Titanium aluminide with about 50 at. % of Al is a promising structural material for use in automotive and aircraft applications due to its low density and creep and a relatively high strength even at the temperature as high as 1200 K, however its resistance to gas corrosion at high temperatures still needs to be improved.
A great improvement of this resistance has been achieved due to nanocrystalline microalloyed magnetron sputtered gammaTiAl coatings. In view of the results obtained by means of SEM, EDS, EBSD, X-Ray and microthermogravimetric analyses at 1173 K of bare commercial gammaTiAl alloy as well as the one with nanocrystalline magnetron sputtered microalloyed gammaTiAl coatings the following conclusions have been drawn in the work:
1. Magnetron sputtered Cr, Nb, Ta, Mo, Ag or Si -alloyed gammaTiAl coatings are well adhering and show a uniform, dense, nanocrystalline structure as opposed to the coarse-grained gammaTiAl substrate
2. The resistance of the gammaTiAl-based nanocrystalline coatings to oxidation is due to the formation of thin, tight Al2O3 layer on the surface of the primary deposit which lowers the oxidation rate of the latter several orders of magnitude
3. Both the nanocrystallinity as well as the effect of the "third element" (Cr, Nb, Ta, Mo, Ag or Si) in the ternary gammaTiAl based coatings are conducive to the increase of the diffusivity of the Al atoms in the coatings and, subsequently, to preferential nucleation of the Al2O3 phase on the coatings’ surface
4. The nanocrystallinity as well as the alloying elements (Cr, Nb, Ta, Mo, Ag or Si) in the gammaTiAl based coatings are conducive to the grain refinement of the oxide scale growing on the coatings’ surface during oxidation
5. Any synergistic effect of the both alloying elements (Cr and Nb, Ta and Nb, Mo and Nb, Ag and Si) in the quaternary TiAl(Cr,Nb), (Ta,Nb), TiAl(Mo,Cr), TiAl(Ag,Si) coatings have not been observed in the work
|