Non-equilibrium Solidification Characteristics And Microstructural Evolution Of Peritectic TiAl Alloys

TiAl-based alloys are one of the important peritectic alloys.Solidification microstructure has high sensitivity in Al content,especially for narrow Al content of peritectic TiAl-based alloys.For the casting TiAl-based alloys,various Al content has an important effect on solidification microstructure of ingots and castings.Casting belongs to the category of the near-equilibrium solidification in general.Because of the fast transition process from liquid phase to solid phase,the edge of ingots or thin wall castings can deviate from the near-equilibrium solidification and is closer to the non-equilibrium solidification.At present,many studies focused on the investigation of the non-equilibrium solidification behavior of binary,ternary and few multiple TiAl-based alloys.However,phase selection and microstructural evolution for non-equilibrium and peritectic TiAl-based alloys with the high Al content are still unclear and limited.On this basis,this thesis was carried out to systematically research the transition point of primary phase,phase composition and microstructure evolution for Ti-xAl-2Cr-2Nb?x=46,47,48,49,50,51,52??at.%?alloys in near-equilibrium solidification conditions.Meanwhile,this work focused on the phase selection and microstructure evolution of commercial Ti-48Al-2Cr-2Nb?at.%?alloy and Ti-50Al-2Cr-2Nb?at.%?alloy near the transition point of the primary phase solidification Meanwhile,the effects of undercooling and cooling rate on phase selection and microstructural evolution were investigated.Moreover,the formation of substructure,widmannst?tten laths,feathery and massive microstructures and factors were also investigated during undercooled solidification.In addition,the precipitation and growth of metastable phase were discussed in the coupling effect of undercooled and rapidly quenched solidification.The major conclusions are as follows:Microstructural characteristics of Ti-xAl-2Cr-2Nb?x=46,47,48,49,50,51,52??at.%?alloys were investigated under near-equilibrium and non-equilibrium solidification conditions.First,under near-equilibrium solidification conditions,when Al content was lower than 50 at.%,the primary phase was ? phase and the microstructure of the alloys was composed of ?₂ phase,? phase and small amounts of B2 phase.When Al content was higher than 50 at.%,the primary phase was ? phase and the microstructure of the alloys was composed of ?₂ phase and ? phase.The primary solidification phase changed from ? phase to ? phase when Al content was higher than 50 at.%.With increasing Al content,the lattice distortion?c/a?of ? phase increased and the lattice distortion?c/a?of ?₂ phase tended to decline.Meanwhile,the primary dendrite arm spacing changed with the crystal temperature range of primary phase.When Al content was 48 at.%,primary ? phase was prior to nucleate,but cannot grow up sufficiently.Peritectic ? phase nucleated on primary ? phase.The solidification path of the alloy was the hyper-peritectic solidification and gradual solidification style.Microstructure refined obviously under nonequilibrium solidification conditions.The growth of the secondary dendrite arm and the transformation of ? ???₂ + ??were suppressed.However,the primary solidification phase changed from ? phase to ? phase when Al content was also higher than 50 at.%.The maximum undercoolings of 370 K and 290 K were achieved for hyper-peritectic Ti-48Al-2Cr-2Nb?at.%?alloy and Ti-50Al-2Cr-2Nb?at.%?alloy by means of electromagnetic levitation?EML?experiments,respectively.In the achieved undercooling range,for Ti-48Al-2Cr-2Nb?at.%?alloy,? phase is always prior to nucleate from the undercooled melts,indicating that the alloy has the high stability.In the low undercooling range of ?T<120 K,for Ti-50Al-2Cr-2Nb?at.%?alloy,? phase is always prior to nucleate from the undercooled melts.Once the undercooling exceeds a critical value,?T*=120 K,metastable ? phase nucleates directly from the undercooled melts,indicating solidification mode is easier to change near transition point of the primary phase solidification.In addition,the relationships between undercooling and nucleation of two phases in Ti-48Al-2Cr-2Nb?at.%?alloy were investigated systematically using the classical nucleation theory.By calculating the interfacial critical nucleation work and steady state nucleation rates of ? phase and ? phase,the competition nucleation of each phase is explained reasonably.The dendrite growth rate at various undercoolings was calculated and the microstructural evolution was anylized by BCT model.Grain refinement at low undercooling can be ascribed to the break-up of dendrites induced remelting.Grain refinement at high undercoolings can be attributed to the dendrite fragmentation,closely related to the internal stress during the undercooled solidification.Various substructures in the undercooled samples were observed,which were composed of the deformation twins,high dislocation density and small amounts of stacking faults.Microstrain tended to increase with increasing undercooling.The process of undercooled solidification in TiAl alloys is equivalent to that of plastic deformation.Mechanism of the releasing internal stress is twin shear deformation.Plastic deformation occurs in solidification microstructure,due to the undercooled solidification.?/? twins?type-P twin?and secondary twins?type-Q twin?form during shear transformation.Shear transformation even evolves into neighboring ? laths,inducing twin intersection.At moderate undercoolings,widmannst?tten laths??_w?were obtained by EML during undercooled solidification rather than heat treatments in most cases.At high undercoolings,a mixture of massive??_m?and feathery??_f?microstructures was observed in Ti-48Al-2Cr-2Nb?at.%?alloy subjected to the undercooled solidification.?_w,?_f and ?_m did not directly nucleate from the undercooled melt but formed during the solid-state transformations.Meanwhile,high dislocation density,stacking faults and undercooling in the solid state phase transformations can provide sufficiently high driving force for the nucleation of them.Post-solidification cooling rates,undercoolings,alloy composition and size of lamellar colony are closely related to the formation of ?_w,?_f and ?_m.Ti-48Al-2Cr-2Nb?at.%?alloy and Ti-50Al-2Cr-2Nb?at.%?alloy conical and rapid solidified samples have been prepared by electromagnetic levitation combined with copper mold casting,corresponding to the cooling rates from 2.9×10³ K/s to 2.6×104 K/s and from 3.6×10³ K/s to 2.6×104 K/s,respectively,as calculated by ANSYS software.The cellular/dendrite arm spacing??₁?corresponding to various cooling rates?T?decreases with increasing cooling rates.Linear fitting of experimental data for ?₁ and T is characterized by the relationship ?₁=282.1150.32?T|-?^-0.32 and ?₁=2.70×10³ 0.53?T|-?^-0.53,respectively.For Ti-48Al-2Cr-2Nb?at.%?alloy,with the cooling rate increasing to 2.3×104 K/s,peritectic transformation?L + ? ? ??and the transformation of ? ???₂ + ??can be restrained.With further increasing cooling rate to 2.6×104 K/s,a large amount of massive ? phase and only few lamellar structures??₂ + ??form,attributed to the strong chilling effect,thereby giving rise to the high undercooling and the high dislocation density during rapid solidification.For Ti-50Al-2Cr-2Nb?at.%?alloy,once the cooling rate exceeds a critical value,4.0×10³ K/s,metastable ? phase nucleates directly from the non-equilibrium melts and the nucleation of primary ? phase and peritectic reaction are suppressed.Experimental and kinetic analysis in the present work demonstrates that cooling rates and Al contents can cause phase selection between the primary ? phase and peritectic ? phase and be beneficial to obtain the required phase and microstructure.Direct nucleation and growth of peritectic ? phase are determined by both enrichment of solute and the nucleated undercooling at high cooling rate.The coupling of undercooled solidification and rapidly quenched solidification is in favour of the nucleation and growth of metastable ? phase.