Mechanism Of Dynamic Strain Aging On Tensile Property Of 15-15Ti Cladding Tube

Lead-bismuth cooled reactor has become the main development strategy of the fourth generation reactors. As the first barrier of reactor core, cladding tubes are designed to keep the integrity of fuel elements, to resist the overflow of fission products and to segregate the coolant and fuel. Therefore, good mechanical property is necessary for the fuel cladding material.15-15Ti stainless steel has been selected as the candidate cladding material for China LEAd-based Reactor (CLEAR), because of its excellent temperature strength, good corrosion resistance and acceptable radiation performance. According to the design conditions of CLEAR, the temperature range of 15-15Ti servicing in CLEAR are 300～500℃, and dynamic strain aging (DSA) would occur for austenitic stainless steel in the temperature range. DSA will change the mechanical behavior of the materials. DSA could enhance the strength property. However, the plastic property would be reduced by DSA. Plastic reduction would cause the breakage of the materials. Therefore, the effect of DSA on the mechanical properties of 15-15Ti cladding tubes must be studied under the operation conditions of nuclear reactors.This thesis focused on prior cold worked 15-15Ti cladding tube. Uniaxial tensile test was performed on 15-15Ti cladding tubes. Tensile behavior of 15-15Ti tubes was investigated from room temperature to 600 ℃, at strain rate of 10-4s-1, by analyzing the tensile test data, microstructure after tensile test. The results showed that,1) in the range of 300～500℃, the tensile strength of cold worked 15-15Ti cladding tubes decreased slowly with increasing temperature, and the deviations of stress were about 15MPa.2) When the temperature reached 600 ℃, the tensile strength decreased evidently with increasing temperature.3) In the range of 300～600 ℃, the yield strength of the cold worked 15-15Ti tubes also decreased slowly with increasing temperature. The elongation of 15-15Ti was maximum at room temperature, while the minimum appeared in 300℃. In the range of 300～500℃, the total elongation increased slightly with increasing temperature. When the temperature reached 600 ℃, the total elongation declined evidently. While in the range of 300～600 ℃, the fracture elongation zone maintaining at 14 to 15% range, did not change significantly with increasing temperature.The tensile behaviour of 15-15Ti cladding tube was committed for further consideration. The effect of DSA on the tensile strength, the yield strength and elongation were analyzed. The results showed that, serrated yielding phenomenon occurred within the temperature range from 400 ℃ to 600 ℃. It is indicated that DSA phenomenon was accompanied in this temperature region. The higher temperature regime for serrated flow could arise from the effect of prior cold work. At 400 ℃-500℃, the A serration type was observed. While, the A+B serration type was observed at 600 ℃. Normal Portevin-Le Chatelier effect was deduced from the decreasing critical strain with increasing temperature. In the temperature regime of DSA, DSA slowed down the trend of the decrease of strength with increasing temperature, but DSA in 15-15Ti does not accompany with a plateau of yield stress.The microstructure of 15-15Ti tubes after tensile test was analyzed by using SEM and EBSD technology. The results showed that the proportion of twinning was relatively higher at room temperature while a gradual degradation of twinning was observed with increasing temperature.60°/<111> twin boundary was formed, and twins turned the grain orientation type from<111>//AD to <001>//AD. However, a fraction of twinning increased again at 500 and 600 ℃ due to the blocking effect of DSA on dislocation movements. To summarize, DSA effect played an important role on hindering the dislocation movements, which would enhance the strength properties and reduce the plastic properties.The thesis studied the effect of DSA on the mechanical properties of 15-15Ti cladding tubes. The results could provide the partial theoretical basis and experimental data for the optimization of cladding material for lead-bismuth cooled reactors.