Anisotropy of the critical resolved shear stress of a superalloy containing 6 vol.% L12-ordered γ′-precipitates

Single crystals of the γ′-strengthened nickel-base superalloy NIMONIC PE16 have been compression tested in the temperature range 683–1143 K. Four different orientations of the specimens have been studied: [0 0 1].[1¯23].[011] and [1¯11]. They were either in the homogenized, single-phase state or in the peak-aged state. The critical resolved shear stress (CRSS) of the homogenized specimens was isotropic at 683 K. The CRSS of the peak-aged specimens, containing 6 vol.% of L1₂-long-range ordered γ′-precipitates of 25 nm radius, was anisotropic at 683 K and at 989 K: the [001]-orientated specimens were the softest ones, the CRSS increased as the orientation moved towards [011] or [11¯1]. This is the same orientation dependence found for the CRSS of single-phase L1₂-ordered materials. The interpretation of the anisotropy of the CRSS of NIMONIC PE16 follows that given for single-phase L1₂-long-range ordered materials.     

The dependence of creep rate on microstructure in a γ′ strengthened superalloy

The principal creep strengthening mechanism in superalloys is the ability of γ′ precipitates to impede dislocation motion. In this report various theories of dislocation creep are reviewed, with particular emphasis on the contribution of precipitates to the ‘friction stress’ resisting dislocation motion. Creep data is presented for the nickel-base superalloy IN-738, which is used as a blading material in the first row high pressure stage of gas turbines. At high stress (315 MPa at 1123 K) it is shown that dislocations may overcome γ′ precipitates by cutting whereas at low stress by-pass of precipitates involves diffusion-controlled climb. A model is developed which considers the contribution to the friction stress of the back stress resulting from the climb process, which is used to explain the decrease in creep resistance of this alloy as a consequence of precipitate coarsening at the operating temperature.     

Tem analysis of square-shaped dislocation configurations in the γ′ phase of a Ni-based superalloy

Straight sessile dislocations are observed in the γ' phase of a Ni-base superalloy deformed at 975°C. They are shown to be prismatic pure edge dislocations, lying along 〈110〉 directions, with a Burgers vector $\text{b}\text{= 2a〈001〉}$. We propose a formation mechanism and a non-planar core structure which accounts for the observations.     

TEM-in situ deformation of beryllium single crystals—a new explanation for the anomalous temperature dependence of the critical resolved shear stress for prismatic slip

The critical resolved shear stress for prismatic slip within beryllium single crystals passes through a maximum in the temperature range of 170–450 K. A valid explanation has not been found till now. In the present paper the experimental results of TEM-in situ deformation of single crystal beryllium are presented. Although the samples were oriented to promote prismatic slip, combined slip mechanisms on prismatic and basal planes were observed at room temperature. However, at 170 K plastic deformation only occurred by prismatic slip. A new model explaining the anomaly of the critical resolved shear stress is proposed which is based on elementary dislocation processes, i.e. formation and motion of salient points on dislocation lines. The same mechanisms might also result for other hexagoanl metals in a comparable anomaly of the plastic behavior.     

Early stages of precipitation of γ′-phase in a nickel base superalloy: An atom-probe investigation

The early stages of precipitation of γ′ phase in a nickel base superalloy have been investigated by means of atom-probe techniques. The microstructure parameters, as derived from concentration profiles, have been studied as a function of the aging time at 650°C. An analytical method is proposed for determining the particle size, the number density as well as the volume fraction from the features of concentration profiles. The results which are observed show for aging times smaller than a few hours that the precipitate composition varies in a large extent while the particle size remains nearly constant. In contrast, in the coarsening stage γ′ precipitates reach their equilibrium composition after the first 25 h. The results are interpreted within nucleation—growth and coarsening theories.     

Investigation of γ/γ′ lattice mismatch in the polycrystalline nickel-base superalloy IN738LC: Influence of heat treatment and creep deformation

The γ/γ′ lattice misfit in the polycrystalline nickel-base superalloy IN738LC has been determined by convergent beam electron diffraction (CBED) technique in a transmission electron microscope (TEM). The influence of heat treatment and creep deformation on the misfit was investigated. The chemical compositions of the γ and γ′ phases were analysed with the help of Energy Dispersive Spectrometry (EDS). The influence of different heat treatments on the γ/γ′ misfit can be explained in terms of the dependence of the chemical compositions of γ and γ′ phases on the heat treatments. The creep deformation causes a tetragonal distortion in γ′ precipitates and leads to considerable changes in the misfit. The tetragonality in γ′ phase and the evolution of the misfit during creep process can be understood on the basis of a dislocation model. An estimation of the local internal stress in γ′ phase generated by γ/γ′ interfacial dislocations formed during creep deformation shows it to be higher than the applied stress.     

