Effect of TiC and TiB2 Content on Properties of TiAl Based Alloys

In this paper, we added different content of TiC and TiB2 particles powder into TiAl based alloy as particle reinforcement, and prepared different components of TiAl based composite. We also studied the influence of the content of TiC and TiB2 particles on the physical and mechanical properties of TiAl based composite. The results show that in a certain range, the physical properties of TiAl based alloy increase with the increase of TiC and TiB2 particle content, and the mechanical properties first increase and then decrease with the increase of TiC and TiB2 particle content. It is proved that the physical and mechanical properties of TiAl alloy can be effectively improved by adding the proper amount of TiC and TiB2 particles.


Introduction
TiAl based alloy is a new type of light and high temperature resistant material. TiAl based alloys have the advantages of low density, high specific strength, high temperature resistance, oxidation resistance and corrosion resistance [1][2][3]. But because of its brittleness and poor tribological properties, it is necessary to strengthen it to achieve the purpose of engineering application.
In order to improve the properties of TiAl alloy, some alloying elements can be added properly in the composition design to improve the strength of the alloy, refine the grains and make the structure uniform, so as to improve the comprehensive mechanical properties of the alloy.
The strength and toughness of TiAl alloy can be effectively improved by adding particles to the alloy matrix. The choice of reinforcement phase should not only ensure the thermal expansion coefficient of TiAl Matrix is similar, but also ensure the chemical reaction between reinforcement and matrix and ensure the interface compatibility [4]. Among the particle reinforcements of TiAl alloy, TiB 2 and TiC have similar thermal expansion coefficient with the matrix of TiAl alloy, and have good chemical compatibility, and can effectively improve the physical and mechanical properties of TiAl based alloy. Therefore, TiC and TiB 2 are ideal particle reinforcements for strengthening TiAl alloy [5,6].
Based on Ti45Al8Nb0.3Y, a new TiAl matrix composite material was prepared by adding TiC powder and TiB 2 powder. The effects of TiB 2 and TiC on mechanical and physical properties of TiAl- 2 based alloys were studied, which provides theoretical guidance and basis for the following research on TiAl-based alloys.
At first, Ti powder, Al powder, Nb powder and Y powder are mixed according to the atomic ratio of Ti45Al8Nb0.3Y, then the corresponding mass fractions of TiC powder, TiB 2 powder are mixed respectively. Then, the mixed powder is milled by a planetary ball mill, and argon is introduced into the ball mill tank to prevent the blended powder from being oxidized during the milling process. The ball milling parameters are ball material ratio of 10:1, milling time of 2h, rotation speed of 240r/min.
After ball grinding, the powder is poured into prepared graphite mold and put into hydraulic test machine for cold pressing at 40MPa and 3min. After cold pressing, the graphite mold made of shaped powder is put into vacuum hot-pressing sintering furnace for hot-pressing sintering. The highest sintering temperature is 1300℃, pressure is 30Mpa, vacuum is 10-2Pa. The Hot-pressing Sintering Process is: from room temperature to 600℃ at the rate of 10℃/min, holding time is 30min, from 600℃ to 800℃ at the rate of 5℃/min, holding time is 30min, temperature rises from 800℃ to 1000℃ at the rate of 5℃/min, holding time is 30min, and holding time rises from 1000℃ to 1300℃ at the rate of 5℃/min. The holding time is 40min and the sintering pressure is 30MPa. Finally, the sample is cooled in an oven. Grind, clean, dry and polish the surface of the specimen. The flow chart is shown in Figure  1:

Density test
The Archimedes drainage method is used in the density test, which has the advantages of simple operation and no special requirements for the shape of the specimen. In order to reduce the experimental error, the density test was carried out five times, and then the average value was taken as the actual density value.

Bending strength test
The flexural strength is measured by the three-point bending method. Before the bending test, the sample was first made into a strip sample of 36mm × 5mm × 3mm. The test is conducted 5 times and the average is taken. The bending test parameters are: loading speed 0.5mm/min, span 30mm.

Fracture toughness test
Before the test, the test piece was made into a "40.0mm × 3.0mm × 4.0mm" long strip sample by wire cutting technology, and then a V-shaped notch with a depth of 1mm was cut at the bottom of the alloy. The test conditions were: span of 30mm, loading rate of 0.5mm/min. Take the average number for 5 times.

Hardness test
The Vickers hardness tester was used as the hardness tester. The loading force is set to 600N. In order to reduce the error, this test uses the average value of the hardness values of 10 points collected at different positions of the test piece as the actual hardness value. Table 1 shows the effect of different contents of TiC and TiB 2 on the properties of TiAl based alloys. The data in Table 1 are arranged into a line chart to study the influence of TiC and TiB 2 content changes on the mechanical and physical properties of materials, as shown in Fig. 2, Fig. 3, Fig. 4 and Fig. 5. TiAl matrix composites containing 0%, 3.5%, 7% and 10.5% TiB 2 were named 0TiB 2 , 3.5TiB 2 , 7TiB 2 , and 10.5TiB 2 .    It can be seen from Figures 2 and 3 that the hardness of the material increases with the increase of the TiC and TiB 2 mass fraction, and the material density increases first and then decreases with the increase of the TiC mass fraction, and increases with the increase of the TiB 2 mass fraction. It shows that adding TiC and TiB 2 particles can effectively improve the physical properties of TiAl alloy, but excessive TiC will reduce the density of the alloy and reduce the physical properties of the alloy.

Test results and analysis
It can be seen from Figures 3 and 4  fraction is 3.5% and the TiC mass fraction is 6% when the material has the best mechanical properties. It shows that proper amount of TiC and TiB 2 particles can effectively improve the mechanical properties of TiAl alloy through grain refinement and dispersion, but excessive TiC and TiB 2 particles will reduce the mechanical properties of TiAl alloy.

Conclusion
In this paper, a new type of TiAl-based composite material was prepared by vacuum hot-press sintering, and the effects of TiC and TiB 2 particle mass fraction changes on the physical and mechanical properties of TiAl-based composite material were studied. The results show that a certain content of TiC and TiB 2 particles can significantly improve the mechanical and physical properties of TiAl-based composites through refinement and dispersion strengthening, proving that TiC and TiB 2 are ideal reinforcing phase materials for TiAl alloys. Too much TiC and TiB 2 will reduce the mechanical properties of TiAl alloy.