Mechanically driven phase transformation in titanium and Ti alloys at high pressure torsion

严重塑性变形(SPD)越来越广泛地用于纳米结构钛和钛基合金的加工，至少在实验室范围内是这样。然而，SPD工艺不仅细化了组织，而且还引发了伴随的相变。根据实验技术和所用压力环境的不同，α-Ti向高压ω-Ti的相变发生在2~12 Gpa之间。此外，外加剪应力可以为马氏体相变提供额外的驱动力。在这方面，高压扭转SPD的应用有利于研究α-Ti到ω-Ti的转变，因为该技术同时施加高压和剪切应变，并且条件可控。在第一个资助期内，发现钛合金中的高温高温热诱发相变依赖于合金元素的数量，特别是稳定高温β-Ti的元素。本项目的目的是研究高温高温热处理在高温和低温下诱发的Ti-Fe和Ti-Co合金中的机械驱动相变，以便能够描述α→ω和β→ω相变过程中的扩散过程和马氏体(无扩散)机制之间的相互作用。根据第一个注资期的结果，我们想要描述微观结构特征，特别是微结构缺陷对亚稳态相稳定性的影响，并解释所获得的纳米结构(局部相组成、颗粒的性质和相界)与所产生的力学性能之间的关系。最后，找到了一种用CALPHAD方法描述亚稳态相稳定性的方法。用X射线衍射仪(包括高温原位X射线衍射仪)、差示扫描量热仪(DSC)、扫描电子显微镜(SEM)、常规和分析电子显微镜(包括ACOM电子显微镜)和原子探针层析(ACOM)来确定样品的微观结构、物相组成和各相的热稳定性。将评估材料的初始状态(合金化、相组成和组织特征)和工艺参数(压力、温度、应变和应变率)对相变的影响。高压相图的计算将作为实验工作的补充。这些研究将有助于加深对钛基合金机械驱动相变的基本认识。所获得的结果不仅对基础材料科学具有重要意义，而且还将有助于阐述提供高强度和高韧性的超细晶粒材料的热机械处理原理。

Severe plastic deformation (SPD) is increasingly intensively used for processing of nanostructured titanium and Ti-based alloys, at least on the laboratory scale. However, the SPD process does not only refine the microstructure, but it also initiates concomitant phase transformations. Depending on the experimental technique and on the pressure environment used, the phase transition of α-Ti to the high-pressure ω-Ti occurs between 2 and 12 GPa. Furthermore, external shear stresses can provide an additional driving force for martensitic phase transformation. In this respect, the application of SPD by high pressure torsion (HPT) is advantageous for studies of the α-Ti to ω-Ti transition, because this technique applies high pressure and shear strain simultaneously, and under well-controlled conditions. During the first funding period it was revealed that the HPT-induced phase transitions in Ti alloys depends on the amount alloying elements, especially of the elements stabilizing high-temperature β-Ti.The aim of this project is to investigate mechanically driven phase transformations in the Ti–Fe and Ti–Co alloys that were induced by HPT at elevated and cryogenic temperatures, in order to be able to describe the interplay between the diffusion processes and the martensitic (diffusionless) mechanism during the α → ω and β → ω phase transformations. Based on the results of the first funding period, we want to describe the effect of the microstructural features, in particular the effect of microstructure defects, on the stability of metastable phases, and to explain the relationship between the nanoscale structure achieved (local phase composition, nature of the grain and phase boundaries) and the resulting mechanical properties. Finally, a way for describing the stability of the metastable phases by using the CalPhaD method should be found. The microstructure and phase composition of the samples and the thermal stability of individual phases will be determined by using XRD (including in situ XRD at high temperatures), DSC, SEM, conventional and analytic TEM (including ACOM TEM), and Atom Probe Tomography. The influence of the initial state of the material (alloying, phase composition and microstructural features) and processing parameters (pressure, temperature, strain and strain rate) on the phase transformations will be evaluated. The experimental work will be complemented by the calculations of the high pressure phase diagrams. The proposed investigations will contribute to the basic knowledge regarding the mechanically driven phase transitions in Ti-based alloys. The obtained results will be important not only for fundamental materials science, but they will also allow to elaborate the principles of the thermo-mechanical treatments of ultrafine grained materials providing the high level of strength and ductility.