原子尺度连续调控钛合金BCC-HCP结构转变及其强化机制研究

Ti alloys generally possess a strength-to-modulus ratio ranging from 1% to 2%, much higher than others metallic materials. Our recent investigations using a multifunctional Ti alloy with high strength and low modulus have revealed that its high temperature bcc phase passes through a nano-scale iphase separation, leading to a nano-scale elastic architecture to restrict the bcc/hcp transition in which the bcc crystal is distorted continuously toward the direction to form the hcp crystal via the coupled atomic shear and shuffle by keeping an orth symmetry. Thus, the original discrete first-order bcc/hcp transformation is tuned into a continuous high-order like transition. The in-situ SXRD measurements showed that the successive atomic shear and shuffle are tunable. As a result, both components outlined by the well-known Burger’s pathway were traced in experiment. Aided by the bcc phase separation, the homogeneously nucleated hcp phase was produced to strengthen the alloy. Compared with the typical two-phase Ti alloys, such homogeneous microstructure causes a 50% higher ratio of strength to modulus. These results demonstrate that the successive tuning on crystal structure at the atomic level causes some novel properties which are unexpected from the discrete phase transformations. In the study, we will tune the atomic shear and shuffle components by the aging treatments in the bcc+hcp two-phase field to obtain several typical intermediate orth states ranged between the bcc and hcp, which are diffusion-controlled but reversible toward the direction to form hcp crystal. Exploring the nano-scale elastic confinement on their reversibility, the novel properties including high strength and good superelasticity at wide temperature range will be available. These results are helpful to unite the structural and functional applications of the alloy. The nano-scale phase separation and its effect on the hcp phase precipitation will be explored, in hope of obtaining the homogeneously nucleated hcp phase to improve both strength and toughness. This provides new strategy to enhance the mechanical properties of the developed Ti alloys.

钛合金是一类典型的高强度金属材料，其强度/模量比值高达1%-2%，显著优于其它金属材料。课题组在研究一种高强度低模量多功能钛合金时发现：BCC相通过纳米尺度等结构相分解产生了约束BCC-HCP相变的弹性环境，在原子尺度实现了晶体结构的连续调控，并原位测量了原子连续移动的伯格斯路径。这些研究结果给出了一种利用相分解连续调控晶体结构的新方法，为探寻钛合金高强属性和提高其强韧匹配关系提供了一种新思路。本课题拟通过BCC+HCP两相区热处理调控相分解和原子位移，研究BCC-HCP连续相变的几种典型中间态及其对合金性能的影响，阐述相分解-连续相变-组织性能之间的关系，构建纳米尺度弹性约束可逆相变模型，探寻高强高韧可逆变形新机制，为其结构功能一体化应用奠定坚实的理论基础；研究HCP相均匀析出的调控方法及其强韧化效应，为其在钛合金的推广应用提供理论指导。

针对Ti2448合金的原子尺度连续调控BCC-HCP结构转变及其强化效应，课题组采用高能同步X射线衍射、小角中子散射、差热分析仪、热膨胀仪、扫描电镜、透射电镜、高分辨电镜等原位与离位表征手段，研究了成分分解调控、微观组织转变、连续约束相变和强化效应之间的关系，取得了如下主要结果：. 1）采用高能同步X射线原位拉伸测试分析技术，表征了成分分解组织的应力响应，建立了非线性弹性金属材料的结构与功能一体化表征模型，较好地解释了该合金高强度、低模量和多功能特性。2）采用高分辨电镜原位拉伸测试分析技术，发现了原子尺度调控可逆连续转变，表征了这种连续转变的强化效应及其宽温域功能化。3）通过透射电镜、高分辨电镜等表征技术研究了该合金的连续扩散位移型相变及其强化效应，为进一步理解该合金的高强度多功能特殊属性奠定了良好基础。4）通过扫描电镜与高分辨电镜发现了冷加工过程中存在多种变形机制并形成了一种特殊的多层次三明治复合微观组织，较好地解决了高强度与高韧性的矛盾关系。5）通过小角中子散射研究了海绵组织-调幅组织的连续转变和演化动力学，建立了利用有序度定量表征微观组织和力学性能的表征方程，揭示了连续晶体重构强化效应的根源，给出了超高强度钛合金的设计原则。.综上所述，Ti2448合金纳米尺度海绵状成分分解和连续晶体重构两种机制具有显著的强化效应，实现了高强度与高韧性的匹配兼容，为新型高强度多功能一体化材料的设计提供重要理论依据。以上研究结果在 SCI 期刊发表论文12篇，其中 Acta Mater期刊 3篇，Scripta Mater 期刊1 篇，被 SCI引用 264 次。