Effect of Ceramic Content on the Compression Properties of TiB 2-Ti 2 AlC / TiAl Composites

In situ synthesized TiB2-reinforced TiAl composites usually possess high strength. However, it is very expensive to use B powder to synthesize TiB2 particles. Moreover, the strength enhancement of TiB2/TiAl composite is generally at the cost of plasticity. In this study, in situ dual reinforcement TiB2-Ti2AlC/TiAl composites were fabricated by using B4C powder as the B and C source, which greatly reduces the potential production cost. The 6 vol. % TiB2-Ti2AlC/TiAl composite fabricated by using the Ti-Al-B4C system shows greatly improved compressive properties, i.e., 316 MPa and 234 MPa higher than those of TiAl alloy and with no sacrifice in plasticity.


Introduction
Composite technology, i.e., introducing stiff and hard particle reinforcements, is an effective approach to improve the strength of TiAl alloy [1][2][3][4][5].TiB2 and Ti2AlC are the two most frequently-used reinforcing particles in TiAl matrix composites [4][5][6].It has been reported that TiB2 particles can effectively enhance the strength of TiAl alloy [7][8][9][10].However, the strength enhancement of the TiAl matrix composite caused by the addition of TiB2 particles is usually at the cost of plasticity [7][8][9][10].In contrast to TiB2, Ti2AlC ceramic combines unusual properties of both metals and ceramics, that is it possesses both plasticity and strength [11][12][13].Although the enhancement effect of Ti2AlC is relatively weaker than that of TiB2, Ti2AlC particles could improve the strength of the TiAl alloy and, at same time, with no great plasticity damage [14].Therefore, introducing TiB2 and Ti2AlC particles simultaneously into TiAl matrix is expected to fabricate the in situ dual reinforcement TiB2-Ti2AlC/TiAl composite with further improved strength and also excellent plasticity.
The most direct idea to fabricate in situ dual reinforcement TiB2-Ti2AlC/TiAl composite is using the Ti-Al-B-C system.However, B powder is very expensive, which limits its potential application in industrial practice.Thus, it is necessary to find another material to use as the B source instead of B powder.As is known, the cost of B4C powder is at least 20-times less expensive than that of B powder.Meanwhile, B4C powder can offer C to synthesize Ti2AlC particles.Thus, it is a more cost-effective way to fabricate the in situ dual reinforcement TiB2-Ti2AlC/TiAl composite by using B4C as the B and C source.
Recently, several methods have been applied to fabricate TiAl matrix composites.Van Meter et al. [7] fabricated 40 vol.% and 50 vol.% TiB2/TiAl composites by the method of powder metallurgy (P/M).The compression strength of the composites was reported in the range of 2484-2866 MPa, and fracture strain was in the range of 0.4%-1.7%.Bohn et al. [9] fabricated Ti5Si3/TiAl composites by the method of hot isostatic pressing (HIP).Compression strength reached up to 2680 MPa for the sample with a mean grain size of 170 nm, while fracture strain was about 1.2%.Yang et al. [15] fabricated Ti2AlC/TiAl composites by the method of spark plasma sintering (SPS).The compression strength of the composites reached 2058 MPa, and the fracture strain was about 0.16%.Compared with these above methods, the method of combustion synthesis and hot press consolidation represents an in situ processing technique for the preparation of composites, which takes advantage of the low energy requirement, cleaner particle-matrix interface, one-step forming process, density and high purity of the products.Consequently, the fabricated composites would possess better comprehensive properties.
In our previous work [16], we focused on the issues of the effect of B4C size on the fabrication of in situ TiB2-Ti2AlC/TiAl composites.It was concluded that just when the size of B4C particles is reduced to 3.5 μm, pure in situ TiB2-Ti2AlC/TiAl composites could be successfully fabricated.However, the effect of the content of synthesized ceramic particles on the compression properties of TiB2-Ti2AlC/TiAl composites was not discussed.Thus, in this work, under the basis of the mentioned research work, the B4C particles with a size of 3.5 μm were directly used to fabricate TiB2-Ti2AlC/TiAl composites with different contents of the synthesized ceramic particles.The effect of the content of synthesized ceramic particles on the compression properties and work-hardening capacity of TiB2-Ti2AlC/TiAl composite was investigated.Moreover, the reinforcing effect of the TiB2-Ti2AlC particulates synthesized from Ti-Al-B4C and Ti-Al-B-C systems was compared.

