MARTENSITIC-TRANSFORMATION IN A DISPERSED TI-AL-V-FE BETA-PHASE AND ITS EFFECT ON FRACTURE-TOUGHNESS OF GAMMA-TITANIUM ALUMINIDE

A comprehensive experimental investigation of the materials microstruc ture (using optical, scanning and transmission electron microscopy), t he crystal structure of the continent phases (using X-ray and selected area electron diffraction) and fracture toughness (using three-point bending tests) has been carried out in a (Ti-40Al-32V-2Fe, wt.%) two-p hase gamma-TiAl-based alloy containing second-phase particles of a b.c .c. phase and in a (Ti-44Al-27V-0.6Fe, wt.%) single-phase gamma-TiAl a lloy. The chemical composition of the two-phase (gamma + beta) alloy w as designed in such a way that the beta-phase meets the following func tional requirement in the alloy: (a) thermodynamic stability of the be ta-phase relative to martensite is sufficiently high to prevent marten site formation at the ambient temperature except for the cases when th e material is subject to high stress and strain levels as encountered in the vicinity of sharp cracks; (b) volume increase due to the beta-p hase --> martensite transformation is maximum and (c) the initial two phases, gamma and beta are in a chemical equilibrium at high temperatu re (greater than or equal to 800 degrees C). The chemical composition of the single-phase gamma-alloy was selected so it coincides with that of gamma-TiAl matrix phase in the two-phase alloy. Experimental chara cterization of the alloys microstructure and fracture toughness showed that a deformation-induced beta --> alpha '' martensitic transformati on takes place within the beta-phase, and that due to the orthorhombic crystal structure of the alpha '' martensite the transformation is ac companied by a similar to 2.8% increase in volume. In addition, the oc currence of the martensitic transformation and the accompanied crack-t ip shielding effect is found to give rise to a nearly two-fold increas e in the fracture toughness relative to that of the single-phase gamma -TiAl alloy, in which no martensitic transformation takes place. (C) 1 997 Elsevier Science S.A.