Microstructure and compression properties of in situ dual phase nanosized (TiB 2 –Ti 5 Si 3 )/TiAl matrix composites fabricated by combustion synthesis and hot press consolidation

Abstract

The dual phase nanosized (TiB₂–Ti₅Si₃)/TiAl composites were successfully fabricated using in situ method of combustion synthesis and hot press consolidation. The effects of proportions of the in situ dual phase nanoparticles (TiB₂ and Ti₅Si₃) on microstructures and compression properties of the composites were investigated. Compared with the monophase TiB₂ and Ti₅Si₃ particles, dual phase TiB₂–Ti₅Si₃ particles exhibit more obvious grain refinement effect. The grain size of TiAl alloy was refined from ∼66 to ∼12 μm with the the generation of dual phase TiB₂–Ti₅Si₃ particles. Moreover, the TiAl matrix composites reinforced with dual phase nanosized TiB₂–Ti₅Si₃ particles exhibit better comprehensive compression properties. When TiB₂:Ti₅Si₃ = 2:1[Σ(TiB₂ + Ti₅Si₃) = 4 vol.-%], the TiAl matrix composite has a high true fraction strain of 26.1%, a true yield strength of 761 MPa and an ultimate compression strength of 1647 MPa.

Keywords

Dual phase Nanoparticles TiAl matrix composites

Introduction

In recent years, with the development of the modern advanced aerospace and automotive applications technology, the TiAl intermetallics will be a candidate for high temperature applications in aerospace automobile industry owing to their low density, high specific strength and relatively good resistance to oxidation at service circumstance.^1,2 However, the properties of brittleness and low strength at room temperature are the main restrictions for their application.^3–5 In previous works, many researchers have tried to solve these problems by introducing the stiff and hard particle reinforcements to TiAl alloy, such as TiB, TiB₂, Al₂O₃, SiC, Ti₂AlC and Ti₅Si₃.^6–10 Nevertheless, the strength improvement by the addition of these particles was usually at the cost of the ductility. Recently, there are some reports that the dual phase ceramics have higher wear resistance and mechanical and electrical properties than the monolithic ceramics.^11–15 The TiB₂ and Ti₅Si₃ ceramic particles are the most frequently used reinforcing particles in the TiAl matrix composites since TiB₂ and Ti₅Si₃ have good orientation relationship and the habit plane with γ-TiAl.^16–18 However, the detailed effects of introducing hybrid TiB₂ and Ti₅Si₃ into the TiAl alloy are still unknown.

Many fabrication methods have been used to produce the TiAl matrix composites, such as combustion synthesis,^19,20 plasma arc melting process,²¹ induction skull melting process²² and hot isostatic pressing process.^23,24 In this study, the combustion synthesis plus hot press consolidation is used to fabricate the TiAl matrix composites reinforced with in situ dual phase nanosized TiB₂–Ti₅Si₃ particles. This method has the advantages of low energy requirement and cleaner particle/matrix interface, and the TiAl composites could be synthesised and densified simultaneously at lower temperatures. Moreover, the influence of the different ratios between TiB₂ and Ti₅Si₃ on the microstructures and compression properties of the composites are also investigated.

Experimental

The starting materials were made from commercial powders of titanium (99.5% purity, ∼25 μm), aluminium (99% purity, ∼74 μm), boron (98% purity, ∼3 and 26 μm) and silicon (99.5% purity, ∼25 μm). As shown in Table 1, sample A is a TiAl alloy, sample B is a 4 vol.-%TiB₂–TiAl composite (B powder with a particle size ∼3 μm), sample C is a 4 vol.-%Σ (TiB₂:Ti₅Si₃ = 2:1)/TiAl composite, sample D is a 4 vol.-%Σ(TiB₂:Ti₅Si₃ = 1:1)/TiAl composite, sample E is a 4 vol.-%Σ(TiB₂:Ti₅Si₃ = 1:2)/TiAl composite, sample F is 4 vol.-%Ti₅Si₃–TiAl composite and sample G is a 4 vol.-%TiB₂–TiAl composite (B powder with a particle size ∼26 μm). Elemental powder blends corresponding to ‘samples A–G’ were mixed sufficiently by ball milling for 8 h and then cold pressed into cylindrical compacts using a stainless steel die. The powder compact with 28 mm diameter and ∼36 mm height was placed in a graphite die, which was put into the self-made vacuum thermal explosion furnace (Fig. 1). The heating rate of the furnace was ∼35 K min^− 1, and the temperature was measured by Ni–Cr/Ni–Si thermocouples closed to the centre of the compact. When the temperature measured by the thermocouples suddenly rose rapidly, indicating that the compact should be ignited, the compact was quickly pressed just when it was in the soft state. The pressure (∼30 MPa) was maintained for 30 s, and then, the product was cooled down to the ambient temperature.

