The interfacial β₀ phase strengthening the creep properties of powder hot isostatic pressing γ-TiAl alloy

Abstract

Interfacial β₀ phase strengthening on the creep properties of TNM alloy has been investigated. The initial fully lamellar microstructure with fine-dispersed interfacial β₀ phase was fabricated by powder hot isostatic pressing (P-HIP) and then carried out ageing treatment. The results indicated that the aged condition presented better microstructural stability and excellent creep resistance than the P-HIPed condition. Moreover, the ω phase precipitated within the β₀ phase by stress-inducing. The β₀/β₀(ω) phase was not only effectively restricted the dislocation motion, twins and stacking faults propagation, but also delayed the α₂ lamellae decomposition to maintain the integrity of the fully lamellar structure.

GRAPHICAL ABSTRACT

IMPACT STATEMENT

The fine-dispersed β₀ precipitated at the α₂/γ lamellar interface by ageing treatment was effectively hindered the dislocation motion and significantly strengthened the creep properties. The strength mechanism was revealed.

KEYWORDS:

• TiAl alloy
• β₀ phase
• hot isostatic pressing
• creep
• phase transformation

1. Introduction

TNM alloys with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (at. %)[1], as the third generation TiAl alloy, are exhibiting a lower density, high specific modulus, good high-temperature mechanical strength, which is considered to be attractive for high-temperature structure materials [2–4]. Creep resistance and structure stability are both significant prerequisites for high-temperature structure TiAl components and determine the service temperature range [5]. Therefore, it is essential to improve the creep property of TiAl alloy.

The mechanical properties of TiAl alloy are shown a stronger dependence on the microstructure and chemical composition [5]. Due to the refractory elements (such as Mo, Nb.) that are generally strong β stabilizer elements in TNM alloy, the ordered β₀ phase with hard and brittle characteristics remains at room temperature and is difficult to eliminate [6,7]. Previous research has been studied on chemical composition on the TNM alloy, the fully lamellar microstructure (FL) of TNM alloy was obtained by adding the 0.3% C and 0.2% Si elements (in at. %), which presented the best creep resistance than the near fully lamellar microstructure (NFL) of TNM alloy, i.e. (NFL + γ_g+ β₀₎ and (NFL + β₀) [8]. Since the blocky β₀ phase with a body-centered crystal structure is located at the colony boundaries, which facilitates the diffusion processes at high temperatures due to the lower dense-packed [9,10]. As for the cast TNM alloy with 1.5 Mo element addition, the β₀ phase was irregular particle and the volume fraction of the β₀ phase could be controlled at a range of 3–18%. However, its effect on the creep properties was uncertain [11]. Especially, the β₀ phase distributed around the colony boundary and presented a continuous net shape acting like a ‘low creep strength film’ to reduce creep resistance or result in local stress concentration [12,13]. In addition, the ω phase was easier precipitated within the β₀ phase and the β₀/β₀ (ω) phase with a brittle characteristic at grain boundary caused deformation incompatibility and resulted in local stress concentration, which was treated as nucleation site of cavity or cracks [14,15]. The crack propagation always took place on the β₀/β₀(ω) phase area and, up to the creep failure. Therefore, a higher volume fraction of blocky β₀/β₀(ω) phase located at the colony boundaries might be a negative effect on the creep resistance.

The conventional peritectic solidification γ-TiAl alloy was easier to obtain fully lamellar microstructure [16]. With W, Mo, Nb β-stabilization element addition, the fine dispersed interfacial β₀ phase precipitated within α₂ lamellar after ageing treatment, and the dispersed β₀ phases provide precipitation strengthen and hindered the dislocation motion [17,18]. Moreover, the dispersed interfacial β₀ restricted the migration of ledges along with the α₂/γ interface, thererby delaying the α₂ decomposition. Hence, the creep resistance is improved. However, the β₀ was precipitated in the single-phased γ areas and in the lamellar colonies, which in turn induces the formation of W-depleted bands in the borders and the reduction of the width of α₂ lamellae, as well as some tungsten segregation in these precipitates. Therefore, the strength and creep resistance was deteriorated [19]. The impact of β₀ on creep performance is controversial and complex, which is stronger dependent on the grain size, distribution, volume fraction, etc.

