近α型高温钛合金微纳层状结构调控机制及组织稳定性研究

This proposal is guided by major research projects of NSFC, i.e. the relationship between structural evolution and performance stability of high temperature materials during preparation and service conditions. A novel laminated structure was prepared with the layer by layer distribution of primary equiaxed αp grains (soft phase) and fine lamellae αs grains (hard phase) in Ti-Al-Sn-Zr-Mo-Nb-Si-Er near-alpha high temperature titanium alloys. Multi-stage compression have realized by controlling deformation temperature, strain and strain rate in (α+β) two-phase region using hydraulic wedge of thermal simulation. Thus, the influential mechanism of thermal deformation parameter on the substructure evolution and dynamic recrystallization in αp and β grains are investigated. A model for calculating and modifying inverse orientation of initial β phase in the (α+β) phase region is established, and then the mechanism of deformation and phase transformation in the β phase, the evolution of microstructures and orientations in the α phase are clarified. The effects of thermal processing parameters on the ratio and distribution of the αp/αs layers in layered Micro-Nano scale structures and micro-orientation on both sides of the interface are studied. Some common scientific problems such as the coordination deformation mechanism between the αp and β grains are also investigated. The micro-structure stability was evaluated under thermal exposure, creep deformation and fatigue testing. This proposal was beneficial for enriching the self-organized composite material-like microstructure model.

本项目以重大研究计划培育项目中的高温材料在制备和服役条件下结构演化与性能稳定性关系为导向，提出在Ti-Al-Sn-Zr-Mo-Nb-Si-Er系近α型高温钛合金中制备初生等轴αp晶粒（软相）和细层片α晶粒（硬相）逐层分布的新型层状结构。利用热模拟液压楔在(α+β)两相区同步多段精确控制形变温度、形变速率、形变量实现多段压缩，并揭示热加工参量对αp晶粒与β晶粒的亚结构及动态再结晶的影响机理。建立在(α+β)相区控制形变及相变过程中初始β相逆取向计算及修正模型，并揭示形/相变控制的α相微结构及取向的演化规律，阐明热加工参量对层状微纳结构中αp/αs层比例、分布及界面两侧微取向的影响规律、以及αp晶粒与高温β晶粒的协同变形机制等共性科学问题。对αp/αs相层状结构材料进行热稳定性、蠕变及疲劳性能测试，研究其热力耦合条件下组织演化机理，进一步丰富高温钛合金自组织型类复合材料的组织模式。

本项目以高温钛合金在制备和服役条件下结构演化与性能稳定性关系为目标导向，研发了4种稀土铒微合金化近α型高温钛合金(Ti5.5~6.2Al-3Sn-2.5~5Zr-0~2.5Hf-0.5Mo-1Nb-0.4Si-0.2Er)，制备了αp/αs相层状结构的近α型高温钛合金并揭示了层状结构的形成机理，获得了热力耦合作用下组织稳定的高温钛合金，具有潜在的应用及理论意义。首先，建立了在α相及(α+β)相区加工过程中预变形对后续β相动态再结晶的“V”型影响规律，通过引入α-Ti的等效柏氏矢量|b|i计算得出800C~950C预变形储存能的“V”型规律，从而揭示了后续1050C形变 β相的“V”型动态再结晶机理，提出了β相形变动态再结晶加工工艺。其次，提出了钛合金“双平行翼”晶体学几何关系控制的β相动态再结晶对α相变体取向选择新机制，基于EBSD数据计算α相平均取向、及β相转变成12种α变体的取向关系、结合最小取向差原则，建立了高温β母相的逆取向计算及修正模型，揭示了动态再结晶β母相与α相之间的相变变体取向选择的“双平行翼”晶体学几何关系，并提出了避免“宏区”的形变及相变新工艺。再次，揭示了引入动态及静态再结晶球化初生αp相方法制备αp/αs相层状结构的调控机制，揭示了降低形变温度来抑制[21 ̅1 ̅0](01 ̅11)锥面滑移、促进(11 ̅00)[112 ̅0]柱面滑移使αp相形变成层状结构，而高温变形β相及相变形成αp/αs相层状结构的形成机理。经650C/100h热暴露测试后，层状组织结构钛合金的抗拉强度由1100Mpa小幅提高至1115Mpa，延伸率由10.5%降至8.5%，对比双态组织其塑性损失显著减少，且αp/αs层状组织的形貌、大小及硅化物变化很小、热稳定性良好。经650C/100Mpa/100h蠕变性能测试表明，层状组织合金的总蠕变应变和稳态蠕变速率为0.207%和9.11×10-10s-1，均优于双态组织合金的0.224%和3.22×10-9s-1，且在蠕变的热力耦合条件下层状组织形貌、尺寸及析出相变化均很小，具有良好的高温服役性能。高周疲劳性能测试表明，疲劳强度550MPa以上，并揭示了疲劳裂纹路径沿αp/αs和αs/αs相界处出现偏折、增加裂纹扩展阻力提高疲劳性能的影响机理。