Experimental Section
Starting materials were made from commercial powders of Ti (99.5% purity, ~25 μm), Al (99% purity, ~74 μm), B4C (99.5% purity, ~3.5 μm), B (98% purity, ~3 μm) and carbon black (99.9% purity).The powders of Ti, Al and B4C (Ti-Al-B4C system) corresponding to nominal 2, 4, 6 and 8 vol.% TiB2-Ti2AlC/TiAl composites were mixed sufficiently by ball milling for 8 h at a low speed (~35 rpm) in a conventional planetary ball-miller.Both the pot and balls were made of stainless steel, and the mass ratio of ball to powders was 20:1-25:1.Then, the mixed powders were cold pressed into cylindrical compacts using a stainless steel die.In addition, the powders of Ti, Al, B and carbon black (Ti-Al-B-C system) corresponding to nominal 6 vol.% TiB2-Ti2AlC/TiAl composite were also mixed and pressed into cylindrical compact, which was used to compare to the composites fabricated by using the Ti-Al-B4C system.Unless specified otherwise, the composites were fabricated from the Ti-Al-B4C system.The powder compact of 28 mm in diameter and approximately 36 mm in height was contained in a graphite mold, which was put into a self-made vacuum thermal explosion furnace.The heating rate of the furnace was about 30 K/min, and the temperature in the vicinity of the center of the compact was measured by Ni-Cr/Ni-Si thermocouples.When the temperature measured by the thermocouples suddenly rose rapidly, indicating that the sample should be ignited, the sample was quickly pressed just when it was still hot and soft.Pressure (~50 MPa) was maintained for 10 s, and then, the product was cooled down to ambient temperature at a cooling rate of ~10 K/min.
The phase constituents of composites were examined by X-ray diffraction (XRD, Rigaku D/Max 2500PC, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation.Microstructures were studied using scanning electron microscopy (SEM, Evo18, Carl Zeiss, Oberkochen, Germany) equipped with an energy-dispersive spectrometer (EDS, Oxford Instruments, Oxford, UK).The density of each sample was measured three times by Archimedes' water-immersion method, and average values are listed in Table 1.The cylindrical specimens with a diameter of 3 mm and a height of 6 mm were used for compression tests, and the loading surface was polished parallel to the other surface.Uniaxial compression tests were carried out under a servo-hydraulic materials' testing system (MTS, MTS 810, MTS Systems Corporation, Minneapolis, MN, USA) with a strain rate of 1 × 10 −4 • s −1 .

Phase Constituents and Microstructures
In order to identify the synthesized ceramic particles, the phase constituents of the composites were examined by XRD. Figure 1 shows the X-ray diffraction results of the composites fabricated from the Ti-Al-B4C system.Actually, due to the limitation of the detection capacity of XRD, no ceramic phases, but the γ-TiAl and α2-Ti3Al phases, were detected in the samples with 2, 4 and 6 vol.% nominal ceramic contents.In the sample with a high content of ceramic particles (the nominal ceramic content is 8 vol.%), the TiB2 and Ti2AlC phases were detected besides the γ-TiAl and α2-Ti3Al phases.The combustion synthesis reaction in the Ti-Al-B4C system is complete, and no trace of residual B4C particles was found, which indicated that TiB2 and Ti2AlC particles could be synthesized simultaneously in the Ti-Al-B4C system and that in situ dual reinforcement TiB2-Ti2AlC/TiAl composites could be successfully fabricated by using B4C as the B and C source. Figure 2a-d shows the microstructures of the TiB2-Ti2AlC/TiAl composites fabricated from the Ti-Al-B4C system.It can be seen from Figure 2a-c that when the contents of synthesized TiB2 and Ti2AlC particles increase from 2-6 vol.%, the ceramic particles distribute more homogeneously in the TiAl matrix.The increasing content of synthesized TiB2 and Ti2AlC particles results in more heat released by the reaction and, thus, a higher temperature.The increased temperature facilitates faster and more complete diffusion of the reactants during combustion synthesis, leading to a more uniform distribution of the ceramic particles in the TiAl matrix [17].However, the ceramic particles begin to sinter together, as shown in Figure 2d, when too many ceramic particles formed in the composites (8 vol.% TiB2-Ti2AlC).In order to examine the distribution of TiB2 and Ti2AlC particles, EDS was used to distinguish the two ceramic particles from each other (as shown in Figure 2c).It can be seen that in 6 vol.% TiB2-Ti2AlC/TiAl composite, Ti2AlC particles are rod-like in shape with sizes of 1-6 μm in length, while TiB2 particles are in a near spherical shape with a size of less than 1 μm in diameter.

