| 其他摘要 | NiAl intermetallic has been paid more attentions as a potential structural material because of its high melting point, good thermal conductivity, moderate density and excellent oxidation resistance at elevated temperatures. However, its poor high temperature strength and serious lack in fracture toughness and ductility at room temperature limit the use of NiAl alloys. Fortunately, NiAl-28Cr-6Mo eutectic alloy shows the better room temperature fracture toughness and high temperature strength than other NiAl based alloys, which might make it into application. But its properties are not good enough compared to Ni-based superalloy. Although appropriate Hf can improve its high temperature strength, Ni2AlHf formed near the phase and cell boundaries weakens the fracture toughness at RT severely. In this paper, in order to obtain the better room temperature and high temperature properties, the effect of minor rare element Gd on NiAl and NiAl-Cr-Mo-Hf, superheating treatment technique and liquid metal cooling directional solidification technique on the NiAl-Cr-Mo-Hf-Ho were investigated. The relationship between the microstructure and mechanical properties was analyzed.
After adding Gd into binary NiAl, a new hard and brittle GdAlNi phase was formed. The GdAlNi phase precipitated in grains and on grain boundaries. Gd addition also refined the grains.The fracture mode from intergranular changed to transgrannular due to Gd addition. Addition of 0.10 at.% Gd can improve the room temperature compressive strength of NiAl due to grain refinement, solid solution hardening and second phase strengthening. When 0.25 at.% and 0.50 at.% Gd were added into NiAl, too much GdAlNi phase precipitated on the grain boundaries which led to the poor room temperature strength. Grain refinement and the interaction between Gd and O, S improved room temperature compressive ductility. NiAl-0.25 at.% Gd showed the best high temperature compressive properties over other alloys. At 1000 ℃ and 1100 ℃, the strength was almost the same for the alloys with different Gd amount because of the weakening of strengthening effects.
Adding 0.01 and 0.05 wt.% Gd into Ni-33Al-28Cr-5.9Mo-0.1Hf (at.%) eutectic alloy, minor Gd atoms solutioned in NiAl and Cr(Mo) phases, and most of the Gd atoms were distributed in Heusler phase. The microstructure was refined including the decrease of the eutectic cell size and of the lamellae spacing between the NiAl and Cr(Mo) plates, and cohesive strength of the phase boundaries was improved due to Gd addition. The alloy was strengthened through solid solution hardening of Gd atoms. Therefore, the compressive strength at room temperature and 1100 ℃ was increased due to Gd addition. The interface of fine lamellae which could provide amount of mobile dislocations are helpful to improve the ductility. Higher cohesive strength of phase boundaries and purification of the alloy by Gd addition were also beneficial to the higher compressive ductility at room temperature. However, the Cr(Mo) phase on the grain boundaries was coarsened and distributed irregularly when 0.1 wt.% Gd was added into the alloy, which resulted in the decrease of properties at the testing temperature.
For NiAl-28Cr-5.9Mo-0.1Hf (at.%)-0.05 (wt.%) Ho alloy, superheating treatment resulted in higher cooling rate, and solidification was close to the eutectic coupled zone. Therefore, the amount of primary NiAl decreased by 30 %, the lamellae spacing decreased and the amount of alloying elements in NiAl and Cr(Mo) phases increased as well. After superheating treatment, the morphology of most primary NiAl changed from sphere to dendrite. At the same time, the cell size was increased about 70%. The shape of Cr(Mo) precipitates in the primary NiAl changed significantly from dendrite for normal alloy to particles for superheated alloy. So the compressive properties at room temperature and high temperature and the fracture toughness at room temperature were improved due to superheating treatment. However, the treatment did not change the deformation behavior and fracture characteristics.
Liquid metal cooling directional solidification technique was used to prepare NiAl-28Cr-5.94Mo-0.05Hf-0.01Ho (at.%) alloy. With the increase of withdrawal rate, the microstructure was refined including the refinement of eutectic cells and lamellae between the NiAl and Cr(Mo) phases, but the regularity of directional Cr(Mo) layers decreased. At the same time, the amount of alloying elements in NiAl and Cr(Mo) was increased. The directionally solidified alloy by liquid metal cooling technique could obtain better compressive properties than that by conventional Bridgeman technique due to the refinement of microstructure and more alloying elements in two eutectic phases. At room temperature, the alloys fractured by cleavage mode after tensile tests, and the withdrawal rate had little influence on tensile strength. At 980 ℃, yield behavior occurred which led to the higher tensile strength and ductility compared to those at room temperature, and the properties were independent on withdrawal rate. At 1100 ℃, tensile properties increased with the increase of withdrawal rate, and both the strength and elongation rate were lower than those at 980 ℃. The fracture toughness depended on the regularity of directional Cr(Mo) layers. With the increase of withdrawal rate, the fracture toughness decreased. |
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