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.
The critical issue of ingot-metallurgy g-TiAl-based alloys is low ductility and a large scatter in other mechanical properties which impedes their wide industrial application. These deficiencies are associated not only with intrinsic brittleness of the constituting g-TiAl and a 2 -Ti 3 Al phases caused by directed type of interatomic bonding but also with a coarse columnar structure, a high level of dendritic segregation and a sharp casting texture which often evolve 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 does not exclude texture and chemical inhomogeneities even if the microstructure is effectively refined. Another issue is a high cost of the hot working procedure. [1,2] One of the effective methods for breaking down coarse grained structures in g-TiAl alloys, at least in small-sized ingot bars, is the ''massive transformation technique,'' which includes quenching from the single a phase field followed by ageing in the temperature range of the (a þ g) phase field. [3][4][5][6][7][8] Quenching leads to the massive a ) g m phase transformation, where g m is the massive g phase, and subsequent ageing in the (a þ g) 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. [3][4][5][6][7][8] As has been recently demonstrated, the ''massive transformation technique'' was effective in the alloys alloyed by elements with reduced diffusivity, particularly in the Ti-46Al-8Ta alloy. [8] Owing to alloying by tantalum the range of cooling rates, over which massive gamma is formed, was significantly shifted towards lower cooling rates; therefore, air quenching instead of usually applied oil quenching was found to be enough to reach the massive g phase in small sample bars. [3][4][5][6][7][8] This finding seems to open up possibilities for manufacturing applications in respect of parts with thin sections. However, to promote this technique towards commercial application a better understanding of the near convoluted microstructure formation during ageing and its dependence on the ageing conditions is necessary. Another critical issue revealed in our previous work was the stability of the near convoluted microstructure obtained by high-temperature ageing. [8] The present paper considers the influence of the ageing conditions on the formation of the convoluted/lamellar microstructure in the Ti-46Al-8Ta alloy. This work continues COMMUNICATION [*] Dr. V. Imayev, R. Gaisin Microstructure evolution after air quenching and variable ageing treatment has been investigated in small-sized ingot bars of the Ti-46Al-8Ta alloy. It was found that air quenching and subsequent ageing at 1200-1260 8C led to the formation of a homogeneous refined near convoluted microstructure. To increase the stability of t...
A newly developed heavily alloyed polycrystalline nickel-base superalloy containing rhenium and intended as a structural material for turbine discs in gas turbine engines has been studied. Homogenisation and heterogenisation heat treatments were developed for the as-cast superalloy, which led to improving the hot workability as compared to that of as-cast material. electron backscatter diffraction analysis of samples isothermally compressed at temperatures near the solvus temperature showed that continuous dynamic recrystallisation occurred in the superalloy leading to the formation of refined recrystallised structure. These results were used for the development of canned forging processing under quasi-isothermal conditions. The superalloy subjected to canned forging followed by aging exhibited superior high strength and high-temperature capability, while retaining reasonable ductility as compared to known disc superalloys.
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