Study Of Microstructure Evolution And Property Of Severe Plastic Deformed Zirconium

Strength and ductility are two important parameters of structural materials, and both ofhigh strength and ductility are desirable for structural applications. Unfortunately, strengthand ductility are exclusive. One metal can be strong, or ductile, but rarely both at the sametime. Although many technologies have been employed to improve the strength of metals,but these metals always exhibit poor ductility due to the limited motion of dislocations.The reason is that dislocations are carriers of plastic deformation and their motion ishindered in these strengthened metals. In the present study, we chose Zr as a model metalto study the microstructure evolution and mechanical properties of the metal withcryorolling deformation, high pressure confinement deformation and thermal annealingparameters, in order to enhance the strength and ductility simultaneously.In the present study, a multimodal grain structure composed of coarse grains (~24%),ultrafine grains (~56%), and nanoscale grains or subgrains (~20%) has been formed in hcppure Zr by employing cryorolling combined with subsequent low-temperature annealing.The multimodal structured Zr exhibits a high ultimate tensile strength (~658MPa) andlarge uniform elongation (~8.5%), demonstrating a good combination of high strength andductility. The formation mechanism of multimodal structure and the deformationmechanism of multimodal structured Zr are discussed in-depth.The effect of strain rate on the microstructures of cryorolled Zr before and afterannealing has been investigated. The dislocation density increases with increasing strainrate, resulting in different microstructures after annealing via increasing recrystallizationnucleation rate and prompting the abnormal growth of grains.The dislocation configuration of cryorolled Zr has been tailored by using various strainrate cryorolling. Dislocation configurations have a significant effect on mechanicalproperties of cryorolled Zr, which can be used to optimization of mechanical properties ofsevere deformed metals, a simultaneous increase in strength and ductility is obtained athigh strain rate of=2.24s-1. The increases in strength can be attributed to high-densitydislocations, and the increase in ductility to the motion of preexisting dislocations in lamellar configuration.An abnormal mechanical annealing behavior, i.e. a significant reduction in dislocationdensity and strength with increasing compression strain, has been observed in thecryorolled Zr suffered high-pressure (HP) confinement deformation. A good combinationof high strength and ductility has been achieved in the mechanically annealed sample. Thisunusual mechanical annealing behavior results from the motion of preexistinghigh-density dislocations in the cryorolled Zr under high stress. The formation mechanismof nanoscale grains and subgrains are discussed deeply.Abundant dislocation clusters formed in the cryorolled Zr after500℃thermalannealing, which can’t be explained by conventional recrystallization theory. Dislocationclusters become smaller and multiple upon tensile deformation, contributing to the strainhardening and plastic deformation of the sample. A good combination of high strength andductility has been obtained in the coarse-grained Zr sample. The formation mechanism,deformation mechanism and the relation between dislocation cluster and mechanicalproperties have been discussed.