Frabication Of Lamellar Microstructure Of TiAl Alloys By Electromagnetic Confinement And Its Properties

TiAl-based alloys have been considered a new generation high-temperature structural material for aviation,aerospace and automotive industries due to their excellent properties,such as low density,high specific strength and modulus,excellent oxidation resistance and high-temperature creep strength and so on.But the relevant investigation and application are significantly limited due to the low ductility at room temperature and poor hot deformation ability.Among various microstructures of TiAl alloys,the fully lamellar microstructure grown from a seed during directional solidification has an excellent balance between the tensile strength and elongation,thus offering a great potential for application in industry.Unfortunately,it is difficult to obtain a TiAl sample without contamination by Bridgman type furnace because there exists inevitable chemical reaction between TiAl melt and various ceramic crucibles.Although optical float zone furnace has been employed to prepare non-polluting fully lamellar microstructure of TiAl alloys,it is only useful for laboratory research to produce samples of small section size.Consequently,more techniques are expected to obtain non-contamination and large-scale samples of TiAl alloy with excellent mechanical properties.The aim of this paper is to combine the “electromagnetic confinement and directional solidification”,which has been developed by our research group,and the “seeding method of TiAl alloys”,and then to form a “seeding method of TiAl alloys by electromagnetic confinement and directional solidification”.Subsequently,the large-scale samples of Ti Al alloys without contamination,in which the lamellae were aligned parallel to the growth direction,were obtained by controlling the process parameters and solidification conditions.The mechanisms during general seeding and quasi-seeding process were systemically discussed,and the solute redistributions and their effects on seeding process were analyzed by two models,respectively.Besides,the mechanical properties of the lamellar microstructure were measured and the effects of lamellar orientation on mechanical properties were investigated.The main conclusions of the present studies can be drawn as follows:During the electromagnetic confinement and directional solidification process,it was important to heat and melt the sample and then confine the molten zone simutaniously by adjusting the parameters,such as material,sample diameter,confinement coil shape,insert depth of screen,etc.Besides,parameters like the shape and stability of molten zone and shape of solid/liquid interface played important roles in the growth of lamellar microstructure.By controlling these experiment parameters,samples of TiAl alloy with diameters of 12.5 and 16-20 were obtained.Based on the general seeding process,a general seed ingot of Ti-43Al-3Si alloy,in which the lamellar microstructure was parallel to the longitudinal direction of the ingot and had excellent thermal stability upon heating and cooling,was obtained by casting.When the growth velocity was low,the lamellar microstructure of Ti-47 Al alloy was aligned parallel to the growth direction by the seed.The tensile testing results showed that the lamellar microstructure had excellent tensile strength(693 MPa)and elongation(10.0%)simultaneously.Besides,the fracture toughness of 34.7 MPa·m1/2 was also measured.All these mechanical properties were higher than the reported properties of similar microstructures,suggesting that the electromagnetic confinement and directional solidification had obvious advantage in the preparation of non-contamination and large-scale samples of TiAl alloy with excellent mechanical properties.Note that,with the increase of growth velocity,stray grains nucleated and grew during the directional solidification,which usually lead to the failure of seeding process.The novel quasi-seeding mechanism was proposed and developed for the first time and then was used to align the lamellar microstructure of TiAl alloys.By controlling experiment conditions during casting,columnar grains within the quasi-seed ingot of Ti-48Al-2Nb-2Cr and Ti-48Al-6Nb-1Cr alloy were solidified as peritectic ? phase rather than the primary ? phase and thus the quasi-seed ingot with appropriate lamellae orientation was obtained.When the heating rate was sufficiently high,the nucleation and growth of ? was significantly restricted and thus the lamellar microstructure could maintain unchanged during heat treatment.Based on the results,the quasi-seeds of Ti-48Al-2Nb-2Cr and Ti-48Al-6Nb-1Cr alloy,in which the lamellar microstructure could maintain unchanged by a rapid heating procedure,were successfully used to align the lamellar microstructure of TiAl alloys with the same alloy composition,suggesting that the lamellar microstructure could be aligned well by a quasi-seed even though the alloy composition did not satisfy the requirements of seed material for general seeding method.However,similar to the general seeding process,stray grains nucleated and grew during the quasi-seeding process with the increase of growth velocity and thus the seeding process usually failed.Two different models were developed to discuss the solute redistributions during general seeding and quasi-seeding process,respectively.Based on the results,the solute distributions during the two seeding processes and their effects on the constitutional undercooling of primary ? phase were investigated,which is benficial to predict the nucleation and growth of stray grains during the seeding processes.Besides,the transformation of solid/liquid interface from planar to dendritic and then to cellular was discussed by analyzing the effects of constitutional undercooling and Si elements on the interface stability.The lamellar orientation had important effects on the mechanical properties of fully lamellar microstructures of TiAl alloys.When the lamellar orientation was parallel to the loading direction,the propagation of crack was significantly resisted by ?2/? lamellar boundary and ?2 lamellae,and thus numerous crack propagation mechanisms and toughness mechanisms occurred in the specimens,such as cract tip blunting,nucleation of microcrack,second crack,crack bridging and so on.All these mechanism could significantly improve the mechanical properties of TiAl samples.However,when the lamellar orientation was inclined or perpendicular to the loading direction,only a few crack propagation mechanisms were observed in the specimen,leading to the poor mechanical properties of the lamellar microstructures.