4-11. Atomic mobility database for titanium alloys and its application in β/α phase transformation

Liu Libin，Bai Weimin，Xu Guanglong，Zhang Ligang

Central South University, School of Materials Science and Engineering

Abstract: Nowadays, with the increasing use of titanium alloys, various of titanium alloys have been developed, resulting in more and more complex compositions and processes. The complicated application scenarios constantly put forward new requirements for the properties of titanium alloys. Mastering the microstructure evolution of titanium alloys under different compositions and processes plays a crucial role in the design of new titanium alloys. As a matured method for computing method, CALPHAD (CALculation of PHAse Diagram) method has been applied to the design and development of a variety of materials. With the thermodynamic and kinetic databases established using CALPHAD method, the phase composition and microstructure evolution in the alloy can be quantitatively simulated to achieve an in-depth study of the microstructure evolution of the alloy in the actual process, establish a quantitative relationship between composition/process, microstructure and performance. However, the current application of CALPHAD method, especially kinetic database, is far from enough in titanium alloys. In particular, due to the lack of diffusion data, it is far from meeting the need for quantitative simulation of microstructure evolution. At present, most of the alloy diffusion coefficients used in the simulation calculation process are fixed values independent of the component or temperature, which are obviously inconsistent with the facts. Based on this situation, in the present work we focus on studying the diffusion behavior in Ti-Al-Cr-Fe-Mo-Nb-Ta-V-Zr system, in order to establish an atomic mobility database of titanium alloys, and apply it in β/α phase transitions in titanium alloys, to provide some theoretical guidance to the design and development of titanium alloys.

Using diffusion couple technique, combining the Whittle-Green and Hall methods, the inter- and impurity diffusion coefficient in bcc phase of Ti-Nb-Ta, Ti-Nb-Zr, Ti-Mo-Zr, Ti-Mo-Nb and Ti-Mo-Ta systems at 1373 K and 1473 K were determined. Base on the experimental results in this work and literatures, a self-consistent atomic mobility database for bcc and hcp phases in Ti-Al-Cr-Fe-Mo-Nb-Ta-V-Zr was established using CALPHAD method. The composition-distance profiles and diffusion paths for diffusion couples in the experiments were simulated and compared with experimental results. The basic consistence between experimental and predicted results demonstrates the effectiveness of the parameters obtained in this work. Furthermore, using moving boundary model of DICTRA software, kinetics of β/α phase transition in Ti-Al-(V, Cr, Mo, Fe, Nb, Ta) alloys were studied, by which the redistribution process of different elements in the phase transition process and its influence on the phase transition were studied.

This work improved the multi-phase kinetic database of titanium alloy, which can provide theoretical support for the study of diffusion-controlled phase transition in titanium alloy.

Keywords: Titanium alloys, diffusion coefficients, atomic mobility, phase transformation, CALPHAD method

Ti合金原子移动性数据库的建立及其在β/α相变中的应用

刘立斌，白伟民，徐广龙，章立钢

中南大学材料科学与工程学院

摘要：现代钛合金的应用日益增加，其种类也日益增多，纷繁复杂的应用场景对钛合金的各种性能不断提出新的要求。掌握钛合金在不同成分和工艺下的微观组织演化规律对新型钛合金设计具有至关重要的作用。CALPHAD(CALculation of PHAse Diagram)方法作为已经发展成熟的计算材料研究方法，已经被应用于多种合金的设计与开发。利用CALPHAD方法建立的热、动力学数据库，可以对合金中的相组成和组织演化进行定量模拟，实现对合金在实际过程中组织演变规律的深入研究，建立成分/工艺-微观组织-性能之间的定量关系。然而，目前CALPHAD方法特别是动力学数据库在钛合金中的应用还不够深入。特别是由于扩散数据的匮乏，远远无法满足对组织演化定量模拟的需要。目前大多数模拟计算过程中使用的合金扩散系数还在使用与成分-温度无关的固定值，这显然与实际组织演化规律不符。本研究选取Ti-Al-Cr-Fe-Mo-Nb-Ta-V-Zr体系，使用实验测定+计算机优化的方法，获取钛合金中元素的扩散信息，并建立可以描述全成分-温度区间内扩散信息的数据库，并用于钛合金中的相变模拟，为新型钛合金的设计提供基础数据。

基于扩散偶方法，结合Whittle-Green方法和Hall方法测定了Ti-Nb-Ta、Ti-Nb-Zr、Ti-Mo-Zr、Ti-Mo-Nb和Ti-Mo-Ta体系bcc相1373 K和1473 K下的互扩散系数和杂质扩散系数。在本研究与文献中的实验结果基础上，利用CALPHAD方法建立了包含bcc相和hcp相的Ti-Al-Cr-Fe-Mo-Nb-Ta-V-Zr原子移动性参数数据库。利用数据库模拟得到的扩散路径与实验值进行对比，结果吻合较好。基于DICTRA软件的移动界面模型，对Ti-Al-(V，Cr，Mo，Fe，Nb，Ta)合金中的β/α相变过程进行了动力学模拟。研究了不同元素在相变过程中的再分布过程及其对相变的影响。

关键词: 钛合金， 扩散系数，原子移动性，相变，CALPHAD方法