基于γ′相孪晶化镍基单晶高温合金高温低应力蠕变行为及机制

The improvement of creep life is one of core objectives in designing Ni-based single crystal superalloys. The research key is to control and adjust the configuration of γ′ phases and the interaction between γ′ phases and dislocations. In recent years, nano-twin boundaries were used to improve the strength of metal materials, which was paid a great attention by researchers. However, the strengthening by means of nano-twin boundaries for Ni-based single crystal superalloys has not yet been reported. In this project, this strengthening concept is introduced, together with the creep mechanism of Ni-based single crystal superalloys, a series of model Ni-based superalloys with different stacking fault energies are designed by the theoretical calculation. Then, some high density stacking faults on {111} plane are first formed in γ′ phases and they are further transformed into the annealing nano-twins by the process controlling. The interaction between nano-twin boundaries and dislocations under the high temperature and low stress creep condition is clarified, and the contribution of nano-twin boundaries for the creep life is also quantitated. Finally, an aim that the creep life is increased can be achieved by controlling and adjusting the configuration of nano-twins inside γ′ phases and the reaction mode of dislocations to block the dislocation further cutting for the γ' phases. Simultaneously, a corresponding structural model based on the twin boundary strengthening of γ′ phases is established. These research results have an important scientific significance in understanding the creep behavior and the corresponding reaction mechanism between γ′ phases and dislocations based on the twinned γ′ phases in Ni-based single crystal superalloys under high temperature and low stress condition. Further, this project will also provide a theoretical foundation and a new idea in designing new Ni-based single crystal superalloys.

镍基单晶高温合金蠕变寿命的提高是其设计的核心目标之一，控制和调整γ′相形态及其与位错之间的交互作用是研究的关键。近年来，借助纳米孪晶界强化金属材料的研究思路得到重视，但目前尚未见到在镍基单晶高温合金中成功应用的报道。本项目基于镍基单晶高温合金的蠕变机制，引入纳米孪晶界强化概念，以层错能为切入点，通过理论计算、模型合金设计和工艺调控，在γ′相内部预形成高密度{111}面孤立堆垛层错，并转变为退火纳米孪晶组态；阐明高温低应力蠕变条件下γ′相内部纳米孪晶界与位错之间的交互作用，量化其对合金蠕变寿命的贡献；通过调控γ′相内部纳米孪晶组态及位错分解方式，阻止位错进一步切割γ′相，最终实现合金蠕变寿命提高的目的，建立γ′相孪晶界强化结构模型。研究结果对认识和理解γ′相孪晶化镍基单晶高温合金高温低应力蠕变行为及其位错反应机制具有重要的科学意义，能为新型镍基单晶高温合金设计提供理论依据和新思路。

镍基单晶高温合金具有优异的高温力学性能，是目前航空发动机高压涡轮叶片的必选材料。通过调控层错能来提高高温合金的性能是一种可行的方式。本项目采用第一性原理计算的方法研究了各合金元素对γ和γ′两相层错能的影响，研究表明Co、Cr元素能够同时降低两相的层错能。通过3D-APT技术研究了各合金元素在镍基单晶高温合金的分配行为，表明Co元素在γ′相中分配较多；在此基础上设计出3种不同层错能的单晶高温合金，通过层错退火时湮灭过程的研究，揭示了层错形成机制为：a/2[10-1]+a/2[01-1]→a/3[11-2]+SF+a/6 [11-2]；研究了两种低层错能高温合金的高温蠕变性能，在1100℃/150MPa条件下，T6和T13合金蠕变断裂寿命分别为301h和215h；T13合金高温蠕变性能高于国外同代次单晶EPM-102和MC-NG，与TMS-138合金相当；T6合金高温蠕变性能优于TMS-138，略低于TMS-196合金，基本达到国外同代次水平。进一步讨论了高温蠕变性能差异的内在本质，发现位错交滑移和攀移是影响高温蠕变性能的主要机制；T6合金高温蠕变性能优于T13合金的主要原因有3个：T6合金基体相和γ′相等效扩散均小于T13合金；T6合金基体相层错能小于T13合金；T6合金高温组织稳定性优于T13合金；最后分析了高温蠕变过程中的组织演化特征：T6合金筏形化速率约为T13合金的40%，筏形化进程受元素扩散控制，筏形化完成时间与最小应变速率出现时间高度一致，筏形化对高温蠕变性能有利；T6合金蠕变初期位错网形成比T13合金缓慢，位错形核和增殖困难是T6合金界面位错网形成受阻的主要原因；研究了γ′相筏化对合金在750ºC/750MPa条件下蠕变性能的影响，揭示了γ′预筏化合金低温高应力蠕变性能降低的原因：γ基体宽度增大，降低了位错运动抗力；γ/γ′两相界面的位错网释放了错配应力，降低了界面强度；γ′相内部存在超位错，降低了γ′相蠕变抗力，最终导致合金蠕变速率提高，蠕变寿命降低。