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 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 – T_β = 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 β matrix consisting of nodular α grains in the powder shell and α lamellae in the powder core. The stress/strain induced martensite formed during the milling associated with the heavier deformation at the powder surface areas contributes highly to the formation of an arrangement of nodular α grains by a recovery/recrystallization phenomenon of β and α"/α phases during the heating.