TiAl alloys are of increasing technical importance for high temperature applications in automotive and aerospace industries due to their low density combined with attractive high temperature properties. However, cast TiAl-based alloys usually suffer from poor room temperature ductility and a large scatter in other mechanical properties that impedes their wide industrial application. It is associated not only with intrinsic brittleness caused by directed type of interatomic bonding in the c-TiAl phase but also with a coarse columnar structure and a sharp casting texture which often evolves in castings during freezing and cooling. To overcome these deficiencies hot working (canned extrusion or forging) is usually applied in order to breakdown the ingot structure and to reach refined microstructure. However, this way is very laborious taking into account very high extrusion/forging temperatures. [1,2] Two approaches are now considered in the literature to refine the microstructures in cast TiAl alloys without involving any hot working: it is through solidification (during freezing and cooling) and through various heat treatments. The first approach can include: i) adding strong b-stabilizers (such as Re) and boron, [3,4] ii) adding boron in a level of about 1 at%, [4,5] iii) the use of b-solidifying alloys doped with b-stabilizing elements (such as Nb, Mo) and small boron additions. [6] The second approach is often associated with the use of the "massive transformation technique", which includes quenching from the single a phase field, followed by ageing in the temperature range of the (a+c)/a phase field. The most attractive advantages of this treatment are its simplicity and excluding boron as a grain refining agent necessary in the case of the first approach. The latter should be beneficial for ductility taking into account that coarse borides reduce the tensile ductility in cast TiAl alloys. [7] Quenching can lead to the massive a 7 ! c m phase transformation, where c m is the massive c phase, and subsequent ageing in the (a+c)/a phase field can provide a desirable refined (convoluted) lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well balanced mechanical properties. [8][9][10][11][12][13] The main problem in developing the massive transformation as a useful processing route for c-TiAl alloys relates to the fact that high cooling rates and a very narrow cooling rate window are commonly required for the massive transformation. High cooling rates can lead to crack formation and is moreover hard to achieve in thick sections. Another problem is to know the temperature range at which c m transforms into a convoluted structure avoiding the re-transformation to a grains because this can lead to coarse lamellar c+a 2 structure during final cooling. Therefore, to utilize the massive transformation as a method of microstructural refinement in c-TiAl alloys special alloying additions such as niobium or tantalum are requested: they reduce the diffusivit...

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Microstructure and hot workability have been considered for a number of -TiAl alloys including -solidifying TNM alloys. All TNM alloys under study showed improved hot workability in cast condition. As was shown for the Ti-45Al-5Nb-1Mo-0.2B alloy, a critical issue of TNM alloys is room temperature ductility in the conditions with lamellar structure.

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