In this work, the fully lamellar TNM alloy was fabricated by powder hot isostatic pressing, and the fine dispersed particle β₀ phase precipitation was obtained by ageing treatment. The influence of the interfacial β₀ phase on the microstructure stability and creep properties were discussed in detail.

2. Materials and methods

The fully lamellar TNM powder ingots (Ti-43.98Al-4.01Nb-1.04Mo-0.17B, actual composition in at.%) with a diameter of 30 mm and a height of 300 mm were consolidated by powder hot isostatic pressing (P-HIP) at 1260°C under 170 MPa for 4 h. Cylindrical samples with a diameter of 12 mm and a height of 80 mm were cut along the central axis and then ageing treated at 1050°C for 2 h with furnace cooling. For evaluating the creep behavior, cylindrical samples were machined by computerized numerical control (CNC) machining to a gauge length of 25 ± 0.1 mm and a diameter of 5 mm. Uniaxial creep experiments were tested on the NCS GNCJ-100E creep tests machine at 750°C under the constant stress of 250 MPa for 140 h. Samples for scanning electron microscope (SEM) and transmission electron microscope (TEM) were cut from the gauge area along the load direction and then prepared by grinding and then followed by electron polishing procedure. The FEI Helios G4 CX SEM was used to observe the microstructure characteristic utilizing the backscattered electron mode (BSE). The phase volume fraction was used Image-Pro plus software to statistic 5–10 SEM pictures. Thin foils for TEM observation were prepared by twin-jet electron polishing from 60 μm thick, 3 mm diameter disks using a solution of 500 ml electrolyte (5% perchloric, 35% butanol, 65% methanol) at −35℃, 30 V. The chemical composition were analyzed by using energy dispersive spectroscopy (EDS) system. The high angle annular dark-field (HAADF) and TEM images were tested on the FEI Talos F200X microscopy at 200 kV.

3. Results and discussion

Figure 1(a) shows a fully lamellar structure of the P-HIPed condition. The insert image depicted γ lamellar in dark gray and α₂ lamellar in light gray and a very small amount of β₀ phase in white located at the colony boundaries. The volume fraction of β₀ and γ phases are about 0.3% and 72.1%, respectively. Moreover, the bright-field (BF) TEM image in Figure 1(b) depicted that there was no precipitation observed within the α₂ lamellae. And the corresponding selected area electron diffraction (SAED) patterns only presented γ and α₂ phase diffraction spots, which obeyed orientation relationship as followed by: <10-1>γ//<11-20>α₂ and {1-11}γ//{0001}α₂. The microstructure characteristic of the aged condition of TNM alloy is shown in Figure 1(c,d). With the α₂ lamellae decomposition during ageing treatment, i.e. α₂ →γ and α₂ → β₀ phase transformation. The proportion of β₀ phase and γ phase increased to 1.4% and 79.1%, respectively. Many interfacial β₀ phase precipitated within the α₂ lamellae marked with red arrows in Figure 1(c), and fewer β phases were located within γ lamellae like ‘isolated island’ because of the α₂ lamellae dissolved. Since the excess and metastable α₂ phase tends to decompose to achieve phase equilibrium after elevated temperature treatment [20,21]. The BF image in Figure 1(d) shows that the β phase marked with red arrows was nucleated at the α₂/γ interface, the corresponding SAED pattern of β phase taken along [001]β₀ direction show that there was no precipitation within the β₀ phase after aged treatment.

Figure 1. (a) BSE image of the P-HIPed condition, the insert image shows high magnification, (b) BF image of α₂/γ lamellae, the corresponding SAED patterns of α₂/γ lamellae, (c) BSE image of Aged condition, the insert image shows high magnification, (d) BF image of the interfacial β₀, the corresponding SAED patterns of β₀ phase along [001]β₀ direction.

