In-situ,study,on,γ,phase,transformation,behaviour,of,γ-TiAl,alloys,at,different,cooling,rates

Zhiping Li ,Lingshun Luo,＊ ,Ynqing Su,＊＊ ,Binin Wng ,Ling Wng ,Tong Liu ,Mengji Yo,Chen Liu,Jingjie Guo,Hengzhi Fu

aNational Key Laboratory for Precision Hot Processing of Metals,School of Materials Science & Engineering,Harbin Institute of Technology,Harbin,150001,China

bAnhui Key Laboratory of High-performance Non-ferrous Metal Materials,Anhui Polytechnic University,Wuhu,241000,China

cLaboratory for Space Environment and Physical Sciences,Harbin Institute of Technology,Harbin,150001,China

Keywords:TiAl based Alloys High-temperature laser scanning confocal microscopy Nucleation and growth Phase transformation

ABSTRACT γ-TiAl-based alloys are promising lightweight high-temperature structural materials,and the transformation from the α parent phase to γ lamellae during the cooling process has a great influence on the microstructures and mechanical properties of TiAl alloys.In this paper,an in-situ observation technique,high-temperature laser scanning confocal microscopy(HTLSCM),was utilized to investigate the continuous cooling transition(CCT)from the α phase in three compositions.The nucleation and growth behaviors of γ lamellae were studied at several moderate cooling rates.In addition,the processes of helium gas quenching for three alloys were investigated,and the massive transformation was observed in Ti–49Al.CCT diagraphs were concluded for three alloys.The Vickers hardness of TiAl alloys subjected to different cooling rates was tested,and it was found that the hardness of alloys was enhanced with increasing cooling rate due to the refinement of lamellar spacing.

γ-TiAl-based alloys are brittle materials at room temperature with strong anisotropy.Four main types of microstructures can be obtained through heat treatment,i.e.,full lamellar,full γ,near lamellar and duplex microstructures.A great number of studies have been carried out on the first because of its balanced performance between strength and ductility[1,2].A full lamellar microstructure is made up of numerous (α2+γ)lamellar colonies in which platelets of the γ phase and α2phase stack based on the Burgers orientation relationship [3].Researching lamellar colony formation behaviors can help to optimize the heat treatment process and further improve the mechanical properties of TiAl alloys.

α phase can transform into (α2+γ) lamellar colonies at moderate cooling rates [4,5].The formation process is the so-called eutectoid reaction:α→α2+γ.However,the existence of the eutectoid reaction in TiAl alloys is controversial because it has not been confirmed to occur as a discontinuous transformation.Even for a near-eutectoid composition Ti–40Al(at.%),(α2+γ)lamellae cannot grow into the parent phase on a moving interface of two product phases like pearlite in steel [6,7].Actually,they form by two types of formation sequences:α→α+γ→α2+γ or α→α2→α2+γ.The formation sequence of the two phases is determined by the Gibbs free energy curves of the α→γ and α→α2reactions,and the crossover is Ti–44Al(at.%).Thus,α2phase in Al-lean alloys(40–44 at.%)or γ phase in Al-rich alloys (44–49 at.%) is the primary phase [8,9].In recent decades,many studies on the CCT of (α2+γ) lamellar colony formation in TiAl alloys have been carried out[5,10–19].In the early stages,ex-situ methods were mainly used to study the CCT of TiAl alloys,such as cooling at different cooling rates and quenching to identify whether the transition occurs or finishes.This is an indirect way.Thus,many details in transition,for example,the process of nucleation and growth,have been ignored.Accordingly,many in-situ research techniques have been utilized in the CCT of TiAl alloys.Using differential scanning calorimetry(DSC) or differential thermal analysis (DTA),the starting and finishing temperatures of the transformation in TiAl can be obtained [20–25].Electrical resistivity measurements have also been adopted to help study the CCT of TiAl,and product phase formation can be identified by the change in electrical resistivity[4,12,26].Similarly,dilatometry has been used to investigate CCT in TiAl alloys [16,17].Moreover,high-energy X-ray diffraction has been utilized to distinguish the phase evolution of TiAl alloys during the heating and cooling process [27].The methodsmentioned above focus on macro-property changes to study phase transformations,but microstructure evolution during the process cannot be displayed.Thus,an in-situ observation technique is urgently needed.HTLSCM is a widely used and promising in-situ observation method with the advantage of a higher testing temperature compared with in-situ SEM and TEM.Currently,it has been used in many fields of solid phase transformation in TiAl alloys.The nucleation and growth of the α phase in the β/α transition were studied,and equiaxed growth at a low cooling rate as well as Widmannstatten growth at a high cooling rate was observed [28–31].Li found that γ lamellae grew in pairs during the formation process in Ti–48Al–2Cr–2Nb alloy [32–35].Yang has investigated grain boundary migration.The results show that β grain boundary migration occurs significantly at 1510°C in Ti–45Al-8.5Nb-(W,B,Y)alloy and α grain boundaries begin to merge into each other evidently at 1423°C in Ti–48Al–2Nb–2Cr alloy[36].Thus,HTLSCM can discover the remaining questions of lamellar structure formation in TiAl alloys.

This paper aims to in-situ investigate the formation process of(α2+γ)lamellar structures in TiAl alloys.Ti–40Al,Ti–45Al and Ti–49Al,which have different predicted formation mechanisms according to current research results,were employed.Specifically,the primary phase of lamellar structure formation is the α2phase in Ti–40Al,but it is the γ phase in Ti–45Al and Ti–49Al.Additionally,there is no so-called eutectoid transus in Ti–49Al.The effect of cooling rates was discussed,and CCT diagraphs were concluded.The nucleation rate and growth rate of γ lamellae were also investigated in-situ.Moreover,hardness tests were carried out to research the mechanical properties of the alloys cooled at different cooling rates.This work will deepen the comprehension of the(α2+γ)lamellar colony formation mechanism,and it is helpful to control microstructures in TiAl alloys to enhance mechanical properties.

