Superalloys are widely adopted in many fields that require materials to serve at elevated temperature such as aircraft engines, gas turbines, chemical plants (Betteridge and Shaw, 1987; Pollock and Tin, 2006; Reed, 2008; Cheng et al., 2012; Qu et al., 2018). Co-based superalloys show better resistance of hot corrosion and oxidation than Ni-based superalloys. They are considered as one of promising candidates for applying at high temperature (Sims et al., 1987). However, compared with Ni-based superalloys, traditional Co-based superalloys are less used due to the inferior high-temperature strength. In Ni-based superalloys, the γ′-Ni₃Al strengthening phase with L1₂ structure precipitates from the disordered γ matrix phase with fcc crystal structure. And the coherent interfaces are formed between γ′-Ni₃Al and γ. This is one of main reasons for the outstanding high-temperature properties of Ni-based superalloys. Traditional Co-based superalloys are strengthened by carbide precipitations and solid solutions of refractory elements. The inefficient strengthening methods make traditional Co-based superalloys only be applied in mechanically low-loaded corrosive environments (Sims et al., 1987).

As early as 1971, Lee (Lee, 1971) discovered precipitation-hardened phenomenon in a 71 wt% Co–4 wt% Al–25 wt% W alloy, in which the ordered γ′ phase (L1₂) precipitated in γ matrix phase (fcc_A1). But almost nobody paid attention to his research results at that time. Until recently, Sato et al. (Sato et al., 2006) observed a γ+γ′ two-phase coherent microstructure in an alloy with the composition of 83.5 at.% Co–9 at% Al–7.5 at.% W, which was similar to that in Ni-based superalloys. The γ′-Co₃(Al, W) phase precipitated from γ matrix and formed the coherent interfaces with γ phase, which made Co-based superalloys possess the increased high-temperature strength. Due to higher melting point of Co than Ni, Co-based superalloys might possess better temperature capabilities. Subsequent studies (Kobayashi et al., 2009; Tsukamoto et al., 2010; Xue et al., 2011; Lass et al., 2014) showed the stability of the γ′ phase was still uncertain. According to the experimental results of Lass et al. (Lass et al., 2014), the γ′ phase existed for thousands of hours at 1173 K in the Co–Al–W alloys but its volume fraction decreased continuously and slowly with annealing time.

The high temperature performances of superalloys can be improved by adding alloying elements (Yokokawa et al., 2003; Shinagawa et al., 2009; Bauer et al., 2010). Ti (Xue et al., 2013; Christofidou et al., 2016; Bocchini et al., 2017; Llewelyn et al., 2017), W (Gupta et al., 2008a; Gupta et al., 2008b; Liu et al., 2010; Balam and Paul, 2011), Cr (Gupta et al., 2008b; Liu et al., 2010), Nb (Balam and Paul, 2011; Xue et al., 2013), Mo (Liu et al., 2010; Balam and Paul, 2011), and Ta (Balam and Paul, 2011; Xue et al., 2013) are important alloying elements in superalloys. The addition of Ti can improve the high-temperature stability of γ′ phase by increasing of its solvus temperature (Xue et al., 2013; Christofidou et al., 2016; Bocchini et al., 2017; Llewelyn et al., 2017) and promote the strength at high temperature of the Co-based superalloys. Furthermore, only Ti can form stable A₃B type compound (Co₃Ti) with ordered L1₂ crystal structure with Co among the above elements. And a two-phase region fcc + L1₂ is formed in the Co–Ti phase diagram. W is one of the most efficient alloying elements for improving high-temperature strength by solid-solution strengthening and increasing the stability of γ′ phase (Gupta et al., 2008a; Gupta et al., 2008b; Liu et al., 2010; Balam and Paul, 2011). Therefore, the Co–Ti–W system is worth being studied in detail.

The ternary phase diagram is an important basis for the design and application of materials (Li et al., 2009; Zhang et al., 2011; Xu et al., 2016). So far, the phase diagram of the Co–Ti–W ternary system has hardly been studied. In the current work, the phase relationships at 1273 and 1473 K of the Co–Ti–W system were experimentally determined. The purpose is to construct the Co–Ti–W isothermal sections and explore composition range and stability of γ′ phase at high temperature.

