Study On Mechanical Properties And High Temperature Oxidation Behavior Of NiAl-Cr-V Alloy

Intermetallic compound NiAl has the advantages of high melting point,low density and good oxidation resistance,but its room temperature brittleness and low high temperature strength limit its application as high temperature structural materials in aerospace and other industries.The results have shown that alloying and preparation of eutectic composites were effective methods for strengthening alloys.For example,the NiAl-Cr eutectic alloy with a large amount of Cr element has both higher room temperature toughness and higher temperature strength,while the NiAl-V eutectic alloy shows the highest room temperature fracture toughness.Therefore,this paper intends to combine the above two strengthening methods to add the V element into the NiAl-Cr eutectic alloy to further improve the comprehensive mechanical properties of the alloy.However,the addition of too much V would cause adverse effects on the high temperature oxidation resistance of NiAl alloy.Therefore,in this paper,different contents of V elements were added into NiAl-Cr alloy.The microstructure morphology,volume fraction of the strengthening phase,room temperature and high temperature compression properties,and high temperature oxidation behavior of NiAl-Cr-V alloys with different compositions were studied.Moreover,the microstructure evolution mechanism,strengthening mechanism,and high temperature oxidation mechanism of the alloy were also discussed.The main research results are as follows:(1)V atoms main solid solution in the Cr phase to form the Cr(V)phase.The solidification microstructure of the alloy changes from the fibrous in NiAl-Cr alloy to the lamellar in NiAl-CrV alloy.Fully eutectic microstructure with cellular morphology and black NiAl+gray-white Cr(V)lamellar is obtained in the NiAl-28Cr-6V alloy.The solidification microstructures of NiAl28Cr-10V,NiAl-28Cr-14V and NiAl-28Cr-18V hypereutectic alloys are composed of primary(?)lamellae.With the increase of V content,the volume fraction of the primary Cr(V)phase increases gradually,and the lamellae at the cell boundary become coarser and more irregular.The addition of V disrupts the formation of lamellar structure in NiAl-Cr-V alloy.(2)After adding the V element,the solution strengthening of V and the large volume fraction of the Cr(V)phase are contribute to the improvements of hardness,room temperature compressive strength and high temperature compressive strength of the alloy.However,the interface between Cr(V)primary phase and the eutectic microstructure is weak,and the degradation of the eutectic microstructure makes the lamellar thickened and irregular,which adversely affect the properties.Therefore,the hardness and room temperature compression strength of the alloy increase first and then decrease with the increasing V addition.Among the all alloys,NiAl-28Cr-10V alloy shows the best room temperature strength and room temperature plasticity.In addition,all NiAl-Cr-V alloys exhibit brittle pseudo-cleavage fracture characteristics at room temperature,and the tear edges,cleavage planes and cleavage steps are observed in the fracture surfaces All alloys exhibit plastic fracture characteristics at high temperature.With the increase of V addition,the high temperature compressive strength of the alloy increases significantly due to the solution strengthening of V and Cr(V)strengthening phase with a large volume fraction.Unlike the room temperature condition,the coarse microstructure at high temperature is more conducive to the increase of the compression strength of the alloy.Therefore,the higher the amount of V added,the higher the high temperature strength of the alloy.(3)The oxidation weight gain curves of all NiAl-Cr-V alloys at different temperatures follow the parabolic rule,and the phase composition of the surface oxide film after oxidation for 24h is basically the same.The addition of V deteriorates the high temperature oxidation resistance of the alloy,but the NiAl-28Cr-6V alloy still maintains excellent oxidation resistance at 800℃.At the initial stage of oxidation,the Al2O3 oxide layer is formed on the surface of the alloy,and a mixed oxide layer composed of Cr2O3 and NiO is formed after further oxidation.As the oxidation proceeds,the V element which is solid solution in the Cr phase diffuses to the surface of the alloy,and first reacts with Ni to form the Ni2V3 oxide layer,and then Ni2V3 reacts with O2 to form Ni3(VO4)2 oxide layer.The oxidation temperature,oxidation time and amount of V addition have significant effects on the growth morphology of Ni3(VO4)2 oxides.Ni3(VO4)2 first grows in a needle-like and granular form,then gradually spheroids into a spherical,pebble-like and finally becomes a pit-like.Among them,the granular Ni3(VO4)2 has the best antioxidant effect.