The Effect of Stacking Fault Energy on Microstructure and Texture Evolution of Cold Drawn FCC Metals

Author:Ma Xiao Guang

Supervisor:chen zuo


Degree Year:2018





Stacking-fault energy(SFE)is an intrinsic property of materials and has significant influence on the microstructure and texture evolution of face-centered cubic(FCC)metals.Although the effect of SFE has been widely studied,there are still many deficiencies.The researches on metals with medium to high SFE are mainly concentrated on pure metals(such as Al,Cu,Ni,etc.),but researches on metals with low SFE are focused on alloys(such as Cu-Zn,Cu-Al alloy,etc.).Meanwhile,the deformation is mainly by rolling,equal channel angular pressing and high-pressure torsion,etc.,and the research on the microstructure and texture evolution of cold-drawn FCC metals is scarce.Alloying elements addition not only can change SFE,may also introduce short-range order(SRO)structure.Since the influence of SRO can not be excluded,the effect of SFE on microstructure and texture evolution in FCC alloy is overestimated.In present investigation,the microstructure and texture evolution of pure Al(166mJ/m~2),pure Ag(22 mJ/m~2),Cu-10at.%Mn alloy(45 mJ/m~2,similar SFE with pure Cu,with SRO)and CoCrFeMnNi high entropy alloy(HEA,18.3-27.3 mJ/m~2,similar SFE with pure Ag)with different SFEs deformed by cold-drawning have been characterized using transmission electron microscopy(TEM),scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD).Furthermore,the work hardening was analyzed using a microhardness tester.The effect of SFE and SRO on microstructure and texture evolution of cold drawn FCC metals has been investigated systemically.Al has the highest SFE among pure FCC metals,and the results show that the main deformation mechanism of cold-drawn polycrystalline Al is dislocation slip.The decrease in deformation temperature does not alter the deformation mechanism,but only suppresses dynamic recovery and increases dislocation density.Meanwhile,the spacing of geometrical necessary boundaries(GNBs)decreases and the grain refines with decreasing temperature.During cold drawing deformation,the grain with<100>//DD orientation has a high dislocation density and fast misorientation angle evolution rate,while the grain with<111>//DD orientation has a low dislocation density and slow misorientation angle evolution rate.Compared with the polycrystalline Al with random initial orientation,the crystal rotation becomes difficult for the polycrystalline Al with initial<100>orientation,and<100>has strong heredity.At high strains,axial transformation of elongate substructure promotes the formation of<100>//DD texture components instead of consuming it.Ag has the lowest SFE among pure FCC metals,and the results show that the deformation mechanism of room-temperature(RT)drawn polycrystalline Ag and a single crystal Ag with<110>orientation is dislocation slip and twinning.At low strains,a large number of discrete dislocations and dislocation cells together with a small amount of deformation twins can be observed.At medium to high strains,GNBs,deformation twins and brass-type shear bands consisting of equiaxed grains form.When the strain is below 0.58,the volume fraction of the complex texture component decreases,and<111>//DD and<100>//DD texture components increases with increasing strain.When the strain is above 0.58,the volume fraction of texture component becomes constant.The comparison of microstructure evolution of drawn polycrystalline Al,Cu and Ag indicates that the decreasing SFE suppresses cross-slip and climb and promotes the activation of deformation twinning.The formation of deformation twins causes the rotation of<111>//DD and<100>//DD,which is beneficial for the formation of complex texture component,thereby decreasing the critical strain at which the stable texture component forms.The Cu-10at.%Mn alloy and pure Cu have similar SFE,but their RT-drawn microstructure and texture evolution have significant difference.This suggests that the effect of SFE on microstructure and texture evolution is overestimated.At low strains,the Lomer-Cottrell dislocations,equiaxed dislocation cells,GNBs and deformation twins can be observed in the RT-drawn Cu-10at.%Mn alloy.However,there are no the Lomer-Cottrell dislocations and twins can be observed in RT-drawn polycrystalline Cu.SRO in Cu-10at.%Mn alloy leads to the transformation from wavy slip to planar slip,which facilitates the activation of deformed twins and the formation of<111>//DD texture component.Although the CoCrFeMnNi HEA and pure Ag have similar SFE,the microstructure and texture evolution of them has some differences.At low strains,the microstructure evolution of RT-drawn CoCrFeMnNi HEA is dominated by planar slip and thus the Lomer-Cottrell dislocation,high density dislocation wall and Taylor lattice form.However,only discrete dislocations and equiaxed dislocation cells can be observed in RT-drawn polycrystalline Ag.At medium strains,the main deformation mechanism of CoCrFeMnNi HEA is dislocation slip and twinning,and a large number of twin/matrix lamella can be observed.With increase of strain,the de-twinning causes the thickness of deformation twins to become narrow.Meanwhile,the dislocation sliding mode transforms from planar slip to wavy slip.At high strains,deformation twinning and shearing are dominant ways to coordinate plastic deformation.Large lattice distortion and high solid solubility have the similar effects on deformation behavior with SRO,and they promote the transformation from complex texture component to<111>//DD and<100>//DD texture components and prohibit the formation of stable texture component.