Nickel-base Superalloy SC16 Research Articles

Precipitation kinetics of γ′ phase in nickel base superalloy SC16: an in situ neutron diffraction study

An in situ study of the kinetics of nucleation and growth of the γ′ phase in the single crystal Ni base superalloy SC16 was carried out by means of neutron diffraction. The investigation was aimed at a better understanding of the fundamental kinetic phenomena, in view of the optimisation of the heat treatments applied prior to the industrial use of the alloy. On heating above the solvus point (1525 K) the γ′ phase decomposes, and when cooling down back towards lower temperatures, it reappears. Measuring superlattice reflections allows the different stages of decomposition, nucleation, growth and concurrent coarsening to be followed. Results show that at temperatures T≤1490 K an increasing volume fraction of γ′ can be found with decreasing temperature, until saturation occurs at around 1100 K. Following classical kinetic theory, an Avrami like growth (time) law was used at each aging temperature and the isothermal reaction rates calculated. They show a maximum at around 1300 K, where the competing nucleation and growth processes find equilibrium. Nucleation and growth can be described with a modified Arrhenius law, analogous to that used for eutectoid reactions. The reaction rate first increases with the undercooling and then decreases. This implies a change in the character of the reaction, which was found to be initially surface and then bulk driven.

Formation and relaxation of coherency strain in the nickel-base superalloy SC16.

An in situ study of the kinetics of relaxation of the gamma-gamma' lattice mismatch in the single-crystal Ni-base superalloy SC16 was carried out by means of high-energy synchrotron radiation diffraction. The high resolution achievable was exploited to measure the time variation of the gamma and gamma' lattice parameters during isothermal ageing at several temperatures, starting from the gamma' solvus point. On cooling from the solvus temperature (1523 K) the gamma' precipitate phase nucleates and grows following concurrent mechanisms. The variations of the gamma and gamma' peak position and integrated intensity could be followed by means of fundamental and superstructure reflections. At each temperature T < 1473 K the integrated intensity follows an Avrami time law. The gamma' Volume fraction increases as a function of time at each temperature. It increases with cooling, until saturation occurs at temperatures as low as 1075 K. The lattice mismatch follows an exponential time decay, while having larger values at high temperatures. The pArticles are born fully coherent, and the coherency strains the set-up. Over time, long-term ageing shows a stabilization of the misfit value, while the precipitates are supposed to lose their coherency to the matrix (within 3-4 h) and the strains relax.

Simulation of solidification and heat treatment of nickel-base superalloy SC16

Abstract Thermodynamic equilibrium and Gulliver–Scheil as well as diffusion calculations using the DICTRA program have been made to simulate solidification and heat treatment of the nickel-base superalloy SC16. These simulations were validated by comparing them with own and literature experiments concerning directional solidification, heat treatment and differential thermal analysis (DTA) cycles. Equilibrium calculations predicted very well the element distribution in homogenized samples. Gulliver–Scheil based on the CALPHAD method and modified to serve multi-component systems proved to deliver sufficiently correct data for the directional solidification of SC16. The element distribution obtained after cooling below the solvus temperature and by DTA heating and cooling cycles was, however, only described with acceptable precision by applying the diffusion model. All these models require a sound multicomponent thermodynamic data set. Since DICTRA needs much more computing time than the other models it shou...

Measurement of the lattice misfit of the nickel-base superalloy SC16 by high-energy synchrotron radiation

High-energy, high-resolution synchrotron radiation diffraction was successfully used to measure the lattice misfit in the single-crystal nickel-base superalloy SC16. A three-peak model, one belonging to the precipitate phase γ′ and two to the matrix γ, was used to fit the diffraction peaks. Room-temperature (RT) contour plots and a temperature scan up to 1170 K revealed that the misfit evolves together with the measured thermal expansion difference between the γ and γ′ phases. This suggests that the origin of the misfit lies in the different thermal expansion coefficient of the two phases. The misfit was found to be positive at RT and to evolve toward negative values as the temperature increases.

? ? nucleation and growth in the nickel-base superalloy SC16

An in situ study of the kinetics of the nucleation and growth of the γ′ phase in the single-crystal Ni-base superalloy SC16 was carried out by means of neutron diffraction. This alloy has industrial applications (turbine blades for power generation) and is of scientific interest because of its modelling properties (small number of components). Upon cooling after solutionising the γ′ phase at Ts=1250 °C (the solvus temperature), the γ′ precipitates reappear. The different stages of the nucleation and growth of γ′, as well as the successive (concurrent) coarsening, could be followed using a superstructure reflection. Results show that at T≤1200 °C an increasing volume fraction of γ′ can be found with decreasing temperature, saturation occurring around 800 °C. Holding the alloy at a constant temperature, an Avrami-like growth law is applicable; the isothermal reaction rates have a maximum around 1050 °C, where the competing processes are in equilibrium. This explains the pseudo-Arrhenius behaviour, which does not agree with an Ostwald ripening (i.e. with a pure Lifschitz–Slyozov–Wagner size-distribution law).

