The Study On Electrolytic Hydrogenation Of The N18 Zircaloy And The Mechanical Behaviors Of The N18 Zircaloy Containing Hydrogen

Zirconium alloys are employed extensively in reactors, because of their low absorption cross-section for thermal neutrons (which is lower 30 times than that of iron), excellent mechanical properties and physical properties, well processing properties and good compatibility with UO₂. However, the zirconium alloy is one kind of materials that easy to absorb hydrogen, will absorb a large amount of hydrogen during the course of their service, but hydrogen solubility in zirconium alloys is limited, so the superfluous hydrogen will precipitate as hydride flakes, which are bad for the mechanical properties of the zircaloy matrix. In the development of zirconium alloys, the first generation are represented by Zr-2 and Zr-4, the second generation is marked by optimized Zr-4, and the third generation includes ZIRLO, M5, E635 and so on. The N18 is one of the zirconium alloys developed by our country (N18, N36, NZ2, etc.).In this paper, the method of electrolytic hydrogenation is applied to charge hydrogen for the specimens of the N18 zircaloy. The influences of hydrogenation time and the current density on the hydrogen content and the distribution of hydrides are studied. The microhardness of the N18 zircaloy with hydrides is measured. The in situ tensile tests are used, and the effects of hydrides which on fracture behavior are studied during the tensile processes. The fracture surfaces are observed. The results show that:The hydrogen content of the N18 zircaloy can be increased by both adding the hydrogenation time or increasing the current density. When the hydrogenation time is short, the hydrogen content is low, and hydrides sparsely distribute in the samples. When the hydrogenation time is long, the hydrogen content is high, and single hydrides aggregate to massive hydride aggregations, and the longer the hydrogenation time sustained, the severer the hydrides aggregated.The microhardness of the N18 zircaloy increases with the hydrogen content of the zircaloy.The tensile strength of the N18 zircaloy increases with the hydrogen content, while the plasticity decreases with the hydrogen content of the zircaloy.During the tensile process, single hydrides can be deformed plastically together with the matrix, and the fracture of single hydrides isn't observed. But it will affect the deformation behavior of the alloys when single hydrides aggregate to massive hydride aggregations. Hydride is case for the cracking of the N18 zircaloy, because the cracks of all the samples initiate at the massive hydride aggregations, then little cracks are connected reciprocally, and propagate to the matrix. There are secondary cracks on the fracture surface, and the morphology and distribution of secondary cracks also change with different hydrogen content as that of hydrides. When the hydrogen content is lower, the hydrides and the secondary cracks are smaller, sparsely distribute on the surface of the samples and the fractures. When the hydrogen content is higher, the hydrides aggregate to massive hydride aggregations, and the secondary cracks on the fracture surface are larger, and the morphology and distribution of hydrides are similar to that of the massive hydrides aggregations on the surface of the samples. Hydrides and secondary cracks on the surface of the samples and the fractures are strips, and their propagation directions are perpendicular to one another, that means the hydrides in the samples are slice shapes with some thickness.