Figure 2(a) displays the creep curves of different conditions at 750°C under 250 MPa constant loading, i.e. the gray curves represent of aged condition and the red curves represent of P-HIPed condition. The ageing treatment was not only stabilize the lamellar structure but also form finely dispersed β₀ precipitates at the α₂/γ lamellar interface. The interfacial β₀ phase of the aged condition showed a significant improvement in the creep resistance. The aged condition with 1.7% creep strain was over three times lower than the P-HIPed sample with 5.4%. Furthermore, the steady-state creep strain rate of the aged condition with 5.64E-9 s⁻¹ was seven times slower than the P-HIPed condition of 8.89E-8 s⁻¹. Compared to the other TiAl alloy at 750°C in Figure 2(b) [8,22], the steady-state creep strain rate of the aged condition presented the lowest creep rate than the fully lamellar TNM and Ti-46Al-2W-0.5Si-0.7B.

Figure 2. (a) Creep curves at 750°C under 250 MPa for 140 h,(b) comparison of steady creep strain rate.

Figure 3 shows the deformed microstructure characterization after the creep test. The microstructure of the P-HIPed condition displays severe microstructure degradation in Figure 3(a). The excess α₂ lamellar decomposition caused massive phase transformation during creep, in which the volume fraction of γ phase increased from 72.1% to 84.9%. It destroyed the accumulated defects and caused microstructure instability. With the creep strain increased, the strain incompatibility at the colony boundaries caused local stress concentration [23,24]. The high magnification of the insert image in Figure 3(a) shows that the P-HIPed condition was released local stress concentration by dynamic recrystallization (DRX) of γ phase and the nucleation of the cavity. It deteriorated the microstructure stability as well as creep resistance. Figure 3(b,c) presents the deformed microstructure of the aged condition, which depicted better microstructure stability. The fully lamellar structure was not damaged, and no DRX occurred. There were some twins observed within the γ lamellae. Compared to the P-HIPed condition, the volume fraction of β₀ and γ phase increased to 2.6% and 84.1%, respectively. Due to the α₂ dissolution, some β₀ phases were located within γ lamellar, which look like ‘isolated islands’ in Figure 3(c,d). The dark-field (DF) TEM image in Figure 3(e) shows that there was some ellipsoid ω phase precipitated within the β₀ phase. Based on the insert image of the SAED pattern which concludes two sets of diffraction spots. The ω phase and β₀ phase obey orientation relationship as <−111>β₀//<11-20>ω and {110}β₀//{0001}ω. A previous study found that the ω phase was also precipitated at β₀ phase at 750–825°C for long-term heat treatments under unloaded conditions, its growth mechanism is dominated by element diffusion [25]. Due to the introduction of the applied stress, it was attributed to accelerating the element diffusion, and the formation of the ω phase was also promoted. According to the HAADF and the corresponding EDS images (Figure 4(f)) and TEM-EDS spot measurements in Table 1, ellipsoid ω phase was enriched Nb element and depleted Al element. As the growth of ω phase consumes Nb elements and rejects Mo elements, it led to a stable β₀ phase by dissolving more Mo elements. However, the γ phase has lower solubility of β-stabilizer elements, i.e. Mo and Nb elements [26]. Consequently, the β₀(ω) phase could not be transformed into γ phase. It was treated as the second particle to reinforce the creep resistance, and the strengthening mechanism will be discussed as follow.

Figure 3. (a) BSE image of the P-HIPed condition, the insert image shows high magnificaiton;(b-c) BSE image of the aged condition;(d) BF of the aged condition, (e) DF image of ω phase, the insert image shows the SAED patterns of β₀ and ω phase; (f) the corresponding HAADF and EDS image of β₀(ω).

Figure 4. The defects interaction of aged sample after creep test using the diffraction vector at g = [1−11]. (a, d) HAADF images of β₀ phase, the insert images is the CBED of the β₀ phase; (b, e) TBBF images and (c, f) WBDF images of the interaction between interfacial β₀ phase and defects, respectively; (g, h) TBBF image of the interaction between grain boundary β₀(ω) phase and defects; (i) TBBF image of the interaction between lamellae and defects.

Table 1. TEM-EDS spot measurements within pure β₀ and ω phase chemical composition (at.%).

