| With the development of the aerospace industry, the requirements for applicabletemperature and mechanical properties of the high temperature structural materialsincrease. Intermetallic NiAl has some advantages, including high melting point, lowdensity, high thermal conductivity and excellent oxidation resistance, which make itan attractive candidate to for the next generation of high-temperature structuralmaterials. However, low plasticity at room temperature and inadequate strength atelevated temperature limit its practical application. Combining second-phasestrengthening with directional solidification to prepare eutectic in-situ composite maybe one promising way.In this paper, different contents of Cr and Mo were added to NiAl alloy. The phaseselection, morphology of solid/liquid interface and microstructure evolution ofdirectionally solidified NiAl-Cr(Mo) alloy with the composition range from eutecticto hypereutectic under high temperature gradient and different withdrawal rates wereinvestigated. In addition, the relationship between the mechanical properties and themicrostructure as well as the strengthening and toughening mechanisms ofNiAl-Cr(Mo) alloy were explored. Finally, fully eutectic microstructure with largevolume fraction of strengthening phase and regular lamellae was obtained in thedirectionally solidified NiAl-Cr(Mo) hypereutectic alloy; the room temperaturefracture toughness and high temperature tensile strength were markedly improved.The main research results are showed as follows:The microstructure of directionally solidified NiAl-xCr-6Mo(x=28,32,36) wascomposed of NiAl and Cr(Mo) lamellae. NiAl-28Cr-6Mo eutectic alloy andNiAl-32Cr-6Mo hypereutectic alloy could obtain fully eutectic microstructure underall process conditions of directional solidification. NiAl-36Cr-6Mo hypereutecticalloy obtained fully eutectic microstructure only at lower withdrawal rate whileprimary Cr(Mo) dendrites+eutectic microstructure at high withdrawal rate. The valueof G/V decreased and the enrichment degree of Mo element ahead of the solid/liquidinterface aggravated with the increasing withdrawal rate, which led to the instabilityof two-phase growth interface. The morphology of solid/liquid interface underwentthe change of planarâ†'cellularâ†'dendritic, and the corresponding solidificationmicrostructure changed from planar eutectic to cellular eutectic and then dendritic eutectic. High temperature gradient could increase the planar/cellular transformationrate and expand the coupled eutectic growth zone of NiAl-Cr(Mo) alloy, which werehelpful to obtain fully eutectic microstructure at faster withdrawal rate. For theNiAl-32Cr-6Mo hypereutectic alloy, when the withdrawal rate was not too high,primary Cr(Mo) dendrites appeared in the initial stage of directional solidification. Asthe directional solidification proceeded, the number of primary dendrites graduallydecreased. When directional growth reached steady state, the Cr(Mo) dendrites wereeventually eliminated, and fully eutectic microstructure was observed. But when thewithdrawal rate was high, dendritic eutectic phase directly nucleated and grew.When the content of Cr+Mo remained unchanged, the relative content of Cr andMo significantly influenced the morphology of solidification microstructure. Whenthe content of Mo was2%, some rod Cr(Mo) phase appeared in the planar eutecticalloy, and the transformation rates of planar/cellular and cellular/dendritic increased.Reducing Mo content could relieve the enrichment degree of Mo element ahead of thesolid/liquid interface, enhancing the stability of growth interface.The eutectic lamellar spacing decreased with the increasing withdrawal rate. Whenthe temperature gradient was250K/cm, the relationship between the eutecticinterlamellar spacing λ and the withdrawal rate V was λ=4.48V-0.40in theNiAl-28Cr-6Mo eutectic alloy while λ=4.82V-0.42in the NiAl-32Cr-6Mo hypereutecticalloy, which showed that J-H model was also applicable for the cellular and dendriticgrowth in the NiAl-Cr(Mo) multicomponent eutectic alloy. The volume fraction ofCr(Mo) strengthening phase increased with increasing deviation degree from eutecticcomposition. In the NiAl-28Cr-6Mo eutectic alloy it was about48.4%, and in theNiAl-32Cr-6Mo and NiAl-36Cr-6Mo hypereutectic alloy about54.3%and59.1%,respectively.When the NiAl-Cr(Mo) alloy solidified with cellular interface, the morphologies ofcellular microstructure did not change consecutively with the increasing withdrawalrate. Perfect cellular microstructure was obtained at a middle rate. The width of theintercellular regions was narrowest, no coarse and short plates existed at the cellboundaries, and the thickness of the lamellae was almost uniform. The reason was thatthe interface undercooling came to minimum compared to the coarse cellular growth,growth interface was more stable and exhibited shallow cellular.With the increasing volume fraction of Cr(Mo) strengthening phase, the roomtemperature fracture toughness and high temperature tensile strength of directionally solidified NiAl-Cr(Mo) alloy gradually increased. The maximum values were26.15MPa·m1/2and513.8MPa, respectively, which were significantly higher than those inthe present NiAl-Cr(Mo) alloy system. Under the same composition, the mechanicalproperties of planar eutectic alloy were higher than those of coarse cellular eutecticalloy. However, perfect cellular eutectic alloy had the comparable fracture toughnessas planar eutectic alloy and even higher tensile strength. This is because the bondingstrength between eutectic cells in the perfect cellular eutectic alloy was high, whichresulted in high crack propagation resistance and harmonious deformation in theintracellular regions and the intercellular regions. The result showed that cellularmicrostructure could also be used in the in-situ eutectic composites, which is helpfulto improve efficiency in the industrial production.All the directionally solidified NiAl-Cr(Mo) alloy failed as brittle quasi-cleavagefracture at room temperature. Cleavage surfaces, cleavage steps and tearing ridgeswere observed on the fracture surfaces. Some toughening mechanisms, such as crackbridging, crack deflection, crack blunting, crack nucleation, interface debonding andshear ligament toughening as well as linkage of microcracks increased the crackpropagation resistance in varying degrees, thus improving the room temperaturefracture toughness. The NiAl-Cr(Mo) alloy mainly failed as ductile fracture at1000℃，some dimples were observed. Refined microstructure, larger volume fractionof Cr(Mo) strengthening phase and dispersion strengthening of second phase particleswere contribute to the improvement of high temperature tensile strength. The bondingstrength of interface was obviously different in the alloys with differentmicrostructure morphologies, which had significant effect on the high temperaturetensile strength. For planar eutectic alloy with regular lamellae, high interface bondingstrength played a part in the improved tensile strength. For coarse cellular alloy,although refined interlamellar spacing had positive effect, low bonding strengthbetween eutectic cells made the crack easily initiate and propagate along the cellularboundary, which had greater adverse effect and resulted in low tensile strength.Benefiting from the good bonding strength between eutectic cells, high temperaturetensile strength of perfect cellular alloy increased significantly. |
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