Effect of mechanical milling on the harmonic structure development during spark plasma sintering of Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe β-metastable titanium alloy (β-Cez alloy)

The formation of a harmonic arrangement of α grains in a beta metastable Ti alloy was realized for the first time. It was obtained by a powder metallurgy route including at first powder mechanical milling and further Spark Plasma Sintering. The harmonic arrangement consists in clusters of α lamellae in large beta grains, i.e. the core, that are surrounded by a continuously connected network (“the shell”) of fine nodular α and β grains of which the size is close to 1 µm. The structure and microstructure evolutions were analysed by electronic microscopy at several step of the process. In addition in situ characterizations using electrical resistivity and synchrotron X-ray diffraction allowed to determine the successive phase transformations leading to the final microstructure. SEM (BSE) observation of harmonic arrangement with α (dark) + β (light gray) phases in the beta metastable alloy (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe). The microstructure is obtained after SPS thermal treatment using a powder previously mechanical milled at room temperature. [Display omitted] The present study focuses on the formation of harmonic microstructures in a metastable β titanium alloy, the β-Cez alloy (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe – Tβ = 895 °C). Previous studies emphasized mainly fabrication and improvements in the mechanical properties of alloys prepared by powder metallurgy and showing a harmonic structure. In this study, the harmonic structure was obtained after mechanical milling of the initial powder, followed by a Spark Plasma Sintering (SPS) process. During the process, phase transformations occur that are dependent on the initial state of the powder. Moreover, their kinetics depend on the thermo-mechanical history of the powder. In order to analyze the microstructure formation, the behavior of milled and non-milled powders was studied during a heat treatment similar to the one applied during the SPS process. In-situ high-energy X-ray diffraction was used to characterize the evolution of phases during the thermal treatment. Additional in situ electrical resistivity measurements were carried out on sintered compact specimens. Characterizations evidenced that the initial powder is in a β metastable state. After mechanical milling, stress/strain induced α” martensite was observed inside the powder’s β grains. The stepwise microstructural characterization revealed the influence of the initial state of the mechanically milled powder on the formation of a harmonic α arrangement in the