Microstructure Evolution and Mechanical Properties of Novel γ/γ′ Two-Phase Strengthened Ir-Based Superalloys

Abstract

Ir-based superalloys are irreplaceable in some specific harsh conditions regardless of their cost and high density. In order to develop a new class of Ir-based superalloy for future ultrahigh-temperature applications, the microstructure evolution, phase relationships, and mechanical properties of Ir–Al–W–Ta alloys with γ/γ′ two-phase structure were investigated. Room- and high-temperature compressions at 1300 °C, and room-temperature nanoindentation for the Ta-containing Ir–6Al–13W alloys were conducted. The results show that the addition of Ta can significantly improve the high-temperature mechanical properties, but does not change the fracture mode of the Ir-based two-phase superalloys. The compressive strength of quaternary alloys can be attributed to the precipitation of γ′-Ir₃(Al, W) phase and solid solution strengthening. The microstructure and mechanical properties of Ir–Al–W–Ta quaternary alloys exhibit promising characteristics for the development of high-temperature materials.

1. Introduction

Structural materials need to be used at even higher operating temperatures than ever before. The temperature capability limit of Ni-based superalloys reaches about 1100 °C, which is approximately 85% of their melting points [1]. Therefore, many attempts have been made to develop intermetallic and refractory alloys which hold higher operating temperatures [2,3,4,5,6]. Platinum group metals are one of the candidates for the development of new refractory superalloys, due to their high melting temperature, exceptional mechanical properties and superior oxidation resistance. In particular, the melting point of Ir is 2447 °C and it is the only metal that still possesses excellent high-temperature mechanical properties above 1600 °C in an oxidizing atmosphere [7].

In recent years, an intermetallic ternary γ′-Ir₃(Al, W) phase with an L1₂ structure was discovered in the Ir–Al–W ternary system by Sato et al. [8]. The coherent relationship between fcc-structure Ir and L1₂-structure Ir₃(Al, W) in Ir–Al–W ternary systems is similar to the coexistence of γ/γ′ phases in Ni-based superalloys [9]. Therefore, the high-temperature strength of Ir alloys can be enhanced not only by solution strengthening, but also coherent precipitation strengthening of γ′-Ir₃(Al, W) phase, which significantly improves the mechanical properties at ultrahigh temperatures [10]. Shinagawa et al. first found that the Co–Al–W ternary system has γ/γ′ coherent structure similar to Ir–Al–W alloys, and can be further strengthened by the addition of Ta and Nb [11]. It is beneficial to the increase of the γ′ solvus temperature. The precipitation temperature of the γ′ phase in a Co–9Al–10W alloy increased from 1000 to 1079 °C by adding 2.0 at.% Ta [12,13]. However, Satoru indicated that adding Ta hardly changes the phase equilibria in the Co–Al–W ternary system [14]. T. Omori et al. investigated the phase equilibria and mechanical properties of the Ir–Al–W systems and found that they showed high hardness and strength at room and high temperatures [8]. Note that the microstructure evolution and mechanical properties in the Ta-containing Ir–Al–W alloy system has not been reported.

The aim of the present study is to investigate the influence of Ta on the microstructure evolution, phase relationships, deformation behavior, and mechanical properties of Ir–Al–W ternary superalloy. The microscopic mechanical characteristics of the novel Ir-based two-phase polycrystalline alloys were also studied and documented. Alloys were compressed at room temperature and 1300 °C, and grains with different orientations were tested via a nanoindentation technique. The role of alloying elements and the strengthening mechanism for Ir-based refractory superalloys are discussed.

2. Materials and Methods

Ir-based button alloys with the compositions of Ir–6Al–13W–xTa (at.%) (x = 0, 2, 5, and 8) were arc-melted under an Ar back-filled atmosphere by using 99.9% Ir, 99.99% Al, 99.9% W, and 99.9% Ta as the raw materials. Each button alloy was melted four times for homogenizing chemical composition followed by rapid cooling. These alloys were then subjected to solution heat treatment at 1900 °C for 1.5 h followed by a water quenching, which retained the microstructures at high temperature.

