Texture Evolution And Formation Mechanism Of Zircaloy-4 Tube During Cold Pilgering

Zirconium alloy is usually used as cladding tubes and structure components in nuclear reactors.Cold pilgering has been considered as the preferred rolling fabrication technology for difficult-to-deform tubular materials such as Zircaloy-4.The texture of cladding tubes is one of key factor influencing the final performance and becomes a hot issue in recent years.However,the correlation between texture evolution and spatial inhomogeneous deformation behavior is still unclear.In this work,systematical works were done to clarify the formation mechanism of texture evolution during cold pilgering.Based on the formation mechanism,the interaction effect of initial texture,Q value and deformation amount on deformed texture were explored during pilgering process.Firstly,a macroscopic hardening model is proposed to describe the hardening behavior of Zr-4 tube for better modeling of cold pilgering process.The model can describe the deformation under a large strain condition.Model parameters have been obtained from uniaxial tension tests coupled with its corresponding finite element analysis.The difference of force-displacement curve between test and finite element analysis is minimized iteratively through adjusting the necking point.In addition,the influence of strain rate and temperature on hardening behavior is also introduced into the hardening model.Then the model is utilized for the finite element analysis of cold pilgering process.The predicted results on rolling force and tube dimensions during cold pilgering are compared with experimental ones.The error of rolling force between simulation and experiment is less than 1%.The conical dimension curve(wall thickness/outer diameter)agrees well with those of experimental measurement,which validated the established hardening model.Complex stress history and deformation law can be obtained.Secondly,mechanical responses of pilgering Zr-4 alloy are measured under uniaxial(tension/compression)loadings.Tension was carried out in three different directions:along axial direction(AD),transverse direction(TD)and 45° to the axial direction(45AD)at quasi static strain rate.Compression was only performed along axial direction.Texture measurement was obtained at intermediate strains with electron backscattered diffraction method(EBSD).For different uniaxial deformation,anisotropic yield stress and strain hardening behavior can be observed from the mechanical results.This complex behavior is explained through the understanding roles of deformation mechanisms using the visco-plastic self-consistent(VPSC)model.In order to calibrate the VPSC model's constants as accurate as possible,it is employed to simulate four mechanical responses and crystallographic characteristics.What's more,the predictive polycrystal yield locus is compared with the one described as Hill 1948 which is calculated from tensile results.The predictive Lankford coefficients are also calibrated with anisotropic tension results,too.The modeling results show that the roles of different mechanisms in plastic deformation change significantly.In order to enclose selection of deformation modes and related texture evolution during one-pass pilgering process,EBSD samples of Zircaloy-4 were cut from a tapered tube with different area reduction and Q value respectively to find the effect on texture evolution.The selection of deformation modes with various reduction amount was also measured with in-grain misorientation axes(IGMA)method.The relationship between texture evolution and deformation modes was then discussed with Effective Schmid Factor(ESF)distribution calculation and activity of slip modes.The results show that basal slip activity indeed exists when deformation strain increases to a sufficiently high level.For low strains,the basal pole is changing from transverse direction(TD)to radial direction(RD)caused by the combination of prismatic and pyramidal slip.For higher strains which is larger than 30%,the cause of the same texture change tendency is primarily related to basal slip with the accommodation of pyramidal slip.Further calculation shows that increasing Q value leads to higher effective Schmid factor which was indicated to increase grain rotation velocity.Taking radial Kearns factor(Fr)as important indicator for evaluating the deformed texture after pilgering,the effect of initial texture,Q value and deformation amount was revealed.Larger initial radial Kearns factor and deformation amount can be considered to obtain the desired near radial texture.For a given initial texture and deformation amount,it shows the preferential alignment parallel to radial direction for Q larger than a critical value and clustering around the tangential direction for Q lower than the critical value.According to the statistics,it is also found that the critical Q value changed with various initial texture.The larger the Fri is,the bigger the critical Q value is.It is necessary to select a larger Q value and deformation amount for further increase of radial texture during the final pass pilgering.