Murray (Murray, 1982) and Cacciamani et al. (Cacciamani et al., 2000) reviewed the experimental data of the Co–Ti system that were published before 1998 in detail. Davydov et al. (Davydov et al., 2001) determined the congruent melting point of the CoTi phase using differential thermal analysis (DTA) and a visual observation of melting (VOM) and optimized the phase diagram of the Co–Ti system. Recently, Wu et al. (Wu et al., 2020) experimentally confirmed that the invariant reaction among liquid, Co₃Ti and αCo₂Ti was a eutectic reaction at 1427 K. The phase diagram of the Co–Ti system in Ref (Wu et al., 2020). was adopted in this work and shown in Figure 1A. In the Co–Ti phase diagram, five solution phases liquid, fcc (Co), hcp (Co), bcc (Ti) and hcp (Ti), and five intermetallic compounds Co₃Ti, αCo₂Ti, βCo₂Ti, CoTi and CoTi₂ are stable.

In the Co–W phase diagram calculated by Kaufman et al. (Kaufman and Nesor, 1978), fcc (Co), hcp (Co), Co₃W, Co₇W₆ and bcc(W) phases were stable. Subsequently, Gabriel et al. (Gabriel et al., 1985) experimentally determined solid-liquid phase equilibria on the Co-rich side of the Co–W system. The temperatures of the liquidus and invariant reactions were determined using DTA with a heating rate of 5 K/min. Sato et al. (Sato et al., 2005) investigated phase equilibria among solid phases using equilibrated alloys and the diffusion couple technique. And the magnetic and martensitic transition temperatures were systematically studied by differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM) and dilatometric measurement. Dmitrieva et al. (Dmitrieva et al., 2005) determined the homogeneity ranges of Co₃W and Co₇W₆ at 1473 K using equilibrated alloys. Ravi et al. (Ravi and Paul, 2011) precisely measured the solubility of W in fcc (Co) by the diffusion couple technique. Recently, Wang et al. (Wang et al., 2019) evaluated the Co–W phase diagram based to the above experimental data. The Co–W phase diagram in Ref (Wang et al., 2019). was adopted in the current work and shown in Figure 1B. The Co–W phase diagram contains two intermediate phases, Co₇W₆ and Co₃W, which are formed by peritectic reaction liquid + bcc(W)→Co₇W₆ and peritectoid reaction fcc (Co)+Co₇W₆→Co₃W, respectively.

The Ti–W phase diagram is relatively simple, and liquid phase and two solution phases bcc (Ti, W) and hcp (Ti) are included. Murry (Murray, 1981) reviewed previous experimental information and evaluated the Ti–W phase diagram. Jin et al. (Jin and Qiu, 1993) optimized the Ti–W system on the basis of the modified Gibbs energy descriptions of pure Ti and W from the Scientific Group Thermodata Europe (SGTE) database. And all relevant experimental data were taken into account simultaneously. Jonsson (Jonsson, 1996) re-optimized the Ti–W system using simpler models and all the experimental data were well reproduced. The Ti–W phase diagram optimized by Jonsson (Jonsson, 1996) was adopted in present work and shown in Figure 1C. In the Ti–W phase diagram, there is a miscibility gap of bcc (Ti, W) below 1523 K, which makes the high temperature bcc (Ti, W) phase decompose into the Ti-rich and W-rich solid solutions.

Although the Co–Ti–W system is one of the most important Co-based superalloy subsystems, only very limited literature on this ternary system (König et al., 2014; Naujoks et al., 2017) was found. König et al. (König et al., 2014) fabricated a Co–Ti–W thin film materials library by magnetron sputtering. The compositions and resistances of various points on the thin film materials library were measured. Since the resistances of single-phase regions are lower than that of mixed phase regions, an unknown ternary phase region was revealed at 1223 K in the Co–Ti–W system according to sudden changes of resistance values at the phase region boundaries. The composition of the new ternary phase was about 60 at% Co–15 at% Ti–25 at% W. However, the new phase was not directly observed in the microstructure of the annealed bulk alloy at 1373 K. Naujoks et al. (Naujoks et al., 2017) proved that the solubility of Ti in Co₇W₆ phase was 18 at% at 1223 K and determined a three-phase region bcc(W)+Co₇W₆+αCo₂Ti in the 1273 K isothermal section of the Co–Ti–W system using thin-film and bulk materials. Besides, no experimental phase diagram data of the Co–Ti–W system is available.

