Slim Zghal

A quantitative TEM analysis of the lamellar microstructure in TiAl based alloys

The microstructure of lamellar TiAl alloys was analyzed by transmission electron microscopy. Two alloys (a PST crystal and a Ti{sub 54}Al{sub 46} alloy) were investigated. The microstructure is formed by {alpha}{sub 2} and {gamma} lamellae which are very long and very thin with respect to their length. The proportion of {gamma} lamellae is higher than that of {alpha}{sub 2} lamellae. The orientation relationships associated with the {gamma}-{gamma} interfaces are examined: they correspond to the order domain, twin and pseudo-twin relationships. Statistical analyses have been performed on the volume fraction of the two phases, on the orientation of the {gamma} lamellae and on the nature of the {gamma}-{gamma} interfaces. These results are used to have a better understanding of the structural transformation (hexagonal phase - > face centered cubic phase) and the ordering process which are active in the process of formation of the microstructure during cooling.

Pinning points anchoring ordinary and shockley dislocations in TiAl alloys

Abstract Pinning points anchoring Shockley dislocations and ordinary dislocations in the γ phase of two TiAl alloys (a single-phase alloy and a lamellar two-phase alloy) are studied by in situ straining experiments performed inside a transmission electron microscope. The overcoming processes of these pinning points are observed. Their densities are measured and compared. The results indicate that the pinning points are extrinsic to the dislocations. The measured densities do not correspond to a homogeneous distribution of the individual oxygen atoms present in the crystal. It is proposed that they are due to the segregation of part of the interstitial oxygen atoms.

Transmission of the deformation through γ-γ interfaces in a polysynthetically twinned TiAl alloy II. Twin interfaces (180° rotational)

Abstract The transfer mechanisms of the deformation at γ-γ twin interfaces in a polysynthetically twinned TiAl alloy are studied by in-situ straining experiments performed at room temperature in a transmission electron microscope. Several situations which differ in the nature of the incident dislocations and in the orientation of the applied stress are analysed. Some cases for which no dislocations are emitted in the second lamella are also presented. The crossing of the interface by ordinary dislocations occurs by cross-slip if they are of screw character when lying in the interface plane. If they are not of screw character when lying in the interface plane, these ordinary dislocations are locked at the interface and no emission of dislocations is observed in the neighbouring lamella. For the case of incident twinning dislocations. twins and ordinary dislocations are emitted in the mirror plane of the neighbouring lamella. All the results of this paper and of the companion paper are then discussed with the aim of understanding the role of these interfaces on the mechanical properties of lamellar TiAl alloys.

Migration of ordered domain boundaries and its effect on the lamellar interfaces in Ti?Al-based alloys

Microstructural investigations have been carried out on a fully lamellar Ti49Al47Cr2Nb2

Intrinsic Misfit Dislocations on 120° Interface in TiAl

The misfit between 120° rotational variants of the lamellar structure in γ-phase TiAl with the ordered L1 0 lattice is analysed. It is shown that such a misfit on the (111) interface plane, which has a shear character, can be accommodated by one, two or three arrays of interface intrinsic dislocations. One example of interface dislocation structure observed by transmission electron microscopy is presented.