| Alloy 718 is a gamma"-strengthened Ni-base disk superalloy used in the aerospace industry, and it has been used prominently for decades. Though there has been extensive research into the processing/property relationships, there is very little known about the intermediate microstructure and mechanisms that are common in commercial 718 that dictate the yield strength. A variety of analytical techniques, including scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) were employed to investigate the microstructure of alloy 718 after various deformed conditions and heat treatment conditions.;The gamma" in alloy 718 following a commercial heat treatment was found to have both monolithic gamma" particles as well as composite particles in which gamma' and gamma" share a planar phase boundary. Isothermal heat treatments were applied to solutionized samples, and it was found that low heat treatment temperatures produced a large percentage of composite particles, but high temperatures led to the formation of a primarily monolithic structure. Additionally, these composite particles were shown to have a desirable stabilizing effect at high temperatures, and they were seen to grow much more slowly than their monolithic counterparts. STEM analysis was able easily show the morphology of any edge-on gamma" particles, and EDS was capable of determining the general morphology of in-plane particles. EDS was also useful in determining a rough volume fraction of each phase in tin foils, and it was found that the volume fraction of gamma' was slightly larger than that of gamma" after commercial heat treatments.;Deformation mechanisms were also characterized using STEM. Diffraction STEM was used on isolated dislocations and it was determined that isolated dislocations do not have contrast consistent with 1/2 dislocations, so some form of dislocation was thought to be operative. Atomic resolution STEM analysis uncovered a variety of mechanisms present in tensile samples. Deformation at 22°C was found to occur primarily through the isolated shearing of gamma" into an ISF configuration while the surrounding matrix is left without a fault by a partial loop forming at the gamma"/matrix interface. At 427°C, stacking faults were seen extending into the matrix, and isolate microtwins began to form in the gamma" phase. At 649°C, microtwins began to extend through the matrix and across large portions of the grain. The partials forming the microtwins are of multiple Burgers vector values, but they do not shear in a regular pattern. This indicates that the microtwin formation is likely assisted by localized reordering in the both the gamma' and gamma" phases. Even so, there was no evidence found for segregation of elements to faults or twins.;The understanding gained through this research has been used to inform precipitation and growth models. The precipitation and growth models will be combined with mechanistically-accurate yield strength models to improve predictions localized properties after a desired heat treatment. |
