镍基合金蠕变裂纹扩展的氧化促进效应及拘束效应

The objective of this project is to achieve a thorough understanding of creep crack growth at elevated temperatures in a Ni-based alloy. Creep crack growth tests and in-situ observations will be carried out under different oxygen partial pressures, temperatures and applied stress levels using compact tensile specimens with different thicknesses in order to investigate the effects of oxygen partial pressure, diffusion coefficient, deformation mechanism and multiaxial stress on creep crack growth rates. This will elucidate the interaction between creep and environment induced damages in the alloy under different testing conditions. Analyzing the effects of creep fracture constraints both in air and in vacuum, the control stress parameter of the creep damage and the oxidation damage will be determined. The critical strain will be correlated with multiaxial stress by using digital speckle correlation and crystal finite element analysis. Based on the combined irreversible thermodynamics and Fick's law, the model of stress assisted grain boundary oxidation will be described. Furthermore, being inspired by the theory of the fracture process zone and mechanical properties degraded by the effect of oxygen diffusion ahead of the crack tip, a multiaxial chemical-mechanics damage model will be developed. This model will incorporate the influence of oxidation on critical strain, when the creep damage equation is used. This comprehensive model will enable more accurate predictions of oxidation assisted creep crack growth rate in a Ni-based alloy, which will lead to better design and assessment of major national equipments.

针对高温环境下镍基合金的蠕变裂纹扩展问题，采用不同厚度紧凑拉伸试样进行不同氧分压、温度、应力水平下的蠕变裂纹扩展试验及原位观测，研究氧分压、扩散系数、变形机制、多轴应力对裂纹扩展性能的影响，揭示不同参数条件下蠕变损伤与环境损伤间的交互作用；通过真空与空气环境下的蠕变断裂拘束效应研究，确定蠕变损伤及氧化损伤的控制应力参量；采用数字散斑及多晶体有限元分析，确定临界应变与三轴应力度间的关系；基于不可逆热力学与Fick定律相结合，建立应力加速下的晶界扩散氧化模型；把蠕变断裂过程区的思想与氧元素扩散导致的裂尖材料性能变化相结合，将裂尖氧化对临界应变的影响纳入蠕变损伤方程中，建立化学-力学的多轴损伤模型。可望实现镍基合金氧化促进蠕变裂纹扩展速率的准确预测，为我国重大装备的设计、评价提供理论支持。

围绕镍基合金蠕变强度高但韧性差这一特点，开展了镍基合金高温裂纹扩展氧化促进效应及拘束效应研究，主要工作结果如下：.1) 设计并制造了氧分压可控的蠕变裂纹扩展性能测试设备、高通量蠕变及蠕变裂纹扩展试验设备，为镍基合金高温裂纹扩展试验开展奠定基础。.2) 制定了镍基合金高温裂纹扩展门槛值试验测定方法及长时数据预测方法。得到了Inconel 783合金、NiCr20TiAl合金在不同热处理条件、温度、氧分压下的裂纹扩展速率及裂纹扩展门槛值。.3) 基于损伤容限设计理念，提出了高温紧固件防断裂设计方法。.4) 对GH3625及Inconel 718合金开展了不同几何试样的裂纹扩展试验，揭示了几何尺寸对蠕变裂纹扩展速率的影响与断裂机制相关。.5) 提出了虑及断裂机制的蠕变拘束参数αc，建立了统一的纳入拘束效应的蠕变裂纹扩展速率预测方程。.6) 建立了蠕变-氧化裂尖损伤模型，实现了镍基合金高温裂纹扩展有限元模拟。.在本项目资助下，共发表论文7篇，申请专利6项，参加国际会议5次，培养研究生1名，在读研究生4名。部分成果取得应用，获得了2018年上海市技术发明一等奖（排名11）。