Design and strengthening behaviour of Ti 2 AlC/TiAl composite by low-temperature hot-pressing process

Introduction

As potential structural materials for high-temperature applications, γ-TiAl alloys suffer from poor ductility and bad fracture toughness.^1-4 Thus, considerable attempts have been devoted to improve their mechanical properties and ductility over the last decades, such as heat treatment, thermal–mechanical processing, alloying and composite technology. Intermetallic-matrix composites have attracted much attention because of the improvement of toughness, oxidation resistance and other properties by introducing ceramic reinforcements. ⁵ Compounds such as TiB₂, Al₂O₃, Ti₅Si₃, Ti₂AlC ^{5 8} as well as their combinations^9,10 have been identified as compatible and thermochemically stable reinforcing agents for TiAl matrix composites. Among them, Ti₂AlC is considered as the most attractive reinforcement for TiAl matrix composites, mainly due to its ternary-layered structure and properties with both metallic and ceramic characteristics. Especially, the thermal expansion coefficient of Ti₂AlC (8.8 × 10^–6 K⁻¹) is close to that of TiAl (12 × 10⁻⁶ K⁻¹). ¹¹ Currently, Ti₂AlC/TiAl composites were mainly synthesised from the mixture powders of Ti–Al–Nb–C, ¹² Ti–Al–CNTs ¹⁰ and Ti–Al–TiC. ¹³

In this paper, it is the first time to prepare in situ Ti₂AlC/TiAl composite by hot-pressing process using Ti₃AlC₂ and Ti–Al alloy powders. The microstructure and mechanical properties of the composite are investigated in detail.

Experimental

Commercial powders of Ti (99.8% purity, ∼35 μm), Al (99.6% purity, ∼55 μm) and TiC (99.9% purity, ∼20 μm) were used as initial materials. First, high-purity Ti₃AlC₂ powders were prepared by in situ reaction from the 2TiC–Ti–1.2Al system, and the experimental details can be found elsewhere. ¹⁴ Second, Ti–Al alloy powders were synthesised by in situ reaction from Ti–48Al (at.-%) system at 900°C for 30 min. Then, 5 wt-% Ti₃AlC₂ and 95 wt-% Ti–Al alloy powders were precisely weighted and milled sufficiently for 2 h. The mixed powders were put into a graphite mould pre-sprayed with BN coatings and then hot-pressed at 1150°C for 2 h with a rate of 10°C min⁻¹ under a pressure of 30 MPa in vacuum.

The phase composition and microstructure of the composite were characterised by X-ray diffraction (XRD) and SEM, respectively. The Vickers hardness of the product at room temperature was measured using a microhardness tester at a load of 9.8 N for 15 s. The flexural strength and the fracture toughness of the composite were measured by the three-point bending method with a dimension of 3 mm × 4 mm × 30 mm using a universal testing machine. The single-edge-notched beam method was employed for determining the fracture toughness of the product. The crosshead speeds for flexural strength and fracture toughness tests were 0.5 and 0.05 mm min⁻¹, respectively.

Results and discussion

Figure 1 shows the XRD patterns of the as-synthesised Ti₃AlC₂ powders, Ti–Al alloy powders synthesised at 900°C for 30 min and Ti₂AlC/TiAl composite fabricated from Ti₃AlC₂ and Ti–Al alloy powders at 1150°C for 2 h. XRD results indicate that Ti₃AlC₂ powders are composed of Ti₃AlC₂ and a small amount of TiC phases (Fig. 1a), and the content of Ti₃AlC₂ is about 99.4 wt-%. Ti–Al alloy powders consist of TiAl, Ti₃Al and TiAl₂ phases, as seen clearly in Fig. 1b. Furthermore, the XRD results of Ti₂AlC/TiAl composite suggest that there are mainly Ti₂AlC phase besides TiAl and Ti₃Al phases, which means that Ti₂AlC phase has been synthesised completely during the hot-pressing consolidation (Fig. 1c). Therefore, we conclude that the following reactions may occur: (1) (2) (3)

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XRD patterns of a Ti₃AlC₂ powders, b Ti–Al alloy powders synthesised at 900°C for 30 min and c Ti₂AlC/TiAl composite fabricated from Ti₃AlC₂ and Ti–Al alloy powders at 1150°C for 2 h

