Fabrication and Characterization of TiAl Alloy by Fields Activated Sintering Technology (FAST)

ZHOU Liming ,WU Mingxia ,QIN Yuan ,YANG Gang ,YANG Yi

(1.School of Mechanical Engineering,Chengdu University,Chengdu 610106,China;2.School of Mechanical Engineering,Sichuan University,Chengdu 610065,China)

Abstract: Ti-47Al alloy was fabricated by multi-physical fields activated sintering technology(FAST).With the coupling effects of electrical and pressure fields as the dominant driving force,a nearly fulldensity (relative density:99.15%) TiAl alloy sample was obtained.The microstructural characterizations,phase transformation at different sintering temperatures and their contributions to mechanical properties were investigated.The results reveal that TiAl3 is the main phase when sintered at 600 and 700 ℃.When the sintering temperature reaches 800 ℃,the main phases are TiAl and Ti3Al.With the increase of sintering temperature,the content of TiAl increases and that of Ti3Al decreases.When the sintering temperature reaches 1 000 ℃,the alloy with fine and uniform microstructure and good mechanical properties can be obtained.

Key words: TiAl alloy;Micro-FAST;Microstructure;Mechanical properties

1 Introduction

TiAl alloys are currently a subject of intensive research because of their potentials for a wide range of technological applications and also because they are of fundamental scientific importance.In fact,the interest toward TiAl alloys is positive.It exhibits low density,high strength,high-specific modulus,good creep and oxidation resistance at elevated temperatures,which render it to be an excellent high-temperature structure material.Therefore,it has a wide application prospect in gas turbine engine,aerospace,automobile industry and other fields.Especially,it can be used as a lightweight high temperature material to replace Ni-based or Fe-based superalloys.However,the TiAl alloy applications are impeded due to the poor ductility and toughness at room temperature.

In order to popularize the applications of TiAl alloy,large scale processing methods such as ingot casting,traditional powder metallurgy processeand ingot forging,sheet production by hot-rollingare used to fabricate TiAl.Although lots of techniques are considered more accurate and realistic,their high cost and time consuming limit the utilization.To overcome such problems net-shape fabrication technologies such as FAST sintering processes have been considered.

In this work,a newly developed FAST,named as micro-FAST (Micro-Fields Activated Sintering Technology),based on the coupling effect of electric and force fields,offers a promising new route for fabricating the TiAl alloys.The rapid rate of densification,no need for preliminary powder preparation steps such as cold compaction,additive use and debinding,short time at high temperatures are some densification characteristics that may make the Micro-FAST process economically competitive.During the process,grain growth may be controlled by reducing time and temperature of sintering and,for coarsening sensitive materials,by applying high heating rates.Thus,compared to conventional methods,the Micro-FAST technique allows sintering at lower temperatures and shorter soaking times,which promotes the working efficiency.

The microstructure and properties of the materials are strongly dependent on the fabrication procedure.Therefore,it was significant to understand how the microstructure evolved during Micro-FAST process fabrication of TiAl alloys.For the purpose of the present paper,the focus was the microstructure variation of TiAl alloy samples fabricated by Micro-FAST process at different sintering temperature and their contribution to phase transformation and mechanical properties.A combined study of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was performed to investigate systematically the microstructural evolutions of the TiAl through the entire sintering process.Furthermore,based on the abundant microstructural and phase transformation information of the TiAl alloy,the densification mechanisms and mechanical properties were discussed in detail.

2 Experimental

Commercial titanium powder (a purity of 99.9%,average particle size of 3 μm) and aluminium powder(a purity of 99.9%,average particle size of 2 μm) were selected for the sintering process,as shown in Figs.1(a)and 1(b).Before sintering,elemental powders were weighed to prepare Ti-47Al powder.And then wet mixed in a ball grinding mixer for 2 h at 300 rpm with absolute ethyl alcohol.Finally,the mixed powders were dried in a vacuum.

Fig.1 SEM images of raw material powders and schematic diagram of Micro-FAST:(a)Ti powder;(b)Al powder;(c)schematic diagram of Micro-FAST

The Ti-47Al powder was put directly into a graphite die with the inner diameter of 8 mm.The sintering experiments were carried out using a Gleeble-1500D thermal simulation instrument (Dynamic System Inc.USA).During the tests,the chamber was maintained under vacuum (10Pa) and a constant pressure of 50 MPa was consistently applied to die before sintering.The mixed powders were rapidly heated up to the desired temperature (600 to 1 000℃) with a heating rate of 25 ℃/s and maintained at elevated temperatures for 3 min.Finally,the applied current was reduced and the pressure was released,and the specimen was cooled down to room temperature with a final cooling rate of 20 ℃/s.The sintering process is shown in Fig.1(c).

The density of sintered compacts was measured by the Archimedes’ method using deionized water,where each sample was measured at least three times.The final relative density was defined as the ratio of the experimental density to the theoretical density.Microstructure evolution analysis was carried out by scanning electron microscopy (SEM).X-ray diffraction(XRD) was used to characterize the phase transition.Moreover,the microstructure was evaluated by transmission electron microscopy (TEM),operating at 200 kV.The microhardness was measured by using a digital microhardness tester under a load of 300 g for 10 s along the building direction in the vertical cross section.Cylindrical compression specimens with a diameter of 8 mm and a length of 6 mm were cut from the sintered specimens and the specimens were compressed at room temperature with a strain rate of 10s.

