具有魏氏组织的钛合金中形变孪晶特征研究

The α + β titanium alloys with Widmanstatten microstructure which show impressive damage-tolerance properties, have been gradually used to make structural components of aircraft by β heat-treatment and casting process. The properties of Widmanstatten structure are related to plastic deformation behaviors in crack tip plastic zone (CTPZ) in which deformation twinning has been widely observed by proposers. The large deformation twinning in Widmanstatten structure not only expands the range of CTPZ, but also enriches the way of plastic deformation. However, the mechanisms of nucleation and propagation of deformation twinning in Widmanstatten microstructure have not been definited until now. . In this project, Ti-6Al-4V alloy with Widmanstatten microstructure will be employed for the investigation on deformation twinning in CTPZ. The EBSD method will be applied for the measurement of shearing parameters of deformation twinning. The influence of grain or colony size on the nucleation and propagation of deformation twinning will be studied. The influence of crystal orientation changing between two adjacent grains or colonies on the nucleation of deformation twinning will also be investigated. The behaviors of twins transiting α/β interface will be discussed with the interactions between dislocation and α/β interface. Finally the plastic deformation of body-centered cubic β phase in deformation twinning will be studied by TEM method. . According to the plan of this project, the mechanism of deformation twinning in titanium alloy with Widmanstatten microstructure will be studied by discussing the influence of prior β grain, α colony, β phase and α/β interface. The results of this investigation are hoped to help understanding the influence of microstructure on strength-toughness properties of titanium alloys, and establish theoretical foundation to develop high strength-toughness titanium alloy.

形变孪晶是魏氏组织钛合金重要的塑性变形方式，对魏氏组织的力学性能具有重要影响，但魏氏组织中的形变孪晶鲜有报道。本项目将以具有魏氏组织的Ti-6Al-4V合金为研究对象，采用透射电镜、电子背散射衍射等实验技术，开展裂纹尖端塑性区内形变孪晶特征研究，主要包括以下工作内容：研究魏氏组织中原始β晶粒、α集束等显微组织的尺寸变化对形变孪晶数量及尺寸的影响，分析显微组织的尺寸效应；研究相邻晶粒或集束的晶体取向差对形变孪晶形核的影响，阐述其影响机理；研究形变孪晶前端位错在α/β界面处的运动行为，揭示两相间特定的晶体取向关系对孪晶生长的作用机制；研究形变孪晶带中β-Ti的塑性变形机制，讨论β-Ti在形成大尺寸形变孪晶中的作用。通过以上研究，明确魏氏组织钛合金中显微结构特点对形变孪晶形核及生长的影响机制，探讨形变孪晶在魏氏组织钛合金强韧化中的作用，为探索改善魏氏组织钛合金强韧性的技术途径，提供理论研究基础。

通常具有魏氏组织的钛合金表现出较高的抗裂纹扩展能力和断裂韧性。与等轴组织相比，魏氏组织单位体积内可开动的滑移数量较少，使得塑性应变需要形变孪晶来补充提供，本实验也证明魏氏组织中存在大量的形变孪晶。然而课题立项前针对钛合金孪晶研究很少聚焦魏氏组织。本课题研究了具有魏氏组织的Ti-6Al-4V合金疲劳裂纹尖端塑性区内的形变孪晶特征，探讨了孪晶变形对力学性能的影响，取得以下主要结果：（1）本实验在CTPZ内观察到的孪晶均为{10-12}模式，随着疲劳裂纹扩展，针状孪晶通过延长、增厚、合并逐渐演变为大范围孪晶；（2）大尺寸形变孪晶的形成主要与两个因素有关：大尺寸α集束在晶体取向上为孪晶生长提供了空间；循环加载的每个载荷增加阶段均有助于孪晶生长；（3）施密特因子是影响孪晶形成和生长的主要的参数，当α晶格c轴与拉伸方向近似平行时，6个{10-12}孪晶变体的施密特因子均较高，可在单个α集束内可激发多个{10-12}孪晶变体；（4）基于长程应力下的低施密特因子的一次及二次孪晶同样可被激发，但其尺寸较小或呈现列岛状；邻近结构单元（α集束/α集束，一次孪晶/基体）协调变形引起的局部应力与长程载荷的叠加是诱发低施密特因子孪晶的驱动力；（5）CTPZ中大范围的形变孪晶扩大了塑性区，丰富了CTPZ内的塑性变形；{10-12}形变孪晶是魏氏组织协调HCP结构沿c轴应变的重要塑性变形途径；形变孪晶导致的晶体转动使得孪晶内α晶格的滑移施密特因子升高，为位错滑移提供了有利的晶体学条件；形变孪晶通过分割晶粒或集束起到了显著的组织细化作用。. 本课题的研究成果丰富了HCP晶体中形变孪晶基础研究，具有一定的科学意义：（1）在国内外首次较为系统地研究了魏氏组织钛合金中的形变孪晶特征，研讨了形变孪晶对魏氏组织钛合金损伤容限性能的影响，对于进一步优化魏氏组织钛合金的强韧性匹配以及发展高损伤容限钛合金具有指导作用；（2）在国内外首次提出局部应力、长程载荷的共同叠加作用是诱发HCP晶体中低施密特因子孪晶的主要外因。