Fig.1 the uncoated g-TiAl specimen. This result characterized

Fig.1 displays back scattered electron image of the silicide-aluminide
coating formed on Ti-45Al-2Nb-2Mn-1B (at. %) alloy. A triple-zone structure can
be examined in the obtained silicide-aluminide coating on g-TiAl alloy by slurry process. The chemical
composition analysis of the selected points in Fig. 1 has been showed in Table 1.

According to Fig.1 the cross-section of the coating shows that the
silicide- aluminide resultant coating on g-TiAl alloy is about 50mm thick, close-packed, and adherent to the substrate. Also, no micro-cracks
or pores can be observed within the obtained coating.

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Result
of XRD phase analysis of the silicide-aluminide coating applied on g-TiAl
Ti-45Al-2Nb-2Mn-1B (at. %) substrate is shown in Fig.2. For determining the
phases of the obtained silicide-aluminide coating, XRD phase analysis was accomplished
in two steps. At the first, an XRD phase analysis was done on the initial surface
layer of the resultant silicide-aluminide coating that is shown in Fig. 3(a).
Then, by eliminating 35 mm from the initial layer of the obtained
coating by grinding, another XRD phase analysis was performed on the second
surface that is presented in Fig. 3(b). Due to the results obtained from the
cross-section and EDS analysis of different points as well as XRD analyses, it
can be judged that the resultant silicide-aluminide coating formed on g-TiAl
substrate by slurry process had a clear triple-zone structure which consisted:

–         
An outside scale composing of a TiAl3
matrix and t2(Ti7Al5Si12)
phases.

–         
An intermediate scale consisted of a
TiAl3 matrix and Ti5Si3 phase in the form of compact
columnar lamellae.

–         
An interior scale of TiAl2
phase.

Isothermal
oxidation test at 1200°C and 400 hours for coated g-TiAl
alloy are demonstrated in Table 3. It can be seen that the silicide-aluminide
coating formation by slurry process remarkably modified
high-temperature-oxidation resistance of g-TiAl alloy. Really, mass
increase of coated g-TiAl specimen by slurry process
after isothermal oxidation at 1200°C for 400 hours was greatly less
than that of the uncoated g-TiAl specimen. This result characterized
that silicide- aluminide coating formation on g-TiAl specimen can significantly modify
its high temperature oxidation resistance.

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