The NiAl–TiC composite with dispersed Al2O3 oxides was successfully fabricated by SHS/HE process. The X-ray diffraction analysis on the composite proves that the elemental powders have been transformed to the NiAl and TiC phases, as shown in Fig. 1a. The peaks of NiAl matrix and TiC phase are strong, but the peak of Al2O3 is weak, which should be attributed to its small amount. The typical microstructure of NiAl–TiC–Al2O3 composite is shown in Fig. 1b. From the OM micrograph, it can be seen that the composite is unsatisfied, and there are still some porosity inside. The enlarged image shows that the composite is mainly composed of NiAl Matrix and white TiC, as shown in Fig. 1c. The average grain size of the NiAl matrix is about 10 lm. Additionally, the NiAl phase is elongated along the extrusion direction, which indicates that the composite experiences deformation during hot extrusion. The TiC particles with several microns mainly distributed along the NiAl grain boundaries, which is beneficial to the grain refinement. The SEM observation on the cavity is shown in Fig. 1d. Obviously, the surface of cavity is full of fine particles, which forms the shell of the cavity. The EDS tests on the particles reveal that most of them are TiC and some are fine Al2O3. As is well known, the TiC and Al2O3 ceramic particles have high strength and high melting point. In the present research, the self-propagation high temperature synthesis procedure cannot generate enough heat to soften the ceramic particles.
Fig. 1. (a) XRD pattern of the NiAl–TiC–Al2O3 composites, (b) optical micrograph of the NiAl–TiC–Al2O3 composites, (c) enlarged image showing the morphology of TiC and cavity, (d) SEM image of Al2O3 and TiC particles arranged in the cavity.
Therefore, if the synthesized TiC and Al2O3 particles are segregated, they may form the stiff shell of cavities. During the selfpropagation high temperature synthesis, the synthesized NiAl is heated to be semisolid and the extrusion force the NiAl to deformation, which can promote the TiC and Al2O3 particles to redistribute and then reduce the amount of cavity. But for the small cavity, its shell constituted by TiC and Al2O3 particles can withstand great pressure, so it is very difficult to eliminate.The TEM observation on the NiAl–TiC–Al2O3 composite is shown in Fig. 2. The NiAl matrix has dual-grain structure, as shown in Fig. 2a. Besides the large NiAl grain shown in Fig. 1c, there are much fine grains with hundreds of nanometers. The TiC particles exhibit two kinds of morphologies. The TiC particles along the NiAl grain boundary agglomerate into big ones, as shown in Fig. 1c. The TiC particle in NiAl grain exhibits fine and polyhedron characteristics, as shown in Fig. 2b. The inset selected area electron diffraction (SAED) pattern gives that TiC particle has an orientation relationship with NiAl matrix of 100_TiC==100_NiAl; e020TTiC==e0 _11TNiAl, which was reported in the former research. However, further observation on NiAl and TiC interface finds that an amorphous transition layer with several nanometers exists between NiAl and TiC, as shown in Fig. 2c. According to the previous researches, the formation of amorphous layer should be attributed to the interface stress between NiAl and TiC phases. Though NiAl and TiC phases are both have the cubic crystal structure, however the crystal parameter of NiAl is 0.2887 nm, while the crystal parameter of TiC is 0.433 nm. Therefore, it is no doubt that the difference in crystal parameter would result in great stress along phase interface. Based on the recent research, the high interface stress can result in the formation of amorphous film. In addition, along the NiAl grain boundary, the Al2O3 particles with irregular shape were found, as shown in Fig. 2d. By the SAED pattern shown in Fig. 3e, it can be confirmed that most of particles are a-Al2O3.
Fig. 2. (a) Bright filed TEM micrograph of fine NiAl matrix (inset picture showing its SAED pattern), (b) morphology of TiC particle in NiAl grain (inset picture showing its SAED pattern), (c) HRTEM of the thin amorphous layer along NiAl/TiC phase interface, (d) morphology of a-Al2O3 particles along NiAl boundary, (e) SAED pattern of the a-Al2O3 particle.
Further observations on the TiC particles find that some TiC particles have the laminate characteristic, as shown in Fig. 3a. The inset SAED pattern reveals that stacking faults and microtwins form inside. The corresponding high resolution transmission electron microscopy (HRTEM) image is shown in Fig. 3b, which reveals that the atoms on the twin boundary are different from the one in the twin crystal. The EDS test also shows that there are more Al and Ti elements in the TiC particle that contains microtwins and stacking faults defects. Moreover, some Ti2AlC ceramic particles are observed, which has hexagonal crystal with lattice parameters of a = 0.34 nm, c = 1.36 nm, as shown in Fig. 3c. Inset SAED pattern reveals that there are stacking fault inside. The corresponding HRTEM image shows that except the stacking fault and microtwins inside there are some special intergrowth plates, as shown in Fig. 3d. According to the recent studies the intergrowth plate should be TiC. The HRTEM image also exhibits that the atoms in the plate are different from the Ti2AlC. It can be inferred that in the whole particle, the growth of Ti2AlC phase leads to the lack of Al elements in neighbor area, which promotes the TiC plate to form. And in reverse, the growth of TiC plate inside promotes the formation of Ti2AlC. Then it is reasonable to understand that the special microtwins in TiC phase, which may be the Ti2AlC plates. The formation of such ceramic particle should be ascribed to the elements segregation and the high temperature during SHS/HE.
Fig. 3. (a) Bright field TEM micrograph of TiC particles with stacking fault and microtwins inside (inset picture showing its SAED pattern), (b) HRTEM image of the stacking fault and microtwins, (c) morphology of the Ti2AlC particle (inset picture showing the SAED pattern of Ti2AlC along [330]), (d) HRTEM image showing the intergrowth of TiC and Ti2AlC.
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