Microstructure and nanohardness of hafnium diboride after ion irradiations

Abstract

In order to simulate helium and lithium appearance originating from the ¹⁰B (n,α)⁷Li reaction, that occurs under neutron irradiation in a PWR, hafnium diboride samples have been irradiated with He⁺ and Li⁺ ions at 350°C. As implanted and annealed materials have been characterised by TEM observations and nanohardness measurements. In spite of high concentration of implanted helium, no helium bubbles were found whereas the TEM images showed a contrast associated with tiny dislocation loops. Observation on samples annealed at 1600°C allowed a more detailed analysis of loops which exhibit an interstitial nature (resp. vacancy) for a {1 1 $\text{2}\text{̄}$ 0} or {1 0 $\text{1}\text{̄}$ 0} habit plane (resp. (0 0 0 1)). The organisation of these secondary defects leads to a strongly anisotropic deformation of grains which is thought to cause intergranular microcracking when conflictual stresses are met at grain boundaries.

Introduction

This study concerns materials for control-rod applications in 100% MOX pressurised water reactors (PWR). Hafnium diboride appears to be a promising candidate since it exhibits a high melting point (T_m=3380°C) [1]and a high neutron absorber efficiency, equivalent to that of boron carbide commonly used in the present PWR [2]. Furthermore the new absorber must be able to withhold the corrosion mechanisms of the PWR core environment, i.e. water, with LiOH and H₃BO₃ in solution, at 350°C and under 155 bars. It has been observed that metal-rich pellets of HfB₂ + 10 vol% Hf fulfilled these requirements [3]. All these observations let us think that HfB₂ based materials could be appropriate neutron absorbers.

Investigations on neutron irradiated boron carbide and borides of transition metals have been carried out at the Vallecitos Atomic Laboratory of the General Electric Company in the 1960s [4]. The first post-irradiation analysis showed that diborides had swelled less and released few helium compared to B₄C. By X-ray diffraction, the authors also demonstrated that the crystalline structure in diborides had collapsed less than that of hexa- and dodecaborides. The better behaviour (helium release, lattice resistance,...) of diborides compared to the others under irradiation is explained by the fact that diborides crystallise in a compact structure with a high bonding strength.

Following these former investigations it seems reasonable to assume that, as it is observed in B₄C, HfB₂ swelling is the direct consequence of the He and Li appearance and the subsequent bubble formation, resulting from ¹⁰B(n,α)⁷Li reaction. To ascertain this assumption and to determine the specific role of each (n,α) reaction product in the HfB₂ structural damaging, He⁺ and Li⁺ have been implanted separately on thin TEM samples for further microstructural observations. In addition, nanohardness tests have been carried out on the thin He implanted layer of the HfB₂ samples.