Effect Of Cooling Rate And Aging Temperature On The Second Phase Precipitation And Corrosion Behavior Of Zr-Sn-Nb Alloys

Multiple characterization and analysis techniques including transmission electron microscopy(TEM),transimission Kikuchi diffraction(TKD),three-dimensional atomic probe(3DAP)and in-situ transmission electron microscope(In-situ TEM)were jointly employed to investigate Zr-1.0Sn-1.0Nb-0.3Fe(N1)and Zr-1.0Sn-0.3Nb-0.3Fe(N2)alloys.Systematic work was conducted on the effects of cooling rate on microstructure and precipitation behavior of second phase particles(SPPs)in N1 and N2 alloys.The effects of aging temperature on the growth behavior and phase structure evolution of SPPs were further analyzed.The effect of supersaturated solid solution Nb content and Nb-containing SPPs structure on the corrosion behavior of Zr-Sn-Nb alloy was discussed by taking aging samples for 10 h as an example.N1 and N2 alloy specimens cooled by water(WC),liquid nitrogen(LNC),air(AC)and furnace(FC)from?-phase present martensitic,Basket-weave Widmanst(?)tten,Parallel-plate Widmanst(?)tten and Lenticular structures,respectively.Their structural characteristics are mainly controlled.Nb can stabilize?and?+?phases(Fe and Nb are less than 10 at.%).Samples of N1 alloy after WC,LNC and AC precipitate?or?+?phases.Low Nb alloy(N2)precipitates?+?phase only under WC condition,and Zr₂Fe and Zr₃Fe under LNC and AC condition.Slow cooling rate promotes the precipitation of?-Zr(Nb content exceeding 20 at.%)and Zr(Nb,Fe)₂ phases in N1 and N2 alloys.Meanwhile,Zr₂Fe phases are also precipitated in N2 alloys.In addition,a large number of{10(?)1}compression twins were found in the WC sample of N1 alloy,and the BPy and Py B step structures of twin interface and twin tip were characterized by TKD and HRTEM for the first time.Under the effect of intergranular thermal residual stresses(ITRS),{10(?)2}tensile twins were formed in the FC samples of N1 and N2 alloys.The precipitation behavior of SPPs during aging is effected by cooling rate,aging temperature and Nb content.In the aging process of N1 alloy at 580°C(below monotectoid reaction temperature),Zr₃Fe is transition phase in WC sample,Zr(Nb,Fe)₂and(Zr,Nb)₂Fe are precipitated later.Zr₂Fe is transformed into Zr(Nb,Fe)₂ in LNC and AC samples.The decomposition of?-Zr occurs slowly in FC sample until 100 h aging is not completed,and SPPs are still?-Zr and Zr(Nb,Fe)₂ in FC sample.Monotectoid reaction and precipitation of?-Zr(Nb content is about 13 at.%)occur in four samples of N1 alloy with different cooling rates during ageing at 650°C(above monotectoid reaction temperature).SPPs precipitated from WC,LNC and AC samples are ?-Zr,Zr(Nb,Fe)₂ and(Zr,Nb)₂Fe phases.Monotectoid reaction occur within 10 minutes in LNC and AC samples,but is greatly delayed in WC sample.SPPs precipitated from FC sample are still?-Zr and Zr(Nb,Fe)₂,and the composition of ?-Zr has changed.Zr₃Fe phase was precipitated in N2 alloy during aging at 580°C,then Zr(Nb,Fe)₂ and(Zr,Nb)₂Fe phases were precipitated in WC sample.In the case of Zr₂Fe and Zr₃Fe phases precipitation first,Zr(Nb,Fe)₂ and(Zr,Nb)₂Fe phases continued to precipitate in LNC and AC samples.At the later stage of aging,?-Zr in FC sample was completely decomposed,and only Zr(Nb,Fe)₂ phases existed in samples.The SPPs precipitation law of WC samples is the same as that of 580°C during ageing process at 650°C.Zr₂Fe disappears in LNC and AC samples at late ageing stage,and only Zr₃Fe and Zr(Nb,Fe)₂phase remain.Monotectoid reaction occurs in FC samples at early ageing stage,which changes the composition of ?-Zr,and ?-Zr is completely decomposes at late ageing stage,Zr₂Fe transforms into Zr₃Fe.SPPs in FC sample is Zr₃Fe,Zr(Nb,Fe)₂ and(Zr,Nb)₂Fe phases.In-situ TEM was used to observe the precipitation behavior of SPPs during aging of N1 alloy under WC condition.It was found that?+?was extremely unstable and transformed into?'at the initial stage of aging,then Zr₃Fe was precipitated.WC sample of N2 alloy in the early aging stage was analyzed by 3DAP.Fe segregates at the grain boundary,and Zr₃Fe,Zr(Nb,Fe)₂ and Zr₃(Fe,Si)precipitated near the grain boundary.In the early stage of aging,Nb and Fe elements precipitate at the twin interface,forming a Zr-Nb-Fe phase which is completely coherent with the matrix of?-Zr on both sides of the twin boundary;in the late stage of aging,the twin interface is swallowed up,and the coherence relationship disappears.Based on the study of the influence of cooling rate and aging temperature on precipitation behavior and phase transition characteristics determined by Nb content,the corrosion resistance of N1 and N2 alloys in 400°C/10.3 MPa superheated steam environment was further tested.The results showed that the corrosion resistance of N2alloys(low Nb)was significantly better than that of N1 alloys(high Nb)under the same heat treatment conditions.Cooling rate and aging temperature can change the content of supersaturated solid solution Nb element in?-Zr matrix of N1 alloy,which is the main reason for the significant difference in corrosion properties of samples in different states of N1 alloy.Compared with Zr(Nb,Fe)₂,the precipitation of(Zr,Nb)₂Fe is more beneficial to corrosion performance.The corrosion properties of low Nb(N2)alloys with non-supersaturated solution mainly depend on the type of SPPs in the sample,and secondly on the size of SPPs.In the corrosion process,the Fe element in Zr(Nb,Fe)₂diffuses into matrix,resulting in its amorphous gradually.