Elsevier

Acta Materialia

Volume 97, 15 September 2015, Pages 29-40
Acta Materialia

A new class of high strength high temperature Cobalt based γ–γ′ Co–Mo–Al alloys stabilized with Ta addition

https://doi.org/10.1016/j.actamat.2015.06.034Get rights and content

Abstract

The present paper reports a new class of Co based superalloys that has γ–γ′ microstructure and exhibits much lower density compared to other commercially available Co superalloys including Co–Al–W based alloys. The basic composition is Co–10Al–5Mo (at%) with addition of 2 at% Ta for stabilization of γ′ phase. The γ–γ′ microstructure evolves through solutionising and aging treatment. Using first principles calculations, we observe that Ta plays a crucial role in stabilizing γ′ phase. By addition of Ta in the basic stoichiometric composition Co3(Al, Mo), the enthalpy of formation (ΔHf) of L12 structure (γ′ phase) becomes more negative in comparison to DO19 structure. The ΔHf of the L12 structure becomes further more negative by the occupancy of Ni and Ti atoms in the lattice suggesting an increase in the stability of the γ′ precipitates. Among large number of alloys studied experimentally, the paper presents results of detailed investigations on Co–10Al–5Mo–2Ta, Co–30Ni–10Al–5Mo–2Ta and Co–30Ni–10Al–5Mo–2Ta–2Ti. To evaluate the role alloying elements, atom probe tomography investigations were carried out to obtain partition coefficients for the constituent elements. The results show strong partitioning of Ni, Al, Ta and Ti in ordered γ′ precipitates.

Graphical abstract

(left) Plot showing effect of Ta occupancy on formation energy for Co0.75Al0.125Mo0.125−xTax intermetallic phase in competing DO19 and L12 ordered structures. (middle) Dark field image taken along [0 0 1] zone axis using 100 superlattice L12 ordered spot and (right) APT reconstruction delineated using 10% Al isosurface showing γ′ precipitates (red color) in Co–10Al–5Mo–2Ta heat treated alloy.

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Introduction

Usually in gas turbines, the cast Ni based superalloys (single crystal/directionally solidified) are used in fabrication of blades where high strength and corrosion resistance is required while wrought Ni based superalloys are used in disk applications [1], [2], [3], [4]. Co is often an important constituent in these alloys, because it increases the stress rupture life at high temperatures as well as the resistance to sulfidation by reducing the rate of diffusion of sulfur in the matrix that prevents the formation of low melting point eutectics. In contrast to the Ni–Ni3S2 eutectic that forms in Ni based superalloy with eutectic temperature of 635 °C, in Co based superalloys Co–Co4S3 eutectic forms during exposure to gases containing sulfur with a eutectic temperature of 880 °C [4], [5]. Co based superalloys are mainly used in static parts such as vanes due to their better hot corrosion, oxidation and sulfidation resistance. Their lower strength values (due to limits in strengthening with solid solution or carbide precipitation) make them unusable for blade and disk applications [4], [6]. Hence, Ni based superalloys with γ–γ′ microstructure equipped with high temperature corrosion resistant coatings are being used for blades. These coatings are expensive and very challenging in terms of stability for long term applications [7], [8], [9].

Recently Co based superalloys with γ–γ′ microstructure has attracted attention as a competent alloy system that can be an alternative to the well-established Ni based superalloys. The increase in the attention of Co based alloys with γ–γ′ microstructure was triggered by the paper of Sato et al. on Co–Al–W alloys that reports formation of a strengthening metastable phase Co3(Al, W) with an ordered L12 structure [10]. The first reported work on L12 ordering in this class of alloys with similar stoichiometry is however, due to Lee [11], who has extensively studied the role of W in the formation of γ′ phase in Co alloys. These ordered precipitates are reported to have a cuboidal morphology, and are distributed throughout the fcc γ-Co matrix similar to L12 ordered stable Ni3(Al, Ti) phase in the fcc γ-Ni matrix. Some reports do exist on metastable L12 ordering in binary Co–X alloys with the stoichiometry Co3X (where X = Ti, Al, Nb, Ta, W), but these phases are not stable at high temperatures (>600 °C) [11], [12], [13], [14]. For example, Co–Nb, Co–Ta and Co–W binary alloys are known to form metastable L12 ordering of Co3Nb, Co3Ta and Co3W respectively, along with hcp martensite plates in fcc-Co matrix at the early stage of heat treatment, but transforms to equilibrium structures thereafter [15], [16], [17]. In Co–Ti binary system, Co3Ti with L12 order structure appears. However, besides having lower melting point, the precipitates have large misfit that lowers thermal stability at high temperatures [18]. In Co–Al system, Co3Al phase with L12 ordering has been found occasionally in some of the grains [12]. This system is more prone to discontinuous precipitation of equilibrium B2 CoAl phase in γ-Co matrix [19], [20]. Thus, addition of W and Al with proper proportion in Co stabilises the γ′ structure with the stoichiometry Co3(Al, W) that are stable upto 900 °C, represent an important milestone. The phase stability of the exact stoichiometry Co3(Al0.5, W0.5) intermetallic compound has been evaluated using first principles calculation [21]. The results show that this phase is not a thermodynamically stable phase at 0 K with respect to possible phase separation into hcp Co, B2 CoAl and DO19 Co3W phases. However, at elevated temperatures, the metastability of the phase reduces due to vibrational, electronic and magnetic contributions to the free energy and also due to the fact that some excess Co occupies Al site. This reduces the free energy, and this conclusion is consistent with the excess Co found in the experimentally determined composition of the phase [21], [22]. The metastable nature of the ordered phase is also revealed through diffusion couple experiments that show decomposition of the γ′ phase to equilibrium phases mentioned above on prolonged aging for 1000 h at 900 °C [23], [24]. Further alloying additions such as Ta, Ti, Mo, Nb, Hf, etc. have been shown to improve solvus temperature, stability and high temperature mechanical properties [25], [26], [27], [28], [29], [30]. Their site preferences in the ordered L12 phase were predicted using first principles calculation, which matched well with compositions obtained by atom probe tomography [31], [32], [33], [34], [35], [36], [37], [38]. Similar to Ni based superalloys, these alloys also exhibit temperature dependent yield stress anomaly above 700 °C. The yield strength increases by 50–90 MPa due to activation of cross slip pinning process in L12 ordered precipitates [39], [40], [41].

