Effect of ion implantation on the grain boundary strength of heat treated Inconel X750

Abstract

Bending tests performed with notched cantilever micro-beams were used to measure the initiate of plastic deformation from grain boundaries in Ni+ and He+ implanted Inconel X750. Whereas the local grain boundary yield stress followed a complex nonlinear dependence upon grain boundary misorientation angle up to about 60⁰, it showed a simpler continuous decreasing dependence upon grain boundary energy. This suggests that the grain boundary strength of the heat treated Inconel X750 alloy, in all conditions tested, is related to the grain boundary energy and decreases as the energy increases. Both Ni+ and He+ ion implantation increased the local grain boundary yield stress over the complete range of boundary misorientation investigated. We attribute this to the strengthening effect resulting from segregation of ion-induced crystal defects and helium bubbles/cavities to the grain boundaries. Our data suggest that the strengthening effect from 5000 appm He+, implanted at 25 °C, is significantly higher than what is produced by 1 dpa crystal damage associated with Ni+ implantation at 25 °C. Analysis of these findings in terms of a dispersed barrier hardening model illustrate that accumulated helium is more effective than irradiation-induced crystal defects, such as stacking faults tetrahedra and small dislocation loops, at preventing dislocation nucleation and/or movement from grain boundaries in the heat treated Inconel X750 alloy.