The transition of surface morphology and cross-section of oxide scale

The evolution of the surface morphology of the oxide scale from top to bottom of the FGM TiAl component is shown in Fig. 4-12. After oxidation testing at 800 °C for 100 h, the entire surface of the specimen turned white, coated with oxides. Partial spallation of brittle oxides can be observed in the top region and middle region of the samples. The main reason for spallation is the poor adhesion between the oxide scale and matrix, caused by their different thermal expansion coefficients. The residual oxide film showed a number of cracks on the edges and was almost peeled off from the base at the top surface (Fig. 9(a)). A white-colored network structure was distributed throughout the spalled area, pointing to Al2O3 according to EDS mapping (Fig. 4-13). It can be seen that the surface of the oxide film was uneven due to the formation of oxide clusters gathered in rows. Such morphology could weaken the oxide scale adhesion because cavities are easier to form between the oxide clusters and ravines [61].

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The oxide scales on the surface have features of high brittleness and hardness, as evidenced by flat particle boundaries caused by exfoliation and spallation [242]. The unpeeled area was covered with a mixture of randomly oriented prismatic shaped rutile particles. Unlike the original microstructure of deposited TiAl FGM, the shape of the surface particles did not show any difference with the change of composition. However, the average size of the particles changes from 1 µm in the top surface to 4 µm near the substrate, as displayed in Fig. 4-12(a-c).

Fig. 4-12. Surface morphologies of oxides formed on different locations of the FGM oxidation sample after 100h oxidation at 800 °C in air.

The overall trend of oxide layer depth versus Al concentration is presented in Fig. 4-14. It can be seen that with decreasing Al content, the oxide depth increases at a growing rate, ranging from 3 µm in the top surface to 56 µm near the Ti substrate. It is worth noting that the oxide depth increased greatly when Al concentration dropped to less than 40 at%. It has been demonstrated by Gil et al. [243] that the microstructure and phase composition play a crucial role in the oxidation behavior of TiAl-based alloys. In the case of low Al concentration, the

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volume fraction of α2-Ti3Al is dominant, which is prone to forming mixed non-protective scales.

However, the sudden drop of measured oxide depth that occurred at 6mm and 8mm to 12mm below the top surface, as marked in Fig. 4-14, is mainly due to spallation.

Fig. 4-13. Elemental distribution maps of the spalled area of WAAM-fabricated FGM TiAl alloy after oxidation for 100 h.

Fig. 4-14. Oxide thickness varies with the distance from the top surface of WAAM-fabricated FGM TiAl alloy after 100h oxidation at 800 °C in air.

Fig. 4-15 displays the cross-section of oxide layers in different positions along the vertical, or deposition, the direction of the WAAM-fabricated FGM TiAl sample. Elemental distribution maps are used to investigate the characteristic of the oxide scale. In addition, the differences in the structure of the oxide layers were analyzed.

In position A, 1.5 mm from the top (49 at% Al), the matrix is composed of a near single -TiAl. The oxide layer can be divided into three parts. The outermost and the innermost layers

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both consisted of mixed A12O3 / TiO2. A compact and continuous A12O3 layer existed between them and played the role of hindering the diffusion of O and Ti. A single and continuous A12O3 layer, as the most desired oxide film in TiAl alloys, is considered to be far superior to a single TiO2 layer or a mixture of A12O3 and TiO2 layers, due to the porous nature of TiO2 [61]. The stabilization of the A12O3 scale can be obtained only on the -TiAl phase during high-temperature oxidation [215]. Presumably, Ti and O ions could permeate through A12O3 moderately even on the single- matrix since the outer layer is a mixture of A12O3 and TiO2. There could be some permeation via intergranular paths, since transporting inside grains is considered to be smaller than along grain boundaries by several orders of magnitude.

In position B (45 at% Al), the phase constituent of the matrix lies in the -TiAl + α2-Ti3Al duplex phase field. The oxide scale formed in this position shows a typical stratified multilayer structure, which consisted of four layers: a TiO2-enriched outer layer, an A12O3-enriched layer, then a TiO2 / A12O3 blended layer and finally a TiO2-enriched innermost layer.

