晶体生长的缺陷机制

Crystal growth is a complicated phase transition process.A perfect mechanism for practical crystal growth process has not been proposed and well recognized up till now.A model,i.e.screw dislocation model presented by F.C.Frank for imperfect crystal growth was adopted during early 1950’s.No systemic research on defects other than screw dislocation has been conducted during a quite long time. Since 1980’s,we have engaged systematically in the investigation of the defect mechanism of crystal growth,and our conclusion is that any defect providing step sources in the growing surface can make contribution to continuous crystal growth.These steps contain both complete(whole)steps and sub steps(incomplete steps).

The determining role of carbon addition on mechanical performance of a non-equiatomic high-entropy alloy

The mechanical properties and deformation mechanism of a C-doped interstitial high-entropy alloy(i HEA)with a nominal composition of Fe_(49.5)Mn_(29.7)Co_(9.9)Cr_(9.9)C_(1)(at.%)were investigated.An excellent combination of strength and ductility was obtained by cold rolling and annealing.The structure of the alloy is consisted of FCC matrix and randomly distributed Cr_(23)C_(6).For gaining a better understanding of deformation mechanism,EBSD and TEM were conducted to characterize the microstructure of tensile specimens interrupted at different strains.At low strain(2%),deformation is dominated by dislocations and their partial slip.With the strain increase to 20%,deformation-driven athermal phase transformation and dislocations slip are the main deformation mechanism.While at high strain of 35%before necking,deformation twins have been observed besides the HCP phase.The simultaneous effect of phase transformation(TRIP effect)and mechanical twins(TWIP effect)delay the shrinkage,and improve the tensile strength and plasticity.What's more,compared with the HEA without C addition,the yield strength of the C-doped i HEA has been improved,which can be attributed to the grain refinement strengthening and precipitation hardening.Together with the lattice friction and solid solution strengthening,the theoretical calculated values of yield strength match well with the experimental results.

Dislocation glide and mechanical twinning in a ductile VNbTi medium entropy alloy

An equiatomic VNbTi medium-entropy alloy with outstanding tensile properties and unique deformation behavior is reported.The screw dislocation glide,deformation twinning,and dislocation accumulation induced kink bands are identified as three deformation mechanisms that contribute to a large elongation above 20%.The{112}<111>twins are activated at the beginning of the yield stage accompanied by sudden stress-drop and pronounced acoustic emission.Dislocations dominate subsequent tensile deformation,and the prevalent multiplanar dislocation slip promotes the formation of complex dislocation configurations(e.g.,debris,dipoles,and loops)and dense dislocation networks.The twin bands and kink bands can further impede the dislocation motion meanwhile effectively alleviate stress concentration.The synergistic activation of these deformation mechanisms provides new opportunities to design ductile refractory medium-and high-entropy alloys.

Tensile deformation behavior of nickel-free high-manganese austenitic cryogenic-temperature steel

Nickel-free high-manganese austenitic Fe–24.4Mn–4.04Al–0.057C steel was produced by smelting,and the homogenized forged billet was hot-rolled.The plastic deformation mechanism was investigated through tensile testing of the hot-rolled sample.Different characterization techniques such as scanning electron microscopy,transmission electron microscopy,electron backscattered diffraction,and X-ray diffraction were used to analyze the microstructural evolution of steel under different strain levels.The steel had a single austenite phase,which was stable during deformation.After hot rolling,annealing twins were observed in the microstructure of the steel.The steel showed an excellent combination of mechanical properties,like a tensile strength of 527 MPa,impact energy of 203 J at−196℃,and an elongation of 67%till fracture.At the initial deformation stage,the dislocations were generated within the austenite grains,entangled and accumulated at the grain boundaries and annealing twin boundaries.Annealing twins participated in plastic deformation and hindered the dislocation movement.As the deformation progressed,the dislocation slip was hindered and produced stress concentration,and the stacking faults evolved into mechanical twins,which released the stress concentration and delayed the necking.