| Dislocations play a predominant role in the plastic deformation of metals and alloys,and thus largely determine mechanical properties and technological applications of the materials.Until now,despite intensive efforts have been taken,the theories of dislocation behaviors are mainly built up based on simple crystal structures and generally believed to be controlled by long-range lattice translational order.For complex structured intermetallic compounds,however,their structures are often closely related to atomic environments,with the short-range configurations possibly deviating from the long-range translational order.Thus,how dislocations move in these crystals is obscure.As the largest group of intermetallic compounds,topologically close-packed(TCP)phases show complicated structural features,such as puckered atomic layers,stacking of groups of atoms.Studies on dislocation mechanism in TCP phases are mostly limited in one kind of close-packed atomic plane in Laves phases,basal planes for C14 and C36 structures or {111} planes for C15 structure.Besides,most complex structured intermetallic compounds show brittleness at room temperatures but ductility at high temperatures.The physics of the brittle-to-ductile-transition,particularly the dislocation behavior,is of great significance to solve the problem of room temperature brittleness of these materials.In this work,we have investigated the shear deformation on nonbasal planes in C14 M2Nb(M=Cr,Ni and Al)phase deformed under high-temperature quasi-static compression and room-temperature high strain rate impact,using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and energy dispersive X-ray spectroscopy(EDS).The deformations are resolved to be dominated by atomic-environment polyhedra(or coordinationpolyhedra).Dislocations there have Burgers vector[(?)[1103])]deviating from slip planes and surprisingly move by switching between two different slip planes.Long-range diffusion and local composition variation assist the dislocation motion,as demonstrated by high temperature quasi-static and room temperature impact deformations.These discoveries demonstrate the defining role of short-range configuration in the deformation of complex structured intermetallics and shed light on the novel behavior of dislocations.Besides,during the two kinds of deformations by high-temperature compression and room-temperature impact,different mechanisms of dislocation motion are found operating on different prismatic atomic planes.In high-temperature deformation,a shuffle mechanism assisted by long-range diffusion mediates the dislocation motion.In room-temperature deformation,not only the shuffle mechanism exists,but also a glide on different atomic planes arises as an important dislocation slip mechanism.Large fluctuating strain field is revealed to accompany with room-temperature deformations,which results to the difficulty of dislocation movement and consequently brittle fracture These findings reveal that there are different dislocation mechanisms and strain fields at the atomic scale in the ductile and brittle deformation regimes of complex structured materials.This work also studies the precipitate evolution in an Al-Zn-Mg-Cu alloy.Via quasi-in situ heating and conventional aging experiments,we have established an atomic-scale precipitation process from a saturated solid solution to 7-layers precipitate(η").Combining with other recent researches,the precipitation sequence of the alloy is revised as:solid solution→ GP zones→η’→η"→stable phase η.The formation and growth of 9-layer precipitates with different configurations were reported and analyzed.It was found that the 9-layer precipitates firstly form a composited C14 and C15 structure,and then grow by connecting a defected long period C15 structure at one end.Five-fold symmetrical structure units are found forming at both sides of 9-layer precipitates,they improve the thermal stability of the precipitates. |
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