was also discussed.