| At present,TiAl-based alloys have been initially applied to the manufacture of aero-engines and automotive heat-resistant parts as an advanced high-temperature structural material,but their disadvantages such as room-temperature?RT?brittleness,relatively low strength and poor formability have been limiting their large-scale industrial applications.Therefore,it's of great importance to further strengthen and toughen the TiAl-based alloys.However,the existing strengthening and toughening methods,mainly including hot working and the preparation of PST single crystal,have problems such as high cost and low technical maturity,and it is difficult to meet the needs of actual production.It's found that nano-lamellar structures can be obtained after proper heat treatment to rapid-cooling non-equilibrium solidified TiAl-based alloy,which is beneficial to the strengthening and toughening of TiAl-based alloy,but there is no further research about this result available yet.Meanwhile,in view of the low cost and easy implementation of casting and heat treatment processes,this paper proposed a method for strengthening and toughening of TiAl-based alloys by combining non-equilibrium solidification with heat treatment,and the microstructural evolution rule,as well as strengthening-toughening mechanism by heat-treatment nanocrystallization involved in this method were researched fundamentally.In this paper,two representative conditions?rapid solidification/conventional non-equilibrium solidification+heat treatments?of the method for strengthening and toughening of TiAl-based alloy by non-equilibrium solidification combined with heat treatment were studied.Firstly,the melt-quenching of peritectically solidified Ti-48Al-2Cr-2Nb alloy and continuous casting of?solidified Ti-45.5Al-4Cr-2.5Nb alloy were carried out by electromagnetic cold crucible,respectively,and then the prepared samples were subjected to heat treatment experiments.The solidification microstructure,evolution of solidification microstructure during heat treatment and mechanical properties of TiAl-based alloys prepared by melt quenching and cold crucible continuous casting,respectively,as well as the formation and strengthening-toughening mechanism of deformation nanotwin were mainly studied,then the strengthening and toughening mechanism of the method combining non-equilibrium solidification with heat treatment was further revealed.A very fine and uniform rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy was prepared by melt-quenching in Ga-In liquid.The cellular microstructure is mainly composed of?2 phase,and its growth length and cellular spacing were 358460?m and0.683.6?m,respectively.The formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga-In liquid on the outermost layer of the alloy droplet,and mainly determined by the heat transfer process.The cooling rate of rapid cellular crystals in the growth process ranged from 2.61×106 to 1.26×105 K/s,and the corresponding growth rate ranged from 163 to 8 mm/s.The microstructure of Ti-45.5Al-4Cr-2.5Nb master alloy mainly included the B2/?lamellar structure and the coupled microstructure composed of massive?and B2 phases,the equilibrium solidification path of the alloy is:L?L+?????+?????+??B2+?+??B2+?2+?.The macroscopic grain size of continuous casting Ti-45.5Al-4Cr-2.5Nb alloys decreased at first and then increased with the increase of the pulling rate,reached a minimum when the pulling rate was 1.5 mm/min.The continuous casting alloys were mainly composed of?,B2 and?2 phases,and their microstructures mainly consisted of lamellar structure matrix and elongated coupled microstructure distributed therein.As the draw rate increased,the width of the lamellae region decreased at first and then increased,and reached a minimum at the pull rate of 1.5 mm/min.With the increase of the pulling rate,the content of B2 phase in the continuous casting alloys decreased at first and then increased,and reached the minimum value when the pulling rate was 1.2 mm/min.The content of B2 phase in the continuous casting alloys mainly depends on the Cr content of B2 phase,and decreases with the increase of the Cr content.The rapid cellular microstructures are metastable in high-energy state,their microstructural evolution law during heating at 6941180? was:rapid cellular microstructure transformed to another cellular microstructure with?2/?nano-lamellae substructures at 694?;when heated at 900?,almost all of the rapid cellular microstructure transformed into?phase,and began to transform into equiaxed microstructure by recrystallization;when temperature rised to 976?,there were some?2 phases beginning to precipitate in or between the equiaxed?phases;when holded at1000? for 30 min,the rapid cellular microstructure was completely decomposed into near?equiaxed microstructures.The temperature interval of the??+??two-phase region of Ti-45.5Al-4Cr-2.5Nb alloy is 11971275?,its continuous casting non-equilibrium solidified alloy?with the pulling rate of 1.5 mm/min?still mainly consisted of?,B2 and?2 phases after holded at 1250? for different hours,but the coupled microstructures were significantly reduced,and almost all of the B2/?lamellae transformed into?2/?lamellar structures;the heat-treated alloys consisted of?original grains?and recrystallized grains,with the increase of holding time,the coupled microstructures in the?original grains?decreased at first and then increased,reached the minimum when the holding time was 2h.There were large numbers of high-density dislocations and other substructures generated in the continuous casting non-equilibrium solidified alloys due to relatively rapid cooling rate and large phase transformation stress.After holded at at 1250? for 2h,the high-density dislocations decomposed into lots of Shockley partial dislocations and stacking faults;the lamellar spacing of the heat-treated alloys decreased first and then increased with the increase of the holding time,and reached the minimum value?77.38 nm?in average at the holding time of 2 h.The nanohardness of rapid cellular microstructure was 8.457 GPa in average,with a significant improvement of about 1560%than the general microstructures,and the dependence of nanohardness on the cellular spacing meets the Hall-Petch relationship.The nanohardness of the rapid cellular microstructure after heat treatments firstly decreased and then increased with the increase of heat treatment temperature,and the maximum nanohardness?8.697 GPa in average?was achieved after heat treatment at700?.The cellular microstructure with nano-lamellae substructures could further improve the strength and toughness of rapid cellular microstructure,thus could realize the strengthening and toughening of TiAl-based alloy effectively.The RT tensile strength of continuous casting Ti-45.5Al-4Cr-2.5Nb alloys increased first and then decreased with the pulling rate increased,reached the maximum?380.34 MPa?when the pulling rate was 1.5 mm/min;the RT tensile strength of the heat-treated alloys increased first and then decreased,then increased again with the increase of holding time,reached the maximum?450.76 MPa?when the holding time was 2h;the RT tensile strength and elongation of the master alloy could be improved by 69.28%and 230.77%,respectively,by the method combining non-equilibrium solidification with heat treatment.Through systematic studies about the tensile fracture behavior and deformation behavior of the Ti-45.5Al-4Cr-2.5Nb master alloy,continuous casting non-equilibrium solidified alloy and heat-treated alloy,as well as the thermodynamic analysis of the nucleation process of deformed twins,the strengthening and toughening mechanism of the method combining continuous casting non-equilibrium solidification with heat treatment was revealed:uniform and refined macro/micro structures could greatly reduce the stress concentration of the TiAl-based alloy during deformation,and made the alloy toughened primarily;the large numbers of Shockley partial dislocations and stacking faults in the heat-treated alloy could act as effective heterogeneous nucleation sites for deformed twins,which facilitated high-density nano-twins generated in?phases during deformation,consequently,the significant strengthening and toughening of TiAl-based alloy was achieved finally. |
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