Microstructural contributions to friction stress and recovery kinetics during creep of the nickel-base superalloy IN738LC

The recovery kinetics and friction stress, σ₀, associated with high temperature creep of the directionally solidified nickel-based superalloy IN738LC have been determined by using a numerical method of analysis to extrapolate data from stress drop experiments. The variations of σ₀ and recovery kinetics with initial particle size, temperature and stress and throughout the creep life have been measured. It was found that during the creep test σ₀ remained fairly constant or decreased slightly while the recovery kinetics increased significantly, scaling with the increasing creep rate. High voltage transmission electron microscopy of specimens stopped at various stages in their creep lives revealed that the dislocation spacing at the semicoherent γ/γ′ interfaces decreased with increasing strain and it is proposed that this affects the creep rate through changes in recovery kinetics.     

Atom-probe microanalysis of a nickel-base superalloy

A FIM atom-probe has been used to investigate the phase compositions in a Nb-Mo bearing nickel based superalloy. The composition of γ′ precipitates in fully heat-treated conditions was found to vary with their mean sizes. The matrix analyses revealed the presence of fine secondary precipitates (30 to 100 Å) which occupy 10 pct of the overall volume of the material. The high spatial resolution of the atom-probe allowed the γ-γ′ interface characterization. Composition profiles show that the transition between the phases occurs within one interplanar spacing. Finally, a long range order study of the ordered γ′ phase has been performed. The analysis of the L1₂ type (Ni, X)₃ (Al, Y) precipitates, made on an atomic plane-by-plane basis, shows how alloying elements substitute for Ni and Al in the γ′ sublattice. The observed results, expressed in terms of occupancy probabilities for both types of sites, indicate that Ti, Nb, and Mo preferentially occupy Al sites while Cr and Co substitute for Ni.     

Modelling of plastic flow in coarse grained nickel-base superalloy compacts under isothermal forging conditions

A constitutive relationship for predicting the flow stress and the evolution of microstructure during isothermal forging of coarse grained P/M Ni-base superalloy compacts, forged below their γ′ solvus temperatures, is derived. These coarse grained compacts gradually transform to microduplex γ−γ′ microstructures during forging and this leads to softening during plastic flow. To model the transformation and the resulting softening, the material is considered as a composite material consisting of hard (untransformed) and soft (transformed to the fine microduplex γ−γ′ grains) regions. A rate equation for this material is written in terms of that for the hard and the soft regions assuming that both carry the same stress but each region is subjected to different strain rates. This is consistent with the occurrence of flow localization in the transformed microduplex regions as observed experimentally. A single grain microstructural model and modified forms of established transformation kinetics relationships for grain boundary nucleated reactions are proposed for modelling the deformation dependent transformation. The constitutive relationship is consistent with necklace structure formation typically characteristic of these materials, and it is suggested that it can be used to predict the development of grain size gradients and shear instabilities in forgings.     

Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue

An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760 °C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the present study.     

Elastic constants of a monocrystalline nickel-base superalloy

The interest in using a monocrystalline superalloy for turbine blades makes it imperative to understand the elastic response of the monocrystalline superalloy under external stress. While the elastic response in the direction of loading is well known for monocrystals, the transverse response under longitudinal loading is not well characterized. The present work presents an experimental measurement and an analytical treatment for characterizing the transverse elastic response under longitudinal loading. The work indicates that the transverse elastic response, in general, is sinusoidal. The in-plane transverse direction, therefore, affects the elastic deformation in the transverse direction under unit longitudinal load, and a proper choice of the in-plane crystallographic direction minimizes the in-plane transverse strain. By fitting the experimental data to an analytical equation, the elastic constants can be obtained from a cylindrical monocrystal of arbitrary axial orientation. Formerly with Corporate R and D Center, General Electric Company, Schenectady, NY     

Fatigue of a glass bead blasted nickel-base superalloy

Room temperature fatigue crack initiation and propagation in the wrought nickel-base superalloy Udimet 700 were investigated with electropolished and glass bead blasted material. Cracks were found to initiate at the surface along coherent annealing twin boundaries oriented for maximum in-plane shear stress in both the electropolished and glass bead blasted conditions even though bead blasting more than doubled the fatigue strength. This increase was found to result from an enhanced crack initiation resistance, but even more importantly from a very pronounced retardation of early Stage I crack propagation by the residual compressive surface stress induced in glass bead blasting.     

Motion and remelting of dendrite fragments during directional solidification of a nickel-base superalloy

The formation of spurious grains during the directional solidification of a Ni-base superalloy is studied by modeling the movement and remelting of dendrite fragments originating in channels inside the mush. Such channels exist because of thermosolutal convective instabilities during solidification and persist as freckle chains in the solidified material. The fragment model is linked to a phase equilibrium subroutine for multicomponent Ni-base superalloys, as well as to a previously developed solidification and convection code. A parametric study is performed to investigate the effects of initial fragment location and size on the fragment paths and survivability in the melt for one of the channels predicted in a typical directional solidification simulation. It is found that only a small window of initial conditions exists which leads to spurious grain formation. This window corresponds to medium-sized fragments originating near the mouth of the channel. Other fragments either remelt completely or sink into the channel. The need for an accurate fragment generation model is discussed.     

