Microstructural characterization of superalloy 718 with boron and phosphorus additions

Boron and phosphorus additions are known to improve the stress rupture properties of IN-718. One possible mechanism to explain this property improvement relies on the boron and phosphorus additions slowing down the growth of gamma " and y' precipitates during high temperature service or aging. However, atom probe analysis found no segregation of boron and phosphorus to gamma-gamma " or to gamma-gamma' interfaces in the alloys with the high boron and high phosphorous levels. No difference in growth rates were found by transmission electron microscopy in the sizes of the gamma " or gamma' in alloys with high phosphorus and high boron as compared to commercial alloys and to alloys with even lower levels of phosphorus and boron. Atom probe analysis further found that much of the phosphorus, boron, and carbon segregated to grain boundaries. Creep curves comparing the alloys with high levels of phosphorus and boron and alloys with low levels of phosphorus and boron show a large difference in strain rate in the first hours of the test. These results suggest that the boron and phosphorus may have a direct effect on dislocation mobility by some pinning mechanism.