退火孪晶界对Fe-Ni基沉淀强化奥氏体合金氢致沿晶开裂的影响

Fe-Ni based austenitic alloys strengthened by ordered coherent γ′ phase are one kind of high-strength hydrogen resistant alloys. These alloys would lose about half of the ductility after hydrogen charging and are not suitable for the application in the high-pressure hydrogen environment. Cracking is mainly induced by the segregation of hydrogen atoms at the grain boundary (GB) due to the blocking of dislocations by GB. Suppression of hydrogen-induced intergranular cracking by using grain boundary engineering is proposed and the key point is to select the optimized twin boundaries (TBs) with a higher hydrogen-induced cracking resistance. However, few studies have focused on the hydrogen-induced cracking behavior of TB. It was found that the occurrence of TB cracking was related to the stacking fault energy in the previous study and we will explore the effect of TBs on the hydrogen-induced intergranular cracking of Fe-Ni based alloys in the present research. Firstly, Fe-Ni based alloys with different stacking fault energies are obtained through composition design to investigate the effect of stacking fault energy on the hydrogen-induced cracking behaviors of general GBs and TBs so as to select the alloy with optimized TBs. Secondly, the density of TBs is controlled by thermo-mechanical treatment and then the hydrogen distribution and GB crack propogation of alloys with different TBs densities are studied to reveal the effect of TBs on the hydrogen-induced intergranular cracking and its relationship with hydrogen resistance properties. The study will be useful to clarify the hydrogen-induced cracking mechanisms of general GBs and TBs and also has a great significance on the improvement of hydrogen resistance properties by using grain boundary engineering in theory and application.

Fe-Ni基奥氏体合金是一种高强度沉淀强化抗氢合金，饱和充氢后合金的塑性损减较大，不利于其在高压临氢环境中的应用。晶界阻碍位错运动造成氢在晶界处富集，是引起此类合金氢致沿晶开裂的主要原因。晶界工程理论为抑制氢致沿晶开裂提供了新的途径，如何获得抗氢致开裂的优化孪晶界是实现晶界工程的关键，而有关孪晶界氢致开裂行为的研究鲜有报道。本项目在前期发现层错能与氢致孪晶界开裂有关的基础上，拟围绕如下两个方面研究退火孪晶界对氢致沿晶开裂的影响：首先，通过成分设计控制合金的层错能，研究层错能对普通晶界和孪晶界氢致开裂行为的影响，筛选出具有优化孪晶界的合金成分；其次，采用形变热处理方法控制孪晶界的数量，研究孪晶界数量对氢分布及沿晶裂纹扩展行为的影响，揭示孪晶界数量对氢致沿晶开裂的影响及与抗氢脆性能的关系。本研究不仅有助于澄清普通晶界与孪晶界的氢致开裂机制，还为晶界工程技术改善材料的抗氢脆性能提供理论基础。

针对沉淀强化抗氢合金在饱和充氢后的氢致沿晶开裂现象，本项目在澄清晶界与孪晶界氢致开裂机制的基础上，确定具有优化孪晶界的合金成分。在此基础上，探索退火孪晶界对氢致沿晶开裂的影响。饱和充氢后，晶界与孪晶界的开裂行为研究发现：晶界会阻碍位错运动，位错将携带的氢原子倾倒于晶界上，诱发沿晶开裂；与晶界不同，孪晶界的开裂行为具有可控性，开裂倾向取决于合金层错能的大小。γ'相数量的调控，可提高基体的层错能，使运动的位错顺利通过孪晶界，从而抑制孪晶界的氢致开裂；Ti、Al含量的匹配决定了γ'相的成分，会提高反向畴界能，加剧了位错的平面化滑移，促进沿滑移带的开裂。界面数量的增加，使得位错源与界面之间的距离减小，有利于减少界面处位错塞积的数量，位错输送至界面的氢浓度降低，抑制了氢致沿晶开裂，改善合金的氢脆敏感性。