The Effect Of Severe Plastic Deformation On Microstructure Evolution And Mechanical Properties Of Zirconium Alloys

Severe plastic deformation(SPD)can refine microstructure,change the crystal orientation of metallic materials and improve their mechanical properties.In addition,annealing treatment after deformation can control dislocation density,grain size,the number and distribution of the second phase,so as to obtain the good combination of strength and ductility.In this paper,the grain refinement mechanism and microstructure evolution of α-Zr and β-Zr alloys with two kinds of crystal structure during friction stir processing(FSP)or high pressure torsion(HPT),have been systematically studied by uniaxial tensile tests,electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).And the effect of subsequent annealing treatment on grain growth behavior,mechanical properties and deformation mechanisms was also studied.The grain refinement behaviors of α-Zr(Zr-4)alloy with three different base material structures were studied after FSP.It was found that the differences in grain size and crystallographic texture of the base material did not affect the microstructure features in the stir zone(SZ)after FSP.The grain refinement in SZ was completed by continuous dynamic recrystallization,and the ultra-fine grained(UFG)Zr-4 alloy with an average grain size of about 1μm was finally obtained in SZ.The special texture with the c axis of most grains distribution along the surface of stir pin was formed,due to the unique material flow pattern in SZ.Accordingly,the c axis of grains was parallel to the processing direction in the central region of SZ.However,the differences of base materials caused the differences in the microstructure characteristics of the samples in the thermo-mechanical affected zone(TMAZ)after FSP.The plate coarsening occurred in water quenched samples under thermal cycling conditions.The formation of twins promoted grain refinement in TMAZ in base material with coarse grain.And the twinning induced dynamic recrystallization(TDRX)was also studied.The microstructure evolution of α-Zr(Zr-4)alloy subjected to room temperature HPT was studied.Different from FSP,the microstructure evolution of HPT at room temperature was mainly controlled by dynamic recovery.After 5 turns of HPT deformation,the average grain size of Zr-4 alloy was about 200 nm,but the ultra-fine grain formed at this time had high-energy non-equilibrium grain boundaries.The HPT deformed samples maintained good thermal stability during annealing treatment because of relatively dispersed grain orientation and more dislocation numbers,which provided favorable conditions for the dispersion distribution of the second phase and the pinning of grain boundaries.However,the strong crystallographic texture in FSPed samples,leading to the phenomenon of grain convergence in the microstructure,so the grain growth was easy to occur during annealing treatment.The effects of annealing treatment after SPD and crystallographic texture on mechanical properties and deformation mechanisms of Zr-4 alloy were studied.For the samples with a similar texture,the strength decreased and the ductility increased after annealing,which was mainly due to the growth of grain and the decrease of dislocations and substructures.For the samples with different textures,due to the critical resolved shear stress(CRSS)of {10(?)2}twins was higher than prismatic slip,the yield strength of the alloy increased when the dominant deformation mechanism causing yield changed from prismatic slip to twins.The Schmid factor(SF)was used to explain the twinning behaviors of different samples,and the strain coordination factor(m＇)was used to explain the formation mechanism of twin pairs and twin chains in UFG Zr-4 alloy.The coarse-grained β-Zr(ZrTiAlV)alloy can be refined by one pass FSP.The grain refinement of β-Zr alloy in the SZ was dominated by continuous dynamic recrystallization and discontinuous dynamic recrystallization simultaneously during FSP.The β-Zr alloy processed by FSP can obtain good comprehensive mechanical properties,with the tensile strength and fracture elongation reaching 1083 MPa and 18.3%,respectively.This was mainly attributed to grain refinement,dislocation strengthening,and TRIP effect caused by martensitic transformation during tensile deformation.The strength of β-Zr alloy can be further improved by annealing after FSP.The α phase precipitated during the annealing process improved the stability of the β phase grain,and thus increased the triggering stress of martensitic transformation of the β phase grain during tensile deformation,thus decreasing the ductility and improving the strength of the alloy.Both α-Zr and β-ZrTiAlV alloys showed obvious microstructure refinement and mechanical properties improvement after severe plastic deformation(SPD),but the mechanisms of strengthening and toughening of α-Zr and β-ZrTiAlV alloys were different due to the difference of chemical composition and crystal structure of alloys.The activation of twins can improve the yield strength of Zr-4 alloy,the deformation induced martensitic transformation can improve the ductility and work hardening rate of ZrTiAlV alloy.Furthermore,the grain coarsening during subsequent annealing treatment after SPD decreased the strength of α-Zr alloy,while the α phase precipitation can further increase the strength of β-ZrTiAlV alloy.