Effect of pore structure on mechanical properties of porous TiAl

Introduction

TiAl intermetallic compound porous materials are attracting more and more attention due to an increasing targeted demand.^1–10 The materials can be expected to use as membrane separation materials, catalyst carriers, thermal insulation materials, lightweight structural materials, etc. due to their outstanding intrinsic features, such as high specific strength and specific stiffness, good corrosion and wear resistance, high temperature resistance and oxidation resistance.^11–16 The efficient use of the TiAl porous material requires a detailed understanding of their mechanical property, even when the primary application is not mechanical but functional. TiAl porous materials are typical brittle porous material that high mechanical strength, especially upper yield strength, is of great importance to meet practical demands. ¹⁷ However, there always exists an inverse relationship between mechanical strength and porosity for porous materials. ¹⁸ As a result, in order to obtain high mechanical strength at the same porosity, numerous studies have been concentrated on tailoring pore structures by changing preparation technique.^19–23 For example, Nakajima fabricated porous TiAl with cylindrical pores oriented along a single direction via unidirectional solidification. It was found that the TiAl porous materials with the directional pores can obtain an increased mechanical strength compared to that prepared by elemental powder metallurgy (EPM).^19,22

In our previous study, a new kind of TiAl porous material with a novel double pore structure has been originally prepared using EPM technique. The mechanism of making pores can be described as a combination of replication of space holders and Kirkendall effect. ²⁴ The suitable sintering temperature was discussed and confirmed as around 1400°C. Similar sintering temperature can also be found in porous Ti–48Al–6Nb (at.-%) alloys manufactured by EPM.^{11,12,25, 26} Although the temperature ranging from 1350 to 1400°C can guarantee a desirable sintering quality, higher sintering temperature means higher requirements to sintering equipment and atmosphere, and also means more product cost and energy consumption. ²⁷ In practice, meeting these requirements is quite difficult in many cases. As a consequence, decreasing sintering temperature is greatly expected and meaningful. Guyon et al.^28,29 found that 100% densification of TiAl powder can be achieved via spark plasma sintering at the same condition, but 200°C earlier for the ultrafine-grained powders additionally mechanically grinded by high energy ball milling than for the coarse-grained atomised powder. In view of the dependence of sintering temperature on powder states, in the present work two kinds of TiAl powder, pre-alloyed powder and composite mixture of Ti/Al elemental powders, were prepared as parent materials. Correspondingly, two kinds of TiAl porous material with different pore structures were, respectively, fabricated by powder metallurgy technique. One is single pore structure (SPS) basing on pre-alloyed TiAl powder, and another with composite pore structure (CPS) was obtained depending on the composite mixture of Ti/Al elemental powder. In addition, the dependence of pore structure on mechanical properties of the TiAl porous materials was investigated to clarify the influence of TiAl powder state on TiAl porous materials.

Materials and experimental details

The nominal chemical composition of the TiAl porous materials is Ti₅₂Al₄₈ (at.-%). Ti and Al elemental powders (≥99.5% purity) were used as parent materials supplied by Sinopharm Chemical Reagent Co., Ltd. The irregular-shaped Ti particles were manufactured through an angular hydriding-dehydrogen process, while the sphere-shaped Al particles were prepared by gas atomisation. Ordinary commercial carbamide particles (>99% purity) with spherical and nearly spherical shape were determined as space holder particles because of their particular features, as they can be completely removed by dissolution in water and decomposition when heating even at low temperature. In order to obtain different pore sizes of the TiAl porous material, the carbamide particles were prior sieved into different particle size ranges.

