Study On Twinning Behavior And Deformation Mechanism Of Zirconium Alloy During Plastic Deformation

Zirconium alloys are widely used as fuel cladding materials and structural materials for nuclear reactors due to their excellent nuclear and mechanical properties.The finished zirconium alloys undergo a series of thermomechanical processes such as forging,beta quenching,rolling and heat treatment during industrial fabrication,in which several plastic deformation parameters such as temperature,strain rate and stress state are included.Therefore,understanding the plastic deformation mechanism of zirconium alloys under different deformation conditions is of great significance for optimizing the production processes,so as to control the micro structure and texture evolutions to obtain finished zirconium alloys with better service performance.As a metal with a close-packed hexagonal structure,twinning is a very important deformation mechanism in the plastic deformation of zirconium alloys.The twinning characteristics,including twinning nucleation and growth,the selection mechanism of twinning variants and unconventional twinning behavior under complex stress,have an important impact on the microstructure,texture evolution and mechanical properties of zirconium alloys.As a result,in this paper,using β solution treated and furnace cooled Zr-4 and Zr702 as the experimental materials,we employ optical microscope,back scattered electron imaging(BSE),electron backscatter diffraction(EBSD)and X-ray diffraction(XRD)techniques to systematically study the microstructural and textural evolution of both materials during rolling at two temperatures(room and liquid nitrogen temperature)and twinning behavior of Zr702 during dynamic plastic deformation.In addition,the abnormal twinning behaviors of the two alloys during plastic deformation are also theoretically analyzed and studied.Finally,the mechanism of deformation kinking band,which is in dispute in zirconium,is characterized and its formation is analyzed and discussed.The microstructure and macro texture evolution of β-cooled Zr-4 and Zr702 during rolling at room temperature and liquid nitrogen temperature were firstly studied.It was found that reducing impurity content promoted the activation of deformation.Rolling at liquid nitrogen temperature also promoted the activation of deformation twinning and delayed the transformation of the dominant deformation mechanism from twinning to slip.At the early stage of room temperature rolling,the main twin types are {10(?)2} and{11(?)1} tensile twins,whose nucleation and growth make the orientation of the parent grains away from ND change to near ND.At the early stage of liquid nitrogen temperature rolling,in addition to activating these two tensile twins,a certain amount of{11(?)2} compression twins are generated in those grains with c axis close to ND,which make the c axis away from ND.Meanwhile,secondary twins are easily activated in these compression twins to make the c axis back to ND again.The difference in twinning types activated at the initial stage of the two temperature rolling is the main reason that leads to the great difference in macro texture evolution.The special twinning structure of β-cooled Zr-4 formed during rolling at room temperature was further studied.EBSD analysis showed that it was {11(?)1}-{10(?)2}double twin,which was never reported in zirconium before.Combined with the Schmid factor and the strain accommodation factor m’,it is revealed that the double twin is generated to accommodate the local strain incompatibility caused by the interaction between the primary {11(?)1} and {10(?)2} twins.It is suggested that the variant selection of the secondary twins does not follow the Schmid’s Law,but is determined by the local strain accommodation effect.Then,the twinning behavior of β-cooled Zr702 during dynamic and quasi-state plastic deformation was comparatively studied.It is seen that a large number of deformation twins are generated in the deformed samples subjected to dynamic deformation,even at a low strain(5%).The twins are detected to be mainly dense{11(?)1} twins and a small number of {10(?)2} twins.As the strain increases,sequential{10(?)2} twins are observed to be generated at the {11(?)1} twin boundaries.These sequential {10(?)2} twins further grow and merge to form coarse,irregular "twin domains",in which {10(?)2}-{11(?)2} double twins and even {10(?)2}-{11(?)2}-{10(?)2}triple twins can be observed.Further studies on the variant selection mechanism of the primary and sequential {10(?)2} twins show that the variant selection of the former follows Schmid’s law,while the latter is governed by the local strain accommodation effect.In addition,compared with quasi-static compressive,the {11(?)1} twins generated in dynamic plastic deformation exhibit abnormally dense morphology,which is mainly due to the suppressed dislocation slip under high strain rate deformation,requiring activation of more.{11(?)1} twins to accommodate deformation.Finally,the special twin-like lamellae of β-cooled Zr702 during dynamic plastic deformation to more than 15%reduction was studied.EBSD analysis showes that the twin-like lamellar structures are characterized by～20°/<10(?)0>orientation relationship with its parent grains and are belonging to deformation kinking bands,which provides a direct experimental evidence for the occurance of kinking bands in Zr.By means of Schmid factor calculation and in-grain misorientation axes(IGMA)method,the formation mechanism of kinking bands is discussed.The results show that they are formed by the activation and accumulation of single or multiple basal slip variants inside the parent grains.In addition,by comparison with other deformation mechanisms,it is found that the formation of these kinking bands is harsh,that is,they may occur only in those grains that are not favored for the activation of both twinning and non-basal slips.In summary,in this paper,the twinning behavior of zirconium alloys under different deformation conditions was studied by comparative experiments.The important role of deformation twinning in the texture evolution of zirconium alloys was discussed,and the mechanisms of the twinning variants selection and the formation of unconventional twinning behaviors in zirconium was clarified.The direct experimental evidence for the novel mechanism of deformation kinking bands was provided.This study enriches the knowledge and understanding of the plastic deformation of zirconium alloys,and provides experimental basis and theoretical references for optimizing the production processes of zirconium alloys.