| Zirconium alloys have been widely used as nuclear fuel claddings due to their low thermal neutron absorption cross-section,remarkable corrosion resistance and excellent mechanical properties.Claddings are corroded by the coolant during normal operation in the reactors.The hydrogen generated by corrosion reaction will be absorbed by the claddings,which may seriously degrade the mechanical properties of the claddings,and thus endanger the reactors' normal operation.Previous researches mainly focused on the effect of hydrogen on the macroscopic mechanical properties of zirconium alloys,while the effect of hydrogen content on the microscopic deformation behavior and deformation mechanism of zirconium alloys are short of in-depth systematic research.In this paper,the hydrogen absorption behavior of zirconium alloys was firstly studied.Furthermore,the effect of hydrogen contents on the mechanical behavior and microscopic deformation mechanism of zirconium alloys were also investigated.To be specific,the electrolytic hydrogen charging behavior of zirconium alloys,the effect of hydrogen contents on the tensile deformation behavior of zirconium alloy sheets,the effect of hydrogen contents on the circumferential deformation behavior of zirconium alloy tubes and the effect of hydrogen contents on the fatigue crack initiation and propagation behavior of zirconium alloy sheets were discussed.The effects of electrolytic hydrogen charging time and annealing process on the hydrogen absorption amount,and hydride type and distribution of SZA-4 zirconium alloy tube were investigated.The results showed that the amount of hydrogen absorption increased with the extension of hydrogen charging time,there was a parabola relationship between the hydrogen absorption amount and hydrogen charging time.After electrolytic hydrogen charging,hydride layers formed on the surface of the samples.The thickness of the hydride layers increased with the extension of hydrogen charging time.The hydride layer composed of ?-ZrH1.66 in the initial stage of hydrogen charging and the content of ?-ZrHi1.66 increased with the hydrogen charging time.When the charging time reached 24 hours,?-ZrH1.801 appeared in the hydride layer.After 400?/6 h annealing,the thickness of the hydride layer increased,and large platelet-like hydrides precipitated.In this process,the ?-ZrH1.801 phase disappeared,and ?-ZrH1.66 transformed into ?-ZrH15 in hydride layers.After 400?/96 h annealing,all the hydride layers disappeared,fine platelet-like hydrides uniformly precipitated in the samples.In this part,the microstructure evolution model of the zirconium alloy tube samples during electrolytic hydrogen charging was established,and the annealing process for removing the hydride layers on the zirconium alloy tube were also obtained.The effect of hydrogen contents on the tensile deformation behavior of Zircaloy-4 sheets was investigated by in-situ tensile tests at room temperature(RT)and 300?.The microscopic deformation mechanism of the samples with different hydrogen contents was analyzed by transmission electron microscope(TEM).The results showed that at RT,with the increase of hydrogen content,the tensile strength increased from 407 MPa to 508 MPa,and the reduction of area decreased from 51%to 7.9%.At 300?,with the increase of hydrogen content,the tensile strength increased from 246 MPa to 318 MPa,and the reduction of area decreased from 66%to 52.9%.The in-situ SEM observations showed that with the increase of hydrogen content,the number of slip lines decreased,the degree of grains deformation was reduced,and the number of cracks increased.Observation of the fracture surfaces indicated that an obvious ductile-brittle transition occurred with the increase of hydrogen content.TEM results showed that,the dislocations were piled up around the hydrides,which caused stress concentration and crack initiation.When the hydrogen content increased to a high level,these hydrides would intersect or collide with each other,causing a significant mechanical property change,i.e.,ductile-brittle transition.At 300?,with the increase of tensile deformation,both the un-hydrided and hydrided samples happened severe plastic deformation,but the cracks initiated not only around the hydrides,but also at the grain boundaries.The fracture surfaces were characterized by ductile fracture feature until the hydrogen content reached 1300 ppm.The hydrides were twisted or kinked and exhibited a good deformation ability.These results showed that hydrogen content had greater effect on the ductility of zirconium alloys at RT than that at 300?.The amount of hydride was the main reason for the influence on the ductility of zirconium alloys at RT.The effect of hydrogen contents on the circumferential deformation behavior of zirconium alloy ring-shaped samples was evaluated by the advanced expansion due to compression(A-EDC)tests and ring tension tests.The failure mechanism of the hydrided samples was investigated by in-situ SEM observation.During A-EDC deformation,the fracture load and ductility of the sample decreased with hydrogen content,especially when hydrogen content exceeded 300 ppm.Microscopic observation showed that cracks distributed along the axial direction appeared on the outer surfaces of the hydrided samples.With the increase of hydrogen content,the number and size of the cracks increased,and the fracture surfaces of the samples gradually changed from ductile feature to brittle feature.In addition,it was found that circumferential hydrides could be deformed together with the matrix,while the radial hydrides were prone to crack.As deformation increased,the number and size of cracks increased,eventually leading to radial fracture.These studies showed that radial hydrides play a major role in the fracture of zirconium alloy ring-shaped sample during A-EDC deformation(ring tension deformation),and a fracture model based on the ring tension deformation of hydrided zirconium alloy ring-shaped sample was established.The effect of hydrogen contents on fatigue crack initiation and propagation behavior of Zircaloy-4 sheets were investigated by in-situ SEM observation at RT and 300?.At RT,with the increase of hydrogen content,the fatigue lifetime increased at first and then decreased(the cut-off point was at about 200 ppm hydrogen),while the fatigue crack growth rate increased monotonously.At 300?,with the increase of hydrogen content,the fatigue lifetime of the sample exhibited the same variations as those at RT.However,the fatigue crack growth rate was less affected by the hydrogen content.This indicated that hydrogen content had different effects on the fatigue lifetime and fatigue crack growth rate.The microscopic mechanism analysis showed that hydrides had a significant effect on the fatigue crack propagation.With the number of hydrides increasing,the crack propagation path increased,and the fatigue crack growth rate accelerated.The hydrogen content had a dual effect on the fatigue lifetime of zirconium alloys.With the increase of hydrogen content,the strength of the sample increased,which improved the fatigue property.However,with the increase of hydrogen content,the number of hydrides increased,the fatigue properties were reduced.In this case,with the increase of hydrogen content,the fatigue lifetime increased at first and then decreased,the cut-off point was associated with the number of hydrides.In addition,the hydrogen content had a significant effect on the fatigue crack initiation mechanism.For the un-hydrided sample,the fatigue crack initiated at the subsurface.With the increase of hydrogen content,the fatigue crack trended to initiate at the surface of the sample,which indicated that hydrogen content had a significant effect on the fatigue crack initiation lifetime of the samples. |
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