Study Of Introducing Gradient Equiaxed Grains Into Zr Plates By Combining Laser Surface Treatment,Rolling And Annealing(LST-RA)

In present work,a typical recrystallized and annealed commercial pure zirconium sheet(Zr702)was subjected to laser surface treatment(LST),50%rolling and annealing at 550°C(10-600 min).Their microstructure and texture evolution in surface layer and center zone before and after different treatments were carefully characterized and compared by EBSD and ECC imaging techniques in the FEG-SEM.Formation for the gradient equiaxed grains by combining laser surface treatment,rolling and annealing(LST-RA)and correlation between microstructural characteristics and mechanical properties after different treatment were explored.In addition,microstructures subjected to conventional rolling and annealing(control group)were conducted to more clearly reveal the influence of LST-RA treatment on zirconium alloys.Main conclusions were obtained as follows:The microstructural and textural evolution during LST,rolling and annealing of pure Zr sheet both in surface layer and center zone have been quantitatively investigated.Results showed that microstructure in surface layer zone evolution during the whole process of LST-RA treatment can be concluded as:primal equiaxed grains?fined?plates(LST)?deformed plates(rolling)?fined equiaxed grains(annealing),while the microstructure evolution in center zone evolves was summarized as:primal equiaxed grains?equiaxed grains(LST)?lamellar structures and less deformed grain(rolling)?equiaxed grains(annealing).After LST-RA treatment,different regions gradually were developed to gradient equiaxed grain structures,namely gradient equiaxed grains with sizes gradually increasing from surface to interior.The formation of gradient equiaxed grain structures was analyzed by the area density of HABs calculation obtained by EBSD results.It was found that due to fine plates in surface layer of pure Zr sheet produced by LST,the area density of high angle boundaries(HABs)in the surface layer is measured to 1.64?m^-1,significantly higher than that in the center(0.22?m^-1)and higher area density of HABs(2.69?m^-1)in the surface layer was further increased after rolling.During annealing,the recrystallization nucleation mechanism dominated by strain induced boundary migration(SIBM)enabled denser preexisting HABs in the surface layer to facilitate more nuclei to be generated than that in center and faster recrystallization completion.It could be subsequently developed to gradient equiaxed grain structures,i.e.,fine grains in the surface layer and coarse grains in the center.Compared textural evolution of pure Zr sheet in surface layer and center zone during LST,rolling and annealing,textural evolution differences was revealed between them.Results showed that due to multiple variants generated by rapid???transformation(high heating/cooling rates help suppress variant selection),the primary bimodal basal texture was replaced by largely scattered orientations.Furthermore,the 50%rolling makes c-axes of most grains be aligned near the ND(ring-like distribution).During subsequent annealing,c-axes of most grains in the surface layer were spread around ND with decreased textural intensity(3.7 times random).In contrast,due to absent of LST effect,the initial bimodal basal texture is well maintained in the center even after rolling.During annealing,no considerable textural changes in the center zone were found.Compared hardness variation pure Zr sheet in surface layer and center zone during LST,rolling and annealing,results showed that hardness in surface layer was increased to 238 HV attributed to grain refinement after LST,while maintained in the center with unaffected microstructure.After rolling,hardness in surface layer and center were increased to 269 HV and 257 HV,respectively due to grain refinement and residual stress increasement.During annealing,ascribed to residual stress relaxation and grain growth,the hardness both in surface layer and center were decreased with annealing time extension,while higher hardness was always belonged to the former.Compared the tensile properties of AR and LST specimens after the rolling and annealing,it is found that former exhibits a low strength and a high ductility while the latter exhibits higher strength with high ductility.After rolling,the strength of both increases considerably and the ductility decreases.During annealing,the strength decreases gradually and the plasticity increases.In addition,LST specimens exhibits higher strength and ductility compared AR specimen at different annealing stages.Note that an exceptional synergy of high strength and high tensile ductility is achieved in LST specimen after rolling and annealing for 10 min.