Correlation between the Dendritic Structure and Lattice Parameter of γ'-Phase in Single-Crystalline Turbine Blades Made of Superalloys.

Jacek Krawczyk,Włodzimierz Bogdanowicz

doi:10.3390/ma15030781

Abstract

The dendritic structure and the distribution of the γ′-phase lattice parameter (aγ′) along selected lines of the longitudinal section in a model single-crystalline blade made of CMSX-4® nickel-based superalloy were studied. It was established that there is a correlation between the value of the aγ′ and the predomination of initial or ending fragments of the secondary dendrite arms. It is most noticed for the areas where the dendrite growth conditions are similar to steady. They are located in the center and near the root’s selector extension (SE) area. The correlation has been related to the dendritic segregation mechanism. It was shown that in the single-crystalline blades obtained by the directional crystallization using a spiral selector, the “walls” of the primary dendrite arms that grow at a low angle to the blade axis are created. It was found for the first time that the value of the lattice parameter aγ′ is decreased near such “walls”. Additionally, it was found that competitive growth of the dendrites may occur at a distance of even several millimeters from the bottom surface of the root. The first-time applied X-ray diffraction measurements of aγ′ made in a single-pass along the line allow the analysis of the dendritic segregation in the whole blade cast.

Highlights

Components of aviation and industrial gas turbines are expected to operate in harsh working conditions, such as high temperature, high pressure, and complex dynamic loading conditions
Ending fragments of the secondary arms located at a longer distance from the primary arms are visualized by the teardrop-like shapes that occur between hourglass-like shapes
Especially in its selector extension (SE) area, there are subareas in which the initial fragments of the secondary dendrite arms predominate, and the dendritic segregation causes a decrease in the lattice parameter of the γ0 -phase

Summary

Introduction

Components of aviation and industrial gas turbines are expected to operate in harsh working conditions, such as high temperature, high pressure, and complex dynamic loading conditions. The casts of these components are widely produced as singlecrystalline using nickel- or cobalt-based superalloys. Due to an impressive combination of high-temperature strength, good phase stability, and resistance to oxidation and hightemperature corrosion during operation, the single-crystalline blades made of CMSX-4®. The single-crystalline casts are commonly produced by directional Bridgman crystallization, during which groups of the γ-phase dendrites are formed. The single-crystalline superalloys contain several alloying additives that segregate into interdendritic regions or into dendrites on a scale of several hundred microns. Complex heat treatment processes are used to reduce heterogeneity that, largely but not completely, eliminate the effects of dendritic segregation [16,17,18,19]. Due to complex chemical composition and high amount of refractory elements, in addition to the dominant phases γ and γ0 (over 70%), there are a small fraction of other phases, e.g., topologically close-packed (TCP) phases such as

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Conclusion