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

双相TiAl合金是近年来研究的热点，用常规的铸锭冶金方法制备的TiAl合金通常具有片层组织。然而，具有这种组织的TiAl合金虽然在断裂韧性和高温强度方面优于任何其他组织的TiAl合金，但其拉伸塑性差，这是TiAl合金广泛应用所必须解决的问题。多晶片层TiAl合金的脆性是由于各片层晶的力学性能存在较大的各向异性所致。在本研究中，我们通过扩散连接PST(多合成孪晶)晶体制备了TiAl的双PST晶体，其中只含有单层片晶，并研究了它们的变形行为，特别关注了粘结界面(即晶界)应变不相容的影响。构成bi-pst的pst晶体是含有…的晶体。更多的取向A_1和B_1，其片层边界相对于加载轴倾斜0和31。，分别为。这些PST晶体在800℃是扩散连接的。C，从而使bi-PST晶体的晶界平行于加载轴。组成晶体中的一个围绕加载轴旋转0-180范围内的θ度。。在分析PST晶体宏观形变的基础上，对晶界应变不相容进行了计算，结果表明，对于A_1-A_1和B_1-B_1双-PST晶体，应变不相容的程度在角为90时最大。。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、加工硬化速率和拉伸伸长率与B_1取向的PST晶体基本相同，而当应变不相容程度较大时(θ接近90)。屈服应力和加工硬化率高于B_1取向的PST晶体，拉伸伸长率低于B_1取向的PST晶体。这清楚地表明，晶界应变不相容对多晶两相TiAl合金的拉伸塑性起着决定性的作用。相反，任何组合的BiPST晶体的屈服应力、加工硬化速率和拉伸延伸率都与A1取向的PST晶体相当。这是因为A_1-A_1bi-PST的应变不相容程度比B_1-B_1bi-PST的小，并且A_1-A_1bi-PST的流动应力足够高，可以产生额外的变形来补偿这种不相容。这一结果对于定向凝固(DS)技术来说是一个重要而令人鼓舞的结果，在DS技术中，所有的片层边界都平行于生长方向。较少

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