Deformation and Damage Mechanisms in Inconel 718 Superalloy

Abstract

The deformation and damage mechanisms in wrought, double-aged, Inconel718 superalloy (AMS 5663D) tested under monotonic tensile strains of 2% and lo%, fully-reversed fatigue, and tensile strain (2% or 10%) followed by fully-reversed fatigue conditions were investigated by examining the microstructures of representative specimens. All tests were conducted in air at room temperature. The specimens were sectioned and examined by transmission electron microscopy to reveal typical microstructures as well as the active deformation and damage mechanisms. Specific mechanistic features addressed include the type of slip, interaction of dislocations with y", y' and the carbides (precipitated during solidification and the subsequent heat treatment received by the superalloy), twinning, and microcracking. In all cases the microstructure of the as-received superalloy is employed as the reference to establish the nature and distribution of the secondary phases before the superalloy is subjected to different types of mechanical loading. Results of the investigation and comparisons of the mechanisms of deformation and damage observed under monotonic tensile strain, fully-reversed fatigue, and tensile strain followed by fully-reversed fatigue in Inconel 718 superalloy are reported.Superalloys 718,625,706 and Various Der~at~ves

Deformation Mechanisms and Fatigue Behavior of Prestrained Inconel 718 Superalloy

“…The evolution of stress ranges for both the tensile and compressive prestrained specimens are nearly the same and stress ranges in both the cases are lower than the stress range observed for the asmachined specimen. A similar result was previously observed by Kalluri et al on a 10% tensile prestrained specimen tested subsequently in fully-reversed fatigue at a strain range of 0.8% (16).…”

“…Another intermetallic phase, the coherent fee y' with a volume fraction of 4% also contributes to the strength of . Needle-shaped precipitates of orthorhombic 6 phase and continuous networks of niobium-rich carbides observed on grain boundaries in the as-received material were also previously reported (16).…”

“…Microstructure of the as-received material was examined with transmission electron microscopy (TEM) and the different phases observed in the alloy were reported in detail previously (16). The main strengthening intermetallic phase in this material is y" with a volume fraction of 15-20%.…”

“…Optical microscopy conducted on the as-received material revealed equiaxed grains with an average grain size of 10 pm. At room temperature, the as-received material exhibited the following average tensile properties: a) 0.2% offset yield strength: 1140 MPa, b) ultimate tensile strength: 1410 MPa, and c) percent reduction in cross-sectional area: 43.3 (16).…”

Prestraining and its Influence on Subsequent Fatigue Life

“…The as-received material contained equiaxed grains with an average grain size of I0 larn.The microstructure and the different phases observed in the as-received material were previously reported (ref. 5). IN 718 derives its strength mainly from an intermetallic phase, y", which precipitates coherently in the facecentered-cubic matrix with a volume fraction of about 15to 20 percent (refs.…”