The microstructures of the as-cast and solid solution alloys were characterized using a Helios G4 CX FIB Electron microscopy (Thermo Fisher Scientific, Hillsboro, OR, USA) attached with a backscattered electron (BSE) detector. To understand the phase structures of the Ir-based polycrystalline superalloys, the constituent phases were identified by a D8 DISCOVER X-ray diffractometer (BRUKER AXS, Karlsruhe, Germany) with Cu Kα (30 kV, 300 mA) radiation over 10° ≤ 2θ ≤ 120°. Specimens for compression tests sized 2 mm × 2 mm × 10 mm were wire cut from these button ingots. Compression tests were conducted at 1300 °C under the initial strain rate of 1.0 × 10⁻⁴ mm/s on a a GNT-1000 Electronic universal material testing machine (NCS Testing Technology Co. Ltd., Beijing, China). All the specimens were held at testing temperatures for 15 min before loading. The load–depth curves, elastic modulus as well as the nanoindentation of the Ir-based two-phase superalloys were processed via a TI980 TriboIndenter (BRUKER, Karlsruhe, Germany).

3. Results

3.1. Phase Relationship

Phase identification in Ir–6Al–13W and Ir–6Al–13W–xTa alloys was conducted using XRD, as shown in Figure 1a,b.

The γ′ phase in the Ir–Al–W ternary compounds identified as Ir₃(W, Al) structure appeared. Also, γ, γ′, and β structures were detected in as-cast Ta-containing Ir–6Al–13W alloys. Axler et al. revealed that the β-IrAl phase with the B2 type crystal structure exists when Al concentration is 48–52 at.% in the binary Ir–Al phase diagram [15]. However, the diffraction patterns with Ir–6Al–13W–xTa contain a small number of peaks of β-IrAl phase which should not appear with design composition. The inconsistency with the previous study may be due to the rapid cooling rates and local components unevenness during solidification process. Therefore, the solidification path of alloys deviates from the equilibrium phase diagram [8], and would promote the precipitation of β-IrAl phase during non-equilibrium solidification. In addition, the peak strength of β-IrAl phase increased with the addition of Ta.

As seen in the Figure 1b, the angles of exhibiting peaks decrease slightly with increased Ta content. This is because the lattice parameters increase with the addition of Ta (the atomic radius of Ir, W, Al, and Ta were 136, 141, 143, and 148 pm, respectively). The lattice parameters of γ/γ′ phase were determined from the X-ray diffraction patterns as shown in Figure 2. It is therefore found that lattice parameters depending on Ta composition are clearly shown in γ/γ′ phase. In addition, we found that the diffraction peak of γ/γ′ phase in Ir–6Al–13W–xTa alloys is much more intensive than that of the Ir–6Al–13W ternary system.

3.2. Microstructural Morphologies

3.2.1. As-Cast Microstructures

Typical microstructures of the Ir–Al–W alloys are shown in Figure 3a–d. Dendritic morphology was observed for all the alloys consisting of γ/γ′ two-phase structure. The element content of different phases are listed in the Ir–Al–W ternary phase diagram (Figure 3f). From the X-ray diffractometer (XRD) results and the composition analysis by Energy Dispersive Spectrometer (EDS), the precipitates with dark contrast were determined as the β-IrAl phase and the phases with bright contrast were determined as the mixed γ-Ir/γ′-Ir₃(Al, W) phase (Figure 3a,f). In particular, component analysis of the EDS shows that the light gray matrix represents the Ir solid solution, which contains Al, W or Ta elements, and the dark gray region represents the γ′-Ir₃(Al, W) phase which precipitate from the interdendritic region. Similar reports also found that the γ, γ′, and β phases were stable in the Ir–W–Al system when the content of iridium is plentiful [8,16]. With the addition of Ta content, the volume fraction of the γ′-Ir₃(Al, W) phase increased. The β-IrAl phase was uniformly distributed with a disk-like or short-rod shape and evolved into a network-like morphology (Figure 3d). We performed semiquantitative analysis of the volume fraction of β-IrAl phase in Ir–6Al–13W–xTa alloys by Image-Pro Plus 6.0 software and found the volume fraction of the β-IrAl was 1.6% in Ir–6Al–13W, 3.1% in Ir–6Al–13W–2Ta, 3.4% in Ir–6Al–13W–5Ta, and 5.0% in Ir–6Al–13W–8Ta. The volume fraction of β-IrAl in Ir–6Al–13W–xTa alloys was very small and increased with the addition of Ta.

