【作者】 吴文平；

【导师】 汪越胜； 郭雅芳； Dietmar Gross； Ralf Mueller；

【作者基本信息】 北京交通大学 ， 固体力学， 2010， 博士

【摘要】 镍基单晶高温合金因具有优异的蠕变、疲劳、氧化及腐蚀抗力等综合性能,而被广泛应用于航空发动机和工业燃气轮机的叶片材料。它有一个引人注意的特征：在高温施加应力的条件下,γ’沉淀相会发生定向粗化形成筏状。这种筏化行为直接影响了合金的蠕变疲劳寿命,而且断裂面通常沿筏化方向发生。所以定向粗化行为一直是镍基单晶高温合金强化机制的研究重点。镍基单晶高温合金是一种两相复合材料,由软的γ基体相(Ni相)和均匀镶嵌在其中的立方状γ’沉淀相(Ni3Al相)组成。其中γ’沉淀相是重要的强化相,其体积分数约为70%。γ/γ’相界面的微观结构以及在载荷和温度条件下的演化决定了其力学性能。本文采用不同尺度的模型和方法研究了镍基单晶高温合金的定向粗化行为,分析了γ/γ’相界面微观结构及其在载荷和温度作用下的演化,探讨了界面微观结构演化与γ’相定向粗化以及合金宏观力学性能的关系,主要内容和结论包括：(1)采用分子动力学方法模拟了镍基单晶合金界面位错网结构,分析了加温和加载条件的影响。模拟结果表明：(100)、(110)和(111)三个不同相界面分别存在三种不同形式的位错网,分别为正方形、矩形和三角形位错网。它们在拉伸载荷和温度的作用下慢慢由规则变得不规则,直至损伤或消失。不同形式的位错网有不同形式和不同程度的变形和损伤。位错网的损伤过程与γ’相定向粗化以及合金的界面力学性能有密切的关系。(2)考虑到镍基单晶高温合金服役过程中复杂的受力形式和失效机制,采用分子动力学方法模拟了不同相界面在不同加载方式(拉伸、剪切和拉剪组合)作用下位错网结构的演化。结果表明：不同加载方式下,不同相界面位错网的损伤过程和损伤难易不同。(100)相界面形成的正方形位错网最不容易损伤和消失,它最能有效地强化界面。这种正方形位错网结构的损伤主要由[100]方向的轴向载荷引起,是镍基单晶高温合金在服役过程中主要由于承受[100]轴向离心载荷而引起失效的原因。(3)基于原子模拟,分析了应变率和温度对界面位错网结构演化和变形机制的影响及其与γ’相定向粗化行为以及宏观力学性能的关系。结果表明：不同应变率和温度下,位错网的损伤程度不同,γ’筏化的难易也不同。只有在相对较低的应变率下,γ’筏化才会发生,而且温度越高,筏化越容易。此外,不同应变率和温度下,位错运动和变形机制不同,从而引起界面宏观力学性能也不同。界面的屈服强度和最大拉伸强度随应变率的增加而增加,随温度的增加而降低；其塑性和延展性能随应变率的增加而减弱,随温度的增加而增加。(4)采用Eshelby等效夹杂原理和Mori-Tanaka平均场方法,计算了不同形状γ’颗粒的总势能,分析了具有不同弹性常数差的两类镍基单晶高温合金γ’相定向粗化行为及其形状稳定性。计算中考虑了基体塑性应变对沉淀形状稳定性的影响,此塑性应变由价’界面位错网形成而引起。结果表明：基体塑性变形对γ’沉淀形状的稳定性以及筏化行为具有重要作用。在外应力作用下,如果基于简单的弹性分析对γ’相筏化和形状稳定性进行预测,其预测结果不总是与实验结论一致；当引入基体塑性应变时,对两类合金都能给出与实验结论一致的准确预测。(5)建立了一个基于Eshelby等效夹杂理论的三相细观力学模型,该模型中将γ基体相看作两相夹杂,而将γ’沉淀相看作基体相,通过计算不同基体通道中的Mises应力、弹性应变能密度和静水压力,预测了不同应力轴取向下的γ’相定向粗化行为。结果表明：当外加应力沿[001]和[110]方向作用时,将发生定向粗化,其粗化方向取决于晶格错配的符号和外加应力的性质；当外加应力沿[111]方向时,不出现定向粗化。(6)采用有限元方法计算镍基单晶高温合金γ基体和γ’沉淀相的Mises应力和弹性应变能密度分布,并结合元素扩散性质探讨了γ’相定向粗化的驱动力。分析了温度和外载对定向粗化行为的影响,给出了启动蠕变位错的临界外加载荷。结果表明：外加应力改变了γ和γ’内Mises应力和弹性应变能密度的分布,其最大值出现在界面附近的基体通道处,该处的蠕变位错和筏化也优先启动。而且随温度的增加,Mises应力和应变能密度增加,蠕变和筏化也越容易。更多还原

