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、抗氧化性和损伤容忍度而被应用于涡轮盘。在使用中，它们在相对高的应力下经历极端的温度和循环载荷条件，因此它们的疲劳寿命在很大程度上受裂纹萌生和扩展的早期阶段的控制。不同尺寸对镍基高温合金裂纹萌生和扩展的影响通常与滑移特性的变化有关。损伤的累积本质上受滑移非均质性程度的影响，这既影响起裂过程，也影响生长过程。因此，温度、环境和与尺寸相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总体疲劳寿命。大多数在役涡轮盘合金具有双模态或三模态尺寸分布，其中初级尺寸（或存在）与固溶热处理的持续时间和温度（例如超溶或亚溶）有关，而二级和三级尺寸与老化温度和冷却速度有关。这些不同大小的种群对滑动特性的影响和相对平衡是一个持续争论的问题。在这项工作中，对RR1000合金进行了非商业热处理，以产生两种不同的单峰尺寸分布。这包括超溶剂热处理，然后是单次时效和缓慢冷却，分别产生细尺寸和粗尺寸。在屈服应力为90%和110%、载荷比为0.1、频率为20Hz的正弦波形条件下，采用三点弯曲法对精粗两种材料的抛光平弯棒进行了短疲劳裂纹试验。采用不间断和间断试验的复制方法研究裂纹演化，并与南安普敦大学先前的工作进行了比较。普通弯杆试验结果表明，具有细变体的RR1000比具有粗变体的RR1000具有稍长的疲劳寿命，这与室温下较慢的I阶段裂纹扩展和更多的平面滑移有关。室温下，裂纹主要在孔隙或滑移带处萌生，细裂纹的扩展速率略高于粗裂纹。通过对南安普顿大学提取的火柴棒进行x射线CT扫描，并通过检测微拉伸样品的裂纹萌生和扩展，提供了三维评估。利用扫描得到的一系列二维薄片，对含粗合金RR1000的内部微观组织进行了分析，包括孔隙率分布和裂纹行为。通过结合3D x射线计算机断层扫描和传统2D观察，可以更深入地了解该模型涡轮盘合金在一系列服务相关应用中的疲劳裂纹萌生和扩展机制。

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.