Development of microstructural observation technique for lessductile materials deformed in tension

为了研究金属间化合物和陶瓷等韧性较低材料拉伸变形时的显微组织演变，对双剪试验技术进行了改进，使其可以作为常规弯曲试验的一个简单应用。本文用分析电子显微术（AEM）测量了Ni_3Al和Ni_3Ge的互扩散系数，这对评价高温力学性能是很重要的。AEM还用于Ni_3Al、NiAl和TiAl中替位合金元素的定位。研究结果如下：1.直径为3 mm，长度为11 mm的双剪切试样可以提供与常规拉伸试样相同的力学行为。这样就可以用双剪切试验技术来检测韧性较低的材料的拉伸性能。Ni_3Al和Ni_3Ge的互扩散系数用Arrhenius关系式表示为D=D_0exp（-Q/RT），其中Ni_3Al的D_0= 1.18 × 10 ~（-3）<-4>m^2/s，Q=276 kJ/mol; Ni_3Ge的D_0= 9.03 × 10 ~（-3）<-5>m^2/s，Q=248 kJ/mol。Mn和Fe是改善Ni_3Al塑性的有效替代合金元素，它们以几乎相等的数量分布在Al和Ni位上。在本实验中，利用无序相位对离域效应进行了校正，并提出了作者自行开发的校正技术.合金化元素Fe在与Ti合金化时更易占据Ni位，且优先占据Al位;与Pd合金化时更易占据Al位，且优先占据Ni位。在这个实验中，作者开发了一个方程，它允许在多组分系统中确定位置占用。在TiAl中添加Mn降低了Ti和Al之间的共价键的强度，并降低了c/a，从而激活了许多滑移系，因此，有人建议，可以改善TiAl的延展性。

In order to examine microstructural evolution with tensile deformation of less ductile materials such as intermetallics and ceramics, a modification has been made of double-shear testing technique which can be used as an simple application of a conventional bending test. Analytical electron microscopy (AEM) has been used to measure interdiffusion coefficients of Ni_3Al and Ni_3Ge which are important for the evaluation of mechanical properties at high temperatures. AEM has been further used to locate substitutional alloying elements in Ni_3Al, NiAl and TiAl. The results are summarized as follows.1. Double-shear specimens with 3 mm in diameter and 11 mm in length can give the same mechanical behavior as expected from conventional tensile specimens. It is then possible to examine tensile properties of less ductile materials with use of the double-shear testing technique.2. Interdiffusion coefficients of Ni_3Al and Ni_3Ge are repesented by an Arrhenius relation as D=D_0exp (-Q/RT) with D_0=1.18x10^<-4>m^2/s and Q=276 kJ/mol for Ni_3Al and D_0=9.03x10^<-5>m^2/s and Q=248 kJ/mol for Ni_3Ge.3. Substitutional alloying elements, Mn and Fe, which are effective to improve ductility of Ni_3Al, are located at both the Al and Ni sites in almost an equal maginitude. In this experiment, delocalization effects have been corrected using disordered phases, while the correction technique being developed by the authors.4. Alloying element, Fe, is more likely to occupy the Ni site in NiAl when alloyed with Ti with the preference of Al site, and to occupy the Al site when alloyed with Pd with the preference of Ni. In this experiment, the authors developed an equation which allows site occupancy determination in multi-component systems.5. Addition of Mn to TiAl reduces the strength of covalent bonding between Ti and Al and lowers the c/a to activate many slip systems so that, it has been suggested, the ductility of TiAl can be improved.