Ni-Fe-Cr基高温合金热膨胀系数的调控机理研究

To save energy and reduce emission, the advanced ultrasupercritical (A-USC) coal power plant technology is being developed by many countries all over the world. Due to the higher operation temperature, some key parts involved in the technology have to be manufactured by superalloys. However, the employment of superalloys in the bran-new field encounters some challenges, for example, the higher thermal expansion coefficient (TEC) makes them difficult to joint ferrite steels. Therefore, reducing the TEC of the alloys becomes the key factor that superalloys can be successfully employed. Based on the above, Ni-Fe-Cr based wrought superalloys were used to explore the effects of chemical composition on the TEC of the alloy. Firstly, the effects of alloying elements (viz. Fe, Cr, Al, Ti, W and Mo) and their interaction on the TEC were investigated; then, the mechanism of alloying elements affecting the TEC was disclosed by examining the close relationship among the alloying elements, microstructures (phases and lattice constant) and TEC; thirdly, a theoretical model for forecasting the TEC of the alloy was established using the interatomic potential function; and finally, based on the foregoing investigations, a new low thermal expansion Ni-Fe-Cr based alloy with outstanding combination properties was developed. This study will develop the TEC theory of superalloys, provide scientific guidance for the composition design of low expansion alloys, and boost the application of superalloys in new wider industrial fields.

为了节能减排，世界各国都在竞相研发700℃超超临界（A-USC）煤电技术。由于工作温度高，该技术的一些关键部件必须使用高温合金。然而，较高的热膨胀系数（TEC），使得它的应用遇到了挑战。降低TEC，成为高温合金能否顺利应用的关键。本项目以此为背景，针对Ni-Fe-Cr基变形高温合金，研究合金元素对TEC的影响及机理。主要内容包括：确定合金元素（Fe、Cr、Al、Ti、W、Mo等）及其交互作用对TEC的影响；探索合金元素、微观组织（相、晶格常数）与TEC的内在关联，揭示合金元素对TEC的作用机理；利用原子间的势函数理论，建立合金TEC的理论预测模型；在以上工作基础上，兼顾力学性能、抗氧化腐蚀性能和微观组织热稳定性，设计综合性能优异的低TEC Ni-Fe-Cr基变形高温合金。本研究将完善高温合金的TEC理论，并将为低TEC合金的成分设计提供科学依据，也将促进高温合金在更广泛工业领域的应用。

本项目针对Ni-Fe-Cr基变形高温合金，重点研究了γ基体元素Fe和Cr对合金热膨胀行为的影响，在线性链模型的基础上提出一种更为准确，高效的计算热膨胀系数（TEC）的新方法；着重研究了γ'相形成元素Al和Ti对合金热膨胀行为的影响，从实验和第一性原理计算两方面分析两元素的作用机理。实验结果表明，Fe和Cr对合金TEC的影响很大，且Fe和Cr之间存在交互作用，能够加强彼此对TEC的增加作用。计算材料的热膨胀行为需要得到晶格振动信息，传统的方法存在诸多缺点，如繁琐耗时，准确率低。本课题仿照布莱克曼线性链模型，从原子间成键角度出发，提出了一种新方法，能够准确的计算不同Fe和Cr含量合金的TEC。根据计算结果，强键（Ni-Ni-Ni、Ni-Fe-Ni和Fe-Fe-Fe）的力常数降低是导致晶体TEC随温度增加的主要原因，弱键(Ni-Cr-Ni和Fe-Cr-Fe)的力常数随温度的增加而上升，实际上起到减小TEC的作用。同时，本课题借助准简谐近似（QHA）和准简谐德拜法（QHD）研究了Al和Ti对γ基体及γ'相热膨胀行为的作用。结果表明，Al和Ti固溶于γ基体增加TEC，参与形成γ′-Ni3(Al,Ti)则降低TEC，两方面共同作用使得合金在600~700 ℃范围内的TEC随Al+Ti总量增加先上升后下降。对于Al/Ti的作用，研究发现，用Ti替换γ′-Ni3(Al,Ti)中的Al原子后，γ'相的TEC显著降低，所以低Al/Ti比的合金具有相对更低的TEC。此外，Fe元素不仅会改变合金的热膨胀行为，也会显著影响其力学性能。Fe含量的增加显著提高了合金的塑性，但由于γ′相体积分数的降低，合金的强度略有下降；Fe元素也会促进TCP相的析出，降低长期时效过程中合金的组织稳定性。通过本项目的研究，明确了主要元素对合金热膨胀行为的影响，对设计综合性能优异的低TEC合金提供了实验指导和理论依据。