Influence of defect dynamics on the nanoindentation hardness in NiCoCrFePd high entropy alloy under high dose Xe+3 irradiation

Abstract

We report the role of irradiation-induced defects and microstructure evolution in the wake of 1.05 MeV Xe3+ ion irradiation on the structural and mechanical properties of single-phase face-centered cubic (FCC) structured NiCoCrFePd high-entropy alloy (HEA). The defect evolution was investigated using positron annihilation Doppler broadening spectroscopy (PADBS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and nanoindentation techniques. Single-phase FCC structured NiCoCrFePd HEA remained structurally stable even upon irradiation to a very high fluence of 9 × 1016 ions/cm2. However, an anomalous reduction in the lattice constant, along with micro-strain relaxation and crystallite fragmentation was observed at the initial ion fluence of 1 × 1016 ions/cm2, which are recovered upon irradiation at the higher ion fluence due to collapse of small-sized point defects into the large-sized dislocations and simultaneously irradiation-induced recrystallization. Ion fluence-dependent PADBS analysis revealed the formation of mono-vacancies at lower fluence which were saturated by recombination and evolution to large-size defects at successive higher ion fluence that is consistent with electron microscopic investigations. Measurement of mechanical properties showed that the hardness was initially increased by 57% upon irradiation at fluence 1 × 1016 ions/cm2 and then it was slightly reduced (∼5%) at a higher fluence of 9 × 1016 ions/cm2 irradiation due to defect recombination and recrystallization which clearly shows the resistance of this HEA towards hardening. Thus, the present study provides a deeper understanding of the defect dynamics and their relation to the mechanical behavior of NiCoCrFePd HEA, which is important for the development of radiation-resistant alloys for nuclear energy systems.