Nanobubble fragmentation: A novel microdamage mechanism in metals

2016-12-07  []

Recently, the research team led byProf. Weizhong Han (韩卫忠) at the State Key Laboratory forMechanical Behavior of Materials, Xi’an Jiaotong University, uncovered thenovel 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 typicalradiation microstructures in metals and alloys, significantly influencing theirdeformation behavior, usually causing severe high temperature heliumembrittlement in bulk metals. However, here we found that helium radiation canactually enhance the room-temperature deformability of submicron-sized singlecrystal copper. In particular, Cu single crystals with diameter of 100 nm to300 nm and containing numerous pressurized sub-10 nm helium bubbles, becomestronger, more stable in plastic flow and ductile in tension, compared to fullydense samples of the same dimensions that tend to display plastic instability(strain bursts). The sub-10 nm helium bubbles are seen to be dislocationsources as well as shearable obstacles, which promote dislocation storage andreduce dislocation mean free path, thus contributing to more homogeneous andstable plasticity. Failure happens abruptly only after significant bubblecoalescence. This findings was published in Nano Letters(2016,16:4117-4124).



Furthermore, the dynamic evolutionof helium bubbles under straining is less explored due to the challengingexperimental 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 thehelium bubble not only can coalesce with adjacent bubbles, but also can splitinto several nanoscale bubbles under tension. Alignment of the splittings alonga slip line can create a bubble-free-channel, which appears softer, promotesshear localization, and accelerates the failure in shearing-off mode. Detailedanalyses unveil that the unexpected bubble fragmentation is mediated by thecombination of dislocation cutting and internal surface diffusion, which is analternative micro-damage mechanism of helium irradiated copper besides thebubble coalescence, see details in Physical Review Letters(2016,117:215501). These findings shed new light on the plastic deformation mechanismof helium irradiated metals.