Growth and Coalescence of γ’-Precipitates in Nickel-Based Alloy 115NC during Slow Cooling for Membrane Manufacturing

By coarsening of the γ’-precipitates and selective extraction of one of the two existing phases, porous structures can be produced from nickel-based superalloys. There are two basic approaches to achieve a bicontinuous γ/γ’-microstructure—directional and incoherent coarsening. Single crystalline superalloy membranes are produced by the so-called rafting of the microstructure, i.e., directional coarsening. Unlike this process, incoherently coarsened membranes lack a detailed understanding of the mechanisms leading to cross-linking of the precipitates. In this paper, the growth and coalescence of precipitates during initial slow cooling from above the γ’ solvus temperature was studied. In addition to the three-dimensional morphological changes of the precipitates, it is also shown that only little coalescence of the particles occurs despite the high γ’ content and, therefore, their very small distance. The loss of coherency that occurs during this part of coarsening must first advance through further aging before a bicontinuous microstructure is formed.

Polycrystalline Superalloy Membranes Produced by Load-Free Coarsening of Incoherent γ′-Precipitates: Microstructure Evolution and Mechanical Properties

Nanoporous superalloy membranes are a functional extension of the use of nickel-based alloys. The material, which is usually used for high-temperature applications, consists mainly of the two phases γ and γ′. Through coarsening of the precipitates and thus forming of a bicontinuous γ/γ′ network, membranes can be produced by removing either of these phases. From the single-crystalline alloy CMSX-4, the bicontinuous network can be formed either thermo-mechanically by directional coarsening of coherent precipitates or by load-free coalescence of incoherent precipitates. Recent investigations have shown that membranes also can be produced from polycrystalline starting material in both ways. In this article, the process route for membranes by load-free coarsening of incoherent γ′ precipitates from a carbon-free version of the polycrystalline alloy Nimonic 115 is presented. This manufacturing method has the advantage of its simplicity and in comparison to single-crystalline membranes it can be realized in larger scales. We discuss the microstructure and show the mechanical properties by means of tensile tests. Despite the grain boundaries as a mechanical weak link, polycrystalline membranes show promising mechanical properties. Their strength even exceeds that of the single-crystalline membranes despite the significantly higher pore volume content.

Crystal Viscoplasticity of a Ni-Base Superalloy in the Aged State

Arising from long-term high temperature service, the microstructure of nickel-base (Ni-base) superalloy components undergoes thermally and deformation-induced aging characterized by isotropic coarsening and directional coarsening (rafting) of the γ′ precipitates. The net result of the morphological evolutions of the γ′ particles is a deviation of the mechanical behavior from that of the as-heat treated properties. To capture the influence of a rafted and isotropic aged microstructure states on the long-term constitutive behavior of a Ni-base superalloy undergoing thermomechanical fatigue (TMF), a temperature-dependent crystal viscoplasticity (CVP) constitutive model is extended to include the effects of aging. The influence of aging in the CVP framework is captured through the addition of internal state variables that measure the widening of the γ channels and in-turn update the material parameters of the CVP model. Through the coupling with analytical derived kinetic equations to the CVP model, the enhanced CVP model is shown to be in good agreement when compared to experimental behavior in describing the long-term aging effects on the cyclic response of a directionally solidified (DS) Ni-base superalloy used in hot section components of industrial gas turbines.

A Review of Rafting in Nickel-Based Single Crystal Superalloys

Nickel-based single crystal superalloys have been widely used in modern aircraft, which is related to its high temperature mechanical strength and creep properties. And the initial cubic γ′ precipitates start to coarsen directionally during high temperature creep, which results in the degradation of the mechanical properties, especially the creep properties. Therefore, it is essential to figure out the mechanism of directional coarsening during the period of high temperature creep. In this article, a broad review of rafting mechanism of nickel-based single crystal superalloys is provided. The major work of this critical review is to introduce several experiments and numerical simulations which are used to analyze the evolution of rafting. For three different numerical simulations, their performance, advantage and disadvantage are discussed in detail. Through methods above, the effect on creep properties is summarized.

Crystallinity Degradation Caused by Alloying Elements Diffusion During Creep of Ni-Base Superalloy

High temperature mechanical properties of Ni-base superalloys are improved by the fine cuboidal γ’ (Ni3Al) precipitates orderly-dispersed in the γ matrix (Ni-rich matrix) because the dispersed texture in a grain inhibits dislocation motion. However, it is well known that directional coarsening of the γ’ precipitates perpendicular to a principal stress occurs not only during creep loading but also during cyclic loading and, the formation of the raft causes the decreasing of high temperature strength drastically. Therefore, it is very important to evaluate the damage of the alloys caused by creep and fatigue loading based on the change of their micro texture. In this study, the change of crystallinity of the Ni-base superalloys (CM247LC) under creep loading was analyzed by applying Electron Back-Scattered Diffraction (EBSD) method. The image quality (IQ) value obtained from the EBSD analysis was used for the quantitative evaluation of the crystallinity in the area where an electron beam of 10 nm in diameter was irradiated. The quality of the atomic alignment of both γ’ and γ phases was found to degrade with increasing creep damage. The degradation of crystallinity suggests that the ordered L12 structure of Ni3Al became disordered and the density of dislocations and vacancies increased. However, KAM (Kernel Average Misorientation) value did not change significantly with increasing creep damage. Therefore, the dominant factor of the creep damage of this alloy is the strain-induced diffusion of elements under loading, and the decrease of the crystallinity.

Nanotexture Change Caused by Strain-Induced Anisotropic Diffusion During Creep of Ni-Base Superalloy

In order to make clear the mechanism of the directional coarsening (rafting) of γ′ phases in Ni-base superalloys under uni-axial tensile strain, molecular dynamics (MD) analysis was applied to investigate dominant factors of strain-induced anisotropic diffusion of Al atoms and nanotexture change of fine dispersed γ′ precipitates. In this study, a simple interface structure model corresponding to the γ/γ′ interface, which consisted of Ni as γ and Ni3Al as γ′ structure, was used to analyze the effect of alloying element on diffusion properties. The diffusion constants of Al atoms were changed drastically by the dopant elements and their contents. When the lattice constant of the γ phase was increased and its melting point was decreased by the addition of Cr or Al atoms, the strain-induced anisotropic diffusion of Al atoms in the γ′ phase was accelerated. On the other hand, the addition of Co decreased the diffusion significantly. Therefore, changes of lattice constant and melting point depending on the chemical composition of the γ/γ′ interface are the dominant factors controlling the strain-induced anisotropic diffusion of Al atoms in the Ni-base superalloy.