Creep-rupture behavior of a directionally solidified nickel-base superalloy

The creep-rupture behavior of the directionally solidified (DS) nickel-base superalloy DZ17G has been investigated over a wide stress range of 60 to 950 MPa at high temperature (923 to 1323 K). In this article, the detailed creep deformation and fracture mechanisms at constant load have been studied. The results show that all creep curves exhibit a short primary and a dominant accelerated creep stage, which results in higher ductility of DS superalloy DZ17G compared to the conventionally cast alloy. From the creep parameters and transmission electron microscopy (TEM) observations, it is suggested that the dominant creep deformation mechanism has a change from gamma prime particles shearing by matrix dislocations in high stress region to dislocation climb process in low stress region. It is found that the fracture mode of DS superalloy DZ17G is transgranular, and it is controlled by the propagation rate of creep cracks initiated at both surface and inner microstructure discontinuities. The creep rupture data follows the Monkman-Grant relationship under all the explored test conditions.     

Dislocation/precipitate interactions during coarsening of a plastically strained high-misfit nickel-base superalloy

The effects of dislocations on the coarsening of γ’ precipitates have been studied in INCONEL* X-750. Using a thermomechanical treatment that includes solution treatment, the addition of approximately 3 pct plastic strain at room temperature, followed by aging at 845 °C for 100 hours, a unique banded microstructure is obtained. The plastic strain results in the formation of intense planar slip bands, and the dislocations in these bands act as preferred coarsening sites by relieving γ’ misfit strains. Precipitates grow on only one side of a slip band, and hexagonal arrays of mixed a/2〈110〉 dislocations form on the precipitate faces in the plane of the slip band. The resulting microstructure consists of interconnected networks of dislocations and precipitates, separated by bands of the γ matrix phase that are relatively free of γ’. The equilibrium dislocation structure has been determined for the γ/γ’ interface by an O-lattice construction. Comparisons with experimental results have been made and interphase boundary dislocation reactions analyzed. A model has also been proposed by which matrix dislocations are incorporated into the hexagonal networks of mixed character. Some fundamental insight into the probable role of dislocations in stress coarsening can be gained from the study.     

Surface recrystallization of a single crystal nickel-base superalloy

The recrystallization behavior of a single crystal nickel-base superalloy was investigated by shot peening and subsequent annealing. Two kinds of recrystallization microstructures, which are intensively dependent on the annealing temperature, are shown in the nickel-base superalloy after shot peening and subsequent annealing. Surface recrystallized grains are obtained when the superalloy is anparticles occurs. Cellular recrystallization is observed after annealing at lower temperatures. Cellular structures induced by high diffusivity of the shot-peened alloy annealed at 1050℃ accords with the Johnson-Mehl-Avrami-Kolmogorov equation. The low Avrami exponent is caused by the inhomogeneous distribution of stored energy, the decreasing of stored energy during recovery, and the strong resistance of boundary migration yb γ' particles.     

Discontinuous cellular precipitation in a high-refractory nickel-base superalloy

A discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy. The reaction transforms the two-phase γ-γ′ parent microstructure into a three-phase cellular structure with a γ′ matrix containing Re-rich P-phase and agglomerated γ lamellae. The reaction has been studied in polycrystalline material and in bicrystals with varying degrees of boundary misorientation at temperatures in the range of T/T _m =0.78 to 0.85. The early stages of the reaction are characterized by heterogeneous nucleation of P-phase precipitates and migration of the grain boundary. At low-angle, near-tilt boundaries misoriented by less than 10 deg, nucleation of P-phase particles was observed, but the cellular reaction did not occur, due to limited boundary mobility and diffusivity. The high degree of supersaturation of Re and W in the initial γ-γ′ alloy appears to be the primary driving force for the reaction. Small amounts of creep deformation did not significantly influence the extent of the transformation. The diffusivity of Re associated with the moving boundary was calculated to be 5×10⁻⁸ cm² s⁻¹ at 1093 °C, which is approximately four orders of magnitude greater than the bulk lattice diffusivity of tungsten.     

Nonuniform recrystallization in a mechanically alloyed nickel-base superalloy

The recrystallization behavior of mechanically alloyed and extruded MA6000 oxide dispersion strengthened (ODS) nickel-base superalloy has been studied as a function of the position of the sample in the extruded bar. It was found that there are profound differences across the cross section of the bar, the edge regions recrystallizing more easily relative to the core. Furthermore, the recrystallized grains tend to be much more anisotropic at the edges. It appears that these differences are caused by inhomogeneous deformation. The oxide particles are more aligned along the extrusion direction at the regions near the surface, presumably because of the more extensive deformation in those regions. This explains the greater anisotropy in the recrystallized grains and the relative case of recrystallization (since the boundary mobility along the extrusion direction is high for aligned particles). Stored energy measurements confirm that the observed effects cannot be explained by differences in the driving force for recrystallization. Formerly Senior Researcher, Research Development Corporation of Japan     

Precision forging of a high strength superplastic zinc-aluminum alloy

Precision forgings can be produced in a high strength zinc aluminum alloy exhibiting superplasticity by means of an isothermal forging cycle which combines an approach at constant speed followed by a dwell at constant load. This study shows that integral threads, sharp corners, and very thin webs can be formed directly in the forging operation. A pressure compensated time parameter is developed which permits forging operations over a range of different conditions to be correlated. This parameter may be used to predict the result of actual precision forging processes from scale model tests. Formerly with St. Joe Mineral Corp.