| Compared with conventional TiAl alloys,high Nb-containing TiAl(Nb-TiAl)alloys have better strength,good oxidation resistance and creep properties at high temperatures.Therefore,they are considered to be potential high-temperature structural materials for aeronautics and astronautics applications.According to the recent reports,an ordering transformation of high temperature β phase results in formation of the βo phase,which transforms to ωo phase within in the service temperature range of the high Nb-TiAl alloys.ωo phase has both ordered hexagonal structure and high Nb content,even has a high hardness and few slip systems.Thus,it can be assumed that the morphology,volume fraction and distribution of ωo precipitations may significantly affect the mechanical properties of these alloys.Many studies focused on the phase transformation and morphology of ωo phase in high Nb-TiAl alloys,and it is indicated that the ωo phase is an equilibrium phase at 700-900℃.However,there are few studies focused on the ωo single-phase regions,the effects of annealing temperature and time on the βo decomposition,as well as precipitation and growth of the ωo and D88-ω)phase.Meanwhile,study on the effects of the ωo phase on the mechanic performances of the high Nb-TiAl alloys is becoming a research hotspot.In this study,the phase transformation of the ωo single-phase regions,decomposition of the βo phase as well as the texture evolution and precipitation of ωo phase during the creep were investigated.The main conclusions and innovations are listed as follows:(1)The phase transformation of the ωo single-phase region was studied.The results show that ωophase can nucleate at the interface of the martensite α2 lath in the nominal composition of ωo phase alloy(Ti-37.5Al-12.5Nb).Stacking faults were observed in α2 lath and the habit plane of the lath was determined as {334} β.The ωo phase precipitated at the α2 lath can cause solute-depletion surrounding theα2 laths,which inhibits the nucleation and growth of new ωo precipitates in the un-precipitated regions.In addition,y phase can precipitated from the interface of the ωo variants as well as the stacking faults within the αo laths during the annealing at 900℃.However,in Ti-34Al-13Nb alloy,the decrease of Al content inhibits the martensitic transformation.The transformion of βo to ωo phase is processed completely during the annealing at 900℃.With increasing the annealing time,various α2 variants can precipitate at the boundary of the ωo phase.(2)The precipitation and growth behaviors of the ωo phase during the annealing were studied.Because of the high driving force of nucleation,homogeneous ωo precipitations formed soon within the βo region at a relative low annealing temperature.With increasing the annealing time,the size of the ωo phases increases.However,ωo phases tend to nucleate at the boundary of the βo phase since the boundary has a low energy barrier of nucleation at high temperature(850℃).With increasing the annealing time,ωo phases filled the βo region fully.(3)The decomposition mechanism of βo phase during the annealing was studied.βo phase can transforme to ωo,α2 and γ phases in the temperature range of 750-850℃.The α2 and γ precipitations can nucleate at the boundary of the ωo phase,and the size of the γ phase increases with increasing the annealing time.In addition,incoherent boundary between the ωo phase and the coarsened γprecipitation was observed.Furthermore,the transformation of the βo(ωo)→γ at the boundary results in boundary migration from γ to βo(ωo).Thus,the ωo pinning can be observed if the ωo phase can not dissolve into the newly γ precipitation during the boundary migration,even entire ωo particle is surrounded by newly y precipitation with the increase of annealing temperature.(4)The precipitation as well as the elements distribution of the D88-ω phase were studied.D8g-ω phase can nucleate at the βo/γ boundary during the annealing at 850℃.The formation process of D88-ω is attributed to net vacancies that flow at the βo/γ boundaries as well as the local enrichment of Nb and W,while Nb and W are the stabilizer elements of the D88-ω phase verified by EDS and HADDF.(5)The microstructure of the ωo phase during the creep was investigated.The ωo strips have larger sizes but more irregular shapes,and the interval βo phase between the ωo strips seems to be larger with the increase of the temperature.Theωo phase depletes W during growth.Thus,the diffusion rate of W can be increased at high temperature and is more concentrated at the βo/ωo boundary,resulting in the formation of the stabilized βo phase with a large size at high temperature.However,high stress can increase the tendency of precipitation and growth of the ωo phase,and the ωo phase eventually filled the βo region fully.(6)The precipitation mechanism of the single ωo variant during the creep was studied.The dislocations can easily cross the interface of βo and ωo1 because the interplanar spacing and atomic arrangement are in good agreement.The interface of βo/ωo1 can be treated as a ’transparent’ interface with a minor barrier resistance.However,the atoms would be shuffled when the dislocations cross the interface of βo and ωo2/ωo3/ω4 variants,which can result in local stress concentrations due to the high lattice resistance.With increasing the strain,the lattices of the ωo2/ωo3/ωo4 variants can be distorted.Thus,these variants can not precipitate.(7)The texture evolutions during the creep were studied.Compared with the initial texture,the strength of the<010>component increases with the increase of creep temperature because high temperature is prone to induce dynamic recrystallization.However,deformation textures,such as the<001>,<111>and<031>components,increase with the increase of stress.(8)The effects of the ωo phase on the creep property were studied.The βo(ωo)precipitation at the triple junction of y grains can retard the growth of recrystallized y grains.Meanwhile,the ωo precipitation can resist and deflect the crack during crack propagation.These two effects can contribute to the improvement of the creep resistance of high Nb-TiAl alloys. |
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