Dispersed titanium-base beta phase transformation toughening of gamma titanium aluminide

The potential and the mechanism for transformation toughening of gamma titanium aluminide by the introduction of a metastable Ti-V-Al-Fe b.c.c. beta phase dispersion are investigated by (a) experimental measurements of the microstructure and fracture toughness in a two-phase {dollar}(gamma+beta){dollar} and in a single-phase {dollar}(gamma){dollar} Ti-Al-V-Fe alloys and (b) atomic-scale simulations of martensitic transformation in a Ti-V b.c.c. beta phase and interaction between a moving crack in the gamma-TiAl matrix and the dispersed beta phase.; A two-phase {dollar}(gamma+beta){dollar} alloy and a single-phase gamma alloy were designed and experimental characterization of the alloys microstructure and fracture toughness showed that a deformation-induced {dollar}betatoalphasp{lcub}primeprime{rcub}{dollar} martensitic transformation takes place within the beta phase, and that due to the orthorhombic crystal structure of the {dollar}alphasp{lcub}primeprime{rcub}{dollar} martensite the transformation is accompanied by a {dollar}sim{dollar}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.; Molecular Dynamics simulations of the martensitic transformation and its interaction with the fracture processes at atomic scale were carried out. Results clearly showed the heterogeneous character of the martensitic transformation and the role, lattice invariant deformation modes play in the progress of the transformation. The occurrence of the crack-tip martensitic transformation was shown to play a major role in causing the crack tip to blunt and, in turn, the crack propagation to cease. This transformation toughening effect was found more effective in opposing the crack propagation when the crack tip resides in the beta phase than when the crack tip is in the gamma-TiAl matrix and is approaching the beta phase dispersion.