Thermally-induced cracking typically occurs during the cooling stage of various manufacturing processes, and is commonly seen in multiphase or the joints of dissimilar materials due to mismatch in their thermo-mechanical properties, such as thermal expansion, elastic-plastic deformation and, in some cases, phase transformation. However, the underlying cracking mechanism associated with local microstructure is still elusive. To improve the mechanistic understanding of thermal cracking, this work uses the diffusion-bonded 9Cr-1Mo steel as an example to study the key microstructural variables, such as interfacial phases, voids, grain boundary migration and crystallographic orientations. Meanwhile, a temperature-dependent crystal plasticity model coupled with a cohesive zone model is developed to provide more insights into the thermal-induced stress distribution at the grain scale. It is found that the stress at the void-free boundary of martensite and ferrite is dominated by shear, and its magnitude is insufficient to nucleate cracks. Whereas voids at phase boundaries can induce significant tensile stress, resulting in cracking at the phase boundaries as well as diffusion-bonded interfaces. Also, the occurrence of interfacial grain boundary migration plays an important role in local stress distribution. These microstructure features and their evolution are experimentally observed and used to verify the developed crystal plasticity models. These findings enhance the understanding of the influence of microstructure features on thermal cracking and provide a guide to designing and fabricating the microstructure with improved thermal crack resistance in various manufacturing processes.

中文翻译：

---

热裂解：通过表征和晶体塑性建模阐明相、空隙和晶粒的影响

热致裂纹通常发生在各种制造工艺的冷却阶段，并且由于热机械性能（例如热膨胀、弹塑性变形）不匹配而常见于多相材料或异种材料的接头中，在某些情况下，热致裂纹通常发生在不同材料的冷却阶段。情况下，相变。然而，与局部微观结构相关的潜在裂纹机制仍然难以捉摸。为了提高对热裂纹机理的理解，本工作以扩散结合 9Cr-1Mo 钢为例，研究关键的微观结构变量，如界面相、空隙、晶界迁移和晶体取向。同时，开发了与温度相关的晶体塑性模型与内聚区模型相结合，以提供对晶粒尺度热致应力分布的更多见解。研究发现马氏体和铁素体无空隙边界处的应力以剪切为主，其大小不足以使裂纹形核。而相界处的空隙会引起显着的拉伸应力，从而导致相界以及扩散结合界面处的裂纹。此外，界面晶界迁移的发生对局部应力分布起着重要作用。这些微观结构特征及其演变通过实验观察并用于验证所开发的晶体塑性模型。这些发现增强了对微观结构特征对热裂影响的理解，并为在各种制造工艺中设计和制造具有改进的抗热裂性的微观结构提供了指导。