Creep of Fully Lamellar Near γ-TiAl Intermetallics

Abstract

ABSTRACTThe influence of the fully lamellar morphology and third phase β on the creep properties of near γ-TiAl intermetallics is presented. Specifically, the effect of improved microstructural control obtainableby a stepped cool, involving furnace cooling and air cooling from the a single phase, on creep resistanceis demonstrated for three near γ-TiAl intermetallics: binary Ti-48AI, ternary Ti-48A1–2W and Ti-47A1–2Nb-IMn-0.5W-0.5Mo-0.2Si. The results indicate that appropriate stepped cooling can be used to reducethe lamellar interface spacing without the formation of Widmanstatten, feathery γ or γM structures, leadingto longer creep life and reduced creep strain rates. A second benefit of stepped cooling is preventionof βformation during cooling from the α phase, allowing controlled β precipitation during aging at 950 C. Creep tests on variously aged Ti-48A1–2W indicate that β precipitation along lamellar grain boundariesimproves creep resistance. Development of a uniform fully lamellar structure in Ti-47A1–2Nb-1Mn-0.5W-0.5Mo-0.2Si significantly improves creep resistance. Applying the stepped cool to this alloy allowsthe precipitation of β and silicides to be controlled during lower temperature aging.