Helium bubble nucleation at grain boundaries and its influence on intergranular fracture

ABSTRACT

An in-depth understanding of the formation of intergranular helium bubbles and its relation to embrittlement is an important issue in the nuclear industry. In this paper, a thermodynamic model is developed to analyze the nucleation of intergranular helium bubbles. Microstructural observation using scanning electron microscopy and electron backscatter diffraction gives a detailed description for the relation between the bubble formation and the grain-boundary (GB) misorientation in helium-implanted nickel and Inconel X750. The theoretical and the experimental results confirm that the nucleation of intergranular helium bubbles is GB structure-dependent, the helium-to-vacancy ratio plays an important role in the bubble precipitation, and the interfacial tension of bubbles cannot be approximated to be the interfacial energy. The bubble-induced intergranular embrittlement in a polycrystal is modelled. The GB misorientation distribution, the intergranular bubble nucleation and growth and the GB connectivity are the key factors affecting the GB fracture toughness. The hoop ductility of the cladding tubes containing helium is analyzed. The hoop stress-induced increase in the GB energy promotes the precipitation of bubbles at the radial GBs and lead to the loss of tube ductility. Based on this work, the complicated correlation among the intergranular helium bubbles, the GB structure, the helium concentration, the applied stress and the helium embrittlement is clarified.

KEYWORDS:

• Nickel alloys
• grain boundaries
• thermodynamics
• mechanical properties

Acknowledgments

We wish to thank Mr. Martin Chicoine and Dr. B. S. Amirkhiz for carrying out the ion implantations at the University of Montreal and analysis. This work is supported by Sylvia Fedoruk Canadian Centre for Nuclear Innovation and CANDU Owners Group.

Disclosure statement

No potential conflict of interest was reported by the authors.