DEVELOPEMENT OF MICROSTRUCTURE-CONTROLLED B2-ORDERED HIGH TEMPERATURE INTERMETALLICS

B2有序金属间化合物的显微组织随化学成分和时效温度的变化而变化。晶体各向异性和偏离化学计量比的成分对时效过程中的显微组织和强度变化起着重要作用。本文用透射电子显微镜（TEM）研究了NiAl（B2）-Ni_2AlTi（L2_1）和NiTi（B2）-Ni_2AlTi（L2_1）赝二元系界面的晶格失配。当Ni_2AlTi析出相的尺寸小于某一临界值时，析出相与基体NiAl和NiTi保持晶格共格。在较长时间的时效后，Ni_2AlTi析出相通过引入界面位错而失去共格性。共格性的丧失是灾难性的，在共格性丧失后，析出物的形状变为球状，但仍保持立方取向关系。位错的形态 关于我们 B_2有序NiTi合金形变产生的含L_2_1有序Ni_2AlTi细沉淀的马氏体相变与时效过程中的屈服强度变化和形变温度有关。滑移位错的Burgers矢量是一<100>种类型，在欠时效状态下剪切H相，但在过时效状态下通过。在未时效状态下，室温变形形成超位错对。随着变形温度的升高，位错开始以波状单位错的形式运动。在含2mol%Hf和5mol%Hf的NiAl中，时效初期在NiAl基体的{111}面上形成细小的L2_1型Ni_2AlHf析出相，并保持与基体晶格的共格性。随着时效时间的延长，Ni_2AlHf析出相失去共格性，变为由错配位错包围的球形析出相。Ni_2AlHf相与NiAl基体的晶格失配度约为NiAl-Ni_2AlTi相的4倍。细小Ni_2AlHf相的析出使合金的高温强度和蠕变强度都有明显的提高。TEM观察表明，颗粒/基体界面位错的相互吸引作用对提高NiAl基合金的高温强度起着重要作用。少

A wide variation of microstructures can be obtained in B2-ordered intermetallicsdepending on the chemical compositions and aging temperatures. The crystal anisotropy and the deviation from the stoichiometric composition play important roles in the microstructure and the strength variation during aging. The lattice misfits at interphase interfaces in NiAl (B2)-Ni_2AlTi (L2_1) and NiTi (B2)-Ni_2AlTi (L2_1) pseudobinary systems have been estimated by transmission electron microscopy (TEM). When the size of Ni_2AlTi precipitates is smaller than a critical value, the precipitates keep their lattice coherency with the NiAl and NiTi matrices. After longer periods of aging the Ni_2AlTi precipitates lose their coherency by introducing interface dislocations surrounding them. The coherency loss occurs catastrophically and the shape of the precipitates changes to a globular one after the loss of coherency, while keeping the cubic-cubic orientation relationship. The morphology of dislocations intr … More oduced by the deformation of B2-ordered NiTi containing fine precipitation of L2_1-ordered Ni_2AlTi is correlated with the yield strength variations during aging and deformation temperatures. The Burgers vectors of the glide dislocations are of a <100> type and shear the H-phase at the underaged state but by pass at the overaged state. In the underaged superdislocation pairs are formed by the deformation at room temperature. With increasing the deformation temperature dislocations start to move as wavy single dislocations. In NiAl contaning 2 and 5 mol% of Hf, fine plate-like L2_1-type Ni_2AlHf precipitates are formed on the {111} planes of the NiAl matrix keeping the lattice coherency with the matrix lattice at the beginning of aging. By longer periods of aging Ni_2AlHf precipitates lose their coherency and change their morphology to the spherical ones surrounded by misfit dislocations. The lattice misfit between the Ni_2AlHf precipitate and the NiAl matrix is about four times as large as that of NiAl-Ni_2AlTi system. The high temperature strength as well as creep strength increase appreciably by the precipitation of fine Ni_2AlHf phase. TEM obsernvations suggested that the attractive interactions for dislocations at the particle/matrix interfaces plays an important role to increase the high temperature strength of NiAl-based alloys. Less