Figure 2 shows the SEM images of the TiAl alloy prepared from Ti–48Al (at.-%) system at 1150°C for 2 h and Ti₂AlC/TiAl composite prepared from Ti₃AlC₂ and Ti–Al alloy powders at 1150°C for 2 h. As shown in Fig. 2a, near-equiaxed γ-TiAl grains with an average size of 25 μm can be obtained. On the contrary, for the Ti₂AlC/TiAl composite, the fine spheroidal Ti₂AlC reinforcing agents with an average size of 2 μm are mainly distributed at the TiAl grain boundaries, and the bonding between Ti₂AlC and TiAl is very tight, which is believed to be beneficial for the enhancement of the strength and toughness of the TiAl composite, as shown in Fig. 2b. And the γ-TiAl grain size decreases to 5–10 μm. Note that some fine Ti₂AlC particulates are dispersed in the matrix, which constitutes intragranular structure and accounts for the significant strengthening and toughening effects. During the process, the Ti₂AlC particles can develop at low sintering temperature, due to the appropriate heat caused by the reaction of 2TiAl₂+ Ti₃Al = 5TiAl, and refine the microstructure of the TiAl matrix.

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SEM images of the TiAl alloy prepared from Ti–48Al (at.-%) system at 1150°C for 2 h and Ti₂AlC/TiAl composite prepared from Ti₃AlC₂ and Ti–Al alloy powders at 1150°C for 2 h: a TiAl alloy and b Ti₂AlC/TiAl composite

The Vickers hardness, flexural strength and fracture toughness of the TiAl alloy prepared from Ti–48Al (at.-%) system at 1150°C for 2 h are 3.2 GPa, 305.8 MPa and 6.2 MPa m^1/2, respectively. Conversely, the Vickers hardness, flexural strength and fracture toughness of the Ti₂AlC/TiAl composite are 5.2 GPa, 937.7 MPa and 7.7 MPa m^1/2, respectively, which are 62.5, 206.6 and 24.2% higher than that of the TiAl alloy. To our surprise, the Vickers hardness is higher than that of Al₂O₃/TiAl, ⁶ Ti₂AlC ¹¹ and Ti₂AlC/TiAl. ¹³ The flexural strength is considerably higher than that of Ti₂AlC/TiAl, ¹³ TiAl–Al₂Ti₄C₂–Al₂O₃–TiC ¹⁵ and Al₂O₃/TiAl.^6,16 The fracture toughness is significantly higher than that of TiAl–Al₂Ti₄C₂–Al₂O₃–TiC, ¹⁵ Al₂O₃/TiAl^6,16 and TiAl/B₄C, ¹⁷ and close to Ti₂AlC/TiAl. ¹³

Typical crack propagation of the TiAl alloy and Ti₂AlC/TiAl composite is shown in Fig. 3. As seen in Fig. 3a, the TiAl alloy exhibits intergranular fracture, and such fracture is a typical brittle fracture. Actually, the strength and the toughness of the TiAl alloy are very lower. Conversely, a fluctuating crack appears in the Ti₂AlC/TiAl composite, and the crack involves more transgranular fracture (Fig. 3b). Moreover, the crack deflection, crack bridging and crack branching are present in the Ti₂AlC/TiAl composite. The above-mentioned phenomena can enhance the crack growth resistance remarkably and result in higher fracture toughness. The crack deflection can extend the effective crack length and absorb more fracture energy during loading. Bridging of the Ti₂AlC particles appeared can facilitate reduction in the crack driving force, as shown in Fig. 3b. Moreover, the bridged Ti₂AlC particles spanned the crack and woke crack bridging tractions over the bridging length, thus shielding the far-field driving force. ¹⁸

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Typical crack propagation of the TiAl alloy a and Ti₂AlC/TiAl composite b

Furthermore, some meaningful results indicated that the interface of Ti₂AlC particles and TiAl matrix belongs to coherence interface.^19,20 Such coherence relationships can be beneficial to improve the mechanical properties of Ti₂AlC/TiAl composites.

Conclusions

In summary, dense Ti₂AlC/TiAl composite was first successfully fabricated by reactive hot pressing at low temperature of 1150°C for 2 h using Ti₃AlC₂ and Ti–Al alloy powders. As reinforcing agent, Ti₂AlC exhibits significant strengthening and toughening effects to the TiAl matrix composite, and the Vickers hardness, flexural strength and fracture toughness of the product are 5.2 GPa, 937.7 MPa and 7.7 MPa m^1/2. Strengthening of the composite is mainly attributed to the fine grains and dispersed Ti₂AlC particles. Toughening of the composite owes to crack deflection, crack bridging, crack branching and pull-out of the Ti₂AlC particles, transgranular cracking as well as coherent interface between Ti₂AlC and TiAl.