Quantification of precipitates and their effects on the response of nickel-base superalloy to shot peening

Abstract

This paper reports on a microstructural study of a nickel-base superalloy, Inconel 718, with a focus on quantifying precipitate density and their effects on conductivity variations. The study is motivated by eddy current (EC) characterization of residual stresses, where observed EC signals are attempted to correlate with stress profiles of shot peened superalloy surfaces. It has been observed that the correlation is less universal than anticipated, and in fact strongly influenced by the material hardness, or the aging conditions. For example, the soft sample surface exhibits significantly stronger EC signals than the fully hardened sample when both are shot peened at the same Almen intensity. Thus, the objective of the present study is to examine this complex material response against aging and shot peening treatments at the microstructure scale, by the use of techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We will describe preparations of a series of Inconel 718 samples that was aged and shot peened at various conditions, and present microstructural data obtained from SEM and TEM images such as precipitate densities, correlated with bulk properties such as the hardness and conductivity. ABSTRACT. This paper reports on a microstructural study of a nickel-base superalloy, Inconel 718, with a focus on quantifying precipitate density and their effects on conductivity variations. The study is motivated by eddy current (EC) characterization of residual stresses, where observed EC signals are attempted to correlate with stress profiles of shot peened superalloy surfaces. It has been observed that the correlation is less universal than anticipated, and in fact strongly influenced by the material hardness, or the aging conditions. For example, the soft sample surface exhibits significantly stronger EC signals than the fully hardened sample when both are shot peened at the same Almen intensity. Thus, the objective of the present study is to examine this complex material response against aging and shot peening treatments at the microstructure scale, by the use of techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We will describe preparations of a series of Inconel 718 samples that was aged and shot peened at various conditions, and present microstructural data obtained from SEM and TEM images such as precipitate densities, correlated with bulk properties such as the hardness and conductivity.

Effect of Microstructure on Electrical Conductivity of Nickel-Base Superalloys

“…As mentioned earlier, the notable difference with the samples I and VII is the significantly higher amount of fine secondary c¢ along with the coarse primary c¢ 510 precipitates, which were difficult to segregate quantita-511 tively. A similar reduction in the hardness with over 512 aging was reported earlier [18,19,29] for different materials, 513 where the effect was attributed to the formation of new 514 phases. SEM micrographs of RR1000 could not reveal 515 any significant new phase formation in samples I and 516 VII.…”

“…The bulk conductivity of IN718 was found to 87 increase, along with the volume fraction of secondary 88 phase precipitates. [18] In a recent study, Pereira et al [19] 89 compared the electrical conductivity and hardness 90 change of IN718 with its lattice constant, in order to 91 examine the underlying mechanisms. The resistivity 92 behavior for varying aging time and temperatures was 93 reported for different nickel-base superalloys including 94 Nimonic PE16 [20] and Waspaloy.…”

Analysis of Surface Properties of Nickel Alloy Elements Exposed to Impulse Shot Peening with the Use of Positron Annihilation

“…As demonstrated by Ortiz, the value and depth of compressive residual stresses in the surface layer of the C-2000 nickel alloy depend on the medium used in the shot peening process and surface mechanical attrition treatment [ 15 ]. As a result of shot-peening objects made of the Inconel 718 nickel alloy, compressive stresses arise and microstructure changes [ 16 , 17 ]. Shot peening also improves the fatigue properties of nickel alloys.…”

Feasibility of in situ controlled heat treatment (ISHT) of Inconel 718 during electron beam melting additive manufacturing

“…[3] Previous transmission electron microscopy (TEM) analysis indicated that peak aging of Alloy 718 sheet (i.e., after an aging at 720°C for 8 hours, 621°C for 8 hours) after solution treatment led to a 17±3% volume fraction of precipitates ( ). [4,5] It is also clear that this measured percentage of precipitates by TEM can be highly variable due to small sample size relative to entire product, if the underlying microstructure is heterogeneous. Such heterogeneous microstructures are usually found in cast and weld samples.…”