Nanobubble fragmentation: A novel microdamage mechanism in metals
Recently, the research team led by Prof. Weizhong Han (韩卫忠) at the State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, uncovered the novel deformation mechanism of helium bubbled copper, which was published in Nano Letters (2016, 16:4118-4124) and Physical Review Letters (2016, 117:215501).
Helium bubbles are one of the typical radiation microstructures in metals and alloys, significantly influencing their deformation behavior, usually causing severe high temperature helium embrittlement in bulk metals. However, here we found that helium radiation can actually enhance the room-temperature deformability of submicron-sized single crystal copper. In particular, Cu single crystals with diameter of 100 nm to 300 nm and containing numerous pressurized sub-10 nm helium bubbles, become stronger, more stable in plastic flow and ductile in tension, compared to fully dense samples of the same dimensions that tend to display plastic instability (strain bursts). The sub-10 nm helium bubbles are seen to be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens abruptly only after significant bubble coalescence. This findings was published in Nano Letters (2016, 16:4117-4124).
Furthermore, the dynamic evolution of helium bubbles under straining is less explored due to the challenging experimental setup. Here, by using a new design of the testing sample geometry, we are able to probe the evolution of singular bubble, and discover that the helium bubble not only can coalesce with adjacent bubbles, but also can split into several nanoscale bubbles under tension. Alignment of the splittings along a slip line can create a bubble-free-channel, which appears softer, promotes shear localization, and accelerates the failure in shearing-off mode. Detailed analyses unveil that the unexpected bubble fragmentation is mediated by the combination of dislocation cutting and internal surface diffusion, which is an alternative micro-damage mechanism of helium irradiated copper besides the bubble coalescence, see details in Physical Review Letters (2016, 117:215501). These findings shed new light on the plastic deformation mechanism of helium irradiated metals.