However, the W content in these alloys varies between 15 and 25 wt%, thereby increasing the density of the alloys (9.3–9.9 g cm−3) significantly, which restricts applications where strength to weight ratio is crucial. Since Mo exists in the same group and also is a strong fcc stabilizer similar to W, reports of attempts to replace W with some amount of Mo to reduce the density of the alloy [42], [43] are available. The extent of replacement in these cases is only upto 3 at%, beyond which equilibrium phase Co3Mo with DO19 ordered structure appears. The morphology of Co3Mo precipitates is needle-like and promotes brittle fracture of the alloy. In case of Co–Al–Mo ternary alloys, L12 ordering does not take place on aging between 600 °C and 800 °C. Instead, equilibrium phases of Co3Mo with DO19 structure and CoAl with B2 structure appear [11], [44]. There are no reports on Co3(Al, Mo) phase having L12 ordering. Recently a new class of W free Co alloys with Al and Mo as alloying additions with γ–γ′ structure was reported [45], [46]. It was shown that small addition of Nb plays a critical role in stabilising γ–γ′ microstructure.

In this paper we first present an ab initio study, which indicates that Ta can stabilize ordered L12 γ′ structure in Co–Al–Mo alloys. Following this we present the experimental results of the effect of Ta addition to basic Co–10Al–5Mo alloy. We also present attempts to further enhance the solvus temperature and volume fraction of γ′ phase by additions of Ni and Ti. A detailed atom probe study has been carried out to understand the role of each alloying additions in the development of microstructures of this new set of alloys. Finally we shall present the mechanical properties of these alloys and endeavor to compare the results with the current generation Co and Ni based superalloys in order to establish the potential of these new alloys.

Section snippets

Effect of alloying elements on the stability of Co3(Al, X) L12 structure: ab initio study

Structure optimizations, total energy and electronic structure calculation of different crystal structures were performed using first-principles density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [47], [48], [49]. The electrons and ions interaction are described by all-electron projector augmented wave potentials [48], [49]. Exchange and correlation potential of electrons is described by Perdew–Burke–Ernzerhof functional (PBE) form within

Experimental

Although we explored large number of compositions, in this paper we present the results of three representative alloys. The three alloys of Co–10Al–5Mo base composition (in at%, throughout the manuscript) with varying amount of Ta, Ni and Ti were arc melted under argon atmosphere for atleast 10 times and cast in the form of 3 mm rods using four nine purity elemental metals. The final compositions of these alloys as measured by electron probe microanalyser are given in Table 1. We also prepared a

γ–γ′ phase evolution in Co–10Al–5Mo–2Ta alloy (2Ta)

The cast rods of the alloy Co–10Al–5Mo–2Ta were solutionised at 1300 °C for 15 h and subsequently quenched in water. The samples cut from the cast rods were aged at 800 °C to get the peak aging time. The peak aging was achieved after 10 h and the measured hardness value at this condition was Hv 400 ± 8. The microstructure of the peak aged sample was examined using transmission electron microscopy.

Fig. 3(a) shows dark field micrograph taken from the {1 0 0} superlattice spot in [0 0 1] zone axis for 2Ta

Conclusion

In conclusion, the paper reports novel superalloys based on Co–Al–Mo–Ta that are W free, exhibit γ–γ′ microstructure, and Ta that plays a crucial role in stabilising γ′. First principles calculation show that the occupancy of the Ta atoms in the stoichiometry Co3(Al, Mo) enhances dd hybridization among the transition metals, thereby ΔHf value becomes more negative for the L12 structure compared to DO19 structure. This leads to the formation of a pseudo gap. By replacing 30% Co with Ni in the

Acknowledgements

All the authors would like to acknowledge the microscope facility available at Advanced Facility for Microscopy and Microanalysis (AFMM) center, Indian Institute of Science, Bangalore. The authors also thank the Supercomputer Education and Research Centre and Materials Research Centre, IISc, for providing the required computational facilities. One of the authors (KC) acknowledges the financial support from Department of Science and Technology in the form of J.C. Bose National Fellowship.

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