Efect of Cyclic Heat Treatment on Cast Structure of TiAl Alloy

The subject of the article is focused on the refining of the cast structure of gamma TiAl–2Nb alloy using cyclic heat treatment and on the analysis of the grain refining mechanism. Microstructure evolution after applied cycles of heat treatment was characterized using light, laser and electron microscopy and using microhardness tests. Application of five heat treatment cycles during which two phase transformations (eutectoid and alfa-recrystallization reactions) repeatedly took place resulted αin refining of the cast columnar structure. The mean grain-lamellar colony size 0.512 mm was transformed to fully lamellar structure containing gamma and alfa2 phases having the mean grain size 0.229 mm. Lamellae thickness of gamma was not changed while the thickness of alfa2 phase decreased to 78 nm. Refining of alfa2 phase resulted in the increase of the microhardness by 20 %. The recrystallized cast structure obtained by cyclic heat treatment and the knowledge on the mechanisms of the refining the structure were compared and discussed with the literature data.