| As a new type of lightweight high-temperature structural material,TiAl alloy,has the advantages of high specific strength,specific modulus,good creep resistance and corrosion resistance,and has a broad application prospect in the field of aerospace and automobile industry.However,poor ductility at room temperature and insufficient oxidation resistance and strength at high temperature have been restricting the further engineering application of tial alloys.Therefore,how to improve the ductility and high temperature properties of tial alloys at room temperature has become a hot research topic of scholars all over the world.In this paper,the microstructure and properties of Ti-48Al-2Cr-2Nb alloy were improved by adding nano-Y2O3 and heat treatment during the melting process.The effect of nano-Y2O3 on oxidation resistance and strength of tial alloy at high temperature and its mechanism of action were also discussed.The effect of nano-Y2O3 addition on microstructure evolution during heat treatment was systematically studied.The high temperature oxidation resistance test showed that Ti4822-42 had better oxidation resistance than Ti4822 alloy.The weight gain of oxidation at 800? 850?and900? for 100 h decreased by 18.31%?19.31 and 30.72% respectively.The mechanism is that the addition of Y2O3 can refine the size of oxide formed during oxidation of tial alloy and inhibit the deterioration of oxide morphology.That is,from nearly hexagonal to porous columnar and square,it can enhance the bonding strength of oxidation layer with matrix,promote the formation of Al2O3,delay the process of oxidation reaction of alloy,and improve the structure of oxide layer.The oxidation resistance of as-cast Ti4822-73 alloy was improved.The average tensile strength at room temperature is 593 MPa at 750 ?,708 MPa at 800 ? and 616 MPa at 800 ?.The average tensile strength of as-cast Ti4822-73 was observed by TEM before and after deformation.It is found that the addition of nano-Y2O3 leads to the formation of a large number of ? twins in the alloy,and a large number of dislocation plug deposits are found between the two twin boundaries,and serious dislocation plug deposits are also found near the Y2O3 particles.This indicates that the strengthening mechanism of nanocrystalline Y2O3 is that a large number of twinboundaries hinder the movement of dislocation in the alloy,and it also hinders the movement of dislocation.The results of heat treatment showed that the microstructure of Ti4822-42 alloy containing nano-Y2O3 was obtained at 1100-1250? for 36 h.The smaller the ?-crystal size was,but the more serious the residual thick lamellar was,the higher the temperature was,the more the near ? transition was.But some ?-crystals coarsened.The near ? transition is mainly carried out by continuous coarsening reaction,which is due to the segregation inside the alloy is favorable to the continuous coarsening reaction.During heat treatment,the volume fraction of ? phase increases and the number of small angle grain boundaries decreases obviously.The addition of nanometer yttrium oxide promoted the formation of ?-crystal,increased the temperature of phase transition and decreased the rate of microstructure transformation.In high temperature and long time heat preservation,the rich Y-phase has the tendency of spheroidization and agglomeration,which is formed gradually by dispersed block,sharp stripe,bony and irregular aggregation,which is composed of block and strip.The spherical particles tend to distribute at the ? grain boundaries,while the bulk Y-rich aggregates are mostly distributed in the grains.The three-step heat treatment and cyclic heat treatment designed in this paper can first obtain the near ? structure with an average grain size of 15?m or 20?m,and the grain size distribution of the latter is uniform,and the whole lamellar microstructure can be obtained by further heat treatment. |
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