Modelling of internal stress distribution and deformation behaviour in the precipitation hardened superalloy SC16

The deformation behaviour, at elevated temperatures, of the single crystal nickel-base superalloy SC16 has been modelled. The constitution of the alloy consists of the precipitates of the ordered γ′ phase embedded in a γ (f.c.c. solid solution) matrix. The material model used incorporates the ideas based on the current understanding of the deformation mechanisms in this alloy at elevated temperatures. The model allows both the γ and γ′ phases to deform inelastically. Simulations were performed for (001) orientation of the specimen. The Finite Element code ADINA was employed for 3D stress/strain analysis. The mechanical data of this alloy have not been used to determine parameters for the model. The model predictions of the creep rate are in fair agreement with the experimental observations and also allow qualitative interpretation of deformed microstructures.

Small-angle neutron-scattering studies on oriented single-crystal superalloys

A single-crystal nickel-base superalloy SC16, recently developed for blade applications in land-based gas turbines, was investigated using the SANS instrument (V4) at the BERII reactor in HMI Berlin. The two-dimensional scattering patterns were measured as a function of the crystallographic orientation and analysed by comparing with iso-intensity profiles simulated on the base of a microstructural model of the SC16. Sizes, interparticle distances, volume fraction and morphology of precipitates were determined. Depending on the heat treatment conditions different scattering patterns were observed corresponding to different morphologies of γ′ precipitates. Additionally some samples showed streaks in the two-dimensional scattering patterns, indicating the presence of precipitates other than γ′. This was also confirmed by TEM, SEM and X-ray diffraction studies.

Mechanical behaviour and microstructural evolution in the single crystal superalloy SC16

Mechanical testing at 1223 K and at constant strain rates (tensile tests) and constant loads (creep tests) of the single crystal nickel-base superalloy SC16 shows that after an initial hardening the alloy softens during deformation under all testing conditions. Within the testing range covered in the present study two different deformation mechanisms are identified to occur in different regimes of testing. In the present paper we analyse the softening behaviour observed in the low strain rate regime. In this regime the softening is accompanied by a morphological change in γ′ precipitates (rafting). We show that the observed microstructural instability leads to an increase in the spacing between the γ′ particles. This effect quantitatively accounts for the observed softening in this test regime.

Tension—compression asymmetry of the (001) single crystal nickel base superalloy SC16 under cyclic loading at elevated temperatures

Fully reversed low cycle fatigue tests were conducted on 〈001〉 oriented single crystals of the nickel base superalloy SC16 at a constant total strain range of 2.0%. The strain rates were varied from 10 −2 to 10 −5 s −1 at 750°C and the temperatures were changed from 650 to 950°C under the strain rate 10 −3 s −1. During cycling under the high strain rates 10 −2 and 10 −3 s −1 at 750°C or under the strain rate 10 −3 s −1 at 650°C, the SC16 single crystals show that the tensile stress ( T) is higher than the compressive stress ( C). During cycling under the low strain rates 10 −4 and 10 −5 s −1 at 750°C or under the strain rate 10 −3 s −1 at 850°C, the cyclic tension—compression asymmetry of C > T was observed. At 950°C under the strain rate 10 −3 s −1, no tension—compression asymmetry ( T = C) was present during fatigue testing. The deformation mechanism corresponding to T > C determined by transmission electron microscopy (TEM) is that the γ' precipitates are sheared by pairs of a/2〈110〉 matrix dislocations coupled by antiphase boundaries (APB) within γ' phase. The reversed asymmetry behaviour ( C > T) was found to be associated with the (111)〈112〉 viscous slip producing superlattice intrinsic/extrinsic stacking faults (SISF/SESF) in γ' precipitates. At 950°C ( T = C) dislocation climbing over the γ' precipitates becomes the dominant deformation mechanism. Models which can explain the tension—compression asymmetry behaviour were discussed.

Characterisation of γ' Precipitates in a Single Crystal Nickel Base Superalloy SC16 using SEM, TEM and SANS as Complimentary Measuring Tools

Characterisation of the microstructure in nickel base superalloys is important for understanding and predicting the behaviour of these complex alloys. Large particle sizes of γ' precipitates in modern alloys, their regular arrangement in periodic arrays in the γ matrix and the complexity of the microstructure evolving under deformation at high temperatures, pose serious challenge to the precise characterisation of the microstructure. This necessitates the use of complementary measuring tools e.g. X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), optical metallography etc. Small angle neutron scattering (SANS) is a useful modern technique, used to characterise fine precipitates and inhomogenities (< 50 nm) in materials. So far, established methodology does not exist for using SANS to characterise large particles like γ' precipitates in superalloys. In this paper we develop a model to analyse SANS intensity profiles from large (400nm) γ' precipitates and together with the help of complementary methods such as SEM and TEM, characterise them in a single crystal nickel base alloy SC16.

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions.

Influence of Heat Treatment on the Microstructure of the Single-crystal Nickel-base Superalloy SC16

The microstructure of the recently developed single-crystal nickel-base alloy SC16 is investigated in the as-cast state and after different heat treatments using a number of complementary experimental techniques. The chemical composition of this alloy is based on that of the polycrystalline IN738LC. The effect of cooling rate after solution treatment on the γ' particle size on ageing is investigated and the growth kinetics of the precipitates of γ' phase during one- and two-step ageing are described quantitatively. The chemical compositions of the matrix and the precipitate phases in various heat treated conditions are determined. The precipitate volume fractions are derived for some heat treated conditions by means of three different methods