In the present investigation, an eutectic microstructure consisting of β-IrAl and γ-Ir solid solution was found in the as-cast ternary and quaternary alloys, as seen in Figure 3b. The coarsened β-IrAl phase, which formed in the interdendritic regions, also precipitated along grain boundaries as shown in Figure 3c. During the unbalanced solidification, Al element concentrated at the grain boundary region since the solid phase continuously ejected Al atoms and the liquid phase was simultaneously enriched in Al element, resulting in the formation of eutectic microstructure and β-IrAl phase. The distribution of these elements in Ir–6Al–13W–5Ta is shown in Figure 3e. Al was prone to segregate in the interdendritic region in solidification, while Ir and W elements concentrated in the dendrite arm. Ta element can not only dissolve in Ir matrix but also in γ′-Ir₃(Al, W) phase, and the diffusivity of Ta atom in β-IrAl is considerably slow compared with that in the γ/γ′ phase.

3.2.2. Heat-Treated Microstructures

Figure 4 shows the BSE images of the Ir-based alloys after homogenization at 1900 °C for 1.5 h. Microstructures of the Ta-containing alloys after heat treatments were similar to those of Ir–Al–W ternary alloy. The homogenized sample exhibits a fine γ/γ′ microstructure with a homogeneous distribution of γ′ particles, as shown in Figure 4a, which indicated that the γ′ phase precipitated during cooling from the γ single-phase field. After homogenization for a short time, β-IrAl phase with a dark contrast, which discontinuously appeared in the eutectic region, still remained at room temperature (Figure 4b).

3.3. Compression at Room Temperature and 1300 °C

Figure 5a,b show the compressive stress–strain curves for the polycrystalline Ir-based two-phase alloys at room temperature and 1300 °C.

The room-temperature compression process can be divided into three stages:elastic deformation stage (I), yield stage (II) and work hardening stage (III), as seen in Figure 5a. The Ir–Al–W–Ta quaternary alloys showed that the stage of uniform plastic deformation is very short at room temperature, suggesting its plastic is not good enough, and the vertical section profile along the loading direction of the samples after the compression test suggested an intergranular fracture. The plastic deformation of Ir–Al–W–Ta alloys are consistent with other fcc metals, but they then undergo brittle intergranular fracture and brittle transgranular fracture. Marc J. Cawkwell indicated that the pure Ir has a high rate of work hardening during neck-free plastic deformation, and clear dislocation multiplication would take place in pure Ir at a rate far beyond that possible in other fcc metals [17]. Our data also illustrate that it continues without breaking during the work hardening stage.

At 1300 °C, the ductility of the alloys was remarkably improved to around 35–40%, further resulting in a steady plastic deformation (II) after elastic deformation stage (I) in Ir–6Al–13W and Ir–6Al–13W–2Ta alloys, as shown Figure 5b. The work hardening stage (III) and the secondary yielding stage (IV) of Ir–6Al–13W–8Ta alloy occurred under large deformation at 1300 °C. The main reason is because deformation induced recrystallization, suggesting that the ductility of the Ir–Al–W system was improved. All tested alloys exhibit a similar temperature-dependent ductility, which is similar to that in Ni₃Al alloys. Compared with the Ir–Al–W ternary alloy, higher strength at elevated temperature has been confirmed when alloying with Ta element, which is attributed by the strong solid solution hardening in the Ir–Al–W–Ta quaternary alloys with the γ/γ′ two-phase structure.

