Gamma double prime precipitation kinetic in Alloy 718 (original) (raw)

Abstract

Gamma double prime (γ′′), precipitation was studied in Alloy 718 using isothermal and isochronal aging heat treatments applied between 943 and 1003 K. It is shown, that the coarsening behavior of γ′′ precipitates follows the coarsening kinetic predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The activation energy for γ′′ growth has been determined as equal to 272

mol−1 and seems to be controlled by volume diffusion of niobium in the matrix. The energy of the γ′′/matrix interface, Γ, has been found to be 95

m−2 by assuming that the γ′′ precipitates adopt a disk shape which minimizes the total energy. This energy includes a volume distortion term calculated from the Eshelby inclusion theory and a surface component which is assumed to be isotropic. This interfacial energy is discussed and compared with the energy of γ′/matrix and γ′′/matrix interfaces in other superalloys. The constant _K_′′ of the LSW law time dependence has been calculated using the value of interfacial energy and the activation energy of γ′′ precipitates coarsening and is found to be in good agreement with our experimental values.

Introduction

Alloy 718 is a nickel base superalloy mainly strengthened by γ′′ precipitates and largely used to produce critical components for turbine engines because of its excellent mechanical properties at high temperature, as well as good corrosion resistance. The γ′′ precipitate particles size controls the mechanical properties of the Alloy 718 especially the tensile and creep properties [1], [2], while the dislocation shearing mechanism in γ′′ is, at elevated temperatures, related to fatigue softening [3].

Paulonis et al. [4] showed that the strengthening particles in the Alloy 718 are precipitates of the metastable γ′′-Ni3Nb phase which crystallizes in the D022 body centered tetragonal (BCT) structure. These precipitates appear as disc-shaped particles with the following orientation relationship to the matrix: (001)γ||{001}γ and [100]γ||〈100〉γ. These authors have also shown the existence of a small amount of γ′ phase Ni3(Al, Ti) appearing as a fine dispersion of spherical particles. These γ′ precipitates contribute also to the strength of the alloy, but to a lesser degree than the γ′′ precipitates, which is due to the observed smaller volume fraction of 4% γ′ in comparison to 15% γ′′. The strengthening mechanism has been reported to be mainly due to the coherency strains between γ′′ and the matrix [5], [6]. The coarsening behavior of γ′′ particles has been studied by Han et al. [7]. These authors showed that the coarsening of γ′′ phase follows the time-law predictions of Lifshitz–Slyozov–Wagner (LSW) theory [8], [9] with a volume diffusion-controlled growth. However, the constant K of the time-law predictions of LSW theory, which includes a number of parameters, has never been calculated for the γ′′ phase due to the poor knowledge of the value of the energy of the γ′′/matrix interface, Γ, in Alloy 718.

This study is devoted to the study of the coarsening behavior of γ′′ precipitates, and to the assessment of the interfacial energy of γ′′/matrix interface, Γ, in order to compare the experimental coarsening kinetic with the theoretical prediction of LSW theory. This assessment of Γ is made through an investigation of the particle shape as a function of their length and by assuming that this shape minimizes the total (volume and surface) energy of particles.

Section snippets

Experimental techniques

The composition of the alloy is given in Table 1. The material was supplied by Aubert and Duval, as a hot rolled 80

mm diameter bar. Specimens blanks were cut along the longitudinal direction prior to heat treatment at 1243

K for 1

h and subsequent oil quenching. This thermomechanical treatment led to a small grain size of 13

μm and a homogeneous δ phase precipitation along the grain boundaries (Fig. 1). The cooling rate of oil quenching was fast enough to avoid γ′′ precipitation. Then, aging heat

Precipitation kinetic of γ′′ precipitates

The LSW theory [8], [9] of volume diffusion-controlled growth predicts that the kinetic equation can be written as:r3−r03=89ΓCeVm2DRTt=Kt,forsphericalparticles.

Ardell and Nicholson [10] modified Eq. (1) for cubical particles:a3−a03=649ΓCeVm2DRTt=K′t,forcubicalparticles,where r and a are the average radius and the half mean edge length of the growing particles at time t, respectively, while _r_0 and _a_0 are the initial size of the particles at the onset of the coarsening process. Γ is the

Conclusion

The coarsening kinetic in Alloy 718 has been identified as volume diffusion of niobium in the matrix and follows the LSW theory predictions. The value of activation energy of γ′′ coarsening is 272

mol−1. The interfacial energy of γ′′(Ni3Nb)/matrix in the Alloy 718 has been evaluated at 95

m−2 and is quite different from the interfacial energy of γ′/matrix and γ′′ (Ni3Ta)/matrix in other alloys because of the differences between chemical compositions. This approach permits to obtain a full

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