Mg-Sc-Li-Nd轻质形状记忆合金体系的相图热力学研究

The newly developed Mg-20 at.% Sc shape memory alloy (SMA) not only exhibits remarkable superelasticity comparable with those of conventional TiNi and Cu SMAs, but also has small specific weight showing one-third less than that of TiNi SMAs. Therefore, the development of Mg-Sc SMA raises the potential for application in self-deployable space habitat frames and damping devices in motors. Unfortunately, the high price of Sc and the low onset temperature of martensitic transformation β(bcc)-α″(orthorhombic) limit the immediate application of Mg-Sc SMA. Following the thermodynamic principle for alloy design, Li and Nd elements are selected as the alloying elements to partially substitute the Sc, such that the cost of Mg-Sc based SMA can be reduced, and the martensitic transformation may be triggered at room temperature. In this project, the phase diagrams of Mg-Sc-Li-Nd system, particularly for the phase boundaries of β, α and α″, are determined by using both the equilibria alloys and diffusion couple methods. The most promising alloy composition for SMA is to be screened, and the scheduled temperature ranges for β solid solution and α″ precipitation are to be determined. The thermodynamic model of α″ martensite in the framework of CALPHAD is developed assisted by First-Principles calculations, in which the elastic energy and interface dissipation are incorporated. With the developed thermodynamic database of Mg-Sc-Li-Nd system, the instantaneous chemical driving force of β→α″ martensitic transformation and the yield stress of SMA are readily predicted. The experimental and simulation results as a whole provide energy parameters for quantitative mediation of microstructure and help to design of the advanced Mg-Sc lightweight SMA.

最新研制的Mg-20 at.%Sc形状记忆合金(SMA)不仅具有与TiNi-、Cu-基等成熟SMA相当的超弹性，且比重小，有望成为新型航空航天用可伸缩设备和汽车连接件材料。但是，Sc高昂的价格和β-α″马氏体相变过低的相变温度限制了Mg-Sc SMA的工业应用。本项目借鉴多元合金化的热力学原理，确立以Li、Nd等元素部分取代Sc，实现Mg-Sc基SMA低成本、常温服役的目标；采用合金法和扩散偶法测定Mg-Sc-Li-Nd体系相图，确定β、α和α″相的相边界，优化β固溶和α″析出的热处理温度-成分范围；在第一性原理计算的基础上，建立马氏体α″相的计算热力学(CALPHAD)模型；开发Mg-Sc-Li-Nd体系热力学数据库；预测β-α″相变中的瞬间化学驱动力和形状记忆合金的屈服强度；进而为新型Mg-Sc基轻质SMA的成分设计提供坚实的理论依据和知识储备。

Mg-20 at.%Sc是新一代轻质形状记忆合金(SMA)，但Sc高昂的价格、β-α″相变组织的不稳定性及过低的相变温度限制了潜在的工业应用。本项目确立了以Li、Nd、Zn部分取代Sc的合金化方案，采用扩散偶法校核了Mg-Sc二元相图并测定了β_bcc和α_hcp的扩散系数随成分、温度的变化；采用平衡合金法，结合高分辨率表征，测定了Mg-Sc-Nd、Mg-Sc-Zn等体系富Mg角的等温截面和部分垂直截面，确定了β_bcc、α_hcp、B2等相的相边界；为克服超轻元素Li微区成分检测的困难，采用数值回归分析，迭代计算Mg-Sc-Li合金中β_bcc和α_hcp相的相对比例和Mg/Sc成分比例，预测并优化了β_bcc和α_hcp两相平衡，拓展了含超轻元素相图的测定方法；融合第一性原理计算和CALPHAD热力学优化技术，建立包含(Mg)基α_hcp、β_bcc、B2等稳定和亚稳相的Mg-Sc-Li-Nd-Zn相图热力学数据库和Mg-Sc扩散原子移动性参数数据库；开发了耦合热动力学数据的β_bcc↔α_hcp相变及屈服强度模拟脚本；基于热力学计算设计了Mg-Sc基多元合金的成分及β固溶-α″析出的工艺；对合金的微观组织结构与力学性能进行了表征。研究表明，Li、Nd、Zn的合金化可有效地提高β_bcc的固溶成分范围，降低Sc的含量，并在特定成分区间诱导出B2、fcc、α″等多种析出相。Li与Sc在富Mg端可形成连续互溶的β_bcc和α_hcp相，β_bcc的热力学稳定性随着Li的添加量先增后减。设计制备的Mg-Sc-Li合金可在较低的650℃实现β_bcc单相固溶，淬火时效的合金压缩时具有较长的零加工硬化区，在纳米压痕实验中显示异常回弹效应，Li的合金化诱导了β_bcc↔fcc等复杂有序相变限制了形状记忆效应的发挥。以上成果，包括测定的相图、建立的数据库及合金组织-性能的研究为新型Mg-Sc基合金的成分设计、组织工艺调控提供坚实的理论依据和知识储备: 建立的热力学数据库可以用于成分工艺设计，标定满足热力学条件下固溶和析出的极限成分和温度，原子移动性数据库为抑制β_bcc↔α_hcp相变提供有效的扩散动力学参数。