Deformation mechanism and in-situ TEM compression behavior of TB8βtitanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals’surfaces and improve their mechanical properties.However,the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied.Here,ultrasonic surface rolling process(USRP)was used to create a gradient microstructure,consisting of amorphous,equiaxed nano-grained,nano-laminated,ultrafine laminated and ultrafine grained structure on the surface of TB8βtitanium alloy.High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing.In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain,in order to reveal the micromechanics behavior of different grain morphologies.The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers;the ultrafine grained layer and single crystal had an excellent strain hardening rate.The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties.Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy.An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain boundaries caused the grain refinement,where twinning could not be found and no phase transformation occurred.

Surface Dislocation Nucleation Mediated Deformation and Ultrahigh Strength in Sub-10-nm Gold Nanowires

The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy （HRTEM） imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-lO-nm gold nanowires.

Single Cr atom catalytic growth of graphene

Single atoms are the ultimate minimum size limit for catalysts. Graphene, as an exciting, ultimately thin （one atom thick） material can be imaged in a transmission electron microscope with relatively few imaging artefacts. Here, we directly observe the behavior of single Cr atoms in graphene mono- and di-vacancies and, more importantly, at graphene edges. Similar studies at graphene edges with other elemental atoms, with the exception of Fe, show catalytic etching of graphene. Fe atoms have been shown to both etch and grow graphene. In contrast, Cr atoms are only observed to induce graphene growth. Complementary theoretical calculations illuminate the differences between Fe and Cr, and confirm single Cr atoms as superior catalysts for sp^2 carbon growth.