【作者】 李聪；

【导师】 沈保罗；

【作者基本信息】 四川大学 ， 材料学， 2003， 博士

【摘要】 由于锆的热中子吸收截面小，并具有优异的耐高温水腐蚀性能、良好的综合力学性能和理想的热导率，因此，锆合金被用作核电站水冷动力堆核燃料元件的包壳材料和堆芯的其它结构材料。反应堆运行时，堆功率的波动和水冷却介质的流动使燃料组件及其它构件经受不同形式的循环变形，在极端情况下容易出现破损。因此，要保障核电站水冷动力堆的安全运行，就要对锆合金的低周疲劳行为有详尽的了解和认识。特别是当要求研制新型高性能的燃料组件以降低核电成本时，对锆合金低周疲劳行为的研究就显得尤为迫切。 本文采用对称拉压循环变形方法(R_ε=ε_min／ε_max=-1)，系统研究了织构、热处理状态、氢对Zr-4合金低周疲劳性能的影响，Zr-4合金、N18合金和N36合金的循环变形行为，并研究了摩擦应力、背应力随循环周次、塑性应变幅的变化规律。结合疲劳亚结构和疲劳断口的分析结果，深入探讨了影响合金低周疲劳行为的作用机制。 Zr-4合金低周疲劳性能的研究表明，不同冶金状态的Zr-4合金在室温和400℃的低周疲劳寿命（N_f）都随塑性应变范围（Δε_p）的增加而降低，并遵循Coffin-Manson关系N_f^βΔε_p=C。但是，冶金状态不同，合金的抗疲劳性能不同。对于冷加工后经再结晶退火处理的Zr-4合金，轧制方向的低周疲劳寿命比横向要大。随着Δε_p的降低，两个方向的低周疲劳寿命的差别相应增加，这是由于合金中存在织构的缘故。冷加工后经再结晶退火处理的Zr-4合金在β固溶处理后，抗疲劳性能明显降低，这主要是由于β固溶处理降低了合金的Schmid因子；β固溶处理后在α相区的退火对疲劳性能有影响，即500℃×1.5h退火的抗疲劳性能要优于750℃×1.5h退火，这主要与500℃×1.5h退火的合金中沉淀相粒子的数量较少有关。240μg／g的氢要改善Zr-4合金在400℃的抗疲劳性能，其主要原因是α-Zr基体中的固溶氢（～200μg／g）弱化了位错与氧等溶质原子、位错与 摘要 ----------------^<sup><sup><sup><sup><sup><sup><sup><sup><sup><sup><sup><sup><sup><sup><sup>月.................................................，，...._<sub><sub><sub>钉扎点的相互作用，促进了位错的发射，增强了位错的活动性。 用单试样法或多试样法，研究了错合金在室温和400℃的循环变形。结果表明，无论在室温还是在400℃，循环饱和应力，、随塑性应变幅二，的变化满足乘幕关系cT、一K“二，”‘。在低应变范围，冷加工后经再结晶退火处理的错合金的循环应力一应变曲线位于单调拉伸曲线的下方，表现为循环软化;在高应变范围则位于单调拉伸曲线的上方，表现为循环硬化。对于室温下恒应变幅的循环变形，在应变幅较低时表现为循环软化直至疲劳破坏;在应变幅较高时，循环变形的初期为循环硬化，随后是循环软化直至疲劳破坏。在400℃，合金要发生动态应变时效，其原因是氧等溶质原子对可动位错的钉扎作用而导致合金的摩擦应力增大。含240林留g氢的zr一4合金在400℃的循环变形行为和无氢zr-4合金是一样的，但是，含氢Zr一4合金的动态应变时效不象无氢Zr一4合金那样明显，因为固溶氢卜200林留g)弱化了位错与氧等溶质原子、与钉扎点的相互作用。 Masing特性的研究表明，错合金具有Masing特性的充分必要条件为:对于任一应变范围的循环变形，在某一给定的应变值，其对应的背应力和摩擦应力都是相同的。经p固溶处理后的错合金在室温下极易具有Masing特性。 用Cottrell方法对滞后回线进行了分析。结果表明，随着循环变形的进行，合金中的晶粒要转动，使晶粒位于具有更高Sch刀11d因子的晶体学取向位置，摩擦应力相应降低。晶粒转动的难易程度与织构和晶粒形态。织构越强，晶粒转动越容易，摩擦应力的降低也就越明显。对于经p固溶处理而得到的板条状晶粒组织，由于a晶粒团的晶界犬牙交错，晶粒不易转动，摩擦应力只是略微降低。Zr-4合金在400℃时的摩擦应力要比室温时的低;由于动态应变时效，摩擦应力。、会随应变幅的增加而缓慢增长;增加合金中沉淀相粒子的数量或a一Zr基体中合金元素的含量要增加摩擦应力;a一Zr基体中的固溶氢要降低Zr一4合金在400℃时的摩擦应力，因为固溶氢弱化了位错与氧等溶质原子、位错与钉扎点的相互作用。晶粒之间变形的不协调是引起背应力的主要原因;背应力。。随塑性应变幅:，的变化满足对数关系，，一K占hi:，+吼。在塑性应变幅二，‘恒定时，织构越强，背应力，，越低;晶粒越细，背应力J。越大。固溶氢要增加Zr-4合金在400℃时的背应力，因为位错与固溶氢之间存在长程的弹性相互作用。错合金在循环变形时，应力响应都是非对称的，压缩时的流变应力可总是大于拉伸时的流变应力司，造成这种差别的主要原因是合金中的晶间热应力。四川大学博士学位论文:镌合金的低周疲劳行为研究口日‘沪尸产 用透射电子显微镜研究了疲劳试样的亚结构。结果表明，在循环变形过程 中，只有部分晶粒发生塑性变形，晶粒之间的塑性变形程度各不相同。随着应 变幅的增加，晶粒的塑性变形程度相应增加，发生塑性变形的晶粒所占的更多还原

