Typical SEM images of the heat-treated (HT) NiAl–Cr(Mo)–Hf alloy are shown inFig. 1(a) and (b). The alloy is composed of eutectic cell, coarse NiAl dendrites and Hf-rich phase which is mainly distributed at the cell boundaries. Within eutectic cells, black NiAl and gray Cr(Mo) plates exhibits a radially emanating pattern from the cell interior to its boundaries. The interlamellar spacing of NiAl and Cr(Mo) near the cell center is finer than that of in the periphery of the cells. Coarser Cr(Mo) rods, primary NiAl phase and white Hf-rich phase mainly distribute at the intercellular zone. Moreover, there are still primary NiAl phases forming in many eutectic cells. As shown inFig. 1(c), a fine array of coherent NiAl precipitates are observed in Cr(Mo) phase.Fig. 1(d) shows the morphology of Hf-rich phase, and according with its selected area diffraction pattern, the Hf-rich phase can be determined as Ni2AlHf phase. The Ni2AlHf phase has the orientation relationship with NiAl matrix of that [ 111 ]NiAl//[1 1 1]Heuslerand (1 0 ¯1)NiAl//(2 0 ¯2)Heusler.
Fig. 1.(a) SEM back-scattered electron images of the HT alloy; (b) distribution of Ni2AlHf phase; (c) the fine array of coherent NiAl precipitates in the Cr(Mo) phase (inset picture shows the SADP of NiAl and Cr(Mo)); (d) TEM micrograph of Ni2AlHf phase (inset picture shows its SADP).
Fig. 2shows the microstructure of the MT1 and MT2 alloys, which are treated in 10 T SMF at 1073 K and 1 173 K, respectively With the SMF treatment, the white phases become fine and dis- tribute more uniformly along eutectic cell boundaries, as shown inFig. 2(a). The EDS test reveals that the white phase is not the Ni2AlHf but Hf solid solution (HfSS). When the temperature of SMF treatment increases to 1 173 K, the original microstructure of alloy is changed greatly, as shown inFig. 2(b). Most of the eutectic cells disappear, which are substituted by irregular massive NiAl and Cr(Mo) phases. The Cr(Mo) phases have an obvious tendency to be spheroidization. In some places there are still eutectic cells left, but they are trend to be disintegration. Except the coarsening of the Cr(Mo) phases, there are obvious necking phenomena in many Cr(Mo) plates and moreover no clear boundaries left between eutectic cells.
Fig. 2.Microstructure of SMF treated alloys: (a) the MT1 alloy and (b) the MT2 alloy.
Fig. 3(a) exhibits the results of XRD test. The peak of Heusler phase in the HT alloy still exists, but the peak of Heusler phase disappears in SMF treated alloy. TEM observations on the MT1 alloy confirm that the Heusler phase is transformed into HfSSphase, as shown inFig. 3(b). Though such phenomenon has been observed in rapidly solidified NiAl–Cr(Mo)–Hf alloy after hot isostatic pressure (HIP) treatment, previous investigation on SMF treated NiAl–Cr(Mo)–Hf alloy have not reported such transformation.Moreover, except the HfSSparticles distributed on cell boundaries, many HfSSphases are observed inside of eutectic cell. With the increase of SMF treatment temperature, in the MT2 alloy some fine HfSSparticles are found inside NiAl phase, with size of about 50 nm, as shown inFig. 3(d)
Fig. 3.(a) XRD of the HT alloy and MT1 alloy; (b) HfSSparticles distributed at the cell boundaries (inset picture shows the SADP of HfSSparticle); (c) HfSSparticles distributed at the cell interior; and (d) HfSSparticles distributed in the NiAl phase
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