Self-organization of helium precipitates into elongated channels under confinement within a metal nano-layer
Michael J. Demkowicz, Department of Materials Science and Engineering, Texas A&M University
Friday, October 6, 2017
Damage induced by implanted helium (He) is a major concern for plasma-facing materials (PFMs) in future fusion reactors. Because it is insoluble in metals, any He forced into solution by implantation rapidly precipitates out into nanometer-scale “bubbles”. Decades of research have mapped out the fate of these precipitates, from nucleation and growth to bursting out through free surfaces. We show that confinement within nano-scale layers causes He precipitates to depart from their classical growth trajectories: rather than expanding continuously while remaining equiaxed, He precipitates confined within nano-layers spontaneously coalescence into elongated, He-filled channels. These channels form via templated nucleation of He precipitates along layer interfaces followed by their growth and coalescence into stable precipitate lines. The total channel line length and connectivity increases with the amount of implanted He, suggesting that these channels ultimately interconnect into percolating “vascular” networks. Vascularized metal composites suggest a potentially transformative solution to challenges facing PFMs by enabling in operando outgassing of He and other impurities while maintaining material integrity.