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    • Fig a shows the samples sintered at different

    2018-10-25

    Fig. 2(a) shows the samples sintered at different temperatures. The samples sintered at 600 °C appear bright (gray), whereas the samples sintered at higher temperatures appear black. This change in luster is attributed to the reaction products formed during sintering. The surface morphology of the sample sintered at 600 °C is presented in Fig. 2(b). The micrograph shows predominantly un-reacted Al (irregular) and Ti–6Al–4V (spherical) particles. On any given Ti–6Al–4V particle, conical structures can be seen growing on the surface and interconnecting neighboring Ti–6Al–4V particles together. EDS analysis indicates these interconnecting amphetamine sulfate have the composition of TiAl3. Fig. 2(c) shows the surface morphology of a sample sintered at 800 °C for 6 h. Clearly, the surfaces of the particles appear different (grainier) than those sintered at 600 °C. The in-set in Fig. 2(c) shows a high magnification micrograph revealing the surface texture. Given the sintering temperature was above the melting temperature of Al, liquid phase sintering occurs. The Ti–6Al–4V particles retain the original spherical morphology and the surface contains reaction product. EDS analysis on the surface confirms the presence of TiAl3. Initially, Ti atoms on the surface will dissolve in liquid Al and the Ti enriched solution will recast onto the sintered particles, resulting in TiAl3. The appearance of small globular features (due to surface tension) on the sintered particle surfaces indicate recasting has occurred. At the melting temperature of Al, the diffusion of Al in Ti is 75 × 10−3 mm/s while the diffusion of Ti in the Al is 66 × 10−3 mm/s [3]. Given the higher diffusion of Al atoms into Ti particles the result is the enrichment of Al, which aids in forming TiAl3 on/below the surface of the sintered particle [2,3,6,9,10]. As time progresses all the liquid Al will be consumed and an Al rich intermetallic layer grows and thickens on the sintered particle. Subsequently, Al diffuses inwards and Ti–Al intermetallics evolve in different layers. The samples sintered at 1000 °C for 6 h and 24 h also showed similar grainy/globular morphology and the presence of TiAl3 on the particle surfaces. Since the samples sintered at 600 °C and 800 °C were fragile, further microstructural characterization was carried out only on samples sintered at 1000 °C. Fig. 3 shows the cross-section SEM back scattered electron (BSE) micrograph of the sample sintered at 1000 °C for 6 h. Fig. 3(a) shows a low magnification micrograph indicating particles being fused together by a mixture of TiAl3+TiAl (Fig. 3(c)). Multiple layers, five in total, within the Ti–6Al–4V particle can be clearly observed from Fig. 3(b). The micrograph shows the variation in contrast caused by the composition changes from layer to layer. At least ten EDS spot analyses were performed on each of these layers. The outer periphery consists of a higher percentage of Al with a dark gray background against a white network (Fig. 3(c)), which corresponds to the mixture of TiAl3 (73%–75%Al, 25%–27% Ti by Wt%) and TiAl respectively. The second layer appears as a lighter gray ring, which has the composition of TiAl+Ti3Al (34%–38%Al, 62%–65%Ti by Wt%). The third layer corresponds to the composition of TiAl3, which is the predominant phase. The fourth layer corresponds to TiAl (48%–50%Al, 48%–52%Ti by Wt%). The inner core has an irregular shape and was identified as Ti3Al. Earlier studies on reaction synthesis of TiAl [9–11] from elemental Ti and Al powders were based on the Ti–Al phase diagram, and the sequence of formation of the intermetallics is as follows Lee et al. synthesized TiAl from elemental powders and also observed multiple layers of intermediate phases in the reacting constituents [10]. However, in the present study, the presence of TiAl3 in the third layer was unexpected according to the Ti–Al phase diagram. This can be explained by the variation in inter-diffusion rates of Ti in Al through the second layer (TiAl+Ti3Al), and also the sintering time was not sufficient for homogenization of the composition throughout the particle. Mishin and Hertzig reported the inter-diffusion of these elements in TiAl, and concluded that Al diffuses faster than Ti through TiAl [10]. Therefore, enrichment of Al % in the close proximity of TiAl, and simultaneous diffusion of Ti toward the outer surface, results in the formation of TiAl3. Hence, to homogenize the composition and to form TiAl throughout, the samples were sintered for 24 h at 1000 °C. SEM-BSE micrographs of the cross-section of a sample sintered for 24 h are presented in Fig. 4.