The composite mixture of Ti/Al elemental powders was prepared by mechanical mixing using drum-type mixer at the rotation speed of 100 rpm for 2 h. The morphology of the composite mixture after mixing is shown in Fig. 1a, where the white particles indicated by the red arrows refer to Ti particles. It can be found that a well-distributed mixture of Ti/Al powders was obtained after mixing. The pre-alloyed TiAl powder was prepared as following procedure. First, the composite mixture of Ti/Al powders was compacted in a die steel mold at the pressure of 300 MPa for 2 min to form green powder compact. Then, the powder compact was subjected to heat treatment consisted of 650°C for 2 h, 1000°C for 2 h under vacuum atmosphere of 10⁻³ Pa, where the heating rate of 3°C min^–1 was applied. The composite mixture after natural cooling to room temperature was converted into TiAl alloy owing to solid phase diffusion reaction between Ti and Al. In fact, the solid phase reaction occurs even at lower temperature than Al melting point of pure Al, so the Al particle exists in the form of Ti–Al alloy.^30,31 Finally, the alloyed TiAl undergoes a mechanical crushing and ball milling process. The mechanical crushing can be quite easily finished due to low mechanical strength and high brittleness of the TiAl alloy at present state. The coarse TiAl particles formed after crushing were mechanically grinded by high energy ball milling (Fritsch-Pulverisette 6) under a vacuum atmosphere. In order to avoid cold welding and powder agglomeration, zinc stearate powder was applied as process controlling agent with the weight percentage of 1%. The ball milling process was carried out as 400 rpm for 2 h and then 300 rpm for 10 h, where the ratio of ball to TiAl powder and the diameter of the ball are 5:1 and φ20 mm, respectively, for former, 10:1 and φ10 mm for latter. After mechanical milling, the ultrafine-grained TiAl powder was taken out in vacuum glove box. The morphology of the pre-alloyed TiAl powder is shown in Fig. 1b. It can be noted that the distribution of pre-alloyed TiAl powder is nearly homogeneous with an average particle size of 10 μm.

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SEM morphologies of parent materials: a composite mixture of Ti/Al elemental powders (Ti particles indicated by the red arrows) and b pre-alloyed TiAl powder after mechanical ball milling

Based on the two kinds of TiAl power prepared above, two kinds of TiAl porous materials were separately fabricated using powder metallurgical route consisting of four stages developed in our previous work. ²⁴ However, it is worth noting that the final sintering temperatures are different for the pre-alloyed TiAl powder and composite mixture of Ti/Al elemental powders, 1275°C for former and 1400°C for latter. Quasi-static compressive tests of the TiAl porous materials were carried out using a universal material testing machine (MTS C43.504) at room temperature with the strain rate of 1.0 × 10⁻³ s⁻¹. The testing porous samples with the dimensions of ϕ16 × 16 mm³ were cut using electro-discharge machine. SEM (Hitachi TM3000) was applied to characterise the morphology of powder particles and microstructure of the TiAl porous materials. Macroscopic morphologies were observed using optical digital camera (Canon EOS 600D). Phase compositions of the pre-alloyed powder and bantered bulk samples were characterised by XRD (Shimadzu XRD-7000), where the Cu Kα radiation was used as an incident radiation and the voltage, current and angle step were 40 kV, 30 mA and 0.02°, respectively. The actual porosity of the TiAl porous material was calculated in terms of Archimedes principle. Measuring the weight and corresponding dimension of the sample, the porosity P can be obtained by (1) where ρ and ρ_s are the densities of TiAl porous material and solid TiAl alloy (3.75 g cm^–3), respectively.

Results and discussion

Figure 2 shows the X-ray diffraction patterns for the pre-alloyed TiAl powder after mechanical milling and sintered bulk TiAl porous material. It can be noted the both the powder and bulk material are consisted of TiAl and Ti₃Al, but the composition of TiAl is dominant. Relative to pre-alloyed TiAl powder, the XRD curve of the bulk TiAl porous material is more smooth, indicating a higher crystallisation degree.

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X-ray diffraction patterns for a pre-alloyed TiAl powder after ball milling and b sintered bulk TiAl porous materials

The micrographs of the two kinds of TiAl porous materials are shown in Fig. 3. It can be observed that the TiAl porous materials have open-cellular structure for both CPS and SPS materials. Figure 3a and b corresponds to CPS material with 75% porosity. It can be found that the CPS material is consisted of two sets of pores. The millimetre pores with the pore size ranges of 0.8–1.0 mm are well-distributed in TiAl matrix, while the micron pores are uniformly embedded in pore walls and pore struts. The pore size ranges of micron pores are from several micrometres to several tens of micrometre obtained by image scanning of Fig. 3b. Figure 3c and d shows the micrographs of the SPS material with 68% porosity and average pore size of 0.7 mm. The SPS material is consisted of TiAl matrix and millimetre pores. However, it should be noted that the pore walls are not completely dense. A number of small voids are distributed in the pore walls, which formation is associated with the contacting points between carbamide particles. The numbers of the void increase with increasing the porosity. The connection of the pores ensures a complete three-dimension open-cellular structure of the TiAl porous materials. Figure 3d depicts the microstructure of pore wall for SPS. Observation of the image indicates that sintering at 1275°C can guarantee a perfect product quality. The low sintering temperature compared to CPS can be understood in terms of the pre-alloyed TiAl powder state. The powder after mechanical ball milling has an ultrafine grain with high specific surface area, which greatly favours sintering process because of shortened diffusion displacement. ²⁷ In addition, the high kinetics of the pre-alloyed TiAl powder induced during mechanical milling contribute to obtaining high sintering driving force and effective nucleation due to the presence of numerous crystal defects and improved stored strain energy, as a result, decreasing the sintering temperature.^28,29

