Study On Low-Cycle Fatigue For N18 Alloys

Zirconium alloys are employed extensively in light water cooled reactors (LWR) as the cladding materials of fuel elements and other structural materials because of the low capture cross-section for thermal neutrons, good resistance to water-side corrosion at elevated temperature, adequate mechanical properties and high thermal conductivity. The power fluctuation and flow-induced vibration make the fuel assembly and other components deformed cyclically under the LWR operating condition, even failed in the severe cases. In order to develop competition ability of nuclear electricity and reduce the cost of nuclear electricity, it must increase burnup of the pellets and heighten temperature of refrigerant. Simultaneity, it must adjust the PH of refrigerant. So many countries manufacture new fuel assembly and new Zirconium alloys. For instance, ZIRLO alloys in America, E635 alloys in Russia, M4, M5 alloys in France, NDA alloys in Japan, N18, N36 alloys inChina. Therefore, it becomes in immediate need of the detailed knowledge about Low-Cycle Fatigue for N18 Alloys.In the dissertation, the effect of temperature(room temperature, 200℃ 300℃ , 400℃) and hydrogen(300μg/g and 450μg/g) on the LCF behavior of N18 alloy and the cyclic deformation behavior for N18 alloy, N18 alloy have been investigated systematically using fully-reversed tension-compression loading under strain control (Rε =εmin/εmax =-1).The effect of temperature and hydrogen on the internal stresses during cyclic deformation, derived from the analysis of hysteresis loop shapes using the Cortrell scheme, has been studies. On this bases, the mechanism of temperature and hydrogen effect on LCF behavior for N18 alloy has been thoroughly discussed combined with the low cycle fatigue lifetime curves, cyclic stress-strain curves and analysis of fracture surfaces , the main results of the investigation is as follow.(1) Hydrided N18 alloy and N18 alloys which at different temperature of the LCF life decreases with increasing the plastic strain range △εp and obeys Coffin-Manson relation: Nfβ△εp = C .At the low strain range, the LCF life of N18 alloy at room temperature is longer than that of one at high temperature, with the increase of the strain range, the LCF life of N18 alloy at room temperature is close with that of one at high temperature. Because Dynamic strain aging (DSA) appears in the process of cyclic deformation for N18 alloy at 400 ℃ .The reason that Zirconium hydrides decrease the LCF life of N18 alloy is Zirconium hydrides deteriorate plastic of N18 alloy.(2) At 400℃, there are sawtooth hysteresis loop .The LCF behavior of N18 alloy at room temperature and 400℃ show that alloy has better LCF property than at 400 ℃, and cyclic hardening at 400 ℃ which could be attributed to the dynamic strain aging (DSA) effects, this phenomenon is "Portevin-LeChateliereffect". With increasing mounting temperature, the intensity becomes low. At room temperature, N18 alloy exhibits cyclic hardening at high strain amplitude, while exhibiting cyclic softening at low strain amplitude. Zirconium hydrides influence cyclic deformation behavior of N18 alloy. There exists a threshold value of hydrogen concentration beyond which the effect of hydrides on cyclic hardening or cyclic softening is negligible(3) The saturated stress and the back stress during cyclic deformation increase with increasing the plastic strain amplitude, which can be expressedas a power relation σs=Ksεnsp and a logarithmic relation σib - Kb lnεp + Cb respectively for both Hydrided N18 alloy and N18 alloys which under different temperature. From low strain amplitude to high strain amplitude, the change range of friction stress is exiguity, but the change range of back stress is evidence. At room temperature, zirconium hydrides influence friction stress of N18 alloy, but don't influence back stress.(4)At high temperature, the crack propagation zone characterized is composed of tearing zone and smooth zone. At room temperature, the crack propagation is composed of fatigue striations and Second-cracks. At room temperature, the fatigue cracks always initiate from one site, but at high temperature, the fatigue cracks always initiate from much sites. At high temperature, the fracture is characterized by dimples and thin second-cracks, furthermore the fracture is main characterized by second-crack at 400 ℃ .there are few fatigue striations on fracture surfaces of N18 alloy, at 400℃ .because N18 alloy is oxygenation at high temperature ,the fatigue striations have been destroyed. The fatigue cracks of hydrided N18 alloy always initiate at the root part of hourglass-like sample. The zone where the crack initiates has no characteristic in the view field of scanning electron microscopy (SEM). The crack propagation zone characterized by intragranular fracture is composed oftearing zone and smooth zone. The tearing zone is characterized by pseudo-cleavage. The fatigue striations or the tyre patterns appear in the smooth zone. Second-cracks exist in the crack propagation zone, which is almost perpendicular to the main crack. With increasing hydrogen concentration, there are more second-cracks and the fracture surfaces of N18 alloy show more brittle manner.