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

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}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}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}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

Comparación De La Entalpía De Formación De Aleaciones Ni-X (X= AL, V, FE, CU, ZR, NB, HF Y TA) En La Estructura B2 Con La Estructura B32 Obtenidas Por Medio De Cálculos De Primeros Principios, Realizado En La Untels.

En este trabajo de investigación se analiza el comportamiento de las diferentes entalpías de formación de las aleaciones Ni-X (X=Al, V, Fe, Cu, Zr, Nb, Hf y Ta) en las estructuras B2 y B32, también se calcula las propiedades mecánicas (módulo de comprensibilidad) y electrónicas (parámetro de red, densidad de estado, momento magnético) de aquellas estructuras y en sus estados fundamentales. Se realizó el cálculo de primeros principios o llamado también ab-initio, utilizando la teoría funcional de la densidad (DFT) en el método de ondas planas aumentas y linealizadas con potencial completo (FP-LAPW), empleando la aproximación de la gradiente generalizada (GGA), implementado en el código computacional WIEN2k. En los resultados se encontró que la estructura más estable es la del NiAl-B2 con una entalpía de formación de -0.65 eV/mol y la más inestable es la del NiTa-B32 con una entalpía de formación de 0.14 eV/mol. La estructura con mayor módulo de compresibilidad es la del NiTa-B2 con 209.36 GPa y la de menor módulo de comprensibilidad es la del NiZr-B32 con 133.20 GPa.Tesi