Microstructures And Properties Of Zirconium-702 And Zircaloy-4 Irradiated By High Current Pulsed Electron Beam

A “Nadezhda-2” type High Current Pulsed Electron Beam(HCPEB) was used for irradiating on the surface of Zirconium-702 and Zircaloy-4. Surface morphologies and microstructures evolution of specimens before and after irradiation were characterized in detail by using X-ray diffraction(XRD), optical microscope(OM), scanning electron microscope(SEM), and transmission electron microscope(TEM). The surface micro-hardness of the two materials and the wear rate of the Zircaloy-4 before and after HCPEB irradiation were tested. In addition, the corrosion resistance properties of the two materials before and after HCPEB irradiation were analyzed by EIS and electric polarization curves, then the corrosion resistance mechanism induced by HCPEB was emphasized on studying.After the surface irradiation of Zirconium-702 by HCPEB, the cross-sectional micrographs of the specimens present three distinct layers: melted layer(the upper), the solid-state phase transformation layer(the middle) and the heat-affected zone(the bottom). In the melted layer, martensitic phase transformation was induced, besides, volcano-like craters, micropores, and ultrafine structures(including ultrafine martensite structure and ultrafine grain structure) were obtained. TEM observations suggested that high density dislocations and deformation twins were formed after HCPEB irradiation. With the increase of pulses, micro-hardness of the specimens was increased from the initial 178 Hv to 254 Hv. The corrosion resistance results showed that, after HCPEB irradiation, all the specimens had better corrosion resistance in 1 mol/L HNO3 solution compared to the initial one, among which the 5-pulsed sample owned the best corrosion resistance. Ultrafine structures, martensitic phase transformation, surface porosities, dislocations and deformation twins are believed to be the dominant reasons for the improvement of the hardness and corrosion resistance.After the surface irradiation of Zircaloy-4 by HCPEB, surface morphologies and microstructures of specimens were similar with those observed on the irradiated surfaces of Zirconium-702, but have some differences: For commercially pure Zirconium-702, the number of volcano-like craters was very low on 5-pulsed specimen surface, after multiple pulses, the crater density increased slightly. But for Zircaloy-4, the density of volcano-like craters decreased gradually with the increase of the pulses. Besides, for Zirconium-702, the micropores were just founded on 5-pulsed specimen surface, but for Zircaloy-4, micropores were also founded on the multi-pulsed specimen surfaces. Meanwhile, nanocrystaline structures were also observed. In addition, the observation results of XRD, SEM and TEM all showed that Zr(Fe,Cr)2 second-phase particles in the specimen surface were dissolved into the matrix after HCPEB irradiation. The results of micro-hardness and wear rate tests showed that, with the increase of pulses, both micro-hardness and wear resistance of the specimens increased gradually. The corrosion resistance results showed that, after HCPEB irradiation, all the specimens had better corrosion resistance in 0.1 mol/L LiOH solution compared to the initial one, among which 15-pulsed specimen owned the best corrosion resistance. Apart from the reasons mentioned in the above paragraph, the dissolution of the second-phase particles had played a vital role in the improvement of the properties.