| 其他摘要 | In order to explore the application of NiAl alloys, the paper investigates the properties of friction, wearing and oxidation at service circumstance. The results indicate that NiAl alloys could be used as wear-resistance and self-lubricating materials at friction and wear conditions.
NiAl-Al2O3-TiC in-situ particle reinforced composite was prepared by hot pressure exothermic synthesis (HPES) plus hot isostatic pressing (HIP). Three kinds of eutectic alloys, NiAl-28Cr-5.8Mo-0.2Hf, NiAl-28Cr-5.6Mo-0.15Hf-0.25Ho and NiAl-28Cr -6Mo-2S，together with NiAl were manufactured by conventional casting. Directional solidification process was used to prepare NiAl-28Cr-5.94Mo-0.05Ho-0.01Ho directional eutectic alloy. With the aids of X-ray diffractometer and scanning electron microscope(SEM), the microstructures of above alloys were observed. The aggregate of irregular spherical Al2O3 and TiC particles are distributed in the NiAl matrix, with the composite density reaches 99.1 %. NiAl alloy was composed of different size of grains. The three eutectic alloys contain grey-black NiAl and grayish white Cr(Mo) phase. In the eutectic alloy, large amounts of fine NiAl particles precipitated from the Cr(Mo) phase, at the same time large amounts of fine Cr(Mo) phase precipitated from the NiAl matrix. Ho and Hf were added to the eutectic alloy, which result in the fine white phases on the phase interface which contain Hf solution (or Ni2AlHf) and Ni2Al3Ho. After S was added to the eutectic alloy, a series of CrxSy [(x/y)=(2/3)-1] -type eutectic compounds including Cr2S3, Cr3S4, Cr5S6, Cr7S8 and CrS precipitated from the phase interface. As for NiAl-28Cr-5.94Mo-0.05Hf-0.01Ho, besides NiAl and Cr(Mo) eutectic phases, there is a little quantity of primary NiAl and Hf solid solution.
The ambient temperature friction and wear experiment results of NiAl, NiAl-Al2O3 -TiC and NiAl-28Cr-5.8Mo-0.2Hf shows that the dry friction and wear process of NiAl-based alloys is dominated by plastic deformation. The wear resistance of NiAl-based alloys was directly proportional to their hardness and fracture toughness. Among the above three alloys, NiAl-Al2O3-TiC composite showed the best friction and wear properties. The wear mass loss is one-fourth to three-fourth of that of NiAl-28Cr-5.8Mo-0.2Hf eutectic alloy or one-twentieth to one-tenth of that of NiAl alloy at the same working conditions, which was attributed to the efficient transferring stress and supporting effect of reinforcing ceramic particles. Wear rate measurements showed an inverse relation between friction coefficient and hardness. The wear mechanism of NiAl-based alloys was related to counterface material. The main wear mechanism is abrasive wear and the friction coefficient and wear rate are relatively lower if the counterpart material is quenched steel ring, while both abrasive wear and adhesive wear play roles in the wear process. If annealed steel ring is used as the counterpart material, the friction coefficient and wear rate are relatively higher.
The wear behaviors of NiAl-Al2O3-TiC composite, NiAl-28Cr-5.6Mo-0.15Hf -0.25Ho and NiAl-28Cr-6Mo-2S eutectic alloys against SiC ceramic were investigated. The results show that NiAl-based alloys exhibit excellent high temperature self-lubricating properties in the temperature range of 700-900℃, which is attributed to the formation of glass-ceramic lubricant film with the thickness of 1-3 μm. During wear testing the lubricant film was partially transferred to the SiC, which avoided the direct tribo-contact between the alloy and SiC ceramic and led to another friction state which friction happened between glass-ceramic and glass-ceramic. The formation of the lubricant film depends on friction surface temperature. When the temperature is high enough to make nickel oxide severely, oxides of element Ni, Al and Cr form on the surface of NiAl-based alloys and from wear debris which divorced from the wear surface. Through friction chemical reaction the oxides form glass-ceramic lubricant film which then overlays the friction surface. As the temperature increases, the strength of NiAl-based alloys and lubricant film decreases, and SiC particles were pressed into the lubricant film, which led to spalling exacerbation and wear rate increasing. When the temperature is higher than 900℃, the friction coefficient steps up.
At temperature below 700℃, NiAl-Al2O3-TiC composite and NiAl-28Cr-5.6Mo -0.15Hf-0.25Ho eutectic alloy undergo severe abrasive wear caused by ploughing effect of SiC particles and oxides of Al and Cr, making friction coefficient and wear rate relatively high. CrxSy-type eutectic compound in the NiAl-28Cr-6Mo-2S eutectic alloy may soften or melt. Between 200 and 400℃, a relatively intact lubricant film composed of nano CrxSy-type eutectic compound overlays the friction surface and it is a self-lubricant film because it can be transferred to the SiC counterpart. At temperature less than 110℃, CrxSy-type eutectic compound cannot form intact film. Between 500℃ and 600℃, CrxSy-type eutectic compound softens and the ploughing effects of SiC particles and oxides make wear performance worse.
In the range of 700℃-900℃, NiAl-28Cr-5.6Mo-0.15Hf-0.25Ho eutectic alloy has a low level of oxidation, which illustrates that it owns a good oxidation resistance. At 700℃ and 800℃, a loose layer of oxidation film which was made up of Cr2O3 and Ni2Mo3O8 formed on the surface of the eutectic alloy. At 900℃, such loose oxidation film was made up of Cr2O3 and θ-Al2O3, and relatively serious internal oxidation occurred along the phase interface of NiAl/Cr(Mo). At the early stage of oxidation, obvious weight loss occurred because of the volatilization of MoO3 at 700℃. But along with the process of oxidation, the concentration of Mo decreased, and the oxidation mass of Cr increased gradually, so that the oxidation mass of eutectic alloy became positive gradually.
The isothermal oxidation behavior of NiAl-28Cr-5.94Mo-0.05Hf-0.01Ho directional eutectic alloy in the temperature range of 900℃-1150℃ was investigated. The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance. During the oxidation, a phase transformation from θ-Al2O3 to α-Al2O3 existed on the surface of oxidation film. While the temperature is low or at the early stage, metastable θ-Al2O3 grows rapidly. α-Al2O3 forms at higher temperature or at the late stage and grows slower. The oxidation morphology changes from needle-like to equiaxed. Abnormal oxidation mass gain happened when the alloy oxided at 1000℃, which was a result of the phase transformation. Trace rare earth element Ho distributed uniformly at the alloy, together with relatively high level of Cr of NiAl and Al of Cr(Mo) phase are beneficial to the formation of continuous and compact Al2O3 scale. Furthermore, the directional solidification process reduces the amount of grain boundary and relieves the grain boundary corrosion. |
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