The crystallographic data of all phases in the three binary systems are listed in Table 1(Magneli and Westgren, 1938; Murray, 1982; Sato et al., 2005).

Pure Cobalt (99.99 wt%), Titanium (99.99 wt%) and Tungsten (99.99 wt%) (China New Material Technology Company, Ltd.) were used as raw materials in the present work. The weight of each alloy sample is about 3–4 g. These samples were melted by using the arc-furnace (MTI MSM20-7) in an argon atmosphere with a non-consumable tungsten electrode. Several titanium rods firstly were melted in order to absorb residual oxygen before melting samples. To improve the homogeneity, every sample was turned around and re-melted at least 4 times. The samples which weight loss did not exceed 1% during arc melting were used in the following experiments. Then, each sample was cut into several pieces for different purposes.

In order to construct the isothermal sections, the Co–Ti–W alloys were annealed to obtain phase equilibria at 1273 and 1473 K. At 1273 K, the samples were annealed for 1000 h. At 1473 K, the time of the heat treatment varied from 5 h to 3 days for the samples with liquid phase. Others were annealed for about 720 h. In the process of annealing, the samples were sealed in evacuated quartz capsules (6 × 10^–2 Pa), and the chamber furnace (MTI KSL-1400) with a temperature accuracy of ±1 K was used. After heat treatment, the annealed samples were quenched into water to keep the phase constituents at 1273 and 1473 K.

Before microstructural characterization, all samples were ground and polished. The phases and element distributions of the all samples were studied using SEM (Carl Zeiss LEO 1450) in combination with EDS (Thermo Scientific UltraDry EDS). In order to increase the precision of the composition measurements, 3–5 points or areas were analyzed for each phase.

XRD analyses were carried out using X-ray diffractometer (Rigaku Ultima IV) with Cu-Kα radiation at 40 kV and 40 mA to identify the crystal structures of the constituent phases. Diffraction patterns were collected over the 2θ range from 20° to 90° with a scan step 0.02°. The software Jade 6.0 was used to assist phase identifications in this process.

The isothermal sections at 1273 and 1473 K of the Co–Ti–W system were constructed by phase equilibrium relationships obtained using SEM/EDS, XRD methods in the current work.

In the isothermal section at 1273 K of the Co–Ti–W system, five three-phase regions, Co₇W₆+Co₃Ti+βCo₂Ti, Co₇W₆+βCo₂Ti + bcc(W), βCo₂Ti + CoTi + bcc(W), CoTi + CoTi₂+bcc(W) and CoTi₂+bcc(W)+bcc (Ti), were experimentally determined, and three three-phase regions, fcc (Co)+Co₃W + Co₃Ti, Co₃W + Co₇W₆+Co₃Ti and Co₃Ti+βCo₂Ti+αCo₂Ti, were derived according to the phase relations. The experimental results showed that Co₃Ti phase could dissolve ∼7.9 at.% W at 1273 K.

The isothermal section at 1473 K of the Co–Ti–W system included eight three-phase regions, fcc (Co)+Co₇W₆+Co₃Ti, fcc (Co)+Co₃Ti + liquid#1, Co₃Ti + liquid#1+βCo₂Ti, Co₇W₆+Co₃Ti+βCo₂Ti, bcc(W)+Co₇W₆+βCo₂Ti, bcc(W)+βCo₂Ti + CoTi, bcc(W)+CoTi + liquid#2 and bcc(W)+liquid#2 + bcc (Ti). The Co₃Ti phase became a stable phase at 1473 K owing to the addition of W, and the homogeneity range of W in the Co₃Ti phase was determined to be ∼7.4–10.4 at.% at 1473 K.