| 其他摘要 | NiAl has been paid more attentions as a potential high temperature structural material because of its high melting point, good thermal conductivity, moderate density and excellent oxidation resistance at all elevated temperatures, but its poor high-temperature strength and serious lack in fracture toughness and ductility at room temperature limit the application of NiAl alloys. Fortunately, the NiAl-28Cr-6Mo fabricated by directionally solidification not only has relative high fracture toughness but also shows improved creep strength. But, the balanced mechanical properties of NiAl-Cr(Mo) eutectic alloy between room temperature fracture toughness and high temperatures creep strength need to improve further in order to put it in practical application. Recent research found that hafnium (Hf) is very effective in improving the high temperature strength of NiAl-Cr(Mo) eutectic alloy. Unfortunately, the Hf addition weakens the fracture toughness and compression ductility at room temperature severely due to the formed Ni2AlHf near the cell boundaries. In this paper, the effect of minor Ho addition, injection casting technique and strong magnetic field treatment on the microstructure and mechanical properties of NiAl-Cr(Mo)-Hf eutectic alloy and their mechanism were investigated, respectively.
The investigations on the effect of Ho addition reveal that the Ho addition changs the microstructure of NiAl-Cr(Mo)-Hf eutectic alloy by influence the undercooling in front of solid/liquid interface. The minor addition of Ho refines the NiAl/Cr(Mo) lamellar and eutectic cell size of the NiAl-28Cr-6Mo-0.15Hf eutectic alloy, and the 0.2 at.% Ho addition coarsens the intercellular microstructure. When the Ho addition increase to 1 at.%, the alloy becomes to lose the original microstructure characteristics. The trace addition of Ho leads to the formation of Ni2Al3Ho phase, which has Hexagonal crystal structure, and along the [124] crystal orientation the micro-twin crystal defects are observed. In addition, some Ni3.5Al1.5Ho phases and Ho2O3 particles are observed in the alloy as well. It is found that the Ni3.5Al1.5Ho phase has the orientation relationship with the NiAl matrix of . The appropriate addition of Ho can improve the compressive properties of the NiAl-Cr(Mo)-Hf eutectic alloy further. But the fracture toughness of the NiAl-28Cr-6Mo-0.15Hf-0.15Ho eutectic alloy improves just a little. The high temperature treatment and hot isostatic pressing (HIP) treatment result in the coarsening of NiAl and Cr(Mo) precipitating particles. Additionally, the high temperature treatment causes the Hf2Ho2O7 precipitate; while the HIP treatment accelerates the diffusion of Hf and Ho elements into the eutectic cell.
The studies on the NiAl-28Cr-6Mo-0.15Hf-0.15Ho eutectic alloy prepared by injection casting reveal that the injection-cast alloy presents a fine microstructure, i.e. the refined eutectic cell size, interlamellar spacing, intercellular zone, Ni2AlHf and Ni2Al3Ho phases, compared with the conventional cast alloy. In addition, the injection casting extends the solid solubility of elements in NiAl and Cr(Mo) phases and decreases the amount of primary NiAl phases. Moreove, Hf solid solution phase and Ho solid solution phase are observed near the eutectic cell boundary. The Cr7Ni3 stick like phase is observed in the intercellular zone, which has stack fault and micro-twin crystal defects inside. The injection-cast alloy owns much better mechanical properties than the conventional cast alloy, which should be attributed to the refined microstructure, extension of solid solubility and homogeneous distributed fine Ni2AlHf, Ni2Al3Ho phases. The HIP treatment results in the coarsening of parts of microstructure of injection-cast alloy, and the transformations of Ni2AlHf and Ni2Al3Ho phases into Hf solid solution and Ho solid solution phases respectively. More NiAl and Cr(Mo) particles precipitate in the alloy and some moved dislocations form in the NiAl matrix. The microstructure optimization caused by the HIP treatment improves the mechanical properties further, especially the elevated temperature properties.
The results of strong magnetic field treated NiAl-28Cr-6Mo-0.2Hf alloy show that the strong magnetic field treatment can accelerate the elements diffusion, refine the Ni2AlHf phase and transform some Ni2AlHf phases into Hf solid solution phases at relative low temperature. Additionally, the strong magnetic field treatment still leads to the formation of a Cr containing NiAl diffusion zone between NiAl and Cr(Mo) phases, and promotes the necking and spheroidizing of fine Cr(Mo) plates. With the increase of treatment temperature, the microstructure of the NiAl-Cr(Mo)-Hf alloy changed greatly. Some eutectic cells are substituted by coarse NiAl and Cr(Mo) phases. The Cr(Mo) plates inside eutectic cell become necking and spheroidizing and aligning along the strong magnetic field direction. Furthermore some fine Ni2AlHf particles form in the NiAl phase. The strong magnetic field treatment improves the room temperature compressive ductility significantly. |
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