Beta型钛合金的宽温域超弹性研究

Thermoelastic reversible martensitic transformation (MT) is a typical first-order diffusionless phase transformation with a strong autocatalytic nature. Such autocatalysis is positive for the shape memory effect to make it be sensitive with the applied temperature, but is negative for the superelasticity and low/negative thermal expansions which requiring stable in wide temperature range.For examples, the stress-induced superelasticity is limited in narrow temperature range of tens Kelvin above its austenic finish temperature while these novel thermal expansions exhibit more sensitive temperature relations than the superelasticity..Our previous investigations of a beta-type multifunctional titanium alloy have found that a unique nano-scale compositional non-uniform microstructure is created by isostructure phase separation. The MT in the alloy is not only continuous like high-order transition but also adaptable to the applied temperatures by a smart mechanism which is stronger at lower temperature to against the MT. Such adaptive MT leads to some exceptional functional and mechanical properties such as the superelasticity between 4.2 and 500 K, the low and negative thermal expansion up to 625 K, the maximum recovered strain reaching 2.5 times of that from the MT, high strength and good ductility. .In the study, we will investigate such unique microstructure by establishing the relation between phase separation and chemical compositions, charactering the phase separation with aid of the stability of high temperature beta phase, and revealing that a precursor diffustion phase transformation from the beta phase to the alpha phase is the origin of such phase separation. The study also focuses on the controlling methods for the phase separation and the confinement of the unique microstructure on the MT, and relates the continuous MT and its strengthening effect. From the above analyes, we hope to provide the fundmental knowledge to develop new solids which unite both structural and functional properties.

热弹性可逆马氏体相变是一类具有强烈自催化特征的无扩散型一级相变，这有利于在窄温区内实现良好的形状记忆效应。然而，这种温度敏感特性导致材料的超弹性和反常热膨胀性能也局限于很窄的温度范围内。例如，超弹性的温区通常只有几十度，反常热膨胀的温区更窄。我们在研究一种Beta型钛合金时发现：该合金发生了等结构相分解，形成了纳米尺度的成分不均匀组织，产生了具有高阶相变特征的连续马氏体相变，使得该合金的超弹性温度区达到了500度，其低膨胀和负膨胀温区达到了625度，并且该合金产生的可逆应变是马氏体相变的2.5倍。本项目拟研究相分解与合金成分的关联性，表征高温Beta相的稳定性与相分解的关系,揭示扩散型预相变是产生这种相分解的根源；研究相分解的控制方法及其对马氏体相变的约束作用，探讨连续马氏体相变的强化机制.通过以上研究,希望为新型结构功能一体化材料的研制提供理论依据。

针对Ti2448高强度低模量多功能钛合金的超宽宽温域超弹性和可调控热膨胀行为，课题组采用三维原子探针、高能同步X射线、小角中子衍射、差热分析仪、扫描透射电镜和高分辨电镜、超低温力学测试等表征手段，对该合金的相分解、可逆变形机制、力学/功能性能和热稳定性进行研究，取得了如下主要结果：.1）通过三维原子探针发现了一种纳米尺度海面状Nb元素调制组织，周期分布的富Nb区和贫Nb区约为2-3纳米，通过晶格无差异的等结构成分分解给予合理解释；2）揭示纳米尺度海面状成分组织是该类高强度低模量多功能钛合金特异性的根源，提出了一种纳米尺度弹性约束可逆相变的新机制，较好地模拟了该合金在600度超宽温域的超弹性和可调控热膨胀等反常性能；3）通过透射电镜、高分辨电镜和高能同步辐射等原位分析表征技术，研究了载荷、温度和循环应变条件下的可逆变形机制，定量表征了新机制在应力和温度作用下的演化规律，构建了有序度-性能之间的本征关系；4）通过小角中子衍射和扫描透射电镜等研究了纳米尺度海面状组织的热稳定性，表征了海绵组织-调幅组织的转变规律，给出了热处理调控力学和功能性能的有效方法；5）建立了Nb元素上坡扩散调控弹性约束的新方法，通过原位升温高能同步辐射证明了晶体结构变化的连续性，测量了BCC-HCP转变的原子剪切和原子挪动两个分量，给出了原子剪切和挪动两个关联有序参数的线性变化关系。.综上所述，通过纳米尺度海面状成分组织构建约束可逆转变可赋予功能材料更优性能匹配关系，解决强度-功能-温度之间的传统矛盾关系，为新型高强度多功能一体化材料设计提供重要理论依据。以上初步研究结果，在SCI学术期刊发表论文13篇，其中Acta Mater 4篇、Scripta Mater 1篇，Science China: Materials 1篇，总计被SCI期刊论文引用154次。.