3.4. Orientation-dependent Nanoindentation Properties

Electron Backscattered Diffraction (EBSD) analysis provided quantitative orientation information of as-cast ternary and quaternary Ir-based alloys and grain orientation of alloys shown in Figure 6a,b. The γ′-Ir₃(Al, W) phase maintained a coherent relationship with the γ-Ir matrix [10], and cuboidal γ′ precipitates were formed in the Ir–Al–W alloys with a lattice misfit of about 0.1% [16]. Given this, the EBSD result is unable to identify γ and γ′ phases in Ir-based γ/γ′ two-phase systems while the γ and γ′ phases cannot be distinguished as with Ni-based superalloys [18]. Figure 6 shows that the oriented texture increased and the grain orientations were close to (100) base texture as the amount of Ta increased. The misorientation of intracrystalline of the ternary Ir–Al–W was displayed more obviously than Ir–6Al–13W–8Ta alloy. Also, the grain size was significally refined by adding Ta element (Figure 6a,b).

Nanoindentation experiments were performed in some selected grains containing mixed γ/γ′ two-phase structure, and the results are presented in Figure 7a,b, which shows micromechanical properties of different orientations in Ir-based alloys. The grains close to (100) exhibit slightly higher nanohardness and elastic modulus than other regions with soft orientations. The maximum modulus generally appears in the direction of dense packing in fcc-metal, but this is not necessarily the case for polycrystalline materials, whose deformation behavior would be changed by adding solute elements.

Preliminary results demonstrated that the hardness in Ir single crystal was inversely proportional to the loading depth [19,20]. However, in Figure 7c, we ascertained that the hardness of (014) orientation does not decrease as loading depth increased in Ir–6Al–13W–8Ta alloy. In addition, from the nanoindentation test, the average nanohardness of Ir–6Al–13W–8Ta alloy was found to be 14.2 GPa, and only 13.1 GPa in Ir–6Al–13W.

4. Discussion

The volume fraction of γ′-Ir₃(Al, W) increased with the addition of Ta (Figure 1 and Figure 3), suggesting that Ta addition promoted the formation of the γ′-Ir₃(W, Al) phase. After homogenization, the microstructural morphology of the β-IrAl phase was inconsistent with that in [8], which was coarsened by annealing at 1900 °C, and the phase boundary was arc-shaped. Due to the addition of Ta, the heat treatment for homogenization did not reach solution temperature. In order to promote the redissolution of the second phase in Ir matrix and obtain a homogenized solid solution, we consider that the homogenization temperatures in Ir–6Al–13W–xTa alloys should be performed at a higher temperature and the holding time needs to be extended.

Figure 5a shows a high rate of work hardening during neck-free plastic deformation and high strength in Ir–Al–W systems at room temperature, but in some cases, this high strength is undesirable, because it is linked with an increase in fracture and low malleability, ultimately limiting the application of these materials. Higher strength at elevated temperatures was confirmed when alloying with Ta element (Figure 5b), which is attributed by the strong solid solution hardening in the Ir–Al–W–Ta quaternary alloys with the γ/γ′ two-phase structure. The volume fraction and the size of γ′ precipitates were the governing factors affecting the strength of Ir-based alloys [21]. According to the results, the volume fraction of γ′-Ir₃(Al, W) increases with the increasing Ta content, which accounts for the high strength of Ir–6Al–13W–8Ta alloy. In summary, Ta seems to effectively stabilize the γ′-Ir₃(Al, W) phase at higher temperature and improve the strength of γ/γ′ two-phase Ir–Al–W system. Recent efforts have also focused on adding strong γ′-forming elements to stabilize the γ′ microstructure and increase the γ′-solvus temperature in Co–Al–W-based system and it was found that Co–Al–W can be further strengthened by the addition of Ta [11,12,13], similarly to Ir–Al–W systems.

