亚稳态β钛合金的低周疲劳损伤机理及循环形变晶体力学分析

Systematic work on the fatigue behaviour of metastable-β Ti alloy is meaningful for fundamental understanding the new material for future aircraft applications. Micro-macro mechanism (multi-parameter) analysis, i.e. tracing the surface morphology evolution and dislocation movement in cycling, the dislocation dynamics simulation,is the research goal to further understand the mechanical cyclic deformation response and microcrack behaviour of the material. In the present work, we take metastable-β Ti alloy Ti-5Al-5Mo-5V-3Cr-0.5Fe (Ti-5553), which is a relatively new material for landing gear manufacturing in Boeing-787 and Airbus-A380, as a typical candidate in order to comparatively investigate the cyclic deformation response and the corresponding micro-mechanical mechanism in low cycle fatigue tests. The b.c.c. Ti-5553 with β-annealed treatment and Ti-5553 alloy with bimodal microstructure are used. The relationship between the mechanical deformation response and dislocation configuration in b.c.c. β-annealed Ti-5553 would be explored. The new criterion: relationship between the SFE (stacking fault energy) and e/a (free-electron-to-atom) ratio would be used to explain the strain localized planar slip behaviour in β-annealed treatment, and the progressive observation of surface morphology evolution would be explored to reveal typical planar slip behaviour and early formation of strain localization-induced fatigue microcracks. The relationship between mechanical plastic deformation response and microstructure evolution in TEM investigation in β-α Ti-5553 alloy would be explored. Micro-mechanism model is attempt to bulid, as a bridge of the special macroscopic responses and microstructure heterogeneity in different loading conditions. The activation and participation in the cyclic deformation of different constituents, namely the soft primary α phase, the higher-strength transformed β phase and the fine embedded secondary α precipitates would be investigated at different strain levels and at different stages of cycling. The research outcomes would be expected to set up experimental and theortical bases for the damage tolerance designing for landing gear applications.

本项目采用物理实验和位错动力学模拟手段对亚稳态β钛合金的循环形变特征和疲劳损伤特性进行微观机制研究，探索b.c.c结构β-Ti纯压缩疲劳加载下的循环软/硬化行为以及微观位错组态演变的相关性；以自由电子浓度和层错能为判据探求b.c.c单相钛的滑移机制，并结合合金表面微结构特征探索该材料疲劳微裂纹萌生机制；研究低周循环加载下双态合金的塑性应变响应，探求疲劳形变带的位错组态及其晶体学特征；类比复合材料强度的混合物定律，建立双态钛合金的延性二相塑变模型，为结构件损伤容限设计奠定实验和理论基础。

Ti-5553合金是以VT22为基础研制出的一种新型高强高韧、亚稳态β钛合金，其名义成分为Ti-5Al-5Mo-5V-3Cr-0.5Fe，已取代传统β钛合金Ti-10V-2Fe-3Al用于制造波音787和空客A380的起落架等飞机关键承力部位。全面、系统地认识周期载荷下Ti-5553的的疲劳损伤机制以及循环形变响应对于航空关键结构件的损伤容限设计具有重要指导意义。本论文模拟起落架可变高幅度疲劳载荷的工作条件并针对钛的同素异构转变特性，分别对单相和双态Ti-5553材料的循环形变特性和微观机理，以及疲劳微裂纹萌生等现象进行了较为系统的研究，从微观机制上分析了不同载荷条件下影响合金循环硬/软化行为的本质原因。揭示了合金的滑移特性以及微裂纹萌生等现象和规律。本项目的研究成果具体包括以下几个方面：1. 从微观机制上对单相Ti-5553合金纯压缩疲劳加载时的循环应变响应进行研究，发现该合金在任意压缩应力加载条件下均表现出循环早期软化和后期饱和的现象。分析表明，合金基体中的位错湮灭和退孪晶现象与共格ω相切变过程联合作用，引起合金的早期循环软化效应，而位错偶极子在基体中的往复运动时造成后期饱和现象的主要原因。2. 以自由电子浓度和层错能为判据，通过分析平面滑移机制和应力集中现象，结合合金表面微观组织的近原位观察和疲劳裂纹的萌生和扩展，较为完善的研究了单相β钛合金Ti-5553的疲劳损伤机制。3. 对室温低周拉伸-压缩疲劳加载条件下双态Ti-5553合金的循环应力和塑性应变响应进行研究，发现该合金宏观循环硬化/软化行为与总应变幅密切相关。4. 运用复合材料强度的混合物定律建立对双态Ti-5553合金的延性二相合金循环形变研究。剖析基体中的不同相态：初生α相、次生α相和β转变基体引起的微观塑性变形非均匀性对宏观循环形变响应的影响。5. 建立双态Ti-5553合金宏观循环硬化/软化行为与微观位错组态的关系。在总应变加载条件下，发现处于有利位向的初生α相是外加塑性功的主要载体，α晶粒中的滑移启动时小应变加载时合金硬化的主因，而位错湮灭和位错结构简化是大应变条件下合金软化现象的原因。