An Approach Toward Understanding Unstable Gamma Prime Precipitate Evolution and Its Effect on Properties

The evolution of gamma prime precipitates generated from supersolvus solution heat treatment and controlled continuous cooling of two powder metallurgy (PM) disk superalloys are characterized and modeled. Variation in cooling rate for these alloys shows a tendency for unstable precipitate growth with slower rates. The degree to which this variation is observed in terms of its effect on particle spacing and deformation-induced defect interaction has not been well characterized previously. To better understand this effect, a series of laboratory heat treatments of varied cooling rates have been carried out and the mechanical response quantified via elevated temperature tensile tests followed by characterization. A phase-field-based approach is used to simulate the growth instability of gamma prime precipitates during slow continuous cooling; the simulated particle morphologies are compared to post-mortem characterization of laboratory-scale coupons using Focused Ion Beam (FIB) serial sectioning and volumetric reconstruction. Phase-field modeling is then used to interrogate the interaction of the particle morphology with planar dislocation evolution. It was determined that the incipient stages of particle evolution are dictated by interface growth instability more so than elastic anisotropy effects. Planar deformation in the presence of these larger, more evolved particles tended to promote Orowan looping, while smaller particles with smaller gamma channel widths showed a tendency for stacking fault formation under the conditions characterized. These observations suggest that particle morphology is a secondary effect to the overall alloy strengthening mechanism.