Development of Widmanstatten laths in a near-(gamma) TiAl alloy

Microstructures composed of Widmanstatten laths embedded in lamellar colonies were obtained by moderate rapid cooling of a quaternary near-(gamma) TiAl alloy, Ti-46.8Al1.7Cr-1.8Nb (at.%). The morphological and crystallographic features of these microstructures were characterized using SEM (with EBSD) and TEM. Identification of different (gamma) variants was done using an original approach based on automatic analysis of Kikuchi patterns obtained by TEM in convergent beam mode. Widmanstatten laths systematically show misorientations of ~ 65° around the axis (left angle bracket)1 1 0 0(right angle bracket) (alpha)2 with respect to the embedding lamellar structure. The inner structure shows similarities with the lamellar structures, i.e., multi-platelet structure with Blackburn relationship between (gamma) and (alpha)2 phases and {1 1 1}(gamma) habit planes. It also presents strong differences in terms of lamellae fineness and the amount and morphology of (alpha)2 phase. Moreover, the Widmanstatten laths contain a lot of internal structure defects. From the results presented, and taking into account that the 65° misorientation between the Widmanstatten laths and the lamellar structures is associated with a twinning system of the hexagonal phase ({1 1 2 2} (left angle bracket)1 1 2 3(right angle bracket)), we suggest a new transformation mechanism for the development of Widmanstatten laths during cooling.