Design of Advanced Titanium-based Alloys with Controlled Microstructures and Enhanced Properties for Structural Applications

具有可調控微觀組織和優異性能的先進結構鈦合金的設計

Student thesis: Doctoral Thesis

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Award date1 Mar 2021

Abstract

Ti-alloys have wide-spread applications in many fields due to their excellent specific strength, corrosion resistance, and biocompatibility. However, further improvement of their mechanical properties has encountered the classical problem of the strength-ductility trade-off, which is still a difficult challenge. Mechanical properties of these alloys are closely related to their microstructures, which are in turn determined by the alloy composition as well as the processing technologies (e.g. heat treatment, deformation, etc.). Thus, a better understanding of the phase transformation mechanism is critical for us to better modulate and design the ideal microstructure for the unprecedented mechanical properties of the Ti-alloys. In this thesis, a systematic study from the basic phase transformation mechanism to the novel microstructure design and supreme titanium alloy manufacturing is carried out with integrated thermodynamic calculations, phase field simulations, and experimental methods.

Firstly, we start from a γ-TiAl system with a unique microstructure-composition relationship, which provides us a new understanding on the phase transformation mechanism and precipitates’ growth process. Based on the combination of thermodynamic databases as well as phase field simulations, we found that the V-shaped microstructure-composition relationship originates from the so-called pseudospinodal decomposition mechanism. Further analyses prove that the generated ultra-high nuclei densities during the initial nucleation stage can impact the later growth processes. It is the interplay between the nuclei density and the frequency of the coalescence events during the later growth stage that results in such a non-monotonic dependence of the average thickness of the γ lamellar on the overall alloy composition. This study provides an in-depth understanding of the composition-processing-mechanism-microstructure relationship, which offers new insights on the design of the desired microstructure for better mechanical and physical properties.

Then, based on the relationship between pseudospinodal decomposition mechanism and microstructure morphology, we further design a novel two-step aging heat treatment strategy to produce highly heterogeneous precipitate microstructure in β titanium alloys for a high strength and good ductility. Using the integrated thermodynamic databases as well as phase field methods, we show that the CM can be produced by (a) precursory spinodal decomposition in the parent β phase and (b) the interdiffusion of multi-layer systems with different solute elements’ concentrations. The designed CM ?? at different length scales can produce effectively gradient and hierarchical α + β dual-phase microstructures, with a mixture of refined and coarse α precipitate regions, or refined α precipitate regions and precipitate-free-zones. This new strategy sheds the light on a novel pathway to design a highly heterogeneous microstructure with the hierarchical and gradient precipitate distributions with tunable size/length and number density as well as the length scale of the spatial heterogeneity, all of which are beneficial to the desired properties.

Thirdly, we propose a computation-aided alloy design strategy and experimentally fabricate a kind of high performance Ti alloys with desired microstructures. Combined the solidification theory and thermodynamic calculations, we efficiently refine the grain structure of cast Ti alloy by tuning their supercooling capacity through microalloying during solidification. As compared to the base Ti-6Al-4V alloy in the same as-cast state, the grain size of the new alloy is refined down to 50%, and the yield strength and ductility are increased by 19.7% and 51.8%, respectively. In addition to the grain-size refinement, the addition of Cr and Fe also reduces the α lath thickness. This new cast alloy is anticipated to be well suitable for various structural applications as well as additive manufacturing.

Our studies provide an in-depth understanding of the composition-processing-microstructure-property relationship in the titanium-based alloys from a synergetic theory-simulation-experiment approach. More importantly, we provide a powerful tool for a rapid design of Ti-based alloys with desired properties for advanced structural and functional applications.