Spheroidization of the lamellar microstructure in Ti–6Al–4V alloy during warm deformation and annealing

Microstructure evolution during warm working of Ti–5Al–5Mo–5V–1Cr–1Fe at 600 and 800 °C

“…The a/b interphase boundaries were sharp and narrow without any visible signs of stresses in contrast to previous results for alloys such as Ti-6Al-4V [6]. This observation may imply a high level of coherency between the a lamellae and the b matrix.…”

“…In several earlier works, the relative increase in diffusivity due to substructure developed during prior warm working was assumed to be of the order of a factor of 10 (e.g. [5,6]), thus leading to Mo diffusivities of 3.2 Â 10 À 4 mm 2 /s at 600 1C and 0.017 mm 2 /s at 800 1C. Substituting these values of D and the other material and geometry parameters into Eqs.…”

“…The thinning of lamellae during deformation of Ti-6Al-4V was discussed in Refs. [6,7]. In these previous efforts, thinning observations were interpreted in the context of three main factors: homogeneous strain itself, a socalled ''stereological effect'' (apparent decrease in thickness due to rotation of lamellae with respect to the cross-section), and strain partitioning between the a and b phases associated with differences in flow stress.…”

“…Hence, the kinetics of recovery, recrystallization, shear band formation, etc. within a lamellae strongly affect fragmentation [6]. In addition, the evolution of (semi)coherent a/b interphase boundaries into noncoherent ones during deformation [11] may enhance diffusion along the interfaces [8,12] but make slip transfer more difficult.…”

“…The mechanisms and kinetics of the spheroidization of lamellar microstructures in a/b titanium alloys such as Ti-6Al-4V have been investigated over a wide range of hot-and warm-working temperatures [5][6][7][8]. At high temperatures, spheroidization is often associated with (i) the generation of transverse boundaries within alpha lamellae, (ii) the subsequent formation of grooves associated with the instability of 901 dihedral angles between interphase a/b boundaries and intraphase a/a boundaries, and (iii) complete lamellar fragmentation by deepening of the grooves from both sides of the a lamellae.…”