Deleterious Phase Formation in Next- Generation Nickel-Base Superalloys Predicted

Nickel- (Ni-) base superalloy single crystals represent the state-of-the-art for turbine engine airfoil applications because they offer the best balance of properties under the high operating temperatures required for efficient engine operation. Current trends in alloy design take advantage of improved creep rupture strength with the addition of higher levels of refractory elements. In particular, the addition of significantly higher levels of rhenium in third-generation superalloys is key for both microstructural stability and creep rupture strength. Although refractories provide strength benefits, alloys tend to be unstable when their refractory content is high because of topologically close-packed (TCP) phases. The formation of these phases in sufficient amount is detrimental to the performance of these alloys because of their brittle nature and because they deplete the Nirich matrix of potent solid-solution strengthening elements

Formation and effect of topologically close-packed phases in nickel-base superalloys

© 2016 Institute of Materials, Minerals and Mining. The formation of topologically close-packed (TCP) phases in nickel-base superalloys is an issue of increasing importance as alloys are designed with higher refractory element contents to meet the requirements of next generation turbine engines. This review considers the factors that affect an alloy’s susceptibility to TCP formation. In particular, the debate surrounding the effect of certain individual elements, such as Co and Re, in promoting or suppressing TCP formation is examined alongside the various mechanisms that have been proposed to account for this behaviour. In addition, the detrimental effects of these phases on the alloy’s mechanical properties are discussed, including crack initiation at precipitates, depletion of solid solution strengthening refractory elements and the effect on γ/γ′ rafting behaviour. This review was chosen as a runner up of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged

Plasticity of topologically close-packed phases in the Fe-Ta(-Al) system

Understanding the structure-property relationships of materials plays a significant role in the development of materials for technical applications. Due to the many possible combinations of two or more elements, intermetallic phases can be very interesting for these developments. High strength up to high temperatures makes intermetallics promising materials for high-temperature applications. However, their complex structure, resulting in a pronounced brittleness, has so far limited their applicability. We focus on the understanding of plastic deformation in topologically close-packed (TCP) phases, which form one of the largest groups of intermetallics. To do this, we use nanomechanical tests that allow us to study plasticity even in the most brittle materials. Here, we consider the Fe-Ta(-Al) system that contains two closely related TCP phases, a C14 Laves phase and a µ-phase. The building block-like structure of these phases enables a systematic investigation as well as a transfer of the findings to other complex crystals. The mechanical properties of the two TCP phases in the Fe-Ta(-Al) system, investigated by state-of-the-art micromechanical testing, are introduced in this work. The influence of the crystal structure and chemical composition on the mechanical properties and the deformation mechanisms of the TCP phases are discussed

Structural pathway for nucleation and growth of topologically close-packed phase from parent hexagonal crystal

The solid diffusive phase transformation involving the nucleation and growth of one nucleus is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nucleation follows a nonclassical nucleation process, and the whole structural transformation is completely accomplished by the shuffle-based displacements, with a specific 3-layer hcp-ordering as the basic structural transformation unit. The thickening of plate-like TCP phases relies on forming these hcp-orderings at their coherent TCP/matrix interface to nucleate ledge, but the ledge lacks the dislocation characteristics considered in the conventional view. Furthermore, the atomic structure of the critical nucleus for the Mg2Ca and MgZn2 Laves phases was predicted in terms of Classical Nucleation Theory (CNT), and the formation of polytypes and off-stoichiometry in TCP precipitates is found to be related to the nonclassical nucleation behavior. Based on the insights gained, we also employed high-throughput screening to explore several common hcp-metallic (including hcp-Mg, Ti, Zr, and Zn) systems that may undergo hcp-to-TCP phase transformations. These insights can deepen our understanding of solid diffusive transformations at the atomic level, and constitute a foundation for exploring other technologically important solid diffusive transformations

Formation of diffusion zones in coated Ni-Al-X ternary alloys and Ni-based superalloys

Coatings are an essential part of the materials system to protect the turbine blades from oxidation and corrosive attack during service. Inter-diffusion of alloying elements between a turbine blade substrate and their coatings is a potential concern for coated turbine blades at ever increasing operating temperatures because this can cause the formation of undesirable Secondary Reaction Zones (SRZs), which may degrade the mechanical properties of coated Ni-based superalloys. Understanding the effects of each element on the SRZ formation is essential in order to understand both the mechanism and inter-diffusion behaviour between coatings and substrates. In this research, a number of simpler aluminized ternary Ni-Al-X (where X is Co, Cr, Re, Ru or Ta) alloys were investigated in order to elucidate the separate effects of each element on the microstructural evolution, especially at the coating/substrate interface. The aluminized ternary alloys developed distinctive diffusion zones, depending on the third alloy element, ‘X’. Specifically, it has been found that both Ni-Al-Re and Ni-Al-Ta alloys developed a continuous SRZ-like diffusion layer. This diffusion zone persisted in the Ni-Al-Re alloys after high temperature exposure, indicating that Re has a stronger effect on SRZ formation than Ta

Dft-cef approach for the thermodynamic properties and volume of stable and metastable al–ni compounds

The Al–Ni system has been intensively studied both experimentally and theoretically. Previous first-principles calculations based on density-functional theory (DFT) typically investigate the stable phases of this system in their experimental stoichiometry. In this work, we present DFT calculations for the Al–Ni system that cover stable and metastable phases across the whole composition range for each phase. The considered metastable phases are relevant for applications as they are observed in engineering alloys based on Al–Ni. To model the Gibbs energies of solid phases of the Al–Ni system, we combine our DFT calculations with the compound energy formalism (CEF) that takes the Bragg–Williams–Gorsky approximation for the configurational entropy. Our results indicate that the majority of the investigated configurations have negative energy of formation with respect to Al fcc and Ni fcc. The calculated molar volumes for all investigated phases show negative deviations from Zen’s law. The thermodynamic properties at finite temperatures of individual phases allow one to predict the configurational contributions to the Gibbs energy. By applying a fully predictive approach without excess parameters, an acceptable topology of the DFT-based equilibrium phase diagram is obtained at low and intermediate temperatures. Further contributions can be added to improve the predictability of the method, such as phonons or going beyond the Bragg–Williams–Gorsky approximation that overestimates the stability range of the ordered phases. This is clearly demonstrated in the fcc order/disorder predicted metastable phase diagramFil: Tumminello, Silvana Deisy Paulina. Universidad Nacional del Comahue; Argentina. Universidad Nacional de San Martín; Argentina. Ruhr Universität Bochum; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia. Instituto de Investigación En Tecnologías y Ciencias de la Ingeniería. Universidad Nacional del Comahue. Instituto de Investigación En Tecnologías y Ciencias de la Ingeniería; ArgentinaFil: Palumbo, Mauro. Ruhr Universität Bochum; AlemaniaFil: Koßmann, Jörg. Ruhr Universität Bochum; AlemaniaFil: Hammerschmidt, Thomas. Ruhr Universität Bochum; AlemaniaFil: Alonso, Paula Regina. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín. Instituto Sabato; ArgentinaFil: Sommadossi, Silvana Andrea. Universidad Nacional del Comahue; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia. Instituto de Investigación En Tecnologías y Ciencias de la Ingeniería. Universidad Nacional del Comahue. Instituto de Investigación En Tecnologías y Ciencias de la Ingeniería; ArgentinaFil: Fries, Suzana G. Ruhr Universität Bochum; Alemani