Research On The Microstructure Of Zr-Sn-Nb-Fe Alloy

Zirconium alloys are generally used as cladding and structural materials in light and heavy water nuclear reactors due to their excellent neutron economy and corrosion resistance, and favorable mechanical properties under irradiation. To meet the requirements of high burn-up and thermal efficiency in nuclear reactors, countries have developed new zirconium alloys. Originated in Zr-Sn and Zr-Nb systems, Zr-Sn-Nb zirconium alloys have been developed so as to meet the requirements of higher fuel burn-up in PWR, incluing the ZIRLO and X5 A of the United States, the E635 of Russia, the NDA of Japan, the HANA-4 of Korea, and the N18 and N36 of China. Furthermore, many researchers focus on the optimization of process in Zr-Sn-Nb zirconium alloys. For instance, some of the studies for M5 and X5 alloys show that homogeneous and fine SPPs could be obtained through the hypothermal process, which is beneficial to improving the combination property of zirconium alloys. Researches show that corrosion resistance, mechanical and physical propertys of zirconium alloys strongly depend on the microstructure and the intrinsic characteristics(crystallographic structure, shape, distribution, size, microchemistries, etc.) of the second phase particles(SPPs). Howere, the alloying elements(Sn and Nb, etc) and processing have direct relationship with the microstructure and the intrinsic characteristics of SPPs. Therefore, to study the effects of alloying elements and process on microstructure and the intrinsic characteristics of SPPs are important ways to acquire zirconium alloys of excellent combination property.In this context, Zr-XSn-YNb-0.3Fe(X=1.2, Y=1; X=0.4, Y=0.65; X=0.4; Y=1) alloys are obtained from changing the content of Sn and Nb in N36(Zr-1Sn-1Nb-0.3Fe) zirconium alloys.The present investigations will focus on the effects of Sn and Nb on microstructure(grain and SPPs) in Zr-XSn-YNb-0.3Fe zirconium alloys, and the influences of the process and thermal treatment upon microstructure of Zr-0.4Sn-1Nb-0.3Fe. The evolution of microstructure and intrinsic characteristics of SPPs during the change of the mechanical processing and composition was investigation with optical microscopy(OM), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). Through the above researches, a large amount of experimental datas obtained, which provides experimental bases and theoretical support for the optimization of mechanical processing and composition.The investigations of the effects of Sn and Nb on microstructure(grain and SPPs) in Zr-XSn-YNb-0.3Fe show that:(1) The effects of Sn and Nb on grain size are not obvious, and grain size is 12 level in all zirconium alloys. With the increasing of Sn, the degree of recrystallization increases.(2) Sn is independent on the formation of SPPs, but the segregation of Sn exists in the SPPs to some extent, while Nb directly involves in the formation of SPPs. The SPPs are mainly composed of Zr(Nb, Fe)2 with a HCP structure in all alloys. When Nb content is 1%, and Sn content increases from 0.4% to 1.2%, the number of β-Nb particles and the areal density of SPPs decrease, while the Nb/Fe ratio of Zr-Nb-Fe particles and the average diameter of SPPs increase. When Sn content is 0.4%, and Nb content increases from 0.65% to 1%, the β-Nb particles are observed, the average diameter of SPPs decreases, and the Nb/Fe ratio of Zr-Nb-Fe particles and the areal density of SPPs increase.Zr-0.4Sn-1Nb-0.3Fe zirconium alloys was first treated by β-quenching at 1050℃ for 0.5 h, and then followed by a series of processes: hot rolling at 620, 650, 700℃ without aging and hot rolling at 700℃ after aging with 10 and 50 hours at 480℃, cold rolling, intermediate annealing at 600℃ for 2 h and final annealing at 580℃ for 2 h, respectively. The investigation of the microstructure(grain and SPPs) during mechanical process in Zr-0.4Sn-1Nb-0.3Fe zirconium alloys show that:(1) The perfect recrystallization grain, homogeneous and fine SPPs could be obtained through the hypothermal process(620℃), and the average diameter of SPPs is within 100 nm, which is beneficial to improving the comprehensivn properties of zirconium alloys.(2) Homogeneous and fine secondary phases distribute dispersively at 620℃, and most of these SPPs are Zr(Nb, Fe)2 with a C14(HCP) structure. When hot rolling at 650 ℃, β-Nb particles are observed besides Zr(Nb,Fe)2 precipitates, and the distribution of SPPs is bunching and clustering. As the hot rolling temperature increases to 700℃, the SPPs present in a wide variety of sizes and non-uniform distribution, and the number of β-Nb particles increases. Aging prior to hot rolling is helpful to improve the uniform distribution of SPPs, as the aging time extends, the average diameter of SPPs becomes larger. With the increasing of hot rolling temperature, the average Nb/Fe ratio of Zr-Nb-Fe particles increases firstly and then decreases at the range of 620℃ to 700℃, aging processing leads to the further reduce of Nb/Fe ratio.The investigation of Zr-Sn-Nb-Fe zirconium alloys by studying literature and analyzing the precipitation and evolution mechanism of SPPs during the processing and heat treatment process show that:(1) The influence mechanism of intrinsic characteristics of SPPs mainly includes the following: temperature determinism, theory of atomic ratio, dynamic theory of SPPs, grain boundary migration and element diffusion theory.(2) The SPPs of Zr-Sn-Nb-Fe zirconium alloys are composed of Zr(Nb, Fe)2 with a HCP or FCC structure, Zr(Fe, Cr)2 with a HCP or FCC structure, Zr(Nb, Fe, Cr)2 with a HCP structure and β-Nb at different process and thermal treatment.(3) The optimized process could be obtained in Zr-Sn-Nb zirconium alloys: firstly, quenched in beta phase; secondly, hot working in alpha phase; thirdly intermediate annealed in the thermal indifference zone of the alpha phase; fourthly, dealt with large deformation cold working process; finally, according to the specific components, application and so on, annealed between 400℃ and 600℃.