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

双相TiAl合金是近年来引起人们极大兴趣的一类合金材料，在采用常规的铸锭冶金方法制备时，通常呈现层状组织。然而，具有这种类型的显微组织的TiAl合金缺乏拉伸延展性，尽管它们在断裂韧性和高温强度方面上级具有任何其他显微组织的TiAl合金，这是TiAl合金广泛应用所必须解决的问题。多晶层片TiAl合金的脆性是由于各层片晶粒力学性能各向异性大所致。本文采用扩散连接PST方法制备了TiAl双PST晶体（多合成孪生）晶体，其中只有一个单一的层状克，并研究了它们的变形行为，特别注意结合界面（即晶界）处的应变不相容性的影响。构成bi-PST的PST晶体是那些具有 关于我们 取向A_1和B_1，其片层边界相对于加载轴倾斜0 °和31 °，分别这些PST晶体在800下扩散结合。C，使得bi-PST晶体的晶界被设置为平行于加载轴。其中一个组成晶体绕加载轴旋转0-180 °范围内的角度θ。通过对PST晶体宏观变形的分析，计算了具有相应取向的PST晶体晶界处的应变不相容性。结果表明，对于A_1-A_1和B_1-B_1双PST晶体，在θ = 90 °时，应变不相容程度最大。对于B_1-B_1bi-PST晶体，当应变不相容程度较小时，其屈服应力、加工硬化速率和拉伸伸长率与B_1取向PST晶体的基本相同，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，而拉伸伸长率较低。这清楚地表明，晶界处的应变不相容性在确定多晶两相TiAl合金的拉伸塑性方面起着决定性的作用。相反，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的应变不协调程度小于B_1-B_1双PST，而A_1-A_1双PST的流变应力足够高，足以产生补偿不协调所需的附加变形。这一结果是一个重要的和令人鼓舞的定向凝固（DS）技术，其中所有的层状边界平行于生长方向对齐。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