Spallation was clearly observed between the innermost layer and the oxygen diffusion zone.

The middle layer was the thickest. Moreover, it can be seen that pores have formed along the top TiO2/second Al2O3 scale interface and in the innermost single TiO2 oxide region.

In position C (35 at% Al), the oxide scale was distinctly separated from the alloy and its mean thickness was increased significantly compared to that of positions A and B, which may be ascribed to the breakaway oxidation behavior. Transverse cracks have formed at the interface, which could contribute to the separation and spallation of the oxide film. Relatively large pores appear to have grown along the boundaries of the TiO2 outmost layer and A12O3-rich region, but comparatively small counterparts existed in the Al2O3-rich region. Additionally, holes were found to be densely distributed in the innermost TiO2 layer. Transverse cracks and several small longitudinal cracks were observed on the matrix.

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Fig. 4-15. Morphology of the oxide cross-section, the corresponding elemental distribution, and the schematic diagram of oxide layers formed on different matrices of WAAM-fabricated FGM TiAl alloy oxidized at 800 ºC for 100 h. Position A~D are marked in Fig. 4-14.

The structure of the oxide scale in position D (16 at% Al) was similar to that of position C, composed of an uneven outer TiO2 layer, middle mixture layer and thickest inner loose TiO2 layer. According to the phase diagram, the phase under this composition consisted of α and α2. This is mainly due to the similar lattice structure and oxygen solubility for α-Ti and α2-Ti3Al, even though the phase constituent is quite different at position C and D. Previous investigations have concluded that the growth of oxide scale of TiAl alloys follows a parabolic rate law

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without breakaway [125, 190]. However, once the matrix phase is single α2 or α2+α, the growth of TiO2 becomes more competitive compared to that of A12O3. As oxidation time is increased, TiO2 could precipitate along the intergranular channels of A12O3 and finally bridge the outer and inner TiO2 oxide scale by forming a TiO2 channel, contributing to the discontinuity of A12O3. Since mass transport can easily occur in TiO2 or along grain boundaries, the transport becomes dominant along TiO2 channels and the parabolic rate growth could be broken, then breakaway occurs. Hence, the multiple oxide scales become thicker at an increasing rate and the adhesion between oxide scales and matrix weakens, which dramatically impairs oxidation resistance. This is due to the α2 phase having a high rate of oxygen and hydrogen absorption, leading to further embrittlement at high temperatures [244].

4.2.4. Conclusions

In this investigation, functionally graded TiAl alloy has been successfully fabricated using the double-wire arc additive manufacturing process. The designed variation in chemical composition can be achieved by separately adjusting the wire feed speeds of the two alloying elements. A smooth compositional gradient from pure Ti to Ti-50 at% Al is achieved along the height of the bulk deposit. The differences in microstructure evolution, mechanical properties and the oxidation behavior at different points in the graded alloy were analyzed. Conclusions can be drawn as follows:

(1) The phase constitution varies depending on Al concentration. From bottom to top, as the Al content is increased, the phase can be characterized in the following sequence: α+β→

α+ α2→ α2→ α2+ →. The volume fraction and the morphology of different phases are closely related to Al content.

(2) With increasing Al content, the microhardness and tensile strength of Ti-Al FGMs both exhibited a similar trend of a clear increase to a peak value and subsequent decrease,

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governed by several combined factors, such as different phase proportions and grain size.

Maximum hardness and tensile values existed in the single α2 field with an Al content range of 30~33 at%. However, the highest ductility was found in the top region because the volume fraction of  is predominant.

(3) The oxidation behavior is strongly related to phase composition. The effect of Al on oxidation resistance was most significant in the single -phase matrix, since a compact and continuous Al2O3 could form. While the matrix consisted of single α2, the breakaway occurred and had a detrimental effect on oxidation resistance.

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Chapter 5 Effects of ternary element introduction on