| Zirconium alloy have excellent mechanical properties, corrosion resistance, and good compatibility with reactor fuel. Therefore it is suitable for nuclear reactor cladding and structural materials, and has been a hot topic of nuclear material. Nowadays countries promote the development of new zirconium alloy by adjusting the composition of alloys as well as improving the process. In this process, type, quantity, size distribution and other characteristics of the precipitates in alloys have significantly changed. These changes have a significant impact on the mechanical properties, high temperature water resistance, creep resistance, anti-radiation properties of the alloys. Therefore, it is important to critically study the feature of precipitates in the zirconium alloy including crystal structure, composition, defects, and growth law. In this work, precipitates of widely used Zr-4 alloy and Zirlo alloy were comprehensively and thoroughly characterised. Precipitates were also melted to further expanding the size of the scope in this study. In addition, a series of experiments also examined the primarily form of zirconium hydride in zirconium alloys. Its internal structure was analyzed combined with its morphology, which drawed some meaningful conclusions.It was found that there are more than one type of second phase in Zr-4 alloy, including C14, C15-type laves phase and Zr-Fe-Ni tetragonal phase. Comparing the orientation relationship and atomic mismatch between matrix and these two laves phase, we hypothesized that the normal stable state of the laves phase is C14 structure. Through the HRTEM analysis of C14 second phase, the long-period structure has been associated with the presence of its internal stacking fault on certain crystal planes. Meanwhile, the effect of a small number of Ni on the stability of Zr2Fe has been confirmed in zirconium alloys.H element has been produced artificially in Zr-4 alloy, and we observed hydride, which was found to be 8 hydrides, of which the major habit plane is (0001}. Mostly δ hydrides and are arranged in parallel in the same grains with needle-like morphology. A small number of hydrides were also found growing the on the plane of{1015}.In addition, we also observed 8 hydride with irregular shapes, and discovered the existence of twinning structure. Combining with twinning symmetrical feature and growing plane deviation, we have raised new model for the growth of hydride.Due to the presence of Nb, different types of precipitates exist in zirlo alloy, mainly C14-type Zr-Fe-Nb and β-Nb. Composition and formation mechanism of Zr-Fe-Nb phase are discussed. The stacking faults exist accompanied by long-period structure with high index plane. The fine spherical β-Nb phases were found unevenly distributed in the grain with coherent phase boundary between a-matrix. Zr-Fe-Nb second phase were melted, from which we found that Nb atoms in the C14 structure will replace Fe location; Fe-rich alloy will form Ti2Ni type second phase and enrichment of Nb in favor of the formation of C14 type second phase. These conclusions provide a reference to the new zirconium alloy design.From the comparison of the second phase in two alloys, it can be found that there both exist laves-type intermetallic compounds.Combining with a series of experiments, we can draw some common features:there are orientation relationship between laves phase and matrix; the nucleation process are related to the sheared crystal, but in the latter part of the grew up process, due to the diffusion rate of different elements, chemical composition will change in the different stages of growth. These findings provide a reference on the selection of alloys preparation process. |
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