超低弹性模量亚稳β钛合金的制备及其超低模量机理研究

The fabrication of β-type Ti alloy and its mechanism of low modulus has been attracted much attention in the field of Ti alloy. Our recent experimental results revealed that with the assistance of cold-rolling and annealing treatment, metastable β-type Ti-33Nb-4Sn alloy with low content of β-stabilizers can exhibit ultralow room temperature elastic modulus of 36 GPa, which is much lower than those of the currently developed biomedical Ti alloys. In addition, it was shown that the elastic modulus of the Ti-33Nb-4Sn alloy firstly decreases with decreasing temperature and then increases with a further decrease in temperature. In such a case, the Ti-33Nb-4Sn alloy exhibits the lowest elastic modulus near its martensitic transformation starting temperature, which is different from pure Ti. In this work, we will fabricate a novel Ti-33Nb-4Sn-M (M stands for modified) alloy, with room temperature elastic modulus close to or lower than 30 GPa, by further optimizing alloy composition and thermo-mechanical treatment procedure. Additionally, the effect of temperature on the elastic modulus of metastable β-type alloys will be particularly concerned, in the hope of revealing the physical origin of ultralow elastic modulus in Ti-33Nb-4Sn-M alloy. Furthermore, the design principles for preparing metastable β-type Ti alloys with ultralow modulus will be clarified. Therefore, the present work is promising to provide some theoretical guidelines for the design and fabrication of β-type Ti alloy with ultralow modulus.

低弹性模量β钛合金的制备及其相关机理研究，一直是钛合金学术研究和应用研究中的热点问题。我们最近通过调低钛合金的β稳定元素含量并对合金辅以冷轧和退火处理，获得了超低模量的Ti-33Nb-4Sn合金（其室温拉伸模量仅36GPa，低于目前报道的钛合金的最低模量值）。此外我们初步发现，与纯钛弹性模量随着温度降低呈现单调增加的趋势不同，Ti-33Nb-4Sn合金的模量随温度的降低呈现先减小后增大的趋势，并在合金的马氏体相变开始温度附近呈现最低值。本项目拟在前期工作的基础上，通过优化合金成分和热-机械处理工艺，制备出室温模量更接近甚至低于人骨模量（~30GPa）的新型Ti-33Nb-4Sn-M（M代表改进型）合金，并通过对亚稳β钛合金弹性模量与温度相关性的重点关注，揭示Ti-33Nb-4Sn-M合金实现超低模量的物理机理，明确超低模量亚稳β钛合金的设计准则，为新型超低模量β钛合金的设计与研发奠定基础。

在本项研究中，我们通过适度降低钛合金中β稳定化元素的含量并对其辅以热-机械处理，制备出具备超低弹性模量的亚稳β型Ti-32.8Nb-3.92Sn（重量百分比，以下简记为Ti-33Nb-4Sn）合金，其室温拉伸弹性模量仅36GPa，与人骨约30GPa的模量近似相当，可望在生物医用领域获得重要应用。通过对亚稳β钛合金中弹性模量与温度相关性的重点关注，发现相变导致的晶体结构失稳会对此类合金的弹性模量产生重要影响。选用Ti-33Nb-4Sn合金为模型合金，采用高能X射线同步辐射技术和Eshelby-Kroner-Kneer弹塑性力学模型，从亚稳β钛合金多晶体中抽取了其单晶体的弹性常数。进一步的研究表明，Ti-33Nb-4Sn合金超低弹性模量的实现主要归因于成分设计和热-机械处理诱发的极低的β相稳定性（即低的弹性剪切常数C'和C44）。这表明，除了已被人们认知的剪切模量C'外，弹性常数C44也会对亚稳β钛合金的弹性模量产生重要影响。基于上述研究结果，对具备超低弹性模量的亚稳β钛合金提出如下设计准则：合理调低亚稳β钛合金中的β稳定元素（如Nb）含量，使合金在固溶状态下呈现轻微的马氏体相变，然后对其辅以激烈的塑性变形（如冷轧）和时效处理（如低温短时时效），借助于上述热-机械处理引入的高密度位错和晶界迟滞马氏体相变的发生，能够将具备较低β稳定元素含量（对应较低的β相稳定性，表现为极低的弹性常数C'和C44）的高温β相稳定至室温，进而实现具备超低弹性模量的亚稳β钛合金的制备。研究了不同热-机械处理状态（主要是固溶态和冷轧态）Ti-33Nb-4Sn合金的力学行为，阐明了此类合金变形行为的相关机制。本研究通过合金化和热-机械处理的手段，制备出具备超低弹性模量的亚稳β型Ti-33Nb-4Sn合金，阐明了此类合金实现超低模量的物理机理，明确了超低模量亚稳β钛合金的设计准则，为新型超低模量β钛合金的设计与研发奠定理论基础。

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