MOLECULAR-DYNAMICS EMBEDDED-ATOM METHOD SIMULATIONS OF CRACK-TIP TRANSFORMATION TOUGHENING IN FE-NI AUSTENITE

Molecular dynamics simulations of the crack-tip f.c.c. --> b.c.c. mart ensitic transformation in Fe-20Ni and Fe-40Ni (at.%) were carried out. Embedded atom method potential functions were used to represent the i nteraction between the atoms in both crystal structures. The results s how that in both alloys the crack-tip stresses are relaxed owing to fo rmation of the b.c.c. phase which is twinned to help accommodate the t ransformation strain associated with the f.c.c. --> b.c.c. transformat ion. As a result of the transformation, the crack tip becomes blunted which, in turn, enhances the material toughness. To quantify the effec t of the crack-tip f.c.c. --> b.c.c. transformation on material toughn ess, components of Eshelby's F conservation integral were evaluated in the two alloys. It was found that the F-1 component of the integral w hich 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 disloc ation emission in addition to the transformation can take place at the crack tip, causing additional crack blunting.