Table 1

Details of elements content and density of cylindrical powder compacts

Samples	Name	Density/g cm^− 3	Relative density/%	Ti/g	Al/g	Si/g	B/g
A	TiAl	2.511 ± 0.022	66.1 ± 0.6	31.98	18.02	…	…
B	TiAl–4%TiB₂	2.508 ± 0.025	65.5 ± 0.7	32.10	17.17	…	0.73 (3 μm)
C	TiAl–4%(TiB₂:Ti₅Si₃ = 2:1)	2.513 ± 0.023	65.7 ± 0.6	32.14	17.18	0.20	0.48 (3 μm)
D	TiAl–4%(TiB₂:Ti₅Si₃ = 1:1)	2.532 ± 0.026	66.2 ± 0.7	32.16	17.18	0.30	0.36 (3 μm)
E	TiAl–4%(TiB₂:Ti₅Si₃ = 1:2)	2.520 ± 0.021	65.9 ± 0.5	32.18	17.18	0.40	0.24 (3 μm)
F	TiAl–4%Ti₅Si₃	2.541 ± 0.024	66.5 ± 0.6	32.21	17.20	0.59	…
G	TiAl–4%TiB₂	2.513 ± 0.022	65.6 ± 0.6	32.10	17.17	…	0.73 (26 μm)

Schematic of equipment for combustion synthesis and hot press consolidation experiment

The phase constituents of the composites were examined by X-ray diffraction (XRD, Rigaku D/Max 2500PC, Japan) with Cu K_α radiation. The morphologies of the ceramic particles were observed using a field emission scanning electron microscope (JSM-6700F, Japan). Deep etching was carried out using Keller's solution (2.5 mL HNO₃+1.5 mL HCl+1 mL HF+95 mL H₂O) for 5 min to reveal the TiB₂ and Ti₅Si₃ morphologies. Nanoparticles (TiB₂ and Ti₅Si₃) were carefully extracted from the sample using a mixed acid solution (20 mL HCl+10 mL HF+70 mL H₂O). The microstructures were investigated by scanning electron microscopy (SEM, Evo18, Carl Zeiss, Germany). The grain size was determined by the average line interception method. The density of the cylindrical powder compacts, TiAl alloy and its composites was measured by Archimedes' water immersion method.

The cylindrical specimens with a diameter of 3 mm and a height of 6 mm were used for compression tests; surfaces were polished place parallel. The uniaxial compression tests were carried out under a servohydraulic materials testing system (MTS 810, USA) with a strain rate of 10^− 4 s^− 1.

Results and discussion

Phases identification and microstructure of composites

The XRD results of ‘samples A–F’ are shown in Fig. 2. Figure 2a shows that the sample without B and Si addition mainly consists of γ-TiAl and α₂-Ti₃Al phases. As shown in Fig. 2b–f, with the increase in addition value of B and Si elements, the peaks of the TiB₂ and Ti₅Si₃ phases begin to appear, and their intensities change with the different proportions of the synthesised TiB₂ and Ti₅Si₃ particles. The XRD results are consistent with the experimental design components. Figure 3 shows the microstructures of the ‘samples A–F’, and Fig. 4 shows their grain sizes. It can be seen that the grain size of TiAl is greatly refined by the synthesised ceramic particles. Moreover, the grain sizes of the TiAl matrix composites reinforced with dual phase nanoceramic particles (Fig. 3c–e) are more uniform, and the refinement effect of the dual phase nanoceramic particles (TiB₂ and Ti₅Si₃) is more obvious than that of the simplex nano-TiB₂ or nano-Ti₅Si₃ ceramics (Fig. 3e–f). As discussed in our previous work,²⁵ the nano-TiB₂ particles in the TiB₂/TiAl composite can refine the grain size of TiAl by obstructing the growth of the TiAl grain. While in the Ti₅Si₃/TiAl composite, the homogeneously distributed fine silicide particles in the TiAl grain can also act as preferred nucleation sites during recrystallisation, inducing fine grains in the TiAl composites. Hence, in the dual phase TiB₂–Ti₅Si₃/TiAl composites fabricated in this work, the abovementioned grain refinement mechanisms act simultaneously, indicating that the synthesised dual phase TiB₂–Ti₅Si₃ ceramics possess more obvious refinement effect. This is the reason why the dual phase TiB₂–Ti₅Si₃/TiAl composites exhibit finer grain sizes. As shown in Fig. 3c, sample C possesses the finest grain size. Its average grain size is ∼12 μm, which is greatly smaller than those of TiAl alloy (∼66 μm), TiB₂/TiAl composites (∼20 μm) and Ti₅Si₃/TiAl composites (∼25 μm).

