The Investigation of Hierarchical α Microstructure in the Metastable β Titanium Alloys Using Advanced Electron Microscopy

Abstract

Metastable β-titanium alloys are increasingly popular due to their excellent mechanical properties and biocompatibilities. They play a paramount role in aerospace and biomedical industries due to being light-weight, high-strength, and corrosion-resistant. The α microstructure in the β grains is crucial to determining the mechanical properties, such as strength and ductility. The morphology, distribution, texture, and volume fraction of α precipitates in metastable β titanium alloys can be tuned to achieve strength-ductility trade-off. Quite a few factors, including grain boundary, twinning boundary, dislocations, and metastable phases are able to assist the nucleation of α through different phase transformation mechanisms and thus can affect the α microstructure. Recently, a novel highly-indexed {10 9 3}<3 ̅31>β type twinning was found in the metastable β Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) commercial alloy, widely used in aerospace and biomedical fields, such as landing gears of Boeing 787 Dreamliner airplane and artificial joints. With the pre-formed of highly-indexed twinning in Ti-5553 alloy, hierarchical α microstructure can be generated via isothermal aging. This kind of α microstructure is related to achieving the strength-ductility trade-off. The hierarchal α microstructure combines coarse α layers, alpha sub-layers, and fine-scaled α precipitates. This study utilizes advanced electron microscopy techniques such as scanning electron microscopy and transmission electron microscopy, combined with machine-learning-based microstructure quantifications, to characterize the hierarchical α microstructure in Ti-5553 alloy. There are three aspects to be discussed: the highly-indexed {10 9 3}<3 ̅31>β type twinning in Ti-5553 alloys, which possesses hierarchical substructure with different metastable phases; the α microstructure without the influence of pre-formed highly-indexed twinning, where heating rates plays essential roles in α phase transformation mechanisms; the hierarchical α microstructure with three types of α precipitates influenced by the pre-formed highly-indexed twinning. Overall, this study presents the hierarchical α microstructure in Ti-5553 alloy, and explores the influence of highly-indexed {10 9 3}<3 ̅31>β type in the α phase transformation pathways. It can help understanding α microstructural evolutions and tuning α microstructures with different pre-formed interfaces/metastable phases in metastable β titanium alloys.