钛合金多种均匀形核弹性变形机制研究

Ti2448 alloy is a kind of multifunctional titanium alloys with high strength and ultra-low elastic mdoulus and exhibits novel nonliear elastic deformtion behavior with large scale recoverable strain and highly localized plastic deformation behavior favoring rapid grain refinement to nano scale by conventional cold processes. Our previous in-situ analyses have found three kinds of homogenously nucleated reversible elastic deformation mechansims of nanodisturbance, dislocation loop and nano-sized martensite with equiaxed morphology, which are triggered in turn with the increase of external applied stress. On the basis of these studies, the project will focus on the novel elastic deformation mechanisms of single crystals, conventioanl micrometer microstructured and nanostructured Ti2448 alloy by in-situ tensile analysis methods of TEM, high resolution TEM and synchrotron X-ray, in hope of characterizing the influnece of crystal orientation, grain size and precipitation on elastic deformation mechanism and revealing the relation of three kinds of novel elastic deformation mechanisms as well as finding possible ways to controll the novel mechanisms by microstructure modification. Inelastic X-ray scattering will also be applied to investigate phonon dispersion of Ti2448 alloy by varing temperature and external applied stresses, in hope of getting phonon softening evidence to explain the formation of nano-sized martensite with equiaxed morphology and finding possible way to depress such special martensitic transformation by strain glass transition mechanism so as to improve strength of Ti2448 alloy further. These results will provide solid evidenc to clarify some confusions related to elastic deformation mechansms of multifunctional titanium alloys with high strength and low modulus and also be crucial to improving mechanical properties of these alloys and design new alloys with multifunctional properties under the condition of high strength.

Ti2448合金是一种具有非线性弹性变形行为的高强度低模量多功能钛合金，前期多种原位拉伸实验发现了三种均匀形核的纳米尺度可逆变形机制：晶格纳米扰动、位错环和马氏体相变。本项目拟在前期工作的基础上，通过透射电镜、高分辨电镜和高能同步X射线等原位拉伸分析测试方法，研究单晶合金、微米组织合金和纳米组织合金的弹性变形机制，表征晶体取向、晶粒尺寸和析出相对合金弹性变形机制的影响，揭示三种弹性变形机制的相互关系，并给出调控变形机制的微观组织控制方法；通过非弹性散射谱研究Ti2448合金的声子软化及其与温度和加载应力的关系，解释均匀形核可逆等轴纳米马氏体组织产生的根源，探寻控制该类玻璃化转变型马氏体相变的方法，期望进一步提高Ti2448合金的强度。通过以上研究，希望能够统一目前对高强度低模量多功能钛合金弹性变形机制的多种认识，并充分利用这些机制优化合金的性能和设计出其它成分的类似合金。

Ti2448是一种具有非线性弹性变形行为的高强度低模量多功能钛合金，前期研究发现该合金具有多种均匀形核的可逆变形机制。在本课题资助下，课题组采用三维原子探针、高能同步X射线、扫描透射电镜和高分辨电镜等表征手段，研究了Ti2448合金的成分分解、亚稳相变、可逆变形机制及其调控方法，取得了如下主要结果：.1）通过三维原子探针发现了一种Nb含量周期波动的等轴纳米团簇组织，利用纳米尺度等结构相分解机制较好地解释了这种特殊组织形貌产生的根源；2）提出了一种纳米尺度弹性约束可逆马氏体相变的新机制，不仅统一了目前对高强度低模量多功能钛合金可逆变形机制的多种认识，而且抑制了一阶马氏体相变的突变性，使其转变为一种具有高阶特性的连续相变；3）提出了一种提高钛合金相稳定性的新方法，抑制了BCC 结构相关联的多种亚稳相变，大幅度拓展了BCC相的成分稳定极限，显著提高了Ti2448合金的超弹性；4）通过动态力学分析仪、透射电镜、高分辨电镜和高能同步辐射等原位分析手段，表征了Ti2448合金在载荷、温度和循环应变条件下可逆变形机制的稳定性，发现了宽温域高强度多功能特性，证实了纳米尺度弹性约束可逆马氏体相变机制在宽温域和大载荷条件下具有良好的稳定性；5）给出一种将突变型相变调节为连续型相变的新方法，通过高能同步辐射和透射电镜原位分析，发现了BCCHCP相变的中间态是一种扩散切变型复合相变，定量测量了BCCHCP晶体学转变过程中的原子剪切和原子挪动，不仅丰富了对相转变Burgers路径的认知，而且为改善金属材料的结构功能一体化性能给出了一种新思路。.以上初步研究结果，在SCI学术期刊发表论文16篇，其中邀请综述1篇、Acta Mater 4篇、Acta Biomater 2篇、Sci Rep和Scripta Mater 各1篇、2篇论文被列入ESI高被引论文(Mater & Design 2016; J Beha Biomed Mater 2016)。 以上论文被SCI期刊引用100余次，其中2016年引用64次。