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Phase transformation studies in a metastable beta Ti-5553 alloy and near alpha Ti-Mo alloys Mendes Rodrigues, Mariana Carla


The control of the solid-state phase transformations between the hexagonal close-packed (hcp) α and body-centered cubic (bcc) β phases of Ti-alloys is of crucial importance to achieve optimal mechanical properties. The present study explores the applicability of laser ultrasonics for metallurgy (LUMet) as an in-situ technique for the measurement of phase transformations in Ti-alloys. Through the monitoring of ultrasound velocity changes during isothermal treatments of a metastable β Ti-5553 alloy (Ti-5Al-5Mo-5V-3Cr, wt.%) it was seen that LUMet can successfully capture the rate of the β → α + β transformations. Quantitative assessment of the transformation kinetics from the ultrasonic velocity requires, however, validation with ex-situ characterization where direct measurements of phase fractions are possible. In view of the challenges associated with most ex-situ analyses, a more systematic methodology was proposed in order to semi-automate phase fraction measurements while decreasing the number of user input parameters. It was observed that ultrasonic velocity changes and transformed phase fractions are nearly linearly correlated to each other, similar to what has been reported for steels. The proposed correlation has been verified with an analytical model which accounts for the temperature dependent macroscopic elastic constants and densities of the α and β phases. LUMet measurements on near α, quasi-binary, Ti-Mo alloys during continuous heat treatments further showed the sensitivity of the transformation behavior to alloy composition. Retardation of phase transformation due to Mo additions has been confirmed with the ultrasonic velocity measurements during both continuous heating and cooling. Further, the wave velocity behaviors observed during cooling at varying rates are consistent with the formation of diffusional transformation products. Nano-scale analyses on selected specimens using atom probe tomography (APT) indicated that Mo tends to be trapped within moving α / β boundaries, which may contribute to slower phase transformations. This work demonstrates that LUMet is an effective tool to monitor phase transformations in Ti-alloys. The results pave the way for further expansion of the use of LUMet during more complex thermo-mechanical treatments in a wide range of commercial Ti-alloys.

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