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of the original manuscript: Imayev, V.; Gaisin, R.; Gaisina, E.; Imayev, R.; Fecht, H.-J.; Pyczak, F..: Abstract Microstructure and mechanical behavior of near eutectic Ti-1.5 wt.% B and hypereutectic Ti-2B wt.% B composite materials obtained by casting have been investigated. Commercially pure titanium was used as a matrix material. Homogeneously distributed TiB whiskers were revealed in the as-cast composite materials. Multiple isothermal 2-D forging of the composites was carried out in the temperature range of the beta phase field. The hot forging led to effective alignment of boride whiskers with retaining a high aspect ratio. Tensile mechanical tests in ascast and forged conditions were carried out at room and elevated temperatures. The composites demonstrated much higher strength in comparison with the matrix material without drastic ductility reduction. The effect of boride orientation and morphology on the tensile properties of the composite materials is discussed.

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...

The hot-working behavior of ␥(TiAl)-based alloys was investigated in order to understand fundamental aspects of the evolution of the microstructure and to establish guidelines for advanced alloy design and processing. The investigations involved a wide range of Al compositions and are based on metallographic investigations of the deformed samples. Particular emphasis was placed on the effects of phase composition and casting texture. It was found that the behavior of dynamic recrystallization was significantly influenced by the Al content of the alloys. Under the same deformation conditions, dynamic recrystallization was fastest for alloys with nearly stoichiometric composition, whereas the recrystallization kinetics decreased for lower or higher Al contents. This result can be attributed to the effect of the Al concentration on the micromechanisms of deformation and diffusion as well as on the initial cast microstructure, which changed from fully lamellar to equiaxed near-␥ microstructures by raising the Al content from 45 to 50 at. pct. Further, it was observed that the casting texture, i.e., the orientation of lamellae with respect to the deformation axis, significantly influenced the recrystallization behavior. In this respect, the development of shear bands due to kinking and bending of lamellae is concluded to play an important role in the recrystallization behavior and seems, in general, to be a particular feature of the microstructural evolution of lamellar alloys on hot working.

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