Microstructure Engineering in Metastable Beta Titanium Alloys by Tuning Highly-Indexed Deformation Twinning

In order to reduce the fossil fuel consumption and greenhouse gas emissions, there is a continued demand for the new generation lightweight materials for better performance. Titanium alloys have attracted significant amount of attention in aerospace, automobile, chemical and bio-medical industries, due to the combination of great properties, such as high specific strength, great toughness, excellent corrosion resistance and good biocompatibility. The mechanical property of the titanium alloys is highly dependent on the microstructure, more specifically the HCP structure alpha precipitate microstructure in the BCC structure beta phase matrix. Recent studies have shown that various fine-scale alpha microstructures can be achieved in titanium alloys with the assistance of different metastable phases, such as hexagonal structure omega phase. For example, our previous study in a metastable beta Ti-5Al-5Mo-5V-3Cr (wt%, Ti5553) has shown that alpha microstructure of three different size scales, namely refined, more-refined and super-refined alpha, can be generated via the selection of different roles of nanoscale omega phase particles.<br/>This presentation is mainly focused on the use of analytical electron microscopy techniques to investigate the fundamental mechanisms in the development of hierarchical microstructure in the metastable beta titanium alloys. An example of designing high-strength titanium alloys for aerospace application via microstructure engineering will be introduced. In this study, the factor of mechanical treatment has been taken into consideration for microstructure engineering in a metastable beta Ti5553 alloy. For the first time, a novel highly-indexed {10 9 3}&lt;3-3-1&gt; type deformation twin has been characterized in the cold-rolled Ti5553. The substructure of this novel high-indexed deformation twin has been studied using aberration-corrected scanning transmission electron microscopy. Nanoscale hexagonal omega phase and orthorhombic alpha double prime phase has been observed in the interior of highly-indexed deformation twin together with nanoscale omega phase layer at the twin/matrix interface. The response of cold-rolled Ti5553 to the subsequent heat treatment was then studied via scanning electron microscopy and transmission electron microscopy. Hierarchical alpha precipitate microstructure composed of coarse alpha plates, alpha sub-layers and fine-scale alpha precipitates has been observed. It is speculated that the coarse alpha plates may form from the pre-formed {10 9 3}&lt;3 -3 -1&gt; twin boundary via the assistance of twin boundary omega phase; alpha sub-layers and fine-scale alpha precipitates may be related to the substructure in the interior of {10 9 3}&lt;3 -3 -1&gt; twin including alpha double prime phase, omega phase and dislocations. The influence of highly-indexed deformation twin and pre-formed omega phase on the formation of hierarchical microstructure in the metastable beta titanium alloys will be discussed in detail. This work was supported by the National Science Foundation under the contract No. 2122272.