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 microstructure (using optical, scanning and transmission electron microscopy), the 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-phase 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 alloy. The chemical composition of the two-phase (gamma + beta) alloy was designed in such a way that the beta-phase meets the following functional requirement in the alloy: (a) thermodynamic stability of the beta-phase relative to martensite is sufficiently high to prevent martensite formation at the ambient temperature except for the cases when the material is subject to high stress and strain levels as encountered in the vicinity of sharp cracks; (b) volume increase due to the beta-phase --> martensite transformation is maximum and (c) the initial two phases, gamma and beta are in a chemical equilibrium at high temperature (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 characterization of the alloys microstructure and fracture toughness showed that a deformation-induced beta --> alpha '' martensitic transformation takes place within the beta-phase, and that due to the orthorhombic crystal structure of the alpha '' martensite the transformation is accompanied by a similar to 2.8% increase in volume. In addition, the occurrence of the martensitic transformation and the accompanied crack-tip shielding effect is found to give rise to a nearly two-fold increase in the fracture toughness relative to that of the single-phase gamma-TiAl alloy, in which no martensitic transformation takes place. (C) 1997 Elsevier Science S.A.