Development of Widmanstatten laths in a near-γ TiAl alloy

Microstructures composed of Widmanst laths laths embedded in lamellar colonies were obtained by moderate rapid cooling of a quaternary near-gamma TiAl alloy, Ti-46.8A1-1.7Cr-1.8Nb (at.%). The morphological and crystallographic features of these microstructures were characterized using SEM (with EBSD) and TEM. Identification of different 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 similar to 65 degrees around the axis (1100)(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 {111}(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 degrees misorientation between the Widmanstatten laths and the lamellar structures is associated with a twinning system of the hexagonal phase ({1122} < 1123 >), we suggest a new transformation mechanism for the development of Widmanstatten laths during cooling. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.