THE EFFECT OF CRACK-TIP MATERIAL EVOLUTION ON FRACTURE-TOUGHNESS - ANATOMISTIC SIMULATION STUDY OF THE TI-V ALLOY SYSTEM

The Embedded Atom Method (EAM) interatomic potentials and Molecular Dy namics simulations were used to study material evolution in a region s urrounding the crack tip in the Ti-V system containing 0-35 at.% vanad ium. 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 inv olve one or more basic processes. In the alloy containing up to approx imately 15% V, material evolution takes place in a larger region surro unding 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 containin g around 25 at.% vanadium, only materials adjacent to the crack tip un dergo 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 pa rent 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 me chanical twinning. To quantify the effect of the aforementioned materi al evolution processes on fracture toughness, the component of the Esh elby'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 vanadi um in the Ti-V alloy system (around 15 at.%), which gives rise to the optimum cooperation of the basic material evolution processes and henc e to the maximum toughness enhancement. Copyright (C) 1996 Acta Metall urgica Inc.