Effect of heat treatment on the microstructures and hardness of Ti-48Al-xCr alloys

Abstract

Microstructural developement during continuous cooling (0.245oC/min to 500oC/min) from the α-Ti phase region and subsequent annealing treatment, 1450oC (15minutes) 1250oC (1 hour) has been investigated using Ti-48Al, Ti-48Al- 2Cr, Ti-48Al-4Cr and Ti-48Al-8Cr (at. %) alloys. In the Ti-48Al alloys and Ti-48Al- 2Cr, nearly fully lamellar transform fully lamellar with the lamellar grain size and lamellar spacing decrease as the cooling rates increases. At slowest cooling rates, a small amount of Widmanstatten-lamellar structure observed appears to be intermediate between the lamellar structure. Meanwhile, addition of chromium up to 4%, at any cooling rates the lamellar grain size remain unchanged. But the formation of β phase is increases at fastest cooling rate (oil quenched). This is due to the precipitation of the β phase at grain boundaries during heat treatment and insufficient time to dissolve to α and γ phase due to fast cooling. Slowest cooling rates (furnace cooled) all β phase completely dissolved as the following transformation β α + γ α2+ γ. The study revealed at 8% of Chromium slowest and fastest cooling rates shows large portion of β phase at precipitated at grain boundaries but at intermediate cooling rates the β phases seem disappeared or dissolved. Microhardness analysis shows that several factors significantly increase the hardness value of Ti-48Al alloys which is the evolution of α2-volume fraction, high cooling rates and smallest lamellar spacing.