New Generation of NiAl-Based Eutectic Composites with Tuneable Properties

Final Report Summary - NECTAR (New Generation of NiAl-Based Eutectic Composites with Tuneable Properties)

Ni-based superalloys have been state-of-the-art material for aeroengine applications due to their excellent creep strength, oxidation resistance, and fracture toughness. This spectacular combination of properties did not come without effort – it required many decades of basic and applied research. However, nickel-base superalloys are approaching one fundamental limitation – their melting point. Some of them are already operating at temperatures of 90% of the melting point. Hence, in order to achieve service temperatures higher than those of nickel-base superalloys, materials with significantly higher melting points are required. Structural metallic materials for applications in turbine blades are attractive not only from industrial, but also from environmental and socio-economical standpoints. Therefore, intermetallic compounds have been considered as promising candidates to substitute some of the Ni-based superalloys.

The main objective of the NECTAR project is to develop NiAl-based in situ composites with tuneable properties, by applying an innovative design and pioneering concepts. As a major tool, a synergetic approach merging state-of-the-art computational thermodynamics (calculation of phase equilibrium in multicomponent systems and phase field modelling of the solidification) with advanced design and processing, introducing novel alloying concept and gradient ingot method. In order to achieve these main objectives a set of specific objectives were set: i) develop thermodynamics database for the A2 and B2 phases of the Ni-Al-based quaternary systems; ii) design hit compositons employing thermodynamic calculations and experimental studies of potential quaternary phase diagrams; iii) model and investigate solidification behaviour; iv) model eutectic microstructures and optimize processing parameters and v) establish microstructure-property relationship and assess the novel alloying concept.

Besides the contemporary concepts of alloy design and processing, some novel aspects are introduced as well. For example, the use of pseudo-binary eutectic trough concept to systematically change the volume fraction of the reinforcement has been presented for the first time to the scientific community. This will contribute to development of other classes non-structural eutectic materials as well. Also, methodical microstructure control by using gradient ingot method also present innovative features in modern alloy development.

During the first 24 months of the NECTAR project, the idea has been to identify most promising systems and define hit compositions required to fulfill the long term project objectives by developing both computational tolos and experimental investigations. The work performed during the first 24 months of the project has focused on: i) development of the thermodynamics database of the Ni-Al-X-Y (X and Y are Cr, W, V and Re) systems and modeling of equilibrium solidification, ii) selection of alloy systems and definition of hit compositions and iii) solidification processing including casting of prealloys, subsequent processing in the directional solidification facility, parameter optimization as well as computational simulation of the solidification process.

At this time of the project, the thermodynamics database was developed for the liquid, A2 and B2 phases of the quaternary systems by using the CALPHAD technique. Also, equilibrium solidification, including both Scheil-type and diffusion-type solidification modeling was performed using the Thermo-Calc and DICTRA software. Besides that, the eutectic systems NiAl-Cr-W and NiAl-Cr-Re were selected, structural maps developed and the hit compositions defined. Furthermore, most of the alloys were successfully processed by casting and/or directional solidification and critical processing parameters optimized. Finally, microstructural characterization of the processed samples has been completed and solidification microstructure, growth morphology, regularity and uniformity of eutectics identified.

Moreover, the control of eutectic microstructure by varying process parameters, such as growth rate and temperature gradient were achieved. After defining the critical parameters for the steady-state growth, the scaling laws between processing parametrs and microstructural features like interphase spacing, fibre diameter and density were established. These were later used to correlate the microstructure with results from mechanical testing.

During the second 24 months of the NECTAR project, the emphasis was on phase equilibria, solidification processing and chaarcterization required to fulfill the long term project objectives by developing both computational tolos and experimental investigations. The work performed during the second 24 months of the project has focused on: i) phase field modeling of eutectic microstructure, ii) phase equilibria of selected systems and iii) solidification processing including casting of prealloys, subsequent processing in the directional solidification facility, parameter optimization as well as computational simulation of the solidification process.

In the second half of the NECTAR project, thermo-kinetic database was assessed to obtain the Gibbs energy and to describe the mobility properties of the liquid, A2 and B2 phases of the Ni-Al-Cr-W quaternary system by using the CALPHAD and computational diffusion kinetic techniques, respectively. Besides, the major model parameters such as interfacial energy coefficients, interfacial mobility and energy barriers were quantitatively determined based on the calculated metastable phase diagram. Also, the phase field code for modeling the eutectic microstructures, with the ability to use the thermodynamics and mobility databases as materials parameters was developed. With the real driving force and materials parameters seamlessly acquired from the database, the solidification of NiAl-Cr-W alloy during isothermal and continuous cooling processes was quantitatively simulated and the eutectic structures consisted of ultra fine fibers of A2 phase embedded in a B2 matrix. In addition, a detailed analysis of the effect of isothermal temperature and cooling rate on morphology is provided.

Moreover, in order to obtain the pseudo-binary NiAl–Cr(W) eutectic trough, the influence of the variation of chromium and tungsten content on the resulting microstructure of NiAl–Cr–W pseudoternary system was investigated. The tungsten content was systematically changed while keeping constant chromium contents. In contrast with what was expected, the eutectic valley showed an anomalous path, exceeding the 1.5 at% W of its binary eutectic by far. Furthermore, for the systems with W and Re it was demonstrated that the temperature gradient influences interface undercooling in the same way as the growth rate and may be used as an additional parameter to control fibre spacing and diameter. Besides microstructural investigations, in both systems the differential thermal analysis (DTA) indicated that eutectic temperature is decreasing with a decrease in W(Re) content and an increase in the Cr content almost linearly.

Finally, mechanical properties of all the systems have been screened. This included the determination of temperature dependence of mechanical properties as a function of volume fraction of the reinforcing phase, fracture toughness and failure mechanisms, as well as creep properties of the alloys. It was concluded that the alloys with lower volume fraction of the fibres have better strength and creep resistance and the alloys with higher volume fraction are more ductile and have better fracture toughness.

The ultimate goal of the NECTAR project was a new generation of NiAl-based in situ composites with tunable properties with better performance at temperatures above 1150ºC than the state-of-the-art Ni-based superalloys. These objectives are funded in the pressing urgency of finding new materials for jet engines and the importance of alloy development for the aerospace industry, the economy and the wider society. The obtained results indicate that these objectives were partially fulfilled as the new alloys have higher strength and corrosion resistance than Ni-base superalloys, similar creep properties and lack behind only in the fracture toughness. After some additional research efforts directed to fixing this drawback, the developed alloys would be ready to realize their potential and significantly contribute to competitivity and sustainability of the European aerospace industry.

Final Report Summary - NECTAR (New Generation of NiAl-Based Eutectic Composites with Tuneable Properties)

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