相变在Ti2AlNb与Ti合金超塑性扩散连接中产生的界面效应研究

In view of microstructure and properties deterioration during conventional diffusion bonding with high temperature and long time, the project proposes to increase the diffusion bonding rate using phase transformation superplastic diffusion bonding technology. For this technology, it is the key that clarifying the interfacial effect induced by phase transformation during the superplastic diffusion bonding, which is investigated deficiently by now. In view of this, this project is to study the interfacial effect induced by phase transformation during superplastic diffusion bonding between Ti2AlNb and Ti Alloys. Mechanism of the action of various types of phase transformation on the interface cavity closure is to be clarified from perspective of plastic deformation and element diffusion. The enhancement of superplastic diffusion bonding by phase transformation is to be revealed. This can help increase the diffusion bonding rate at lower temperatures. Through research on the interface structure and mechanical properties of joints under various types of phase transformation, the intrinsic relationship among phase transformation types, interface structure and mechanical properties is to be clarified so that the structure and properties of the joints can be designed and controlled. This project has important implications for the promotion of Ti2AlNb alloy as well as its transformation superplastic diffusion bonding technology in China's aerospace industry.

针对Ti2AlNb与Ti合金常规扩散连接温度过高、时间过长而造成的组织性能恶化问题，本项目拟采用相变超塑性扩散连接技术提高其扩散连接速率，而明晰相变在扩散连接中产生的界面效应是成功运用该技术的关键，但目前对此研究尚不充分。基于此，本项目拟对相变在Ti2AlNb与Ti合金超塑性扩散连接中产生的界面效应进行研究。从界面材料塑性流变和元素扩散角度阐明不同相变模式对两种合金超塑性扩散连接界面空洞闭合的作用机制，揭示相变促进两种合金超塑性扩散连接的机理，以实现两种合金的低温快速扩散连接；研究不同相变模式下两种合金超塑性扩散连接界面组织结构及接头力学性能，阐明相变模式、界面组织结构与接头力学性能之间的相关性，以实现接头组织性能的设计与控制。该项目的研究对于促进Ti2AlNb合金及其相变超塑性扩散连接技术在我国航空航天工业中的应用具有重要意义。

高超音速飞行器某些关键部件需要钛铝金属间化合物和传统钛合金连接以实现两种合金优异性能的适当匹配。本项目采用超塑性扩散连接技术实现了钛铝金属间化合物Ti-22Al-25Nb与传统钛合金Ti-6Al-4V在空气中的高质量扩散连接，阐明了相变对超塑性扩散连接过程的促进作用，表征了扩散连接接头的组织结构和力学性能，建立了二者及其与扩散连接条件的相关性。950℃/100min/15MPa条件下对Ti-6Al-4V和Ti-22Al-25Nb合金在空气中进行扩散连接可以获得高质量扩散连接接头。两种合金间形成了明显的扩散层：Ti-22Al-25Nb侧，扩散层仅由B2相组成；Ti-6Al-4V侧，扩散层近界面处为不连续的α/α2相层，界面和Ti-6Al-4V母材之间为项链状β+α’马氏体组织。所有元素垂直界面发生下坡扩散，扩散区域内，元素在某一点的浓度取决于该点到界面的距离以及该点所处相的类型。扩散不仅可以横向进行，也可以纵向进行。Ti-6Al-4V侧β+α’组织的维氏硬度为385HV，αp相与β+αs晶团的显微硬度则分别为347HV、308HV。Ti-22Al-25Nb侧B2相扩散层的硬度由母材的324HV下降至309HV。接头抗拉强度可达894MPa，与Ti-22Al-25Nb抗拉强度相当。接头两侧断口部分区域为平整面，断裂沿连接界面发生，为脆性断裂，Ti-6Al-4V侧有众多形状不规则的凹坑，对应于Ti-22Al-25Nb侧不规则凸起，断裂发生在β+α'扩散层和初生αp晶粒间或αp晶内。在Tmax=950 ℃，Tmin=850 ℃，p=15MPa，N=4，ts=100 min相变超塑性扩散连接条件下所获得的连接接头力学性能和上述950℃恒温条件下所获得的接头性能相当，但其平均连接温度降低100 ℃。该项目研究成果有望在高超音速飞行器某些关键部件制备中获得应用。