Development of Widmansta ¨tten laths in a near-c TiAl alloy S.R. Dey a , A. Hazotte a, * , E. Bouzy a , S. Naka b a Laboratoire dÕEtude des Textures et Application aux Mate ´riaux, UMR CNRS 7078, Ile du Saulcy, 57045 Metz Cedex 1, France b Office National dÕEtudes et de Recherches Ae ´rospatiales, 29 avenue de la Division Leclerc, 92322 Cha ˆ tillon Cedex, France Received 30 July 2004; received in revised form 5 April 2005; accepted 6 April 2005 Available online 1 July 2005 Abstract Microstructures composed of Widmansta ¨tten laths embedded in lamellar colonies were obtained by moderate rapid cooling of a quaternary near-c TiAl alloy, Ti–46.8Al–1.7Cr–1.8Nb (at.%). The morphological and crystallographic features of these microstruc- tures were characterized using SEM (with EBSD) and TEM. Identification of different c variants was done using an original approach based on automatic analysis of Kikuchi patterns obtained by TEM in convergent beam mode. Widmansta ¨tten laths sys- tematically show misorientations of 65° around the axis h1  100i a2 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 c and a 2 phases and {1 1 1} c habit planes. It also presents strong differences in terms of lamellae fineness and the amount and morphology of a 2 phase. Moreover, the Widmansta ¨tten laths contain a lot of internal structure defects. From the results presented, and taking into account that the 65° misorientation between the Widmansta ¨tten laths and the lamellar structures is associated with a twinning system of the hexagonal phase ðf11  22gh  1  123iÞ, we suggest a new transformation mechanism for the development of Widmansta ¨tten laths during cooling. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Titanium aluminides; Widmansta ¨tten laths; Misorientation; Twin relationship; Crystallographic orientation relationship 1. Introduction Despite a number of attractive properties, TiAl alloys are not used in a wide range of applications. In particu- lar, their development for near-shape casting of turbine components is hindered by their final mechanical anisot- ropy due to the strong solidification texture [1–7]. One of the ways of reducing these detrimental effects on mechanical properties is by making appropriate changes in the microstructures. A number of microstructural modifications through thermal treatments have been widely reported [8–14]. To ascertain the physical proper- ties, the crystallographic characterization of these microstructures is necessary. However, amongst many experimental studies of the wide range of structures available from different thermomechanical treatments (see [15] for a review), Widmansta ¨tten laths are clearly the least investigated [16,17], due to the very specific conditions required to obtain them and to their fineness and complexity. The Widmansta ¨tten laths, which comprise multi- lamellae and always accompany the lamellar structures in the microstructure, can also be considered to have similar features to the lamellar structures. The lamellar structures, which form during slow cooling from the a-phase (disordered hexagonal structure, A3), consist of a small amount of a 2 phase (ordered hexagonal struc- ture, DO 19 ) and mostly c phase (ordered tetragonal structure, L1 0 ). The orientation relationship between these phases is given by the Blackburn relationship: {1 1 1} c //(0 0 0 2) a2 and h1  10 c ==h11  20i a2 [18]. During cooling, the a-hexagonal phase transforms to the inter- mediate face-centered cubic (fcc) lattice having two 1359-6454/$30.00 Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.04.007 * Corresponding author. Tel.: +33 387 31 5644; fax: +33 387 31 5377. E-mail address: hazotte@letam.sciences.univ-metz.fr (A. Hazotte). Acta Materialia 53 (2005) 3783–3794 www.actamat-journals.com