Formation of Serrated Grain Boundaries and Their Effect on the Mechanical Properties in a P/M Nickel Base Superalloy

Abstract

To improve the crack propagation resistance of disk alloys, a study of the influence of the different heat treament parameters, which control the grain boundary geometry, was performed on Astroloy fabricated by a powder metallurgy process. Grain boundary serrations, associated with the precipitation of adjacent coarse y ' precipitates, were obtained by heat treating above the solvus for the alloy followed by slow cooling. The size of these coarse y ' precipitates can be controlled by the heat treatment parameters which include: homogenizing temperature, cooling rate and cooling range.The presence of grain boundary precipitates creates serrations by discontinuous precipitation of the y ' phase in the grain boundary region. The precipitates responsible for serrations were uniformly distributed on both sides of the boundaries. The amplitude of the serrations increase with the misorientation between grains. Tensile tests at 750°C showed no real difference between the microstructures, with and without serrations; however the creep fatigue crack growth rate in the alloy with serrated boundaries was found to be significantly slower than that observed in the unserrated material.

The Effects of Heat Treatment and Microstructure Variations on Disk Superalloy Properties at High Temperature

“…The changes in various mechanical properties due to heat treatment in similar disk superalloys have been recently related to effects on microstructural features including morphology of the γ ′ precipitates [7], grain boundary serrations [8], and grain boundary carbides [9]. Processing-microstructure models are in active development [10,11] that could potentially model how processing paths can tailor such microstructural features.…”

Evaluation of the IN 939 Alloy for Large Aircraft Engine Structures

“…The slow cooling rate from 1160 °C down to 900 °C is, as already mentioned, beneficial to the mechanical properties due to the serrated grain boundaries formed, a feature which reduces the tendency for oxidation assisted intergranular high temperature creep crack growth in both ' [13] and " [14] forming superalloys.…”

The effect of environment and structure on creep fatigue crack propagation in a P/M astroloy

“…For this study an Astroloy elaborated with rotating electrode powders compacted by extrusion was used. Three different microstructures have been tested in air and in vacuum : a fine-grained material (20-30 pm) with smooth grain boundaries, a coarse-grained material (65-90 pm) with smooth grain boundaries and a coarse-grained material (45-65 pm) with serrated boundaries ; procedures used to elaborate such materials are detailed in [4].…”