| Research Abstract |
Two-phase TiAl alloys, which have attracted a considerable interest in recent years, usually exhibit lamellar microstructures when produced by usual ingot-metallurgy methods. However, TiAl alloys with this type of microstructure lacks in tensile ductility, although they are superior to TiAl alloys with any other microstructures in terms of fracture toughness and high-temperature strength and this has to be solved for the wide-spread applications of TiAl alloys. The brittleness of polycrystalline lamellar TiAl alloys is considered to be due to large anisotropy in mechanical properties of each lamellar grain, In the present study, we have produced bi-PST crystals of TiAl through diffusion-bonding PST (polysynthetically twinned) crystals, in which only a single lamellar gram is contained, and investigated their deformation behavior, paying special attention to effects of strain incompatibility at a bonding interface (i.e. grain boundary). PST crystals that constitute bi-PST are those with
… More
orientations A_1 and B_1, whose lamellar boundaries are inclined with respect to the loading axis by 0 and 31。, respectively. These PST crystals are diffusion-bonded at 800。C so that a grain boundary of bi-PST crystals is set parallel to the loading axis. One of the constituent crystals are rotated about the loading axis by a degree theta in the range of 0-180。. Calculation of strain incompatibility at a grain boundary based on the analysis of macroscopic deformation of PST crystals with the corresponding orientations indicates that for both A_1-A_1 and B_1-B_1 bi-PST crystals, the extent of strain incompatibility is largest at the angle theta = 90。. For B_1-B_1 bi-PST crystals, values for yield stress, work-hardening rate and tensile elongation are virtually the same as those obtained for B_1-oriented PST crystals when the extent of strain incompatibility is small, whereas when the extent of incompatibility is large (with theta closer to 90。), values of yield stress and work-hardening rate are higher and values of tensile elongation are lower than those for B_1-oriented PST crystals. This clearly indicates that strain incompatibility at grain boundaries plays a decisive role in determining tensile ductility of polycrystalline two-phase TiAl alloys. In contrast, bi-PST crystals of any combinations exhibit values of yield stress, work-hardening rate and tensile elongation comparable with those for A_1-oriented PST crystals. This results from the facts that the extent of strain incompatibility for A_1-A_1 bi-PST is smaller than that for B_1-B_1 bi-PST and that the flow stress for A_1-A_1 bi-PST is high enough for additional deformation, which is needed to compensate for the incompatibility, to occur. This result is an important and encouraging one for directionally-solidification (DS) techniques in which all lamellar boundaries are aligned parallel to the growth direction. Less
|