X-ray diffraction patterns of TiAl alloy and their composites

Microstructures of a TiAl alloy, b 4 vol.-%TiB₂/TiAl composites, c 4 vol.-%M(TiB₂:Ti₅Si₃ = 2:1)/TiAl composites, d 4 vol.-%M (TiB₂:Ti₅Si₃ = 1:1)/TiAl composites, e 4 vol.-%M(TiB₂:Ti₅Si₃ = 1:2)/TiAl composites and f 4 vol.-%Ti₅Si₃/TiAl composites

Grains size of γ-TiAl and α₂-Ti₃Al in TiAl alloy and their composites

Figure 5 shows the nanosized TiB₂, TiB₂–Ti₅Si₃ and Ti₅Si₃ particles extracted from the TiB₂/TiAl, TiB₂–Ti₅Si₃/TiAl and Ti₅Si₃/TiAl composites respectively. It can be seen that the sizes of the TiB₂–Ti₅Si₃ particles generated in the dual phase TiB₂–Ti₅Si₃/TiAl composites are smaller than those of TiB₂ and Ti₅Si₃ particles generated in the TiB₂/TiAl and Ti₅Si₃/TiAl composites.

Images (FESEM) images of a extracted TiB₂ particles from 4 vol.-%TiB/TiAl composite, b extracted TiB₂ and Ti₅Si₃ particles from 4 vol.-%M(TiB₂:Ti₅Si₃ = 2:1)/TiAl composite and c extracted Ti₅Si₃ particles from 4 vol.-%Ti₅Si₃/TiAl composite

Compression properties

Figure 6 shows the true stress–strain curves of ‘samples A–F’, and Table 2 summarises their compression properties and density. The figure shows that the yield strength and ultimate compression strength of ‘samples B–F’ are all higher than those of the TiAl alloy; thus, they are evidence that the strength of TiAl matrix composites is significantly enhanced by nanosized TiB₂ and Ti₅Si₃ particles. For the TiB₂/TiAl composite (sample B), it is significantly enhanced by nano-TiB₂ particles; the of the 4 vol.-%TiB₂/TiAl composite is 414 MPa higher than that of the TiAl alloy, but the average fracture strain of the TiB₂/TiAl composite is weaker from 17.3 to 15.9%. In order to confirm whether the particle size of B has an effect on the mechanical properties of the composites, we have fabricated the TiB₂/TiAl composite (sample G) using B powder with a particle size ∼26 μm and tested its compression properties. Compared with the TiB₂/TiAl composite (sample B) using B powder with a particle size ∼3 μm (as shown in the Table 2), it can be seen that the size of B particle has little effect on the yielding strength, compression strength and fracture strain of the TiB₂/TiAl composites. As to the Ti₅Si₃/TiAl composite (sample F), the average of the TiAl alloy is significantly improved by nano-Ti₅Si₃ particles, the average is increased from 17.3 to 20.9% and the and are 613 and 1586 MPa respectively. These suggest that the TiAl alloy is better strengthened by the nano-TiB₂ ceramic particles, while the ductility of the TiAl alloy is improved more significantly by the nano-Ti₅Si₃ ceramic particles. Compared with the simplex nano-TiB₂/TiAl and simplex nano-Ti₅Si₃/TiAl composites, the dual phase nanoparticles (TiB₂ and Ti₅Si₃)/TiAl composites exhibit better compression properties. In addition, the reinforcement ability of dual phase nanoparticles (TiB₂ and Ti₅Si₃) is related to their proportions. As shown in Fig. 6c–e, the average and of the composites gradually increase with increasing content of TiB₂, and their average is almost the same as the Ti₅Si₃/TiAl composite. When the TiB₂:Ti₅Si₃ = 2:1[Σ(TiB₂ and Ti₅Si₃) = 4 vol.-%], the TiAl matrix composite (sample C) has the best compression properties, of which the is 26.1%, the is 761 MPa and the is 1647 MPa.

