Inconel 617镍基高温合金（简称617合金）被广泛视为（超）高温气冷堆中间换热器的首选结构材料，其将在高温（850–1000 °C）、低应力（3–8 MPa）以及潜在腐蚀性气氛（含杂质的氦气/氮-氦混合气体）下服役数十年之久。因此，掌握617合金在长期服役过程中的蠕变断裂规律与失效机制对保证中间换热器可靠运行具有重要意义。本论文对617合金开展了10组不同温度与应力组合条件下的高温蠕变试验，针对其蠕变性能与断裂模式、微观组织演变规律与断裂机制、内氮化环境腐蚀行为与相应微观机理等方面进行了研究。本文研究内容与所得结论主要如下：（1）分析了617合金的宏观蠕变性能与断裂行为，探明了典型的微观组织特征与断裂模式。结果表明，617合金在不同温度与应力条件下蠕变断裂后形成了三种微观组织特征（动态再结晶、蠕变孔洞与析出相粗化），并分别引发了三种断裂模式（颈缩式韧性断裂、沿晶断裂与脆性断裂）。根据断裂模式与其相应的试验条件，建立了617合金的温度-应力-断裂模式图。发现了当应力水平降低至一定程度，脆性断裂成为主导模式后，Wilshire方程蠕变寿命外推曲线将出现转折。（2）研究了617合金在蠕变过程中的微观组织变化，阐述了不同断裂模式下的微观组织演变规律以及相应的失效机理。结果表明，当应力较高时（850 °C/52.5 MPa/60 MPa/80 MPa、950 °C/38 MPa以及1000 °C/30 MPa），位错快速增殖并重排形成高度有序的亚结构，动态再结晶由此发生，导致试样产生明显塑性变形，最终引发颈缩式韧性断裂。当应力降低（950 °C/27 MPa与1000 °C/23 MPa），沿晶碳化物大量溶解，晶界由于缺乏钉扎作用而发生弱化，孔洞与裂纹即在弱化的晶界处萌生并扩展，最终引发沿晶断裂。当应力进一步降低、蠕变时间随之延长（950 °C/19 MPa/24 MPa以及1000 °C/16 MPa），大量服役气氛中的氮元素扩散进入至试样内部，引发内氮化现象，脆性氮化物大面积析出，最终造成脆性断裂。（3）探究了617合金在蠕变过程中的内氮化行为，讨论了内氮化对蠕变性能演化的重要影响，阐述了各类氮化物析出顺序与形成机理，探究了内氮化的影响因素。结果表明，内氮化造成了材料变形加速与过早失效。蠕变过程中，四种氮化物TiN、AlN、π相与Cr2N依次析出，并由于晶体学特性与热力学稳定性的差异在不同区域形核并呈现不同形貌。发现了蠕变是加剧内氮化的重要因素，所形成的蠕变孔洞与裂纹为氮气进入试样内部提供了连续通道，缩短了其扩散时间与距离。

Inconel 617, a Ni-based superalloy (abbreviated as alloy 617), has been considered the preferred candidate structural material for the intermediate heat exchanger of the (very) high-temperature gas-cooled reactor. Alloy 617 is anticipated to serve for several decades under low stresses of 3–8 MPa, at high temperatures of 850–1000 °C and in a potentially corrosive atmosphere (helium/nitrogen-helium mixture containing impurities). Therefore, it is essential to fully understand the creep rupture laws and fundamental failure mechanisms of alloy 617 during long-term service to guarantee the reliable operation of the intermediate heat exchanger. In this present dissertation, creep tests under 10 different combinations of temperature and stress were conducted on alloy 617. The creep behaviors, rupture modes, microstructure evolution behaviors, and failure mechanisms, as well as internal nitridation and corresponding formation mechanisms, were investigated. The main contents and conclusions of this dissertation are as follows.(1) The creep properties and fracture behaviors of alloy 617 were analyzed. The typical microstructure characteristics and fracture modes were ascertained. The results indicated that three typical microstructure characteristics were formed in alloy 617 after creep rupture under different temperature-stress combinations. The microstructure characteristics were dynamic recrystallization, creep voids and precipitate coarsening, respectively causing three rupture modes including necking ductile rupture, intergranular rupture and brittle rupture. A temperature-stress-rupture mode map of alloy 617 was established according to fracture modes and their corresponding test conditions. It was found that when the stress levels were low and brittle rupture became the dominant mode, a turning point appeared on the creep-life extrapolation curve based on the Wilshire equation.(2) The microstructure changes of alloy 617 during creep were investigated. The microstructure evolution behaviors and corresponding failure mechanisms under different rupture modes were described. The results indicated that when the stress was high (850 °C/52.5 MPa/60 MPa/80 MPa, 950 °C/38 MPa and 1000 °C/30 MPa), dislocations multiplied rapidly and rearranged to form highly-ordered substructures, resulting in dynamic recrystallization. Significant plastic deformation was thus produced, finally causing the necking ductile rupture. With the stress decreased (950 °C/27 MPa and 1000 °C/23 MPa), intergranular carbides were greatly dissolved, which made grain boundaries free of pinning and weakened. Therefore, voids and cracks were initiated and propagated along those weakened grain boundaries, leading to intergranular rupture. As the stress was further reduced and the creep duration was extended (950 °C/19 MPa/24 MPa and 1000 °C/16 MPa), plentiful nitrogen diffused into the alloy from the external atmosphere, causing internal nitridation. Brittle nitrides precipitated in a significant population, eventually triggering brittle rupture.(3) The internal nitridation behaviors of alloy 617 during creep were investigated. The influences of internal nitridation on the evolution of creep properties were discussed. The precipitation sequence and formation mechanisms of various nitrides were described. The influence factors of internal nitridation were investigated. The results indicated that internal nitridation caused accelerated deformation and premature failure. During creep, four types of nitrides, TiN, AlN, π phase and Cr2N precipitated sequentially. These nitrides were nucleated at different locations and exhibited different morphologies due to their different thermodynamic stabilities and crystallographic characteristics. It was found that creep was a significant contributor to aggravating internal nitridation. The formed creep voids and cracks allowed a continuous path for nitrogen to enter and greatly shortened diffusion times and distances.