Data rich imaging approaches assessing early fatigue crack growth mechanisms in Ni base superalloys

镍基高温合金因其优异的高温强度、抗氧化性和损伤容限而被应用于涡轮盘中。在服役中，它们在相对较高的应力下经历极端的温度和循环载荷条件，因此它们的大部分疲劳寿命受裂纹萌生和扩展的早期阶段控制。镍基高温合金中不同尺寸对裂纹萌生和扩展的影响往往与滑移特性的变化有关。损伤的累积本质上受到滑移不均匀程度的影响，这既可以影响起始过程，也可以影响生长过程。因此，温度、环境和与尺寸有关的滑移特性的综合影响通过对起裂过程和裂纹扩展机制的不同影响来控制整体疲劳寿命。大多数使用中的涡轮盘合金具有双峰或三峰‘尺寸分布，其中一次’尺寸(或存在)与固溶热处理的持续时间和温度(例如超级或亚固溶度)有关，而二次‘和三次’尺寸与时效温度和冷却速度有关。这些不同大小的种群对滑移性状的影响和相对平衡是一个持续争论的问题。在本工作中，对RR1000合金进行了非商业性热处理，以获得两种不同的单峰尺寸分布。这包括超级Solvus热处理，然后是单一时效和缓慢冷却，分别产生细小和粗大的尺寸。采用三点弯曲法，在屈服应力为90%和110%、载荷比为0.1、频率为20 Hz的正弦波形下，对粗细材料的抛光平弯杆件进行了室温疲劳短裂纹试验。用复制法进行了连续和间断试验，以研究裂纹的演化，并与南安普顿大学以前的工作进行了比较。平弯试验表明，RR1000的疲劳寿命略长于RR1000的疲劳寿命，这与RR1000的第一阶段裂纹扩展较慢，室温下的平面滑移有关。疲劳裂纹在室温下主要萌生在气孔或滑移带上，细裂纹扩展速率略高于粗裂纹扩展速率。通过对南安普顿大学提取的火柴进行X射线、CT和微拉伸试件中裂纹萌生和扩展的检查，提供了三维评估。通过扫描获得的一系列二维薄片，研究了RR1000粗大内部组织结构，包括气孔率分布和裂纹行为。通过结合三维X射线计算机层析成像和传统的二维观察，对该模型涡轮盘合金在一系列服务相关应用下的疲劳裂纹萌生和扩展行为有了更深入的了解。

Nickel base superalloys are used in turbine disc applications because of their excellent high temperature strength, oxidation resistance and damage tolerance. In service they experience extremes of temperature and cyclic loading conditions at relatively high stresses and so much of their fatigue life is controlled by the early stages of crack initiation and growth. The effect of varying' size on crack initiation and growth in nickel based superalloys is often linked to variation in slip character. The accumulation of damage is intrinsically affected by the degree of slip heterogeneity and this can affect both initiation and growth processes. Thus the combined effects of temperature, environment and slip character linked to' size control overall fatigue life through varying effects on initiation processes and crack growth mechanisms. Most in-service turbine disc alloys have a bi-modal or tri-modal' size distribution, here primary ' size (or presence) is linked to the duration and temperature of the solution heat treatment (e.g. super or sub-solvus) whilst secondary' and tertiary ' size are linked to the ageing temperature and the cooling rates. The effect and relative balance of these different ' size populations on slip character is a matter of continuing debate. In this work a non-commercial heat treatment has been used on RR1000 alloy to produce two different unimodal ' size distributions . This comprised a super-solvus heat treatment followed by a single age and slow cool to produce fine and coarse' sizes, respectively. Short fatigue crack tests have been conducted on polished plain bend bar of fine and coarse' materials by three point bending method under 90% and 110% of yield stress, load ratio of 0.1 and a sine waveform of 20Hz frequency at room temperature. Uninterrupted and interrupted tests with replication ethod to investigate crack evolution were carried out and compared with the previous work done by the University of Southampton. Plain bend bar tests showed that the RR1000 with fine variant had a slightly longer fatigue life than RR1000 with coarse ', linked to slower Stage I crack propagation linked to more planar slip at room temperature. Cracks mostly initiated at pores or slip bands at room temperature and fatigue crack growth rate of fine ' is slightly higher that of coarse'. 3D evaluation has been provided by X-ray CT of extracted match-sticks at University of Southampton and by inspecting crack initiation and growth in micro-tensile samples. Internal microstructure analysis of RR1000 with coarse' including porosity distribution and crack behavior has been investigated from a series of 2D slices obtained by scanning. This provided a deeper understanding of the mechanisms of fatigue crack nitiation and propagation behaviour in this model turbine disc alloy under a range of service relevant applications by the combination of 3D X-ray computed tomography and traditional 2D observations.