On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique
Graphical abstract
Introduction
Ni-base superalloy single crystals (SX) are used to make first stage blades for modern gas turbines which are used in power plants and aero engines, e.g. [1], [2], [3], [4], [5], [6], [7], [8]. There are four aspects, which are important and well established: First, SX have no grain boundaries and are therefore less prone to cavitation damage [7], [8]. Second, Ni-base superalloys feature chemical and microstructural heterogeneities which have their origin in the solidification process (prior dendritic and interdendritic regions [9], largescale heterogeneity: several hundred micrometers) and in the subsequent precipitation heat treatment (γ/γ′-microstructure, small scale heterogeneity: 1 μm). Superalloy SX represent technical single crystals which contain two phases, the ordered γ′-phase (L12 crystal structure, cuboidal particles of a typical edge length of a few hundred nanometers, typical volume fraction of 75%) and the γ-phase (fcc solid solution, thin channels which separate the γ′-cubes, typical volume fraction: 25%). The <001> direction represents a natural solidification direction, but deviations of up to 15° from this solidification direction are not uncommon [3], [6], [10]. Third, Ni-base SX show elastic anisotropy, where the Young's modulus in the <111> direction is significantly larger than in the <001> direction, e.g. [11], [12]. Orienting turbine blades along the <001> direction minimizes thermal stresses associated with fast heating and cooling. Fourth, superalloy SX are produced using the well-known Bridgman directional solidification process [1], [2], [3], [5], [6], [7], [8], [13]. A detailed overview on industrial scale production of SX turbine blades was recently given by Kubiak et al. [13]. Two different techniques are applied to eliminate grain boundaries in superalloys during directional solidification (DS). First, one can use a narrow spiral-shaped crystal selector through which the solid/liquid front must pass during solidification. The selector only allows one single grain to pass through. The second option is to use pre-oriented seed crystals. The seed partially melts back in the early processing stages. Later, epitaxial growth during crucible withdrawal results in the formation of a solid/liquid interface which proceeds through the molten material.
The processing of SX has been a research topic for several decades [13], [14], [15], [16]. A first summary of techniques has been given in the seminal collection of research papers edited by Gilman in 1963 [14]. The research area has always been closely related to the field of solidification of melts [17], [18], [19], [20], [21]. As gas turbine technology developed, the casting and the directional solidification of SX superalloys have received considerable attention, e.g. [6], [10], [22], [23], [24], [25], [26]. However, the production of high quality superalloy single crystals has remained challenging [25], [26]. The formation of microstructures in Ni-base superalloys during SX solidification is governed by various thermodynamic, kinetic and other constraints and involves different length scales [9], [27], [28]. The governing elementary processes are more complex than what is envisaged in classical crystallization theories, e.g. [29], [30]. High contents of refractory elements (e.g. Re, W and Ta) promote segregation during solidification, which results in heterogeneous microstructures. Even when solidification conditions are precisely controlled, there is a risk of defect formation [6], [7], [25], [26], [28].
Today there is a good understanding of the coupling between thermodynamic driving forces and elementary kinetic processes during SX solidification, e.g. [22], [31], [32], [33]. Progress has been made in the area of process modeling, e.g. [26], [34], [35], [36], [37], [38]. However, there are open questions related to microstructural and chemical homogeneity across length scales which the present work addresses in four points. First, it is shown how microstructures and local chemical alloy compositions evolve during different stages of a seeded Bridgman solidification process with special emphasis placed on the early stages of partial seed remelting and crystal growth. Second, an effort was made to demonstrate that finite element based calculational procedures can predict temperature gradients in the solid/liquid region which agree well with experimental data derived from primary dendrite arm spacings. Third, it is important to explain the nature of internal interfaces which are inherent features of cast microstructures and which evolve during solidification. Fourth, there is a need to evaluate the extent of microstructural scatter which is observed when a series of single crystals is produced following the same procedure. All findings are discussed in the light of previous results reported in the literature.
Section snippets
Bridgman seed technique (BST)
In the present work we use a Bridgman furnace of type KZV-A40-400/161G-V from Gero GmbH (Neuhausen, Germany). The lower part of the furnace is shown in Fig. 1. The furnace has three separately controlled graphite heating elements (one of which is highlighted with “1”). These heat a central graphite tube (“2”) which houses the cylindrical crucible, shown in red in Fig. 1. The crucible contains the spacer (“3”), the seed (“4”) and the feedstock (“5”). During solidification, the crucible/holder
Calculated temperature profile across solid/liquid interface
In Fig. 5 we present results for the temperature distribution in the furnace. In Fig. 5a, temperature levels are represented by colors (see corresponding color code on the right). The horizontal dotted lines in the center of the figure represent calculated isotherms for 1653 and 1593 K, which correspond to the liquidus and solidus temperatures of our alloy, as experimentally determined by Quested et al. [49]. In Fig. 5b we show the calculated temperature profile along the dashed central vertical
Bridgman seed technique (BST)
In the present work we use a Bridgman technique with a seed crystal, Fig. 1. The seed crystal partially melts, Fig. 6a. Its remaining solid part provides a starting point for epitaxial growth of the new SX which forms during solidification when the furnace is removed at a rate of 180 mm/h, while the cylindrical Al2O3 mold (inner diameter: 12 mm, containing: spacer, seed crystal and feedstock, Fig. 1) remains fixed. It is well known that the temperature gradient across the solid/liquid interface
Summary and conclusions
In the present work we use a Bridgman seed technique (BST, withdrawal rate v: 180 mm/h, temperature gradient G across liquid/solid interface region: 14 K/mm) to cast cylindrical ingots (length: 120 mm, diameter: 12 mm) of a single crystal (SX) Ni-base superalloy. A number of open questions were studied which address the evolution of local alloy chemistry and microstructure in the technical single crystal. From the results obtained in the present work, the following conclusions can be drawn:
(1) When
Acknowledgement
The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through projects B7 and C5 of the collaborative research center SFB/TR 103 on single crystal superalloys (see: www.sfb-transregio103.de). The authors also appreciate advice and helpful discussions with Katarzyna Matuszewska and Ralf Rettig (Friedrich-Alexander-University Erlangen-Nürnberg) on single crystal growing. The authors also appreciate help from RWP GmbH for their assistance with the FEM calculations.
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