| NiAl alloy has some advantages of high melting point,low density,high thermal conductivity and excellent oxidation resistance at high temperature.But poor plasticity at room temperature and low strength at high temperature hinder its application in high temperature parts of aeroengine.In order to make it reach the requirements of industrial application as soon as possible,this paper combines the advantages of alloying and preparing composite materials,using vacuum non-consumable arc melting to prepare NiAl-Cr(Mo)-Ta alloys with different compositions.The microstructure morphology,phase composition,room temperature and high temperature compression properties and high temperature oxidation properties of alloys were studied,and the microstructure evolution law,strengthening mechanism and high temperature oxidation behavior of the alloy were analyzed.The main research results are as follows:The microstructure of NiAl-28Cr-6Mo alloy is composed of eutectic cells formed by NiAl and Cr(Mo)lamellae,and Cr2Ta phase will be formed after the addition of Ta element.The microstructure of NiAl-28Cr-6Mo-4Ta alloy is composed of primary NiAl dendrites and threephase eutectic of NiAl+Cr(Mo)+Cr2Ta.With the increasing Ta con,tent,the volume fraction of Cr2Ta phase increases,and the primary phase of the alloy microstructure changes from NiAl dendrite to Cr2Ta dendrite.After increasing the content of Cr,the alloy changes into hypereutectic composition,but the phase composition of NiAl-32Cr-6Mo-xTa alloy does not change.The primary dendrites of Cr(Mo)appear in NiAl-32Cr-6Mo alloy,and the primary dendrites of Cr2Ta appear in NiAl-32Cr-6Mo-8Ta alloy.Meanwhile,NiAl+Cr(Mo)two-phase eutectic and NiAl+Cr(Mo)+Cr2Ta three-phase eutectic exist in NiAl-32Cr-6Mo-xTa(x=4,6,8)alloys.Moreover,the addition of Ta disturbs the formation of lamellar microstructure in NiAl-Cr(Mo)alloy,and the lamellae between eutectic cells become coarse and irregular in shape.After the addition of Ta element,the hardness and room-temperature compressive strength of the alloy increase first and then decrease.Among the all alloys,the room-temperature compressive strength and compressive strain of NiAl-28Cr-6Mo-6Ta alloy are the highest,which are 2380 MPa and 16.3%,respectively.With the increasing Cr content,the strength of NiAl32Cr-6Mo-based alloy decrease to different degrees due to excessive primary phase and degraded eutectic microstructure.The room temperature compression fracture of all alloys presents typical brittle cleavage fracture,tearing edges,cleavage planes and cleavage steps are observed in the fracture.All alloys exhibit plastic fracture characteristics at high temperature.The compression strength of NiAl-28Cr-6Mo-4Ta alloy reaches 535 MPa at 1000℃,which is nearly 10 times of the binary NiAl alloy,and 2 times of NiAl-28Cr-6Mo alloy.Besides,the cracks do not propagate along the eutectic cell boundary and the interface between the primary phase and the eutectic microstructure during high temperature compression.The oxidation weight gain of NiAl-28Cr-6Mo-xTa alloy increases with the increase of time and temperature,and the addition of Ta element reduces the oxidation weight gain of the alloy at each temperature,indicating that Ta has a positive effect on the high temperature oxidation resistance of the alloy.The growth of NiO is inhibited by the formation of continuous Al2O3 and Cr2O3 oxide films on the alloy surface.At the same time,the resulting oxides react to form small amounts of spinel oxides,namely CrTaO4,NiCr2O4,NiAl2O4,etc.,in the intermediate layer.The diffusion of O element is inhibited by the internal Al2O3 and CrTaO4 layers,and the growth of oxide film slows down and the oxidation rate decreases,the high temperature oxidation resistance of the alloy is improved.Applied in aerospace field,its high temperature service time is extended. |
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