Multimodal Precipitation in the Superalloy IN738LC

Abstract

ERCAN BALIKCI and DINC ERDENIZIN738LC is a polycrystalline, nickel-base superalloy, which is used in aggressive environments at high temperatures. The required strength is provided by precipitate strengthening. Both unimodal and multimodal precipitate distributions are observed in IN738LC. After reaching a critical size, a unimodal precipitate microstructure transforms to a bimodal one. This transformation is controlled by the precipitate-matrix interface, which is under compression in IN738LC. As the unimodal precipitates grow, the strained interface, due to differential lattice parameter of the matrix and the precipitate phase, stops solute diffusion into the growing precipitates. Hence, the solute atoms, entrapped in the matrix, saturate the matrix and form new, fine precipitates. Dissolution of some large precipitates also supplies solute to supersaturate the matrix. On the other hand, a multimodal precipitate distribution tends to become unimodal at low aging temperatures and bimodal at high aging temperatures. Interestingly, the activation energy is calculated for the coarsening of large precipitates in multimodal distribution and is found to vary with aging time.

Fine Precipitates in Nickel Base Superalloys

“…Alternatively, the fine precipitates might already form during an aging [9] as a result of an isothermal saturation of the matrix channels between the primary precipitates. This may happen because of a mode change from a diffusional to interface and/or mixed controlled growth for the primary precipitates and an increase in the matrix channel width.…”

“…The multimodal precipitation may also occur during an isothermal aging [9]. It has been previously reported [10][11][12] that the γ' precipitates in the superalloy IN738LC grow isothermally in a unimodal cuboidal morphology up to a critical size, beyond which a bimodal size distribution sets in, where visibly large and small size precipitates coexist.…”

Phase Evolution and Thermal Expansion Behavior of a γ′ Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

“…The closer the precipitate is to the larger precipitate, the more likely it is to dissolve, so there are more and smaller precipitated phases around the large precipitated phase. Secondary, as we all know, just the cooling rate is fast enough to cause a sufficient undercooling, the smaller cooling γ′ precipitates will burst nucleate from supersaturated γ channels instead of the original particles growth [22]. The cooling rate after in-situ heating is about 50 °C /min, so there are finer cooling γ′ particles (<10 nm) nucleation.…”

Effect of Hot Isostatic Pressing on Microstructures and Mechanical Properties of IN738LC Superalloy

“…IN738LC superalloy is a cast nickel-based polycrystalline superalloy and has been mainly used for blades and vanes in aircraft engines and stationary gas turbines due to its high strength and creep resistance at high temperature in oxidizing environment [1][2][3]. Investment casting (also known as 'lost wax casting' or 'precision casting') is known for its ability to produce components of excellent surface finish, dimensional accuracy and complex shapes.…”