Although the volume fraction of γ′-Ir₃(Al, W) increased and the Ir–6Al–13W system has been further strengthened by the addition of Ta, Ir–6Al–13W–xTa systems undergo brittle intergranular fracture and brittle transgranular fracture in compression tests. The intergranular brittleness in Ir and its alloys were considered to be intrinsic and tended to occur when the grain boundaries were much more brittle than the intragranular structure. Meanwhile, the addition of Ta could not change the fracture mode of brittle intergranular. Therefore, the grain boundary brittleness should be also considered in these polycrystalline alloys which show some potential applications as a new class of refractory superalloy, as shown in Figure 8.

Fracture at high temperature in Ir–Al–W systems tends to be intergranular, as shown in Figure 8a. According to the results, the segregation of Al element at the grain boundary (GB) resulted in the formation of β-IrAl phase, which formed along the GBs in two-phase refractory superalloys. It reduced the adhesion to the matrix and increased the brittleness and, thus, fracture along the GBs occurred. The thick strip β-IrAl phase inside the grain also governs the transgranular fracture, as shown in Figure 8b. This indicates that when Ta is alloyed, the tendency of cracking along the GBs decreases and the fracture mode changes from intergranular expansion to brittle transgranular cleavage. Cracks can be nucleated by the stress concentration around the β-IrAl phase, which is brittle, and the stress concentration can be quickly relaxed following the crack extension. Thence crack propagation is always along the brittle β-IrAl phase in intracrystalline.

The nanoindentation experiment reflects that the addition of Ta can improve the hardness of γ/γ′ two-phases microstructure. Nanoindentation is a useful tool to investigate plasticity on small scales [22]. The maximum nanohardness did not appear in the dense packing direction (111). The crystallographic should be considered when explaining variations in nanohardness of different orientations in Ir-based alloys. We suspected that the deformation behavior for pure metals would be changed by adding solute atoms. Trinkle studied the direct interaction of solutes with dislocations in molybdenum and indicated solute-induced softening is controlled by both of them [23]. Therefore, the addition of solid solution element Ta resulted in the reduction of deformation resistance of dense packing direction (111) in some cases. Meanwhile, we found that the average nanohardness of all orientations in Ir–6Al–13W-8Ta alloy presents a relatively high value compared with that of Ir–6Al–13W alloy (Figure 7a,b). However, the distribution rule of orientation-dependent hardness is similar. The results further verified the strengthening effect of Ta on the γ/γ′ matrix in Ir–6Al–13W systems.

5. Conclusions

The results showed that Ir–6Al–13W γ/γ′ two-phase refractory superalloys alloyed with Ta are superior to the ternary Ir–6Al–13W alloy as an ultrahigh-temperature structural material in terms of strength, fracture behavior, and ductility.

(1)

The addition of Ta contributes to the change of volume fraction, size, and lattice parameters of the γ′-Ir₃(Al, W), and the angles of exhibiting peaks slightly decrease.

(2)

The γ/γ′ two-phase microstructure can be further enhanced by the addition of Ta, and these crystallographic orientations in matrix phase close to (111) have lower nanohardness compared with other regions, which could be caused by solute-induced softening.

(3)

Ta effectively improves the high-temperature mechanical properties of Ir-based superalloys, and the increased compressive strength of Ir–6Al–13W–xTa quaternary alloys can be attributed to the precipitation of γ′-Ir₃(Al, W) phase and solid solution strengthening. The Ta-containing Ir–Al–W two-phase quaternary alloys show some potential in applications as a new class of refractory superalloy.