Incorporation of gadolinium and boron into Zirconium alloy: Surface alloying of immiscible materials using an intense pulsed ion beam

We investigate the potential for incorporation by surface alloying of two elements, gadolinium (Gd) and boron (B), into Zr-alloy substrates, by the application of an intense ion beam pulse to a conventionally pre-applied Gd or B thin film coating to the substrate. The beam is produced by the Repetitive High Energy Pulsed Power-1 (RHEPP-1) ion beam facility at Sandia National Laboratories. Surface alloying is desirable in this case for two reasons: (1) conventional alloying is not possible because both Gd and B have negligible solubility in Zr at room temperature, and (2) a conventionally applied coating without surface alloying may be expected to delaminate in the harsh end-environment where the elements are used (e.g. fission reactors). While surface alloying has been a topic of investigation both by the present and prior researchers, the goal of the present work is the detailed study incorporating heat flow simulations and a full complement of materials analysis tools to characterize surface alloying of Zr-alloy substrates with Gd and B. Use of code simulations is essential for predicting appropriate film thickness and ion beam treatment fluences, which are specific to a given film-substrate system. Characterization after ion beam surface treatments confirmed successful alloying of both Gd and B by significant extension of their solid solubility in Zr-alloy substrates. While Gd surface alloying of Zr-alloys resulted in unacceptable oxidation in thermal and environmental conditions mimicking a nuclear reactor environment, B surface alloying resulted in acceptable corrosion resistance comparable to the as-received Zr-alloys.