3 Results and discussion

3.1 Microstructure evolution

Fig.2 is the SEM images of Ti-47Al at different sintering temperatures.It can be seen that the pores in the sintered alloys apparently decrease.The relative density of the samples sintered at the temperatures of 600,700,800,900,950,and 1 000 ℃ are measured to be 90.37%,92.94%,94,26%,95,51%,98.31%,and 99.15%,respectively.When sintered at temperatures of 600,700 ℃ (Figs.2(a)-2(b)),there is still a clear boundary between the particles in the sample,and no typical structure is formed.However,the newly formed phase is locally enriched.When the sintering temperature is up to 800 ℃,the island like structure is formed,which is surrounded by the dispersed phase of fine grains and the matrix phase with large grains.When sintered at 800 ℃,the average size of islandgrains is between 10-20 μm.With the increase of sintering temperature,the island grains become larger and larger,and the grain size is between 20-35 μm when sintered at 1 000 ℃.This is due to the increase of temperature,the driving force of grain growth,and the growth rate of grain is faster.

Fig.2 Microstructure of TiAl alloy at different sintering temperatures

3.2 Phase transformation

Fig.3 XRD patterns of TiAl alloys at different sintering temperatures

Fig.3 shows the XRD patterns of TiAl alloys at different sintering temperatures.The XRD patterns confirm that the phases in the alloys sintered at 600 and 700 ℃ is TiAl.Compared with the sample sintered at 600 ℃,the peak strength and width of the samples sintered at 700 ℃ are higher and narrower,indicating that the amount of TiAlis more and the crystallinity is better.In addition,α-Ti phase still exists in the samples due to the low sintering temperature.When sintering temperature is higher than 800 ℃,the main phases of the alloy are TiAl and TiAl.With the increase of temperature,the diffraction peak intensity of TiAl increases,and the diffraction peak intensity of TiAl decreases,which indicates that the content of TiAl decreases and that of TiAl increases when the sintering temperature increases from 800 ℃ to 1 000 ℃.Moreover,the crystallinity is the highest at 1 000 ℃,indicating the best sintering performance.According to the Ti-Al phase diagram and XRD patterns,the sintering reaction process of Ti and Al powders can be divided into the following three steps:

According to the free energy of formation of Ti-Al compounds in Ti-Al binary system calculated by M Sujata

et al

,it is found that TiAlhas the lowest free energy of formation,so TiAlis the primary product,and then a series of reactions related to the transition phase occur.Finally,TiAl with the highest free energy is formed.With the increase of temperature,the faster the atomic diffusion rate is,the more sufficient reaction(3) is,which leads to more reaction transformation of TiAl to TiAl,and the content of TiAl decreases and the content of TiAl increases.

Fig.4 TEM images of TiAl alloy sintered at 600 ℃

TEM images of the TiAl alloy sintered at 600 and 1 000 ℃ are exhibited in Fig.4.For the alloy sintered at 600 ℃,there are many fine grains in the matrix grains (Fig.4(a)).As shown in Fig.4(b,c),area A shows the fine TiAlgrains,and area B shows the normal matrix grains of Ti.With the increase of sintering temperature,TiAl phase transforms into TiAl phase.Therefore,from the sample sintered at 1 000℃ (Fig.4(d)),it can be seen that the microstructure is basically homogeneous TiAl phase.

3.3 Mechanical properties

Fig.5 shows the compressive strength and compression ratio of TiAl alloys at different sintering temperatures.It can be seen from Fig.5 that the compressive strength of TiAl alloy ranges from 713 to 1 530 MPa,the compression ratio of TiAl alloys can reach 7.12% to 22.45%.And the rang of variation is large,which indicates that sintering temperature has an important influence on compressive strength and deformation.

Fig.5 Compressive strength and compressibility of TiAl alloys at different sintering temperatures

For TiAl alloys,the mechanical properties at room temperature have been reported to strongly depend on the proportion of alloy phase,the homogeneity of microstructure and the density of alloy.Therefore,with the increase of sintering temperature,on the one hand,the proportion of TiAl phase increases,and the room temperature plasticity of TiAl is better than that of TiAl,and the comprehensive mechanical properties are better.On the other hand,the microstructure is more uniform with the increase of temperature,so the compressive strength increases.Temperature affects the compressive strength through the alloy phase ratio,density and other factors,which shows that the value of compressive strength has fluctuation.From the experimental results,the effect of temperature on the deformation is single.The higher the sintering temperature is,the greater the deformation is,which has nothing to do with porosity and other factors.In conclusion,it has the best compression properties when sintered at 1 000 ℃.

Fig.6 shows the morphologies of the fracture surface of the samples sintered at different temperatures.It may be noted that the fracture modes of the TiAl based alloys are mainly related to the microstructure,including the volume fraction of TiAl,TiAl and TiAl.The dominant fracture mode in the alloy sintered at low temperatures is found to be both transgranular fracture with visible river patterns and interlamellar fracture in the lamellar grains.But for the alloy sintered with high temperature,a dense surface with only a few microcracks is revealed.With the increase of grain size and fraction of TiAl grains,the mode of fracture has changed to a complicate pattern composed of both transgranular fracture and intergranular fracture.

Fig.6 SEM images of fractural microstructures of the samples sintered at different temperatures

4 Conclusions

In summary,we successfully fabricate the nearly full-density micro-cylinder parts from raw Ti and Al powders using the Micro-FAST technique,which is a new method and employs the coupling effects of electrical and pressure fields as the dominant driving force.TiAlis the main phase when sintered at 600 and 700 ℃.When the sintering temperature reaches 800 ℃,the main phases are TiAl and TiAl.With the increase of sintering temperature,the content of TiAl increases and that of TiAl decreases.When sintered at 1 000 ℃for 3 min,the compression ratio is 22.45%,and the compressive strength is 1 530 MPa,which has the best comprehensive mechanical properties.