Phase	Al	Ti	Nb	Mo
ω	30.8 ± 1.5	60.5 ± 1.6	8.2 ± 0 .6	0.5 ± 0.2
β0	30.7 ± 1.2	55.7 ± 1.8	6.7 ± 0.5	6.9 ± 0.5

Figure 4 shows the HAADF, two-beam bright-field (TBBF), and weak-beam dark-field (WBDF) TEM images of the defects interaction of the aged sample by using the diffraction vector at g = [1−11]. As can be seen from Figure 4(a,d), the HAADF images depicted that the particle precipitations in white were enriched by the high β-stabilizer elements (Ti-30.5Al-6.8Nb-6.9Mo, at. %), and the insert image shows that the precipitates identified by the convergent beam electron diffraction(CBED) of the β₀ phase along the [−111]β₀ direction. Consequently, these precipitates were proved to be β₀ phase. Moreover, a high-density dislocation was observed at the α₂/γ lamellae interface in Figure 4(a,b). Since the interfacial dislocation emission is related to the α₂ lamellae decomposition during creep [27], the interfacial β₀ phases could effectively pin the dislocation motion and result in dislocation tangled at the phase boundaries in Figure 4(b,c). With the α₂ lamellae decomposition, the interfacial β₀ phase with bulged morphology was attributed to hindering the dislocation and delayed the rate of α₂ lamellae dissolved. Moreover, the stacking faults (SFs) presented in two directions parallel and inclined to the γ lamellae, it was also found that the interfacial β₀ phase hindered the SFs propagation in Figure 4(d–f). The fine dispersed interfacial β₀ phase provided precipitation strengthen for creep resistance. Furthermore, the grain boundary β₀(ω) phase hindered the dislocation movement, twins, and stacking faults propagation in Figure 4(g,h). Since the ω phase precipitation stabilizes the β₀ phase and improved effectively the hardness up to high temperature [28]. The grain boundary β₀(ω) phase was acting as a secondary phase particle strengthening. Moreover, the partial dislocations were transferred into the β₀(ω) as a slip dislocation marked with red circles in Figure 4(h). Compared to the P-HIPed condition, the occurrence of DRX was related to the lamellar cylinderization and edge spheroidization [29]. After the ageing treatment, the volume fraction of phases were reached a relative equilibrium condition, and the precipitation of interfacial β₀ phase was formed and stabled the α₂ lamellae. Figure 4(i) shows that the intact lamellar could retard the motion of dislocation during creep and provided obstacles for SFs and twin propagation. Conventionally, the interfaces presented obstacles for dislocation and served as dislocation sources. SFs that are highly stable conditions can block the dislocation motion [30,31]. A higher density of defects was observed within the lamellar colony, and the dislocation was pile-up at the lamellar interface and SFs conjunction area marked with yellow arrows. Moreover, the SFs were transferred into the fine (α₂+γ) lamellae as a slip dislocation marked with red circles in Figure 4(i). These were attributed to strengthening the creep resistance.

The schematic diagrams of the deformed microstructure after the creep test is shown in Figure 5. The deformed microstructure of the P-HIPed condition showed severe microstructure degradation, such as massive phase transformation, deformed lamellae, DRX of γ phase, and the nucleation of cavities. On the contrary, The aged condition was not only stabilize the lamellar structure but also formed a fine dispersed β₀ phase at the γ/α₂ lamellar interface. The interfacial β₀ phase has been shown to significantly improve the creep resistance by hindering dislocation motion, twin, and SFs propagation. It also delayed the α₂ lamellae decomposition and maintain the integrity of the lamellar structure.

Figure 5. The schematic diagrams showing deformed microstructure.

4. Conclusion

A fine and uniformly dispersed interfacial β₀ phase were precipitated within α₂ lamellae after age treatment. The β₀ phase effectively blocks the dislocation motion, twin and stacking faults propagation. Moreover, the interfacial β₀ phase also delays the α₂ lamellae decomposition and stable the lamellae integrity. Finally, the aged condition exhibited better microstructure stability as well as creep properties.

Acknowledgement

The authors would like to thank the Analytical & Testing Center of NPU.

Disclosure statement

No potential conflict of interest was reported by the author(s).This work was supported by the Research Fund of the State Key Laboratory of Solidification Processing (NPU), China under grant number 2020-TZ-03.

Additional information

Funding

This work was supported by the National Science and Technology Major Project of China, (No.J2019-VI-0011-0125), and the Research Fund of the State Key Laboratory of Solidification Processing (NPU), China under Grant No. 2020-TZ-03.