On the formation of solute depleted zones on the surface of a gamma-TiAl alloy during heat-treatment

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Prasad, Uttara
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An investigation was carried out on Ti-45Al-2Mn-2Nb alloy, to study the mechanism of solute depletion from the surface of the specimen and formation of solute depleted damaged layer during heat treatment. Small specimens were wrapped in tantalum foil and encapsulated in vycor capsules which were evacuated and backfilled with 160 torr of commercial argon. The encapsulated specimens were annealed at 1200$\sp\circ$C for various time periods, viz. 15 hrs, 30 hrs and 50 hrs. A solute depleted damaged layer was observed to form in all the specimens which were heat treated at 1200$\sp\circ$C. The thickness of damaged layer was found to have parabolic relation with time. The mechanism involved in solute depletion was proposed. The phases present in the damaged layer were identified by detailed transmission electron microscopy. The damaged layer in the specimens heat treated at 1200$\sp\circ$C, was found to consist of $\alpha\sb2$ phase near the interface between the undamaged lamellar bulk and the damaged layer, B2phase with $\omega$-type precipitates ($\omega\sp{\prime\prime}$) in the central region of the damaged layer and B2 phase with $\omega$-type phase having B8$\sb2$ structure near the outer dge of the specimen.