Microstructure evolution and XRD analysis of as-received FGM materials

The microstructure evolution of the deposited material with progressively higher Al content along the Z direction is illustrated in Fig. 4-6. The sample displayed good cohesion between the layers of deposition since no macro cracks were observed in the bulk. The formation of convex layer bands is due to the partial remelting of previously deposited layers and repeating thermal cycles that have occurred during each subsequent deposition pass [25,26]. The cross-section can be divided into different regions according to the brightness of the etched surface, due to the variation of Al content. Fig. 4-6(a) exhibits typical α–β duplex microstructure (15 at% Al), showing a typical Widmanstätten lath-like morphology, with more laths appearing to originate from the α grain boundary. The transformation of β leads to co-oriented α lamellae separated by retained β ribs. The morphology in Fig. 4-6(b) (30 at% Al) exhibits sinuous grain boundaries accompanied by fine α2 plates inside the grains. A similar characteristic microstructure is found in Fig. 4-6(c) (37 at% Al) manifesting interlacing tangled grain boundaries as well as several short acicular and rod-shaped α2 plates within grain interiors. In addition, the average grain size in Fig. 4-6(c) is much smaller than that in Fig. 4-6(d) due to the cycling thermal heat treatment during the deposition process. When the Al content exceeds

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40 at%, the microstructure significantly changes, displaying coarse α2 grains with fine  laths more or less precipitating at grain boundaries and within the grains. Fig. 4-6(d) shows a large-grain morphology consisting of α2+γ lamellae structure with grain boundaries covered with dark fine  phase representing the typical composition at 43 at% Al content. A typical lamellar structure composed of  and α2 surrounded by sporadic  interdendritic phases for Ti-47 at%

Al alloy is shown in Fig. 4-6(e). The angle between the lamellar direction and the dendrite surface is around 45° indicating that the primary phase is still β under this composition [236].

With increasing Al composition up to 50% in the uppermost zone, the microstructure presents long dark dendrites possessing lamellar nature encapsulated within a white interdendritic γ-phase (Fig. 4-6(f)). According to the TiAl binary γ-phase diagram shown in Fig. 4-7, the solidification should be completed with α dendritic structure when the average Al content is 50%, which is quite different from alloys in the range of 40–49 at% Al [236]. The formation of the interdendritic α phase requires nucleation during α formation, with consequent Al enrichment of the liquid, which will finally solidify as γ phase. The α dendrites transform into α2+γ lamellae structure during solid-state cooling and the γ phase remains interdendritic.

Fig. 4-6. Images of microstructure evolution with progressively higher Al content from the bottom to the top.

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Fig. 4-7. TiAl binary phase diagram [237].

The results of XRD analysis performed at different deposition heights are presented in Fig. 4-8 for identifying the phase structure transition along the composition gradient. It can be seen from Fig. 4-8(a) that the region near the fusion line, containing 12.1 at% Al, mainly consists of α phase and slight β phase, indicating that this region is dilution-affected. From samples S1 to S6, all the peaks are pointing to the α2 phase. Interestingly, the peaks of α and α2 phase occupy almost the same positions in the 2θ scan, owing to the fact that α and α2 phase have the same space group. It is known that the intensity of the diffraction peaks corresponds to the crystallization degree, the grain size, and the order of the corresponding crystal plane.

Therefore, the intensity changes of the α2 peaks from S1 to S6 could be ascribed to the combined factors of the width of α2 lamellae, the preferred orientation of columnar crystals, thermal cycling and the cooling rate after deposition of each layer. Although a similar microstructure can be obtained in the single α2 field, the α2 phase forms via different transformation routes and the alloys’ respective site occupancy are quite different in terms of the deviation from stoichiometric composition [238]. For Ti3Al alloys containing less than 25 at% Al, the final α2 phase takes shape complying with the ordering reaction α → α + α2 → α2. Whereas, when Al content lies in the range of 25 at% to 36 at%, another solid-state reaction β+α →α2 would take place instead of α →α2 or α+ →α2 [239]. It is worth noting that the

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composition of Ti- 37.3 at% Al alloy (S6) exists in two-phase α2+ field according to the phase diagram even if the volume fraction of -phase is too small to be detected via XRD.

Fig. 4-8. XRD diffraction spectra for WAAM-built FGM in different regions from bottom to top of deposit (a) XRD diffraction patterns of fusion line area (shown in Fig. 2(c) ), S1 to S6; (b) XRD diffraction patterns of S7 to S10; (c) average volume fraction of α2 phase of phase coexistence region (S7 to S10). The volume fraction of different phases shown in (c) were obtained by Rietveld analysis on the four experiments in (b), and the error bar shows one standard deviation.

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When Al content reached approximately 40 at%, the phases were composed of -TiAl and α2 -Ti3Al (Fig. 4-8(b)). With the increase of Al content, the intensity of α2 phase peaks becomes weaker while the intensity of the γ phase grows stronger. This trend can be confirmed from the volume fraction of the α2 phase on different samples (Fig. 4-8(c)). The volume fraction of different phases mainly relies on the Al content but is also influenced by thermal cycling and cooling rate during the deposition process. From S7 to S10, the volume fraction of the α2 phase decreases from over 40% to less than 15%. In comparison with other regions, the top region (S10) has the highest γ phase volume fraction of 88±3%.