The Effect Of Cerium And Zirconium Addition On The Microstructure And Mechanical Properties Of V-4Cr-4Ti Alloy

V-4Cr-4Ti has been widely investigated by many researchers for over twenty years and is expected to be applied as structural material in future fission reactors and fusion reactors due to its potentially low-induced activation characteristics,low ductile brittle transition temperature,and high resistance to irradiation-induced swelling.It has been widely recognized that Ti-rich precipitates were common in V-4Cr-4Ti alloys and tended to have a nonuniform distribution and result in an inhomogeneous grain structure.Furthermore,its mechanical properties and the ability to resist embrittlement at higher temperature need to be improved.In order to solve these problems and optimize the mechanical properties,the microstructural evolution and mechanical properties of V-4Cr-4Ti alloys with yttrium addition and zirconium addition have been investigated systematically.Especially,the formation mechanism of YOx and ZrOx particles has been revealed.A reasonable heat treatment process was designed and resulted in an enormous reduction of both Ti(C,N,O)precipitates in grain interiors and V(C,N,O)compounds on grain boudaries and a formation of YOx and ZrOx particles with a relatively uniform distribution.Main conclusions are as follows:(1)Yttrium addition reduces the Ti(C,N,O)particles in grain interiors and V(C,N,O)compounds on grain boudaries enormously.YOx particles with tens of nanometers to several microns in size are observed in V-4Cr-4Ti-0.3Y specimens.YOx inclusions of micrometer level are observed in ingots.The majority of finer YOx precipitates is considered to form by reaction of solid solution Y and O during annealing or thermo-mechanical treatment at high temperatures(above 1000?).(2)Zirconium addition reduces the Ti(C,N,O)and V(C,N,O)particles,too.ZrOx particles with tens of nanometers to several microns in size are observed in V-4Cr-4Ti-xZr(x=0.2,0.4,0.6)specimens.The finer ZrOx particles are considered to form by reaction of solid solution Zr and O during annealing or thermo-mechanical treatment at high temperatures and their volume fraction becomes largest in V-4Cr-4Ti-0.4Zr specimens.ZrOX,YOx and complex yttrium/zirconium oxides are observed in V-4Cr-4Ti-0.3Y-xZr(x=0.2,0.4,0.6)specemens.The content of the complex yttrium/zirconium oxides increases with the Zr/Y ratio increasing.(3)The formation of the yttrium oxides inclusions is found to be responsible for the grain refining effect of yttrium addition on the as-cast microstructure.On the contrary,zirconium addition has no grain refining effect on the grain structure of ingots.The enormous reduction of nonuniform Ti(C,N,O)particles results in a homogeneous recrystallized grain structure and the formation of the finer yttrium-enriched or zirconium-enriched precipitates inhibits the abnormal growth of grains at relatively high annealing temperatures.(4)The rolled V-4Cr-4Ti sheet shows a macro rolling texture of a-fibre and y-fibre.After annealing,an apparent decrease of the a-fibre texture and an invariability of the ?-fibre texture are observed.Both the micro rolling textures and the Ti-rich precipitates have a nonuniform distribution.The volume fraction of the Ti-rich precipitates affects severely the evolution of the recrystallization textures.The rolled V-4Cr-4Ti-0.3Y sheet has a similar macro rolling texture with V-4Cr-4Ti.But it has a more homogeneous micro rolling texture and recrystallization texture due to the reduction of Ti-rich precipitates with nonuniform distribution.(5)Both the yttrium and zirconium addition is beneficial to mitigate the danamic stain aging(DSA).Yttrium addition has no obvious effect on the tensile strength of V-4Cr-4Ti alloys at room temperature(RT)and higher temperatures.The tensile strength of V-4Cr-4Ti-0.1Y specimens is smaller than that of V-4Cr-4Ti due to the reduction of impurities content and Ti-rich precipitates.While the tensile strength of V-4Cr-4Ti-0.3Y specimens is slightly larger than that of V-4Cr-4Ti because of the formation of the finer yttrium-enriched precipitates.According to the values of the tensile strength(including RT and high temperatures),the order is like this:V-4Cr-4Ti-0.3Y<V-4Cr-4Ti-0.3Y-xZr(x=0.2,0.4)<V-4Cr-4Ti-xZr(x=0.2,0.4).Among these alloys,V-4Cr-4Ti-0.4Zr specimens have the highest tensile strength mainly attributed to the formation of finer ZrOx precipitates with largest number density.The tensile strength of all the specimens decreases with testing temperature increasing.Embrittlement phenomenna caused by oxygen absorption from environment at high temperatues become more obvious with temperature increasing,which can be mitigated by yttrium and zircondium addition.It is noting that the best properties resistance to embrittlement caused by oxygen uptakes are observed in V-4Cr-4Ti alloys with Zr additions.