Molecular dynamics embedded atom method simulations of crack-tip transformation toughening in FeNi austenite

Molecular dynamics simulations of he crack-tip f.c.c. → b.c.c. martensitic transformation in Fe-20Ni and Fe-40Ni (at.%) were carried out. Embedded atom method potential functions were used to represent the interaction between the atoms in both crystal structures. The results show that in both alloys the crack-tip stresses are relaxed owing to formation of the b.c.c. phase which is twinned to help accommodate the transformation strain associated with the f.c.c. → b.c.c. transformation. As a result of the transformation, the crack tip becomes blunted which, in turn, enhances the material toughness. To quantify the effect of the crack-tip f.c.c. → b.c.c. transformation on material toughness, components of Eshelbyʹs F conservation integral were evaluated in the two alloys. It was found that the F1 component of the integral which represents the force acting to propagate the crack tip has become negative owing to the transformation. As a result, crack propagation has ceased. Comparison of the two alloys showed that in Fe-40Ni dislocation emission in addition to the transformation can take place at the crack tip, causing additional crack blunting.