Grain boundaries and deformation in two-phase TiAl alloys-Approach with bi-PST TiAl-

两相TiAl合金是近年来引起广泛关注的一种合金材料，采用常规的铸锭冶金方法制备时，其显微组织通常为片层状。然而，具有这种组织的TiAl合金在断裂韧性和高温强度方面优于具有其他组织的TiAl合金，但其拉伸延展性较差，这是TiAl合金广泛应用必须解决的问题。多晶层状TiAl合金的脆性被认为是由于各层状晶粒的力学性能具有较大的各向异性。在本研究中，我们通过扩散键合的PST（多合成孪晶）晶体制备了仅含有单一层状克的TiAl的双层状晶体，并研究了它们的变形行为，特别注意了在键合界面（即晶界）处应变不相容的影响。组成双PST的PST晶体是那些有…更多取向的A_1和B_1，其片层边界相对于加载轴倾斜0和31。,分别。这些PST晶体在800℃时是扩散键合的。C使得双pst晶体的晶界与加载轴平行。其中一个组成晶体在0-180的范围内围绕加载轴旋转一个度。通过分析PST晶体在相应取向下的宏观变形，计算晶界应变不相容，结果表明，无论是A_1-A_1还是B_1-B_1双PST晶体，在θ = 90角处应变不相容程度最大。对于B_1-B_1双PST晶体，当应变不相容程度较小时，屈服应力、加工硬化率和拉伸伸长率的值与b_1取向PST晶体的值几乎相同，而当应变不相容程度较大时（θ接近90）。与b_1取向的PST晶体相比，其屈服应力和加工硬化率较高，拉伸伸长率较低。这清楚地表明晶界应变不相容对多晶两相TiAl合金的拉伸延展性起决定性作用。相比之下，任何组合的双PST晶体的屈服应力、加工硬化速率和拉伸伸长率与a_1取向的PST晶体相当。这是由于A_1-A_1双pst的应变不相容程度小于B_1-B_1双pst，并且A_1-A_1双pst的流动应力足够高，足以产生补偿不相容所需的附加变形。这一结果对于所有片层边界平行于生长方向的定向凝固（DS）技术来说是一个重要的和鼓舞人心的结果。少

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