Monitoring Microstructure Evolution in TiAl Using TEM

Abstract

The phase transformation, involving the ordering of disordered α-Ti phase with formation of ordered α 2 (Ti 3 Al) and γ (TiAl) phases in TiAl-based alloys is of major importance in manufacturing these perspective lightweight high-temperature materials, as it controls the grain size and spacing of γ and α 2 lamellae, and hence the yield strength and other mechanical properties. The paper presents the results of the transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS) experiments conducted on a Ti-46Al-1.9Cr-3Nb alloy before and after heat treatment. Heat treatment of the alloy at 1450°C followed by furnace cooling has led to the formation of a fully lamellar microstructure, which consists of γ lamellae mostly and of small volume fraction of α 2 lamellae. Microdiffraction and elemental composition data prove that the dark and the bright lamellae in the TEM bright field image belong to the ordered α 2 and γ structures respectively. The lamellar thickness of both α 2 and γ phases varies significantly, falling into the hundreds of nanometre range and is refined with increasing cooling rate. Based on the results obtained, the mechanisms of the transformation of the α 2 structure to γ structure are discussed. Microstructural AnalysisFurnace cooling from the single phase (disordered α phase) or two-phase region (disordered α and β phases) generally results in a fully lamellar structure where the lamellae are mostly γ intermixed with α 2 lamellae. Slow cooling (with cooling rates of 5-10°C/min) results in a two phase lamellar structure consisting of different orientations of lamellae of γ and some α 2 lamellae. Fast cooling simply refines the lamellar structure.The results of microdiffraction study of the fully lamellar structure of the Ti-46Al-1.9Cr-3Nb samples after the heat treatment are shown in Fig. 1 by the example of the sample cooled at 5°C/min.The diffraction proved directly that the dark and the bright lamellae in the TEM bright field image belong to the different crystal structures -ordered α 2 (Ti 3 Al with the DO 19 structure based on hcp lattice) and γ (near-cubic face-centred tetragonal L1 0 crystal structure), respectively. This is in contradiction with [1], stating that bright and dark lamellae have the same L1 0 lattice but with different orientation, but in strict correspondence with [2][3][4][5].

Nanometer-scaled lamellar microstructures in Ti–45Al–7.5Nb–(0; 0.5)C alloys and their influence on hardness

“…Thus, the dark lath in the DF image ( Fig. 3(b)) could be both, a single g lath or g variants in twin-relation, as pointed out by Novoselova et al [27]. The more precise description of the dark appearing laths is therefore 'the spacing between a 2 laths'.…”