Direct transformation of equilateral hexagonal Frank vacancy loops to stacking fault tetrahedra under thermal fluctuation.

Stacking fault tetrahedra (SFTs) are highly interesting three-dimensional vacancy defects in quenched, plastically deformed or irradiated face-centered-cubic metals and have a significant impact on the properties and subsequent microstructural evolution of the materials. Their formation mechanism and stability relative to two-dimensional vacancy loops are still debated. Equilateral hexagonal Frank vacancy loops (faulted, sessile) observed in microscopy have been considered unable to directly transform to SFTs due to separation of Shockley partial dislocations as well as embryonic stacking faults. Here using sufficiently long (up to tens of nanoseconds) molecular dynamic simulations, we demonstrate that such a transformation can in fact take place spontaneously at elevated temperatures under thermal fluctuation, reducing potential energy of defected atoms by <0.05 eV/atom. The transformation becomes easier with increasing temperature or decreasing loop size.

Inverted core-shell potential energy landscape of icosahedral clusters in deeply undercooled metallic liquids and glasses and its effect on the glass forming ability of bcc and fcc metals.

Current understanding of the origin of icosahedral clusters or icosahedral short-range ordering in undercooled metallic liquids or glasses is based on Frank's consideration of an isolated icosahedron whose core has lower potential energy than the shell. Using large scale atomistic simulations and statistical analysis of several bcc (body-centered-cubic) and fcc (face-centered-cubic) metals, here we show that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with fcc metals, bcc metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of bcc metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation provides a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability.

Impact of family history of high myopia on level and onset of myopia.

PURPOSE: To investigate the impact of a positive family history of high myopia on the level and onset of myopia and its ocular components. METHODS: A cross-sectional study was conducted. The participants (aged 17 to 45 years) were categorized into four groups: normal, mild, moderate, and high myopia. The age of first glasses for myopia was used as the onset of myopia. The impact of the family history on the level and the onset of myopia was quantified. Parental effect on corneal curvature (CC), anterior chamber depth (ACD), and axial length (AXL) was analyzed. RESULTS: The study included 185 normal subjects, 170 mild, 140 moderate, and 392 high myopes. Family history was strongly associated with the probands' status (P < 6 x 10(-12)). When there was >or=1 highly myopic parent, the odds ratios (ORs) of developing mild or moderate myopia were between 2.5 and 3.7 (95% CI: 1.1-6.5) and the ORs of having high myopia were > 5.5 (95% CI: 3.2-12.6). A strong association (P = 2 x 10(-6)) between parental myopic state and the AXL in the subjects was also found, but there was no statistical relationship for ACD or CC. There was an association between high myopia in parents and the onset of myopia in children. Siblings had a weaker association with the level of myopia and had no effect on the onset of myopia. CONCLUSIONS: This study found strong familial effects on the level and onset of myopia even after adjusting for environmental factors. The parental effect on ocular components in their offspring was primarily on AXL.