The effect of crack-tip material evolution on fracture toughness - An atomistic simulation study of the Ti-V alloy system

The Embedded Atom Method (EAM) interatomic potentials and Molecular Dynamics simulations were used to study material evolution in a region surrounding the crack tip in the Ti-V system containing 0-35 at.% vanadium. The results show that depending on the amount of vanadium, which controls the relative thermodynamic stability of various phases in the Ti-V alloy system, material evolution in the crack-tip region can involve one or more basic processes. In the alloy containing up to approximately 15% V, material evolution takes place in a larger region surrounding the crack tip and dominated by the b.c.c.-->h.c.p. and the b.c.c.-->f.c.o. martensitic transformations and by a slip-type deformation process within the product (martensite) phase. In the alloy containing around 25 at.% vanadium, only materials adjacent to the crack tip undergo the b.c.c.-->h.c.p. transformation, and no formation of the f.c.o. martensite is observed. Deformation by slip, due to emission of the dislocations from the crack tip, still takes place but only in the parent b.c.c phase. When the amount of vanadium is 35 at.% or higher, no martensitic transformation takes place in the region surrounding the crack tip. Instead, the parent b.c.c. structure is found to undergo mechanical twinning. To quantify the effect of the aforementioned material evolution processes on fracture toughness, the component of the Eshelby's conservation F integral in the crack propagation direction was calculated for each of the alloys under investigation. This component of the F integral, which is equal to the force acting on the crack tip trying to extend it, was found to decrease as a result of each of the aforementioned material evolution processes in the region around the crack tip in each of the alloys studied, rendering the alloys tougher. The result further suggests that there is an optimum amount of vanadium in the Ti-V alloy system (around 15 at.%), which gives rise to the optimum cooperation of the basic material evolution processes and hence to the maximum toughness enhancement. Copyright (C) 1996 Acta Metallurgica Inc.