Quantitative Investigation Of Microstructure And Texture Evolution During Fabrication Of N18Zirconium Alloy Sheets

This thesis reports a quantitative investigation of microstructure and textureevolution during fabrication of N18zirconium alloy sheets, which are used as structuralmaterials in nuclear power reactors. Microstructure evolution was observed by electronchanneling contrast (ECC) images and electron backscatter diffraction (EBSD)technology in a scanning electron microscopy (SEM). Local crystallographicorientations were measured by EBSD technology and the macro-texture was examinedby X-ray diffraction technology. Second phase particles were identified usingtransmission electron microscope (TEM). Combining experimental investigations withcrystal plasticity simulation using a relaxed constraint (RC) model, two sheets withdifferent initial orientations were selected to study the effect of the initial orientation onthe deformation mechanism and the texture formation during cold rolling.Recrystallization behavior during primary recrystallization of cold rolled sheets after50%and85%reductions was also presented.In order to identify activated slip systems in the N18zirconium alloy, a methodcombining in-grain misorientation axes (IGMA) distributions with Schmid factoranalyses was employed, based on detailed studies of the effect of each slip system onthe IGMA distribution.The microstructure and texture evolution during fabrication of N18alloy sheetshave been quantitatively investigated. It is found that the microstructure evolves from aweave Widmans tten structure of β quenching stage to heterogeneous deformationstructures associated after hot and cold rolling and then to a fully recrystallized structureafter final annealing. The texture evolution can be summarized that a random textureformed by β quenching transforms into a tilt basal texture after hot rolling, which keepsstable during the following fabrications. The texture of the rolling sheets is mainlycharacterized as <1010> direction parallel to rolling direction (<1010>//RD) while thetexture of the annealing sheets is <1120> direction parallel to rolling direction(<1120>//RD).The effect of the initial orientation on the deformation mechanism during coldrolling has been analyzed. The results show that the0°sheets were deformed mainly byprismatic slip,{1011} pyramidal <c+a> slip and basal slip. Prismatic slip and {1012}tensile twinning were the dominant deformation mechanisms for the90°sheets at low strain while {1011} pyramidal <c+a> slip and basal slip were also observed in higherstrained samples. The slip identification results indicate that {1011} pyramidal <c+a>slip and basal slip were enhanced with increasing deformation strain in both sheets.Texture evolution of the0°and90°sheets during cold rolling was simulated usinga relaxed consistent model. The ratio of critical resolved shear stress of prismatic slip,{1011} pyramidal <c+a> slip, basal slip and {1012} tensile twinning was indentifiedas1:5:9:2. The formation of the double tilt basal texture was ascribed to {1012} tensiletwining,{1011} pyramidal <c+a> slip and basal slip, while the <1010>//RD texturewas interpreted as a result of prismatic slip.After primary recrystallization, the final texture of the50%cold-rolled sheetschanges into a dominant texture with <1120> direction parallel to the rolling direction(<1120>//RD), while the85%cold-rolled sheets display a mixed texture with<1120>//RD and <1010>//RD. The texture differences of the two materials werediscussed based on the preferred nucleation of <1010>//RD grains and preferredgrowth of <1120>//RD grains.