The microstructural morphology and phase compositions were characterized by SEM/EDS. The typical microstructure of the as-cast alloy is shown inFig. 1(a). The microstructure can be characterized that the coarse NiAl phases are separated by the coarse Laves phase into cells with average size of about 30mm. Along the NiAl phase boundaries the NiAl and Laves phase grow into eutectic structure. While in the NiAl phase a lot of needle-like precipitates forms, but between precipitation region and the boundary, there is an area that almost has no precipitates, as shown inFig. 1(b). In addition, some small white phases are found along the phase boundary and EDS test shows it contains much of Ni and Dy elements. TEM observations on the NiAl phase boundary reveal that there are two kinds of phases, as shown inFig. 1(c). The observation on the Laves phase reveals that besides of Cr and Nb elements it also contains Ni and Al. Its corresponding selected area diffraction (SAED) pattern (Fig. 1(d)) exhibits that the Cr2Nb Laves phase has the hexagonal C14 structure, which is different with the previous research[11] that the Cr2Nb phase with C15 crystal structure is more stable than that with C14 structure at ambient temperature. May be the solid soluted Ni and Al elements influence the structure of the Cr2Nb phase. The observation on the Dy rich particle reveals that it contains much Ni element. Based on its corresponding SAED patterns (Fig. 1(e)), the particle is determined as Ni5Dy phase which has the hexagonal crystal structure with a¼0.4856 nm, c¼0.3969 nm and the space group of P6/mmm. Such a phase has been found in the former researches[18]. TEM observation on the precipitation region reveals that the precipitates are mainly long needle-like phases with several micrometers and some precipitates have the short-rod shape, as shown inFig. 1(f).
Fig. 1.(a) SEM image of the as-cast NiAleCr(Nb)/Dy alloy; (b) Morphology of needle-like Cr2Nb precipitates in NiAl phase and NiAl/Cr2Nb eutectic structure along NiAl phase boundary; (c) Bright-field TEM image of the Cr2Nb and Ni5Dy phases; (d) SAED pattern of Cr2Nb particle; (e) SAED pattern of Ni5Dy particle; (f) Bright-field TEM image of Cr2Nb precipitates with long-needle shape.
The HIP treatment changes the microstructure of the as-cast NiAleCr(Nb)/Dy alloy obviously, as shown inFig. 2(a). Firstly, the NiAl/Cr2Nb eutectic structure along the NiAl phase boundary disappears and is already replaced by the massive Cr2Nb phase in the HIP treated alloy. In addition, the average NiAl grain size of HIP treated alloy is about 40mm, which is bigger than that of the as-cast alloy. Secondly, the original long needle-like precipitates in the as-cast alloy are changed into the small precipitates, as shown inFig. 2 (b). Moreover, some gray phases are found along the NiAl/Cr2Nb phase boundary. TEM observation on the precipitates in NiAl phase reveals that most of them have short rod-like shape and some have sphere shape, as shown inFig. 2(c). The average size of the precipitates is about four hundred nanometers, which is much smaller that that of the as cast alloy. Moreover the precipitates have a tendency to be sphere. Observation on the gray phase proves that it isa-Cr phase with some Ni and Al elements, as shown inFig. 2(d).
Fig. 2.(a) SEM image of the as-cast NiAleCr(Nb)/Dy alloy with HIP treatment; (b) Morphology of Cr2Nb phases; (c) Bright-field TEM image of Cr2Nb precipitates with short-rod shape in NiAl matrix; (d) Bright-field TEM image ofa-Cr phase (Inset picture showing its SAED pattern).
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