On the Role of Tertiary γ' Precipitates in the Creep Behaviour at 700�C of a PM Disk Superalloy

Abstract

The creep behaviour of the PM NR3 nickel-based superalloy has been studied at 700°C in a wide stress range as a function of its microstructure. It was first confirmed that at this critical temperature a coarse-grained microstructure was preferable to a fine-grained one to obtain the highest creep resistance. The role of the tertiary γ ′ phase precipitates in the creep behaviour of coarsegrained NR3 was investigated at two stress levels typical of both low stress and high stress creep regimes. Careful identification of dislocation mechanisms by transmission electron microscopy was performed in creep strained specimens in order to correlate the macroscopic behaviour with the creep controlling mechanisms. Prior elimination of the tertiary γ ′ precipitates using an adequate overageing heat treatment promotes easy glide of a/2<110> perfect matrix dislocations between the secondary γ ′ precipitates and therefore confers to the alloy a poor creep resistance. On the contrary the presence of fine tertiary γ ′ particles impedes propagation of the perfect matrix dislocations which are forced to cut the γ ′ precipitates through two different modes of dissociation depending on the local precipitate distribution. Reduction of the γ phase channel width promotes the decorrelation in the matrix of the two a/6<112> partial dislocations constituting an a/2<110> perfect dislocation. This mechanism which leads to stacking fault configurations extending through both γ and γ ′ phases evidences the strong resistance of the microstructure against creep deformation. Coupled analysis of the creep behaviour and of the dislocation structures in the low stress creep regime indicates a growing involvement of the grain boundary areas in the macroscopic creep deformation.

Deformation Mechanisms in Ni-Base Disk Superalloys at Higher Temperatures

“…The spacing between the secondary γ′ is relatively wide which allows for the a/2<110> dislocations to pass between the secondary γ′ with relative ease. There is however a high volume fraction of tertiary γ′ precipitates that are present in the γ channels between the secondary γ′ precipitates that do provide resistance to dislocation motion and can ultimately affect the overall creep behavior as shown by Locq et al [13]. Shearing of these fine precipitates by a/2<110> dislocations will create anti-phase boundaries (APBs) in the precipitates.…”

Gamma Prime Precipitate Evolution During Aging of a Model Nickel-Based Superalloy

“…As before, the γ lattice parameter was obtained from the fitted peak positions using the Nelson-Riley function, Eq. [4]. Conversion of the measured lattice parameters to lattice misfit was performed using Eq.…”

“…The associated lattice parameter was determined from the fitted superlattice positions using the Nelson-Riley method, in which the equivalent lattice parameters from each peak were plotted against the associated absorption error, (δa/a) abs , Eq. [4], and the lattice parameter was taken as the linear intercept. [15] The positions of the γ peaks were obtained by fitting the fundamental reflections with two Gaussian functions, one of which was constrained to the position associated with the previously determined γ′ lattice parameter.…”

“…[2] However, a unimodal distribution of such fine precipitates is difficult to achieve in practice and has been associated with reduced ductility [3] and creep performance. [4,5] As such, a multimodal particle size distribution is often utilized, comprising secondary (∼100 to 250 nm) and tertiary (∼5 to 50 nm) intragranular precipitates. Larger, primary (∼1 µm) intergranular precipitates may also be present, depending upon the solution heat treatment temperature.…”

Decorrelated Dislocation Movement in the γ-Matrix Channels of a Ni-Based Superalloy: Experiment and Dislocation Dynamics Simulation

“…This alloy processed by the powder metallurgy route contains a bimodal precipitation of γ '-Ni 3 (Al, Ti) particles embedded in a γ fcc Ni-based solid solution. In a previous study, the influence of the tertiary precipitates on the creep behaviour has been enlightened [2]. In order to provide a thorough understanding of the tertiary precipitates role in the deformation micromechanisms, transmission electron microscope (TEM) post mortem observations as well as in situ straining experiments on crept samples have been carried out.…”