Role of interphases in the deformation mechanisms of an α/β titanium alloy at 20 K

Two-phase alpha/beta titanium alloys are used in cryogenic applications owing to their high strength and ductility at such temperatures. The alloy studied here is a Ti-6Al-4V made using a powder metallurgy technique. The alpha phase exhibits two morphologies some grains have a globular shape whereas others are lamellar. Five or ten parallel laths form colonies of similarly oriented variants. The deformation mechanisms at 20 K have been studied by Atomic Force Microscopy, post mortem and in situ straining Transmission Electron Microscopy (TEM). The relevant point of the present work is that the nature of alpha slip systems depends on the grain morphology. In globular grains. the main slip systems are prismatic as commonly observed in Ti alloys. On the contrary. in lath colonies the basal slip system is activated. This difference is attributed to the presence of the beta phase between laths. It is clearly shown that the alpha/beta interphases play a key role as they act as an alpha glide system filter. As a consequence, a lath colony behaves as a single grain. which suggests that ductility may be controled by the colony size. (C) 2001 Elsevier Science B.V. All rights reserved.