Study Of Microstructure Control And Mechanical Properties Of Severe Plastic Deformed Zirconium At Ambient Temperature

High strength and high ductility are critical for structural materials under high load conditions.Generally,reinforced metal materials are achieved by refining grains or increasing dislocation density.However,there is a trade-off between strength and ductility.The heterogeneous structure formed by introducing coarse crystals in the fine-grained matrix has a plastic deformation mode with multiple and large strain gradient,which improves the ductility of the metal material while ensuring the strength.In the present work,the hcp Zirconium is selected as the model material to study how to improve the strength and ductility of the metal.The effect of rolling process on the microstructure evolution and mechanical properties of Zirconium have been studied by using ambient temperature rolling deformation.The main research contents and results are as follows:Firstly,the heterogeneous structured Zirconium with excellent properties was prepared by the combination of large strain ambient temperature rolling with low temperature annealing.Its tensile strength and uniform elongation are 669 MPa and 9.9%,respectively.When the true strain is less than 15%,the work hardening rate is higher than that of the coarsegrained counterpart and eventually lasts to 17.5%.The high strength of the heterogeneous structured Zirconium is attributed to the presence of nano-and ultrafine grains and high back stress,the high ductility is attributed to the presence of back-stress hardening.Secondly,the effects of strain on the microstructure evolution and mechanical properties of Zirconium at room temperature were studied.The experimental results show that with the increase of the strain,the grain refines and the dislocation density increases.After annealing at 450 °C,the tensile strength and uniform elongation increase synchronously with the increase of the strain,and when the strain is 3.9,a typical heterogeneous structure is formed.Grain refinement and high back stress increase the strength of the material,and back stress hardening and dislocation hardening increase the ductility of the material.Thirdly,the effects of annealing process on the mechanical properties of Zirconium was investigated.When the annealing temperature is lower than 450 °C,the rolled Zirconium exhibits small grain size and exhibits low ductility of 4.5%.After annealing at 450 °C,the strength and ductility are well combined.After a two-step annealing above 450 °C forms a coarse crystal structure,and exhibits low strength of 560 MPa.Fourthly,the effects of pulse parameters on the mechanical properties of Zirconium was studied.With the increase of pulse frequency from 300 Hz to 500 Hz,the tensile strength and uniform elongation increase synchronously.With the increase of pulse duration from50 ?s to 120 ?s,the tensile strength and uniform elongation increase simultaneously.The reason for these changes is that high frequency and large pulse duration promote the electroplastic effect,beneficial for the movement of dislocations and the occurrence of dynamic recrystallization,further leading to the high strength and good ductility.