Evoluzione della relazione cristallografica tra la fase alfa e la fase beta in una lega Ti-6Al-4V compressa a 800°C

The mutual relationship between phases in two phase titanium alloys, αHCP and βBBC, is such to have: {0001}α || {110}β e {11-20}α|| {111}β, which in literature are known as Burgers orientation relationships (OR). The coherency of the two phases is controlled by this crystallographic mutual relationship. Loss of coherency between phases during deformation can originate from a non parallelism between the two boundary crystallographic planes. The present work focuses on lamellar αHCP / βBBC interface coherency evolution in a Ti-6Al-4V alloy subjected to hot compression at 800°C. Strain rate was 10-3 s-1 and deformation was carried out to average true strains ε = 0.29, 0.69, and 1.20. Loss of coherency was found at strains ε ≥ 0.40. For these strains, the lamellar α+ βmicrostructure also evolved toward spheroidization. The loss of interface coherency was thus associated with an acceleration of the lamellar microstructure dynamic spheroidization which not only regarded the α phase, but, eventually, also the β phase (i.e. at ε > 0.69). To date, quite little data have been published on the titanium α-β alloys OR modifications during deformation. Thus, the aim of the present work was to evaluate deviations from the α/ βBurgers OR in a Ti-6Al-4V alloy with respect to alamellae spheroidization process during deformation at 800°C. The extent of bending / shearing was found to be relevant in either the vicinity of colony boundaries, due to deformation incompatibility, and in colonies that are approximately parallel to the compression axis. The alloy microstructure evolution during deformation was essentially associated with α platelet pinch-off / fragmentation process, which, in turns, resulted in spheroidization of the remaining lamellae. In the case of two-phase titanium alloys with a lamellar microstructure, continuous dynamic recrystallization (CDRX) within each phase is only the first step toward spheroidization. One of the effects of CDRX is the formation of unstable triple points formed at the interphase and α-to-α or β-to-β boundaries. Equilibrium between the various surface tension forces at the interphase triple junctions leads to a diffusive flux to the interphase surface and groove formation along the α-to-α boundary (the so-called boundary splitting mechanism). In the present case CDRX appeared to play a relevant role in the fragmentation process of both phases. Since diffusivity along inter-phases strongly depends on the energy of the interface boundaries, the kinetics of spheroidization process should increase when interphase boundaries lose their coherency. In this paper, this assumption was considered to describe and follow the lamellae spheroidization processes with strain.