Study On Hot Deformation Behavior And Processing Parameters Optimization For Zr-4 Alloy

Zr-4 alloy has been widely used in the nuclear industry,aerospace,chemical and medical industries for its advantages of high melting point,low density,high specific strength,low thermal neutron absorption cross section,good corrosion resistance and moderate mechanical properties.In this paper,the hot deformation behavior and microstructure evolution of as-cast Zr-4 alloy are studiedin the range of temperature from 750℃to 1000℃,strain rate from 0.001s^-1to 10s^-1and the maximum deformation70%by means of isothermal constant strain rate thermal compression tests.The research results are of great significance to the selection of process parameters and improvement of the properties of the Zr-4 alloy in the future.The flow stress of Zr-4 alloy decreases with increasing temperature and decreasing strain rate.In the initial stage of deformation,the flow stress increases sharply with the increase of strain and reaches a peak around the strain of 0.05;then the flow stress exhibits the characteristics of steady-state flow within the low strain rate range of 0.001¹s^-1.At a high strain rate of 10s^-1,a discontinuous stress yield phenomenon occurs.Based on the stress-strain data,the strain-compensated Arrhenius phenomenological model and the BP artificial neural network were used to establish the constitutive model.The results show that the latter is more suitable for constructing the constitutive model of the Zr-4 alloy with an average relative error AARE of 4.83%and the correlation coefficient between the predicted and actual values fits to R=0.99.The effects of deformation temperature on the microstructure of Zr-4 alloy are as follows:（1）The spheroidization rate of original lamellar microstructure increases with the increase of temperature within a certain temperature range when derfomed at strain rate of 0.001s^-11 or 0.01s^-1.（2）The flow localization,kinking and other defects in the organization when deformed at strain rate of 10s^-1.As the temperature rises further,the spheroidization appears in the boundary of lamellar microstructure.The effects of strain rate on microstructure of Zr-4 alloy are as follows:（1）The effect of temperature change on the microstructure of the Zr-4 alloy is small when the deformation temperature is 750℃.（2）The spheroidization rate of Zr-4 alloy decreases with the increase of strain rate when the deformed at temperature of 800℃or 900℃,the original lamellar microstructure is spheroidized within a certain strain rate range,and the spheroidization rate decreases with the increase of the strain rate.（3）The grains change from near equiaxed grains to elongated grains with the increase of strain rate when the deformation temperature is 1000℃,there are a large number of fine acicular microstructure.Based on the Prasad instability criterion of the dynamic material model,the Zr-4alloy processing maps under different strain conditions are plotted.It can easily seen that the area of instability in the processing map increases as the deformation increases.Integrated processing maps and microstructure observation,it can be concluded that the best processing area of the Zr-4 alloy are in range of 775⁸13℃and 0.18⁰.41s^-11 inαphase zone,965⁹75℃and 0.28⁰.47s^-1in β phase zone.