【Abstract】 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. It is therefore necessary to have an accurate knowledge of low-cycle fatigue (LCF) behavior demonstrated by zirconium alloys in order to run the reactor safely, and it becomes in immediate need of the detailed knowledge when the new fuel assembly in request is being developed to reduce the cost of nuclear electricity.In this dissertation, the effect of texture, heat-treatment, and hydrogen on the LCF behavior of Zircaloy-4 and the cyclic deformation behavior for Zircaloy-4, N18 alloy and N36 alloy have been investigated systematically using fully-reversed tension-compression loading under strain control (R =min/max =-1), while the evolvement of the friction and back stresses versus the number of repeated working cycles and the plastic strain amplitude has been studied, and the thorough discussioncombined with the analysis result of fatigue sub-structure, friction and back stresses, and fatigue fracture has been given to the mechanism underlying the LCF behavior of alloys.The investigation for the LCF behavior of Zircaloy-4 with different metallurgical conditions at room temperature and 400 indicates that the LCF life(Nf) always decreases with increasing the range of plastic strain p and obeys the Coffin-Manson relation Nf sp = C. However, different metallurgical conditions of the alloy contribute to different LCF properties. Zircaloy-4 sheet, which was cold-worked followed by recrystallization annealing, exhibits longer LCF life in the rolling direction than that in the transverse direction, and the fact that difference in LCF life between both directions becomes larger as the range of plastic strain becomes lower can be attributed to the texture effect, p-solution treatment deteriorates the alloy’s LCF property because the treatment lowers the average value of alloy’s Schmid factors, and the subsequent annealing-treatment in a-phase range has a impact on the LCF properties, i.e. the subsequent annealing-treatment at 500癈 for 1.5h results in better property than that at 750 for 1.5h, which comes mainly from the fact that the alloy annealled at 500 for 1.5h has lower amount of the precipitate particles than the alloy annealled at 750 for 1.5h. 240 g/g hydrogen improve effectively the alloy’s LCF property because the solute hydrogen in the a-matrix (~200g/g) cheapens the interaction between dislocation and solute atoms, and pinning points, thus leading to speedup of dislocation emission and enhancement of dislocation mobility.The cyclic deformation behavior for Zircaloy-4, N18 alloy and N36 alloy at room temperature and 400 has been investigated in a single-sample test or multi-sample test. The results indicate that the relationship between the saturated stress (,.) and the plastic strain amplitude (sp) both at room temperature and 400 can be expressed by a power law relation s = Ksepns . The alloys, which were cold-worked followed by recrystallization annealing, display cyclic softening in the range of low strain because the cyclic stress-strain curve lies below the monotonic stress-strain curve, and show cyclic hardening in the range of high strainbecause the cyclic stress-strain curve lies above the monotonic stress-strain curve. For the cyclic deformation under a given strain at room temperature, cyclic softening is usually displayed till to failure in the lower range of strain; However, cyclic hardening is displayed in the early stage in the higher range of strain, subsequent cyclic softening in the later stage and till to failure. Dynamic strain ageing (DSA) appears更多还原