Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

Abstract

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read-Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T ¼ 800 C.

Influence of deformation on the Burgers orientation relationship between the α and β phases in Ti–5Al–5Mo–5V–1Cr–1Fe

“…At the beginning of deformation, dislocations are likely able to penetrate the semi-coherent interphase boundaries [11]. During further straining, however, the coherency of the interfaces is reduced due to the interaction of the boundaries with lattice dislocations [7]. The transition of the semi-coherent interfaces into incoherent ones can then significantly accelerate spheroidization of the lamellae via the processes of recovery and/or recrystallization within the phases, platelet grooving, etc.…”

“…Spheroidization is associated with a series of microscopic processes consisting of the development of transverse low-angle boundaries within the  phase; the formation and growth of grooves at the  interphase boundary; fragmentation of the lamellar platelets by the grooves; and final spheroidization and coarsening of the resulting small-aspect ratio  particles by diffusional processes [e.g., [3][4][5]. As shown in [6,7], the nature of interphase / boundaries can have a major influence on the kinetics of spheroidization.…”

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

“…Input data required to calculate the time t p needed to complete fragmentation via grooving in Ti-5Al-5Mo-5V-1Cr-1Fe comprise the slope m g ( $ 0.35), the molar volume of the a platelet material (V M ¼10,440 mm 3 ), and the a/b surface energy, taken here to be g ab ¼0.26 J/m 2 per Ref. [11]. The a-lamella thickness, d a , was set equal to $ 1.1 mm at both temperatures (Fig.…”

“…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 deformation and spheroidization of lamellar microstructures in titanium alloys with large amounts of the b-stabilizing elements (and hence b phase at a given processing temperature) have been studied to a lesser degree than for Ti-6Al-4V.…”

“…Lamella thickness, deformation temperature, and the nature of the intragranular and a/b interfaces (e.g., boundary energy) are the main factors influencing the mechanism and kinetics of spheroidization [8,10,11]. Hence, the kinetics of recovery, recrystallization, shear band formation, etc.…”