Characterization of the Hot Deformation Behavior and Microstructure Evolution of a New γ-γ′ Strengthened Cobalt-Based Superalloy

Recently, cobalt-based γ-γ′ microstructured superalloys have attracted attention. However, studies on their processing behavior [i.e., processing maps (the variation of strain rate sensitivity (m) with temperature)] are limited. Thus, the high-temperature flow behavior of a γ-γ′ Co-30Ni-10Al-5Mo-2Ta-2Ti-5Cr (at. pct) superalloy was investigated using isothermal compression tests between 1348 and 1498 K at strain rates from 0.001 to 10 s⁻¹. The m contour map was generated using the experimental flow stress values, which were used to locate the optimum hot workability and desired microstructural processing range. A strong dependence of m on the deformation parameters (temperature, strain rate, and strain) was observed. A maximum m value of around 0.3 at 1460 K to 1498 K and strain rates of 0.01 to 0.5 s⁻¹ was found. The deformed samples show a fully recrystallized microstructure at high m. Unstable domains showed the formation of cavities at the grain boundary triple points and cracks along the grain boundaries at high strain rates (1 to 10 s⁻¹), corresponding to m < 0.10. A constitutive model was developed using an Arrhenius hyperbolic sine function, yielding an apparent activation energy of 540 ± 30 kJ mol⁻¹ for hot deformation. This study indicates reasonable formability under certain conditions below the solvus, thus opening possibilities for further thermomechanical treatment.

     

Surface stress and the chemical equilibrium of small crystals—II. Solid particles embedded in a solid matrix

The equilibria of small solid particles embedded in a solid matrix are considered. Three interface quantities are of significance; an interfacial free energy representing the work of creating the interface and two interfacial stresses. One represents the work of stretching the interfaces while the other represents the work of stretching one crystal holding the other fixed and thereby altering the structure of the interface. The isotropic case is developed in detail. Several new effects result from the partial accommodation in the matrix of the stress field arising from interfacial stress. An elastic accommodation factor modifies the capillary contribution to the pressure in the particle, the chemical potentials and the Gibbs-Thomson effect. Diffusion potentials but not chemical potentials are constant throughout the system. Coherent and incoherent nucleation is reexamined.     

Effects of the precursor size on the morphologies and properties of γ-Ce_2S_3 as a pigment

Ceria spheres with different sizes and sulfurized products with corresponding morphology were prepared by hydrothermal and gas-solid reaction method at 600–800 °C under CS2 atmosphere for a short time, respectively. Dimensional effect in preparation of γ-Ce2S3 was firstly investigated by means of techniques such as scanning electron microscopy(SEM), X-ray diffraction(XRD), thermal gravimetric analysis(TGA) and spectrophotometer. The results showed that when ceria nanoparticles with small size were used as precursors, the γ-Ce2S3 could be prepared at the lower temperature and the badly sintered products were obtained; when ceria nanoparticles with large size were employed as precursors, pure γ-Ce2S3 was difficultly obtained even if the temperature was up to 800 °C and the products tended to keep their original size. The heat-resistance property of the γ-Ce2S3 with large size was better than the smaller one, and the pure γ-Ce2S3 prepared from precursor with small size had a good pigmentary performance.     

Dwell-Fatigue of Ni-Based Superalloys with Serrated and Planar Grain Boundary Morphologies: The Role of the γ′ Phase on Strain Accumulation and Cavitation

The effect of grain boundary (GB) morphology on the cavitation behavior in a Ni-based superalloy, RR1000, was studied during elevated temperature dwell-fatigue at 700 °C. Following a solution heat treatment, the material was control cooled at two different rates, resulting in high angle GB morphologies that were tailored as either serrated or planar. The resulting γ′ precipitate structures were characterized near GBs and within grains. Along serrated GBs coarsened and elongated γ′ precipitates formed and consequently created adjacent regions that were denuded of γ′ precipitates. Cyclic dwell-fatigue experiments were performed at low and high stress amplitudes to vary the amount of imparted strain on the specimens. A combination of electron backscatter diffraction and digital image correlation were used to resolve strain localization relative to the GBs, in which strain accumulation was found to precede cavity formation. Additionally, the regions denuded of the γ′ precipitates were observed to localize strain and to be initial sites of cavitation. These results present a quantitative strain analysis between two variants of an RR1000 alloy, which provides the micromechanical rationale to assess the increased proclivity for serrated GBs to form cavities.