增材制造高强低模量钛合金纳米尺度相分解及其对马氏体相变影响

The investigation on the microstructure and mechanical properties of additive manufactured biomedical titanium alloys has recently received extensive attentions and becomes a hot topic for the development of biomedical metallic materials. Titanium alloy with high strength and low modulus possess high strength-to-modulus ratio (~2%), nonlinear elastic deformation, superelasticity and high damping across a wide temperature range (from below 4.2K to 573K). Our previous investigations of a Ti-Nb based alloy have found that nanoscale elastically confined martensitic transformation (MT) mechanism because of a nanoscale Nb modulation created by spinodal decomposition is the origin of these unprecedented properties. In this project, the titanium alloy with high strength and low modulus will be fabricated by electron beam melting (EBM) technique. The temperature distribution in powder bed during EBM process will be described using finite element calculations. The alloy elements migration, phase separation and its effect on MT under unique solidification condition and complex heating history during EBM will be clarified. The relation between processing parameters and phase separation will be established and their influences on martensitic transformation and mechanical properties will be elucidated. The effect of post aging treatment on the HCP phase precipitation in EBM product will be revealed. Above results will provide the theoretical guidance for optimizing the microstructure and mechanical properties of EBM titanium alloy products with high strength and low modulus through adjusting EBM processing parameters.

医用钛合金增材制造技术及其组织性能调控是金属医用材料领域研究的热点。高强度低模量β型多功能钛合金是一类结构与功能性兼备的新型医用金属材料，其强度与模量比值约为2%，显著高于传统的晶态金属材料；具有非线性弹性变形行为，在高强度条件下宽温域范围内具有超弹性和高阻尼性能等。课题组前期结果表明，纳米尺度相分解和晶体结构连续可逆变化的马氏体相变是产生该类合金特殊性能的根源。本课题拟采用3D打印技术制备高强度低模量β型钛合金，结合理论计算和实验，研究电子束增材制造过程中的温度场特性，阐明超常凝固条件及复杂热历史下Ti2448合金元素迁移规律、相分解机制及其对马氏体连续相变的影响，建立电子束工艺参数-相分解-连续相变-力学性能之间关联，揭示时效处理后处理对电子束增材制备构件的HCP相均匀形核及生长影响机制，为通过调整增材制造工艺参数优化高强度低模量β型钛合金组织和性能提供理论依据。

Ti-24Nb-4Zr-8Sn（Ti2448）合金是一类结构与功能性兼备的新型医用金属材料，其强度与模量比值约为2%，显著高于传统的晶态金属材料；具有非线性弹性变形行为，在高强度条件下宽温域范围内具有超弹性和高阻尼性能等，受到了各国学者的广泛关注。本项目采用电子束增材制造技术（EBM）制备Ti2448合金，并对其组织和性能开展了研究。结果表明，EBM制备样品致密度随着入射能量的增加呈抛物线趋势，表明能量过高或过低都不利于成品的致密度。在速度函数（Speed function）为8~45，焦距补偿（Focus offset）为9~20范围内，能够制备出致密度99%以上的Ti2448合金构件。样品组织主要由β相构成，预热工艺使粉床温度为~500℃，导致少量α''相在β基体中析出。在与基板成不同堆积方向（0º，45º，90º）的样品中，发现β晶粒存在较强的立方织构{100}<001>，其形成机制类似于单晶。强度和塑性均随堆积方向发生变化，其中0º样品力学性能更贴近高硬化、高塑性、低模量的<100>单晶，45º样品强度最高而塑性最低。以上强度和塑性的差异主要与不同取向样品的织构与拉伸轴夹角不同，导致其不同的滑移系运动特性有关。通过EBM技术控制<100>择优取向，能够有效调节样品的强度和弹性模量，获得具有优异的强度-模量匹配关系的生物医用材料。在400 ℃~500 ℃时效处理后，EBM样品中由于纳米尺度的相分解引发了连续的微观组织演化，由原始态的“海绵状”组织演化为时效态的“板条状”的α''+β双相组织，导致材料拉伸强度提高，塑性降低，其中在400 ℃~450 ℃时效EBM样品强度和塑性匹配最优。