Compression Properties and Work-Hardening Capacity
Figure 3 shows true compression stress-strain curves of the TiAl alloy and the TiB2-Ti2AlC/TiAl composites fabricated from the Ti-Al-B4C system.The compression properties are summarized in Table 1.The yielding strength (σy) and ultimate compression strength (σucs) of TiB2-Ti2AlC/TiAl composites increase with increasing content of synthesized ceramic particles.The fracture strain (εf) of the composites does not change significantly when the content of synthesized TiB2-Ti2AlC particles increases from 2-6 vol.%.However, when the content of synthesized TiB2-Ti2AlC particles increases to 8 vol.%, εf decreases significantly, due to too many ceramic particles being synthesized and segregated together in the composite.This also can be confirmed by the fracture surface of 6 vol.% TiB2-Ti2AlC/TiAl and 8 vol.% TiB2-Ti2AlC/TiAl composites shown in Figure 4a,b.It can be compared to the fracture surface that the 6 vol.% TiB2-Ti2AlC/TiAl composite exhibits with a tear ridge, while the fracture surface of the 8 vol.% TiB2-Ti2AlC/TiAl composite is relatively flat.In 8 vol.% the TiB2-Ti2AlC/TiAl composite, the bad interface combination between aggregated particles and TiAl matrix would play the role of crack initiator during deformation and facilitate the fracture of the composite.The above results indicate that lower content (2-6 vol.%) TiB2-Ti2AlC particles could effectively improve the strength of TiAl alloy without sacrificing plasticity.The 6 vol.% TiB2-Ti2AlC/TiAl composite possesses the best compression properties.The average σy and σucs of the 6 vol.% TiB2-Ti2AlC/TiAl composite fabricated by using the Ti-Al-B4C system are 781 MPa and 1649 MPa, respectively, which are 316 MPa and 234 MPa higher than those of TiAl alloy.The enhancement of strength is mainly due to the reinforcing effect of stiff TiB2 and Ti2AlC particles.The uniform distribution of in situ TiB2 and Ti2AlC particles would be the reason for the maintenance of the high plasticity.Moreover, as discussed in our previous study [14], the metallic property of Ti2AlC particles and the coherent interface between Ti2AlC and TiAl would also contribute to the high plasticity.
It can be seen from the true stress-strain curves of the composites fabricated by using the Ti-Al-B4C system shown in Figure 3 that these curves all show a clear work hardening.The work-hardening capacity of composites is calculated according to the formulary (Hc) (Hc = (σucs − σy)/σy) [18].The results are listed in Table 1.The Hc of TiB2-Ti2AlC/TiAl composite decreases with the increase in the content of synthesized ceramic particles.The onset of plastic deformation with an obvious strain hardening in composites represents a deformation mechanism of dislocation activity.The strain hardening of composites after yielding is mainly due to dislocation multiplication, accumulation and interaction [18][19][20].As mentioned above, the σy of composites increases with increasing content of synthesized ceramic particles, which means that dislocation-nucleation threshold stress increases with the increase in ceramic content, that is the activation of dislocations becomes more difficult.Consequently, the dislocation interactions during plastic deformation would become weak with the increase in ceramic content, leading to a decrease in Hc.The stress-strain curves also exhibit a low modulus in the elastic region, which is similar to other researchers' work [21,22].It is speculated that porosity might be playing a part in this.The porosities in the composites evaluated by the method of image analysis are approximately 1.9%, 1.5%, 1.8% and 1.4%, respectively.In addition, the 6 vol.% TiB2-Ti2AlC/TiAl composite was also fabricated from the Ti-Al-B-C system, which was used to compare to the composites fabricated from the Ti-Al-B4C system.Figure 5 shows the true compression stress-strain curves of the in situ dual reinforcement 6 vol.% TiB2-Ti2AlC/TiAl composites fabricated by these two systems.The compression properties and their work-hardening capacity are summarized in Table 2.The results indicate that the compression properties and work-hardening capacity of the TiB2-Ti2AlC/TiAl composites fabricated from these two systems are similar.Thus, from the economic point of view, the TiB2-Ti2AlC/TiAl composite fabricated by using the Ti-Al-B4C system could be widely used in practical production.

Conclusions
The content of TiB2-Ti2AlC particles significantly influences the compression properties and work-hardening capacity of TiAl matrix composites.With the increase in the content of synthesized TiB2-Ti2AlC particles, the σy and σucs of TiB2-Ti2AlC/TiAl composites increase, while Hc decreases.When the content of synthesized ceramic particles is lower (2 vol.%-6 vol.%), εf does not change significantly.However, when the content of ceramic particles comes to 8 vol.%, εf decreases significantly.The synthesized TiB2 and Ti2AlC particles could effectively improve the σy and σucs of TiAl alloy due to the reinforcing effect of stiff TiB2 and Ti2AlC particles.The uniform distribution of in situ TiB2 and Ti2AlC particles and the special characteristics of Ti2AlC would be the reasons for the maintenance of the high plasticity.The compression properties of the TiB2-Ti2AlC/TiAl composites fabricated by using the Ti-Al-B4C system are similar to those of the TiB2-Ti2AlC/TiAl composites fabricated by using the Ti-Al-B-C system.From the economic point of view, it is better to fabricate in situ dual reinforcement TiB2-Ti2AlC/TiAl composites by using the Ti-Al-B4C system.The σy and σucs of the 6 vol.% TiB2-Ti2AlC/TiAl composite fabricated by using the Ti-Al-B4C system are 316 MPa and 234 MPa higher than those of the TiAl alloy, without sacrificing plasticity.

Figure 3 .
Figure 3. True compression stress-strain curves of TiAl alloy and the in situ dual reinforcement TiB2-Ti2AlC/TiAl composites fabricated by using the Ti-Al-B4C system.

Table 1 .
Compression properties and work-hardening capacity (Hc) of TiAl alloy and the in situ dual reinforcement TiB2-Ti2AlC/TiAl composites fabricated by using the Ti-Al-B4C system.

Figure 5 .
Figure 5. True compression stress-strain curves of the in situ dual reinforcement 6 vol.% TiB2-Ti2AlC/TiAl composites fabricated by using different systems.

Table 2 .
Compression properties and work-hardening capacity (Hc) of the in situ dual reinforcement TiB2-Ti2AlC/TiAl composites fabricated by using different systems.