【作者】 李海燕；

【导师】 石守稳；

【作者基本信息】 天津大学 ， 动力工程及工程热物理， 2023， 硕士

【摘要】 镍基合金690以其优异的抗晶间腐蚀和晶间应力腐蚀性能,广泛应用于压水堆核电站中。实际服役期间,核电构件会承受高温和周期性载荷,其几何结构的复杂性又会使之承受多轴应力状态,这种严苛环境下的多轴疲劳会严重威胁构件的安全使用。本文基于国产镍基合金690开展了350℃下的高温多轴低周疲劳试验,基于多种微观表征手段与动态应变时效(DSA)定量分析方法,系统的研究了其循环力学响应、变形微结构、裂纹萌生和断裂行为,揭示了其多轴疲劳失效机理,建立了基于应变能的双临界面多轴疲劳寿命预测模型,为其实际工况下材料选型设计、疲劳失效机理分析和疲劳寿命评估提供理论借鉴。基于锯齿数量、最大应力降和平均应力降,实现了对多轴疲劳DSA活性的定量分析。明确了DSA活性与应变幅值、加载方向和加载路径的相关性:随应变幅值增大而增大;压缩加载方向大于拉伸加载方向;随加载路径轴向应变幅值的增大而增大,且受加载路径交滑移的削弱。基于平均应力降这一综合参数,明确了同一等效应变幅值下不同应变加载路径的DSA活性顺序:单轴拉压路径>菱形路径>圆形路径>扭转路径≈比例路径≈方形路径。DSA对位错的钉扎削弱了交滑移的发生,增强了对平面滑移的促进作用。揭示了变形微结构对不同加载路径循环力学响应的影响机理。显著的单滑移平面位错特征提供了稳定的位错增殖速率,是单轴和比例路径下持续循环硬化的原因。非比例加载促使单滑移平面位错向多滑移平面位错转变,更高的位错增殖速率导致了显著的非比例附加强化效应;随着进一步的循环加载,部分多滑移平面位错转变成波状交滑移位错,同时短程有序(SROs)结构逐渐被破坏,位错增殖速率和相互作用减弱,致使循环应力响应有所下降。而非比例路径表现出的强平面滑移位错特征与DSA和SROs的存在有关。提出了氧化辅助的滑移带开裂高温多轴低周疲劳失效机理。微裂纹萌生于驻留滑移带(PSBs)处,扩展中会受到孪晶界、DSA、夹杂物和晶粒取向的影响;高温会致使PSBs分布的基体侵入挤出物优先氧化,加剧损伤的累积。非比例路径激活了大量多滑移开裂,氧化作用会进一步加剧损伤累积,使其寿命大幅下降。基于泊松效应、多滑移开裂机理以及最大剪应变分布特征,提出了一种基于正交双临界面应变能的多轴疲劳寿命预测模型。该模型以剪应变和剪应力最大平面为临界面,以临界面及其正交面上的剪应变能和正应变能作为损伤值。该模型对所有数据有86.67%的预测点在2倍误差带的保守侧,仅略次于FS模型;但对多轴路径的预测效果(100%)强于FS模型(85.71%)。对于316LN不锈钢,由于材料敏感性,本文模型对所有数据有83.33%的预测点在2倍误差带,弱于CXH模型的95.83%和FS模型的100%;但对多轴路径的预测效果(91.67%)与CXH模型(91.67%)和FS模型相当(100%)。更多还原

【Abstract】 Nickel-based alloy 690 is widely used in pressurized water reactor(PWR)nuclear power plants for its excellent resistance to intergranular corrosion and intergranular stress corrosion.During actual service,nuclear power components are subjected to high temperatures and cyclic loads,and the complexity of geometrical structure subjects components to multiaxial stress states,which can seriously threaten the safety of components.In this paper,multiaxial low cycle fatigue tests of nickel-based alloy 690were carried out at 350℃.Based on a series of microstructure observations and quantitative analysis of DSA,the cyclic mechanical response,deformation microstructure,crack initiation and fracture behavior were systematically investigated,which revealed the multiaxial fracture failure mechanism.Then a double critical plane fatigue life prediction method based on strain energy was established.This paper provides theoretical reference for material design,fatigue failure mechanism analysis and fatigue life assessment under actual working conditions.Based on the number of serrations,the maximum stress drop and the average stress drop,quantitative analysis of multiaxial fatigue DSA activity was realized.The correlation between DSA activity and strain amplitude,loading direction and loading path is clarified:it increases with strain amplitude;the compressive loading direction is greater than the tension loading direction;it increases with the axial strain amplitude and is weakened by cross-slip of different loading paths.Based on the comprehensive parameter of average stress drop,the order of DSA activity of different loading paths under the same equivalent strain amplitude is defined:uniaxial tension-compression path>rhombus path>circular path>torsional path≈proportional path≈square path.The pinning of DSA on dislocation weakens the occurrence of cross slip and enhances the promotion of planar slip.The influence mechanism of deformation microstructure on cyclic mechanical response of different loading paths is revealed.The significant single-slip planar dislocation feature provides a stable dislocation proliferation rate,which is responsible for the continuous cyclic hardening under uniaxial and proportional paths.Non-proportional loading promotes the transformation from single-slip planar dislocation to multi-slip planar dislocation,which leads to higher dislocation proliferation rate and significant non-proportional additional strengthening effect.With further cyclic loading,some multi-slip planar dislocations are transformed into wavy cross-slip dislocations,and the short-range ordering structure(SROs)is gradually destroyed.Then the dislocation proliferation rate and interaction are weakened,resulting in a decrease in cyclic stress response.However,the strong planar slip dislocation feature of non-proportional paths is related to the existence of DSA and SROs.The multiaxial low cycle fatigue failure mechanism at evaluated temperature of oxidation-assisted slip band cracking is proposed.Microcracks initiated at the persistent slip bands which also affected by twin grain boundaries,DSA,inclusions and grain orientation.High temperature will make the extrusions and intrusions of matrix distributed along the persistent slip bands preferentially oxidized,which will aggravate the damage accumulation.Large number of multi-slip cracks activated by non-proportional loading path will further aggravate damage accumulation and greatly reduce fatigue life under the effect of oxidation.Based on Poisson effect,multi-slip cracking mechanism and distribution features of maximum shear strain,a multiaxial fatigue life prediction model with orthogonal double critical plane strain energy is proposed.The plane with maximum shear strain and maximum shear stress are taken as the critical plane,and the shear strain energy and normal strain energy on this plane and its orthogonal plane are taken as damage values.And 86.67%of the prediction points based on this model are on the conservative side of the scatter band of 2 times,which is only second to FS model.However,the prediction effect of multiaxial path(100%)is stronger than FS model(85.71%).For316LN stainless steel,due to the material sensitivity,there are 83.33%prediction points of this model falling in the scatter band of 2 times,which is weaker than CXH model(95.83%)and FS model(100%).However,the prediction results(91.67%)for the multiaxial paths is equivalent to CXH model(91.67%)and FS model(100%).更多还原