The tested specimens parallel and perpendicular to the extrusion direction were cut from the SHS/HE synthesized composite. Their compressive properties at room temperature and high temperature together with NiAl–TiC composite fabricated by HPES technique are shown in Table 1. From the data, it can be seen that the specimens parallel to the extrusion direction have better compressive properties, compared with those perpendicular to the extrusion direction. Moreover, the yield strength, compressive strength and compressive strain at room temperature of the SHS/HE synthesized composite parallel to extrusion direction is better than that of HEPS synthesized NiAl–TiC composite, which increases by 20%, 19% and 30%, respectively. But at 1273 K, the compressive properties of the SHS/HE synthesized composite parallel to extrusion direction are almost similar with those of HEPS synthesized composite. However, the compressive properties of SHS/HE synthesized composite perpendicular to the extrusion direction at room temperature are just a little higher than these of the HPES synthesized NiAl–TiC composite, while at 1273 K, the compressive properties is some lower. The different properties of the specimens with different directions should be ascribed to the distribution of TiC and Al2O3 particles. From the SEM images, it can be seen that these ceramic particles have an obvious trend to distribute along the extrusion direction. Such a phenomenon is mostly similar with the directionally solidified eutectic alloy, which can make fully use of the strengthening effect from second phase. Therefore, the yield strength and compressive strength of the specimen parallel to extrusion direction are better than those of the specimen perpendicular to extrusion direction.
The typical fracture morphologies of SHS/HE synthesized NiAl–TiC–Al2O3 composite compressed at room temperature are shown in Fig. 1. From the overview on the fracture, two kinds of failure mode can be observed, such as bulk smooth cleavage and small decohesion, as shown in Fig. 1a. Further observations reveal that the bulk cleavage exhibits river like pattern, which starts from S point and expands to the whole phase, as shown in Fig. 1b. The phase with bulk cleavage is the block NiAl, which has more than 30% proportion in the fracture. While in the small decohesion zone, there are many fine cavities. Based on its size and distribution, it can be concluded that the cavity should be the place where the TiC particle stay. Due to its higher strength, TiC particle will not fracture before NiAl failure, so the TiC particle is pulled out from the NiAl matrix and the cavity is left. That will consume more energy during deformation and increase the compressive strength.
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Fig. 1. (a) SEM images of the NiAl–TiC–Al2O3 composites fracture surface after compression deformation at RT; (b) enlarged image of the square area drawn in the dotted line in (a), which shows cleavage fracture of NiAl and decohesion TiC particles.
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