Effect Of Nb And Ti On The Microstructure And Mechanical Properties Of NiAl-Cr(Mo) Alloy

NiAl alloy has the advantages of low density,high thermal conductivity,high melting point and excellent oxidation resistance.However,the room temperature brittleness and low high temperature strength seriously affect its industrial application.Previous studies have shown that alloying and preparing composite materials are effective methods to improve the mechanical properties of alloys,and hypereutectic alloys can increase the proportion of Cr(Mo)reinforcing phase in the alloy,thereby significantly improving the overall properties of the alloy.On this basis,different contents of Nb and Ti are added to the NiAl-Cr(Mo)hypereutectic alloy,observes the as-cast and directional solidification structure of the alloy,and then conducts heat treatment and mechanical performance tests on the alloy.The main research results of the alloy composition,solidification process parameters and heat treatment on the microstructure and mechanical properties of the alloy and the fracture behavior of the alloy are as follows:The microstructure of the as-cast NiAl-32Cr-6Mo hypereutectic alloy consists of primary Cr(Mo)dendrites and NiAl+Cr(Mo)eutectic.After adding Nb and Ti elements,the formation of eutectic cell and eutectic lamellar in the as-cast alloy is disturbed,and bulk Cr2Nb phase and Ni2AlTi phase are distributed at the cell boundary.As the amount of addition increases,the number.of precipitation phases will gradually increase.For the directionally solidified NiAl-Cr(Mo)-Nb alloy,when the pulling rate is 6 ?m/s,through the competitive growth of the primary phase and the eutectic,the primary phase is finally eliminated during steady-state growth,and the fully eutectic microstructure is obtained crystal solidification structure.Because the alloy composition deviates too much from the eutectic point,the fully eutectic microstructure cannot be obtained in the directionally solidified NiAl-32Cr-6Mo-3Nb-3Ti alloy,and the primary phase content increases with the increase of the pulling rate.The phase composition of the alloy remained unchanged after heat treatment.The eutectic lamellar of the as-cast alloys were significantly coarsened and spheroidized after heat treatment,but the microstructure of the directional solidified alloy was relatively stable after heat treatment.In addition,the distribution of precipitated phases in the alloy becomes more uniform.After adding Nb and Ti,the Cr2Nb phase and Ni2AlTi precipitation phase will significantly increase the hardness,room temperature compressive strength and high temperature compressive strength of the alloy.However,the increase of Cr(Mo)primary phase and the deterioration of the eutectic structure are detrimental to the performance.The strength and hardness of the solid alloy increase first and then decrease.However,the regularly arranged full eutectic structure in the directionally solidified alloy significantly improves the strength and hardness of the alloy.Due to the redistribution of the precipitated phase in the alloy after the heat treatment,the room temperature fracture strength and fracture strain of the alloy are improved to varying degrees.Cleavage surfaces and tearing edges can be observed in the compression fracture of the alloy at room temperature,and all alloys exhibit typical brittle fracture.After heat treatment,the coarsening of NiAl particles in the Cr(Mo)phase and the increase of dislocations reduce the hardness of the alloy to a certain extent.Dimple-like fractures can be seen in the fracture at room temperature.After high-temperature compression,almost all of the co-chip layer appeared wave-shaped deformation bands,and the layer mismatch increased significantly.In addition,a small piece of Cr(Mo)lamella fell off,and micro cracks occurred at the cell boundary.Similar to the changes in the properties of the alloy at room temperature,the addition of an appropriate amount of Nb can significantly improve the high temperature compressive strength of the as-cast alloy.Excessive addition of Nb will increase dendrites and affect the formation of eutectic cells and co-crystal layers,thereby reducing the high-temperature strength of the alloy.Compared with the as-cast alloy,the high-temperature strength of the directionally solidified alloy with a regularly and fully eutectic microstructure is significantly improved.