Compression true stress–strain curves of TiAl alloy and their composites

Table 2

Room temperature compression properties and density of TiAl alloy and its composites

Samples	Name	Density/g cm^− 3	Relative density/%	/MPa	/MPa	/%
A	TiAl	3.697 ± 0.008	97.3 ± 0.2	465 ± 41	1415 ± 20	17.3 ± 0.43
B	TiAl-4% TiB₂	3.718 ± 0.011	97.1 ± 0.3	734 ± 5	1829 ± 17	15.9 ± 0.58
C	TiAl-4%(TiB₂:Ti₅Si₃ = 2:1)	3.707 ± 0.012	96.9 ± 0.3	761 ± 11	1647 ± 19	26.1 ± 0.75
D	TiAl-4%(TiB₂:Ti₅Si₃ = 1:1)	3.702 ± 0.011	96.8 ± 0.3	665 ± 9	1625 ± 15	20.7 ± 0.63
E	TiAl–4%(TiB₂:Ti₅Si₃ = 1:2)	3.678 ± 0.008	96.2 ± 0.2	625 ± 13	1510 ± 23	20.5 ± 0.31
F	TiAl–4%Ti₅Si₃	3.687 ± 0.007	96.5 ± 0.2	613 ± 7	1586 ± 7	20.9 ± 0.47
G	TiAl–4%TiB₂	3.722 ± 0.013	97.2 ± 0.3	743 ± 21	1833 ± 35	15.2 ± 0.37

One of the most significant consequences of the generation of in situ nanoparticles is grain refinement, leading to the generation of a high volume density of grain boundaries to impede the movement and propagation of dislocations to adjacent grains, which is one reason for the strength enhancement of the composites. It is usually described by the Hall–Petch equation 1 where σ₀ is the friction stress, d is the average grain diameter and k_y is the Hall–Petch slope. In our work, as shown in Fig. 4, the grain sizes of ‘samples B–F’ are smaller than that of TiAl alloy; this is one reason why their average and are all higher than those of the TiAl alloy. Moreover, ‘sample C’ with the smallest grain size among them possesses the highest .

The other important consequence is the different proportions and distribution of nanoparticles (TiB₂ and Ti₅Si₃), which can enhance the composites by effecting the interaction between the dislocations and particles. Dispersion strengthening is governed by either the Orowan dislocation bypassing or the dislocation shearing mechanisms.²⁶ The critical shear stress τ_c to bypass a particle is defined as 2 where L is the spacing between precipitates, D is the precipitate diameter, r₀ is the line energy cutoff radius defining the elastic dislocation nucleation size, A and B are coefficients and is a quantity as . Mohles and Fruhstorfer had investigated the effect of randomness of the particle arrangement on the critical resolved shear stress by computer simulations.²⁷ They found that the fluctuations of obstacle density and the distances between the nearest neighbours had significant effects on the numerical value of L and , thereby reinforcing composites. In our work, the different proportions of dual phase nanoparticles (TiB₂ and Ti₅Si₃) have significant effects on randomness arrangements of particles, thereby strengthening composites by effecting the numerical value of L and . When TiB₂:Ti₅Si₃ = 2:1[Σ(TiB₂ and Ti₅Si₃) = 4 vol.-%], the dual phase nanoparticles (TiB₂ and Ti₅Si₃) are distributed more uniformly; thus, the TiAl matrix composite has the best combination of mechanical properties.

Conclusions

The dual phase nano-(TiB₂–Ti₅Si₃)/TiAl composites were successfully fabricated using the in situ method of combustion synthesis and hot press consolidation. With the generation of nanoceramic particles, there is an increase in average and of TiAl alloy due to the reinforcement of stiff nanoceramic particles and the grain refinement. Comparing with the nano-TiB₂/TiAl and nano-Ti₅Si₃/TiAl composites, the TiAl composites reinforced with dual phase nanosized TiB₂–Ti₅Si₃ particles exhibit better comprehensive compression properties, owing to the more significant influence of the dual phase nanosized TiB₂–Ti₅Si₃ particles on grain refinement. When TiB₂:Ti₅Si₃ = 2:1[ΣM(TiB₂ and Ti₅Si₃) = 4 vol.-%], the TiAl matrix composite has the best combination of compression properties among these TiAl matrix composites with a high true fraction strain of 26.1%, a true yield strength of 761 MPa and an ultimate compression strength of 1647 MPa.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (NNSFC, grant no. 51171071), National Basic Research Program of China (973 Program, grant no. 2012CB619600), NNSFC (grant no. 50971065), the Research Fund for the Doctoral Program of Higher Education of China (grant no. 20130061110037), the Innovation Experimental Training Project of Jilin University in 2014 (grant no. 2014A43152) and the Project 985 – High Properties Materials of Jilin University.

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