Development of Widmanstätten laths in a near-γ TiAl alloy

Microstructures composed of Widmanstätten laths embedded in lamellar colonies were obtained by moderate rapid cooling of a quaternary near-γ TiAl alloy, Ti–46.8Al–1.7Cr–1.8Nb (at.%). The morphological and crystallographic features of these microstructures were characterized using SEM (with EBSD) and TEM. Identification of different γ   variants was done using an original approach based on automatic analysis of Kikuchi patterns obtained by TEM in convergent beam mode. Widmanstätten laths systematically show misorientations of ∼65° around the axis 〈11¯00〉α2 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 γ and α2 phases and {1 1 1}γ habit planes. It also presents strong differences in terms of lamellae fineness and the amount and morphology of α2 phase. Moreover, the Widmanstätten laths contain a lot of internal structure defects. From the results presented, and taking into account that the 65° misorientation between the Widmanstätten laths and the lamellar structures is associated with a twinning system of the hexagonal phase ({112¯2}〈1¯1¯23〉), we suggest a new transformation mechanism for the development of Widmanstätten laths during cooling.