Investigation On The Control Of Microstructure And Mechanical Properties Of Binary ZrBe Alloy

Zirconium(Zr)and Zr alloys have been widely used in the nuclear industry because of their excellent physical and chemical properties such as high specific strength,small thermal neutron absorption area,corrosion resistance,oxidation resistance and radiation resistance.As study going on,researchers found that Zr alloys had an intensive application prospect in the structural materials.Generally speaking,four kinds of equiaxed,bimodal,basket-wave,and widmanstaten microstructures are the most common in Zr alloy.Equiaxed microstructure and bimodal structure rendered the alloys with a high strength and good ductility among four typical microstructures of Zr alloys.In this paper,phase composition and microstructure were optimized by adjusting composition,hot deformation and heat treatments,resulting in increasing the strength and the ductility of ZrBe alloys.A series of ZrBe alloys with different Be contents were prepared using a vacuum non-consumable electro-arc furnace and the effects of Be contents on the microstructure and mechanical properties of the alloy were investigated.The results showed that the strength of pure Zr was greatly increased by trace Be addition.With the increase of Be content,the strength of the Zr Be alloys increased,while the ductility of the alloys decreased at the same time.As the Be content was 1.0wt.%,the alloy had the highest ultimate tensile strength.When the Be content was 0.5wt.%,the most excellent mechanical properties of the alloys were obtained with a ultimate tensile strength of 700 MPa and a ductility of 12.42%.The microstructure and the mechanical properties of Zr0.5Be alloy with different deformation parameters were investigated.The results showed that the alloy obtained a small amount of β phase after rolling at the lower temperature range near α+β phase followed by quenching,while bimodal structure was obtained after rolling at the higher temperature range near α+β phase.The strength of the ZrBe alloys decreased with increasing rolling temperature,while the ductility of the alloys increased at the same time.After hot rolling at 870°C followed by quenching,the tensile strength of the alloy was 786 MPa and ductility was 11.82%.Compared with the mechanical properties of the alloy processed at 570°C(tensile strength: 919 MPa,ductility: 6.93%),the strength of the alloy decreased slightly(decreased by 14.5%),but the elongation of alloy was significantly improved(increased by 70.6%).In addition,it was found that the mechanism of deformation of Zr0.5Be alloy processed by cold rolling was dominated by dislocation slip.When reduction of the alloy was greater than 50%,the alloy undergone partly dynamic recovery process,and the dislocation was gradually transformed from the stripe like entanglement to the cellular sub-grain.The effects of annealing temperature and annealing time on the microstructure and mechanical properties of Zr0.5Be alloy were investigated.The results showed that the volume fraction of recrystallizationed grains increased with the increase of annealing temperature and the annealing time.After the recrystallization stage was completed,the recrystallizationed grains began to grow up with the further increase of annealing temperature or annealing time.On the contrary,the dislocation density decreased gradually with the increase of annealing temperature and annealing time.The completely equiaxed recrystallized grains,uniformly distributed compounds,and the lowest density of dislocation(7.22×1011m-2)could be observed in Zr0.5Be alloy which annealed at 850°C for 480 min.Compared with the deformed alloy,dislocation density of the alloy annealed at 850°C for 480 min decreased by four orders of magnitude.The effects of solid solution strengthening,second phase strengthening and grain refinement on the strength of ZrBe alloys were studied.It was found that second phase strengthening and grain refinement played an important role in the strengthening ZrBe alloys.The influences of Be contents and hot working process on the strain hardening exponent of ZrBe alloys were investigated.The results showed that the strain hardening exponent of ZrBe alloys could be improved obviously by the addition of Be element and the decrease of deformation temperature.