Creep Behavior Of Ultra-Fine Grained Commercial Purity Zirconium At Room Temperature

The creep behavior of ultra-fine grained commercial purity zirconium?UFG CP Zr?processed by ECAP+rotary swaging was studied at room temperature by transmission electron microscope?TEM?,scanning electron microscope?SEM?,tensile testing and constant stress creep test.The influences of annealing treatment and ECAP+rotary swaging on CP Zr were analyzed.The main research contents and conclusions are as follows.After ECAP+rotary swaging deformation,the microstructure of CP Zr composes of refine grains with an average grain size of about 250 nm.Dislocation density in the grains obviously increases.There are a large amounts of dislocations cells and dislocation tangle zones.It can be observed from TEM photos that there are some subgrains in the grains.The results of tensile test show that ECAP+rotary forging deformation can obviously improve the mechanical properties of CP Zr and the yield strength is increased by 185%.The results of annealing experiment indicate that the dislocation density decreases with the increase of the annealing temperature.When annealing temperature is in the range of 150?³00?,the recovery occurs in UFG CP Zr and the microhardness value of UFG CP Zr drops slowly with the increase of annealing temperature.When annealing temperature is 400?,the grains grow up significantly and the microhardness value of UFG CP Zr decreases obviously.Creep tests were conducted on UFG CP Zr with different stress level at room temperature.The results show that there is a creep saturation phenomenon when creep stress is low.The creep strain of UFG CP Zr gradually increases with the increase of creep stress.The creep mechanism of UFG CP Zr is dominated by dislocation motion at room temperature,and its creep fracture type is a ductile fracture.A new creep equation on the basis of Norton formula was established,which was?=?-₀?⁹).The fitting results were A=7.211×10^-15,n=5.3 and₀=622 MPa.It is demonstrated that there is a creep stress threshold which makes the steady creep rate and creep stress increment follow the exponential relationship.The creep rupture data of UFG CP Zr was analyzed according to the modified Monkman-Grant relationship.The creep damage capacity factor,?in the modified Monkman-Grant relationship is 1.98.It is demonstrated that the creep deformation in the third stage and the creep rupture is caused by the voids.The annealing treatment at 250?can improve the creep properties of UFG CP Zr,weaken the creep stress sensitivity and increase the creep resistance by comparing the creep behavior of UFG CP Zr,UFG CP Zr annealed and CG CP Zr at room temperature.The combined refining process of ECAP+rotary swaging has a positive influence on the creep property of CP Zr at room temperature.