【Abstract】 Ni-based single crystal superalloys are widely used as advanced aircraft turbine blade materials for their excellent creep resistance behavior. They exhibit a remarkable character that theγcubic particles will transform into flat shapes (which are named rafts) under the combined influence of stresses and temperatures. This rafting behavior directly affects the creep fatigue life of Ni-based superalloys. The fracture surface is usually along the direction of rafting. Therefore, the directional coarsening mechanism is a key rule of precipitation hardening in Ni-based single crystal alloys.Ni-based single crystal superalloys are two-phase materials. The material is strengthened by a high volume fraction of hard cubicalγprecipitates (Ni3Al phase) embedded coherently in a softerγmatrix (Ni phase), where the volume fraction ofγphase may reach as high as 70%. The microstructure of theγ/γinterface as well as its evolution under external loading and temperature determines the mechanical properties of Ni-based superalloys. In this thesis, we study directional coarsening behavior of Ni-based superalloys as well as the interface microstructure and its evolution under the influence of temperatures and stresses, by using models and methods in different scales. The relationship between the microstructure evolution and the macroscopic mechanical behaviors is discussed. The main contents and results include:(1) The interface microstructure and its evolution under the influence of tensile loading and temperature are simulated by molecular dynamics. From the simulation we find that three dislocation network patterns, namely square, rectangle and triangle appear on the (100), (110) and (111) phase interfaces respectively. The three patterns of dislocation network change from regular to irregular or disappear under the influence of tensile loading and temperature. Different patterns of dislocation network show different degrees and patterns of damage. Theγrafts and mechanical properties are closely related to the damage process of the dislocation networks.(2) Taking into account the complex stress and failure mechanisms in-service of Ni-based single crystal superalloys, the structural evolution of dislocation networks are simulated under different ways of loading (tensile, shear, combined tensile and shear) by molecular dynamics. The results show that the damage processes of the dislocation networks are different for three different phase interfaces under different ways of loading. The square dislocation network at (100) phase interface is the most difficult to damage and disappear, so it is the most stable and strengthen interface effectively. The damage of this square dislocation network is mainly caused by [100] axial loading, which is the reason for the failure of Ni-based single crystal superalloys.(3) Based on atomistic simulation, the effects of strain rate and temperature on the structural evolution of interface dislocation networks and deformation mechanisms are investigated. The results indicate that the dislocation networks show different degrees of damage at different strain rates and temperatures, and the difficulty level ofγrafting is also different. Theγrafting occurs only in the smaller strain rate, and become easier at higher temperature. Moreover, dislocation motion and deformation mechanisms are different at different strain rates and temperatures, which lead to the change of maro-mechanical properties at interfaces. The yield strength and ultimate tensile strength increases with the increase of strain rate, while decreases with the increase of the temperature. The plasticity and ductility decreases with the increase of strain rate, while increases with the increase of the temperature.(4) The Eshelby’s inclusion theory and Mori-Tanaka’s mean field method are used to evaluate the elastic energy caused by a change in the shape ofγprecipitates. The shape stability and directional coarsening ofγprecipitate in Ni-based superalloys with two types of elastic constant are investigated. Moreover, the plastic strain caused by the formation of dislocation networks at theγ-γinterface is taken into account. The effect of matrix plastic strain on the shape stability of y precipitates is considered. The results suggest that the plastic strain plays an important role on the shape stability and rafting behavior. The predictions based on a simple elastic analysis do not always agree with the experimental results regarding shape stability and rafting behavior. However, if the plastic matrix strain is introduced, the results are perfectly consistent with experimental observations for alloys with two types of elastic constant.(5) A new three-phase micromechanical model based on the Eshelby’s equivalent inclusion method has been developed to study the directional coarsening behavior in Ni-based single crystal superalloys. In this model, theγmatrix as two-phase inclusion, while theγprecipitate as the matrix phase. The von Mises stress, elastic strain energy density and hydrostatic pressure in different matrix channels are calculated to predict the directional coarsening behavior of different stress axis orientations. The calculated results indicate that the directional coarsening ofγprecipitate occurs when the external stress is applied along the [001] and [110] directions, respectively. The rafting direction depends on the sign of the misfit and the type of the external stress. However, no rafts occurs when the external stress is applied along the [111] direction.(6) The finite element method has been applied to calculate the Mises stress and strain energy density distributions of theγandγphases in Ni-based single crystal superalloys. The analysis of directional coarsening behavior ofγparticles and driving force are performed based on the element diffusion behavior. The effects of the temperature and external loading on the directional coarsening behavior are considered. The critical external loading for start-up creep dislocations is obtained. The results show that the application of an external stress leads to differential levels of Mises stress and strain energy density, and the largest value of the Mises stress and strain energy density appears at the corners of the matrix near the interface. Creep dislocations penetrate preferentially into the most highly stressed matrix channels where theγphase rafting is also enlarged. The Mises stress and strain energy density of theγandγphases increase with the temperature increasing, thus the creep and rafting becomes easier at a higher temperature.更多还原