The typical microstructure of the NiAl/Cr in-situ composite without Hf and Ho addition is shown in Fig.1 (a) and (b). It can be seen that the composite is composed of eutectic cell and coarse NiAl dendrites along cell boundary. Within eutectic cells, NiAl and α-Cr phases form the lamellar structure radiating from the cell interior to the boundaries. The interlamellar spacing of NiAl and α-Cr near the cell center is finer than that near the cell boundary. Further observations on the NiAl and α-Cr phases find that fine arrays of coherent NiAl and α-Cr particles precipitate in α-Cr and NiAl phase respectively, as shown in Fig. 1(c) and (d). In addition, the interface dislocations are also observed along the NiAl/Cr(Mo) phase boundary.
Fig.1. (a) SEM micrograph of NiAl/Cr in-situ compoiste, (b) TEM image of the lamellar structure and eutectic cell, (c) Precipitates of Cr(Mo) in NiAl phase and the morphology of interface dislocation, (d) Morphology of NiA precipitates in Cr(Mo) phase
The typical microstructure of the NiAl/Cr(Hf,Ho) in-situ composite is shown in Fig.2 (a). It can be seen that the Hf and Ho addition changes the microstructure of the NiAl/Cr composite obviously. Firstly, the NiAl/Cr lamella structure in the eutectic cell is refined greatly. The interlamellar spacing in the eutectic cell is much smaller than that of the composite without Ho addition, but more coarser NiAl and Cr(Mo) phases form in the intercellular region. Secondly, many white phases are precipitated along the eutectic cell boundary. The EDS tests on the white phases show that they are Hf-rich phase and Ho-rich phase. TEM observations on the composite exhibit that they are Ni2AlHf phase and Ni2Al3Ho phase, respectively. Fig.3 (c) shows the selected area electron diffraction (SAED) pattern of the Ho-rich phase along [111] zone axis. It certify that the Ho-rich phases is Ni2Al3Ho, which has a hexagonal crystal structure with a=0.902 nm, c=0.4052 nm and the space group of P62/mmm. Fig.3 (d) shows the SAED pattern of the Hf-rich phase along [110] zone axis. It certify that the Hf-rich phase is Ni2AlHf, which has a cubic crystal structure with a=b=c=0.6081 nm and the space group of Fm3m. What is interesting is that the Ni2AlHf is semi-coherent with the NiAl phase but incoherent with Ni2Al3Ho phase, as shown in Fig.2 (b). The high resolution TEM (HRTEM) observation shows the NiAl/Ni2AlHf interface is straight and clear. No nano particles and amorphous layer are found along the interface.
Fig.2. (a) SEM micrograph of NiAl/Cr(Hf,Ho) in-situ composite, (b) Bright-field TEM micrograph of Ni2AlHf and Ni2Al3Ho precipitates along NiAl phase boundary, (c) SAED pattern of Ni2Al3Ho phase along [111], (d) SAED pattern of Ni2AlHf along [110], (e) HRTEM image showing the NiAl and Ni2AlHf phases interface
Fig.3. (a) SAED pattern of Ni2Al3Ho phase along [120], (b) HRTEM image of NiAl and Ni2Al3Ho phases, (c) SAED pattern of Al17Ho2 phase along [123], (d) Bright-field TEM micrograph of Al17Ho2 phase precipitated in Ni2Al3Ho phases, (e) HRTEM image of microtwins in the Al17Ho2 phase, (f) Bright-field TEM micrograph of Ho2O3 particles (Inset picture showing it SAED pattern along [211])
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