Effect of Nb on lamellar orientation control of single crystal gamma TiAl alloys with fully lamellar microstructure

Lamellar orientation control of Ti-48Al-8Nb unidirectionally solidified using a seed of y single phase Ti-57Al single crystal has been examined by electron backscatter diffraction (EBSD), and the results are compared with those of binary Ti-48Al grown crystal. Both alloys become single crystal with fully lamellar microstructure (PST), although several lamellar grains are formed above the seed crystal. While the binary PST crystal does not always follow the seed orientation, the ternary grown crystals always follow the seed orientation with < 110]gamma="" direction="" parallel="" to="" that="" of="" the="" seed.="" addition="" of="" nb="" improves="" the="" fitness="" of="" the="" lattice="" parameters="" between="" the="" grown="" and="" seed="" crystals,="" and="" also="" suppresses="" the="" peritectic="" reaction="" (l+alpha-="" /> gamma). The liquidus projection in Ti-Al-Nb ternary system is calculated based on lattice stability of beta-Ti, alpha-Ti, gamma-TiAl and liquid phases, and it is found that the alloys with compositions to be alpha solidification in average by mixture of the seed composition could follow the seed orientation. Addition of Nb drastically decreases the fraction of energetically unstable variant interfaces, thereby stabilizing the lamellar microstructure at elevated temperatures. (AU)