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SEM micrographs of cross-sections of two kinds of TiAl porous materials: a CPS material, b local magnification microstructure of pore wall circled by the rectangular area in a, c SPS material and d local magnification microstructure of pore wall circled by the rectangular area in c

Compressive stress–strain curves of the TiAl porous materials are shown in Fig. 4. It can be observed that the curves are different for CPS and SPS materials. The curves of the CPS material show linearly elastic region at low stress and a long collapse plateau with nearly constant stress. In addition, the plateau regions are not smooth but exhibit obvious jagged fluctuation, indicating a brittle nature of the material. Regarding the SPS material, the compressive stress rapid decrease even to zero just after linearly elastic region, indicating complete collapse and failure of the material. It is also seen from Fig. 4 that the CPS material has higher ductility than that of SPS material. The reason was believed to ascribe to the grain size of the TiAl porous materials, for the CPS material has larger grain size than that for SPS material due to the difference of original TiAl particle sizes. Figure 5 shows the dependence of upper yield strength on relative density of the porous materials. It is apparent that the SPS materials can obtain higher upper yield strength than CPS material at the same porosity. For instance, the strength is 64 and 51.2 MPa, respectively, for SPS material and CPS material at the porosity of 58.6%. The difference is probably associated with different pore structures and compressive deformation process. Figure 6 shows the optical images of the TiAl porous materials at different deformation degree. It can be noted that the images of Fig. 1a and b are well in accordance with the compressive stress–strain curves for the CPS materials. The compressive deformation process can be described as follows. The local expansion first takes place in some fragile fields as marked by white ellipse, leading to the decrease of stress. Further increasing the strain, the stress anew increases until local break in expansion field. With continuously increasing the strain, the broken fields will propagate until thorough collapse of the CPS materials. As a consequence, the alternatively upping and downing of the stress during compressing are responsible for the jagged fluctuation of the compressive stress–strain curves. For the SPS materials, the deformation process is different from that of CPS materials. The materials will be thoroughly broken and crushed once the compressive stress exceeds the upper yield strength, as a result, the stress rapidly decrease even to zero as exhibited in Fig. 6c. Of course, further work is needed to deeper understand the mechanical properties of the TiAl porous materials.

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Compressive stress–strain curves of the TiAl porous materials with SPS and CPS

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Dependence of upper yield strength on relative density of the TiAl porous materials with SPS and CPS

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Optical photographs of the TiAl porous materials at different compressive deformation degrees: a and b 3 and 5%, respectively, for CPS material with the porosity 67.9%; c 2% for SPS material with the porosity 62.1%

Conclusions

Using power metallurgical processing route, two kinds of TiAl porous materials were prepared basing on two sets of TiAl powders, pre-alloyed TiAl powder and composite mixture of Ti/Al elemental powders. The TiAl porous materials have three-dimension open-cellular structure. The SPS TiAl material was fabricated using pre-alloyed TiAl powder, the millimetre pores of which are well-distributed in matrix. The CPS TiAl material is arisen from the composite mixture of Ti/Al elemental powders. The pores are consisted of millimetre pores distributed in matrix and micro pores embedded in pore walls and pore struts. The desirable sintering temperature is around 1275°C for SPS material, which is lower than that of CPS material, due to increased defect density and improved strain energy of ultrafine-grained TiAl powder induced by mechanical ball milling. Compressive mechanical tests indicate that the SPS material has higher upper yield strength than that of CPS material at the same porosity. It was suggested that the difference of mechanical properties for the two kinds of TiAl porous materials is related to the variety of compressive deformation process due to various pore structures.