Effect Of Rare Earth Element La On Microstructure And Mechanical Properties Of TiAl-based Alloys Manufactured By Powder Metallurgy

TiAI-based alloys had low density,high specific strength and specific elastic modulus,and could maintain sufficiently high strength and stiffness at high temperatures,while having good resistance to creep and oxidation.These advantages made it the most competitive material for aerospace engine and automotive heat-resistant structural parts.The TiAl-based alloy prepared by the powder metallurgy method not only had a uniform structure but also could effectively avoid the casting defects such as looseness,shrinkage pores,and component segregation.Rare-earth elements had long been considered as reinforcing agents for black and non-ferrous metals,and its addition not only refined the grains,but also purified the matrix and increased the mechanical strength of the material.In this paper,Ti48Al2Cr2Nb-x La?x=0,0.1,0.3,0.5,0.7,1.5??at.%?alloys with different La contents were prepared by the Ti48Al2Cr2Nb powder and rare earth La powder.The effect of La content on the microstructure and mechanical properties?hardness,compression,three-point bending,tensile,fracture toughness?of the alloy was analyzed.The results shown that the Ti48Al2Cr2Nb-x La alloy could be successfully sintered by hot isostatic pressing at1150?for 4 h.Its organization was equiaxed near gamma mainly composed of equiaxed?phase and a small amount of fine?₂ phase distributed in the grain boundary.The La-rich second phase mainly existed around the original alloy particles.After 1420?and a holding time of 2 h,the microstructure of the alloy was transformed into a full lamellar structure of?₂+?.The addition of elemental La could effectively control the grain size of the alloy,but after the La content reaches 0.3 at%,the effect of grain refinement was not obvious.When La was added in a small amount?less than 0.3 at%?,the La-rich second phase existed in finely dispersed form in the grain boundary of the alloy and in the interior of the crystal lamellar sheet.When the addition amount of La exceeded 0.5 at%,the La-rich second phase existed in a net-like and massive formed at the boundary of the cluster,and there were some holes around it.By XRD and EDS analysis,it was concluded that the La-rich second phase was mainly composed of LaAlO₃ and a small amount of La₂O₃.The doping of La would firstly seize the O in the alloy,not only purified the matrix,but also the formed LaAlO₃ and La₂O₃phases could play a role in grain refinement and dispersion strengthening,making the alloy compressive strength,three-point bending strength,tensile strength strength had been improved.When La was added at 0.3 at%,the lifting effect was most pronounced.However,the excessive addition of La would result in a decrease in performance due to the network distribution and the large La-rich second phase.Excessive addition of La resulted in decline of performance duo to the presence of a La-rich second phase that was distributed in a network and bulk.Although the addition of appropriate amount of La could increase the strength of the alloy,it also caused damage to some properties.For example,as the La content increased,the hardness and toughness of the alloy decreased.