Microstructure And Properties Of Laser Surface Modification On Nuclear Zr-1Nb Alloy

With the rapid development of Chinese nuclear power industry, nuclear reactorstructural material becomes more and more important and it takes our attentiongradually. Especially, zirconium alloys are important structural components andcladding materials, which are commonly used in nuclear reactors. In order to improvethe corrosion property of zirconium alloys in actual working conditions, the surface ofthe Zr-1Nb alloy was modified by laser processing technology.Scanning electron microscopy (SEM), transmission electron microscopy (TEM),energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman and othertesting methods were used to characterize microstructure morphology, compositionand phase constituent of zirconium alloy. Thermal expansion instrument, probe typesurface profiler and autoclave were performed to characterize thermal expansioncoefficient, roughness and corrosion resistance.Dense, smooth, flat oxidation films were formed without cracks by means ofmicroarc oxidation (MAO)+Laser to zirconium alloys. Compared to MAO samples,the samples after Hybrid treatment had lower surface roughness and thermalexpansion coefficient. It was found that all the oxidation flms consisted of monoclinicZrO2and tetragonal ZrO2phases with the thickness of6μm. This Hybrid processingreduced m-ZrO2significantly and increased t-ZrO2relatively. Under the sameprocessing parameters, the corrosion resistance firstly increased with increasing laserpulse energy substantially, after reaching a certain value, and the corrosion resistancedeclined. The optimized parameters were500mJ single-pulse power,2400pulsesnumber. Therefore, the lowest weighted gain of the laser Hybrid processing sampleswas about29mg/dm2in autoclave experiments on the conditions of360℃,18.6MPa,0.01mol/L LiOH and94days, which better than Zr-1Nb alloys of43.5mg/dm2.According to experiment results of the Zr-1Nb alloy samples after CO2laserremelting process. The surface microstructure of Zr-1Nb alloy was mainly of wattleα-Zr. Second-phase was dispersed in the matrix in a large number, in whichprecipitated sphericity was Cr0.5Fe1.5Zr, whereas rod-shaped phase was Fe3Zr. Incontrast, The dislocation density of laser remelting samples was relatively high.Matrix phase was α-Zr, and the second-phase was Zr6Fe3O. When laser power rangedfrom420W to540W, the lower the laser power was applied, the higher corrosion resistance was got.ANSYS software was used to simulate the CO2laser remelting process onzirconium alloy plate. After laser remelting zirconium plate was divided into tworegions in which the upper part was melted area, the lower part was hardened zone. Asthe power increased, the melted layer gradually became larger. The effect of rapidheating and cooling in the laser remelting process improved the microstructure of thematerial, thereby affecting its corrosion resistance.