Precipitation Behaviour of Wrought and Additively Manufactured Ti-5553 β-Ti Alloy

  • Deepak Sharma

    Student thesis: Doctoral ThesisDoctor of Philosophy


    This dissertation provides important insights into the phase transformations occurring in the wrought(conventionally produced) and the additively manufactured(AM)β-Ti alloy as a function of different heat treatment parameters. High-end characterisation techniques such as small-angle neutron scattering(SANS)and electrical resistivity measurement were performed to study the in situ nucleation and growth kinetics of phase transformations as a function of heat treatment parameters. Scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, and X-ray diffraction were used to understand the variability in the microstructural features. These techniques were also used to complement the nucleation and growth kinetics study. A Vickers’ microhardness tester was used to measure the microhardness of the samples to establish the relationship between microstructure and the associated mechanical properties. The availability of microstructure-property relationship data at different heat treatment conditions will benefit β-Ti users. The critical evaluation and understanding of additively manufactured β-Ti alloy will support the potential application of additive manufacturing in the aerospace sector. Further, a better understanding of the precipitation kinetics in the material will enable precise control and optimisation of the alloy properties. Ti–5Al–5V–5Mo–3Cr wt% (Ti-5553), a metastableβ-Ti alloy system, was selected for this work due to its desirable properties for high strength applications and increasing use in the aerospace industry.

    The influence of different ageing approaches (single ageing with different heating rates, and duplex ageing) on the microstructure evolution and the associated property of the wrought alloy was studied extensively. The observations indicated that single samples with slow heating rate and duplex aged samples produced approximately similar microstructures and microhardness at all ageing conditions. This was attributed to the same precipitation mechanism in both cases as observed via in situ electrical resistivity measurements. Both these samples showed extremely fine and uniform microstructures in comparison to single aged samples with a fast heating rate. The single aged samples with slow heating rate and duplex aged samples generally produced higher microhardness in comparison to single aged samples with fast heating rate. However, the difference in the microhardness tended to decrease with ageing time at higher temperatures (600 °C and 700 °C). A correlation (𝐻𝑣=348+247/√𝑑) between the microhardness values and the width of α-phase (𝑑) was established for all the aged wrought samples. This study also supported that the precursorisothermalω-phase(ωiso) precipitates have a direct influence on the nucleation of the α-phase leading to better alloy properties.

    Since the nanoscale ωisoprecipitatesassisted α-phase nucleation led to the higher microhardness, therefore, an investigation was done to study the precipitation kinetics of the ωisoprecipitates during low-temperature ageing (300 °C and 325 °C) up to 8 h to better control the mechanical properties. The precipitation kinetics were studied as a function of cooling rate (air cooling and water quenching) after β-solutionising because, for structural applications in the aerospace industry, the section thickness may determine the achievable cooling rate and therefore limit the mechanical properties. A combined in situ SANS and electrical resistivity measurement approach was used for this purpose. The SANS modelling was consistent with ellipsoid shaped particles for the ωisoprecipitates, for both air-cooled and water-quenched samples. The precipitates attained a maximum size (equatorial diameter) of~21 nm and ~17 nm after 2h and 4h of ageing the water-quenched and the air-cooled samples, respectively. Although the air-cooled samples showed delayed nucleation in comparison to the water-quenched samples, the volume fraction became approximately the same (~11 %) after ageing for 8h. The average value of the activation energy for ωisonucleation from the β-phase matrix was determined as ~122 kJ mol–1 from electrical resistivity data using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The aged sample after water quenching showed higher hardness than the aged samples after air cooling, with the hardness values trend with ageing time for both the cooling rates.

    Additive manufacturing of titanium alloys for aerospace applications is attractive due to design and near net shape benefits. However, the formation of solely β-phase after direct laser deposition of β-Ti alloy does not provide the desired properties. Hence, the ageing response(after single ageing with different heating rates, and duplex ageing)of the AMTi-5553 β-Ti alloy was studied to obtain the bimodal (β+α) microstructure providing improved properties. The as-built samples showed the presence of elemental segregation due to the formation of sub-structures within the β-phase matrix. The duplex aged samples and single aged samples with slow heating rate showed higher hardness when compared to single aged samples with fast heating rate, similar to the wrought alloy. This is attributed to the extreme refinement of microstructure due to ωisoprecipitates assisted α-phase nucleation. The single aged samples with a fast heating rate showed refined intragranular and discontinuous grain boundary α-phase when aged at 500 °C and 600 °C. This is important to get higher strength and higher ductility at the same time. The as-built and aged samples showed similar final microstructures with respect to the wrought samples. This boosts confidence in exploring the possibility of the use of the direct laser deposition method in applications. Similar to wrought alloy, a correlation (𝐻𝑣=348+168/√𝑑) between the microhardness values and the width of α-phase (𝑑) was established for all the aged AM samples. The solution-treated AM sample showed coarser intragranular α-phase precipitates(width = 139 ± 22 nm) with continuous grain boundary α-phase precipitates (thickness = 126 ± 12 nm)after ageing (600 °C/0.5 h).However, the AM sample directly aged (600 °C/0.5 h) after deposition exhibited refined intragranular α-phase precipitates with an average width of 10520 nm and discontinuous grain boundary α-phase precipitates with an average thickness of 46 11 nm. Refined intragranular α-phase precipitates in the AM sample led to increased hardness from 400 ± 5 Hv to 424 ± 4 Hv (~6% increase)in comparison to the solution-treated sample. The average activation energy for ωisoprecipitation from the β-phase matrix of as-built and solution-treated samples was calculated as ~130 kJ/mol using the modified JMAK model for electrical resistivity data.
    Date of Award2022
    Original languageEnglish
    Awarding Institution
    • Coventry University
    • Deakin University
    SupervisorMichael Fitzpatrick (Supervisor), Sitarama Raju Kada (Supervisor), Bo Chen (Supervisor), David Parfitt (Supervisor), Matthew Robert Barnett (Supervisor) & Daniel Fabijanic (Supervisor)

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