TiAl ingots with nominal compositions of Ti–40Al,Ti–45Al and Ti–49Al (at.%) were produced by vacuum arc melting utilizing sponge titanium (purity 99.7%) and aluminum (purity 99.99%).The oxygen contents of the samples were evaluated by Harbin Welding Institute using oxygen-nitrogen-hydrogen mensuration equipment (EMGA-830),and they were not above 700 ppm.The actual compositions of the alloys were examined by inductively coupled plasma–optical emission spectrometry(Thermo iCAP7400),and the results are shown in Table 1.X-ray diffraction (PANalytical X"Pert) was employed to identify the phase constitutions in the alloys.The microstructure analysis of TiAl alloys was performed by optimal microscopy after the mechanically polished samples were etched by Kroll"s solution (HF: HNO3: H2O=1: 1: 18 in volume).DTA experiments were conducted on SETSYS Evolution DTA/DSC.Both the heating and cooling processes were studied at a scanning rate of 10°C/min.High-temperature in-situ observation experiments were carried out on VL2000DX–SVF18SP.Mechanically polished samples with a size of φ 6 mm × 2 mm were set in alumina crucibles,and a thermocouple was attached to the bottom of the crucible to record the temperature of the samples in real time.To calibrate the device with respect to temperature,the melting process of pure iron samples was observed by HTLSCM,and thermocouple calibration was carried out by the melting point of pure metals [37].Before experiments started,the furnace chamber was cleaned 3 times with Ar gas to avoid oxidation.Then,the samples were set in a dynamic atmosphere of high purity Ar.The samples were heated to the stabilized temperature,and held for 10 min to make them transform into α phase completely.Then,they were cooled at moderate cooling rates,including 0.3°C/s,1°C/s and 3°C/s.To quench the samples in-situ,helium gas was chosen to cool the samples rapidly.In general,the maximum cooling rate can reach 100°C/s at 1500–1000°C,27°C/s at 1000–500°C and 7.5°C/s at 500–200°C.Vickers hardness measurements were carried out under a load of 1 kgf and a dwell time of 20 s,and every hardness value was an average of 10 measurements.

Fig.2.Microstructure evolution of TiAl alloys during the heating process.

3.1.Phase and phase transformation of as-cast alloys

Fig.1 displays the microstructures and phase constitutions of the ascast Ti–40Al,Ti–45Al and Ti–49Al alloys.The microstructures of the three compositions are all full lamellar structures with different sizes of lamellar colonies.With increasing Al content,the sizes of lamellar colonies become significantly larger,and they increase to 50–100 μm in Ti–40Al,200–1000 μm in Ti–45Al and over 2 mm in Ti–49Al.This indicates that Al addition has a strong coarsening effect on lamellar colonies.In addition,according to the XRD patterns,all alloys consist of the γ phase and α2phase.The peaks of the γ phase are of strong intensity in Ti–45Al and Ti–49Al,but they are weak in Ti–40Al.The main peaks of Ti–40Al turn to the α2phase from the γ phase.This suggests that Ti–45Al and Ti–49Al possess more γ phases,although all three compositions have the same full lamellar microstructure.This agrees with current results that Al-rich compositions will form more γ phase and a higher ratio of γ/α2[38,39].

Fig.3.In-situ observation of the annealing process to completely transform the microstructure into the α phase in TiAl alloys.

The results of the DTA experiment on the as-cast TiAl alloy are presented in Fig.1(e).According to the curves,the α→γ transition occurs first during the cooling process in all alloys,and Al addition can push the transus to higher temperatures.It is interesting to find that the α→γ transition appears in Ti–40Al.This is contrary to current results that α2is generally regarded as the primary phase when Al content is less than 44 at.%[9,40].After a weak peak of the α→γ transition appears,α→α2and α2→γ occur as expected.For Al-rich compositions of Ti–45Al and Ti–49Al,α→α2also occurs subsequently even if the α2phase cannot exist in Ti–49Al theoretically.By comparing the transus of α→α2,it is found that Ti–40Al and Ti–45Al have similar ones,but Ti–49Al alloy,which is free of the α2phase in the phase diagraph,has a higher one.The addition of Al is proven to increase the transus of α→γ and α→α2,and at a slow cooling rate of the DTA test,an undercooling of α phase in a large degree appears.Then,the retained α phase is ordered into the α2phase by α→α2,which accounts for why the α2phase is preserved in Ti–49Al.The phase transformation temperatures of the three alloys measured by DTA and their comparison with the phase diagraph calculated by Pandat are presented in Fig.1(f).The Ti–40Al and Ti–45Al data obtained from the DTA experiment are in good agreement with the phase diagraph.However,the tested value of Ti–49Al shows a discrepancy.Notably,γ precipitation starts at a higher temperature than the one at which the α phase completely transforms into a lamellar structure.

3.2.In-situ observation of (α2+γ) lamellar microstructure formation of TiAl alloys

Fig.4.In-situ observation of γ lamellae precipitating during CCT in TiAl alloys at different cooling rates.

Before observing the formation process of TiAl alloys in-situ,the Ti–40Al,Ti–45Al and Ti–49Al samples are heated to the stabilized temperature at which they transform into the α phase completely.Then,they are annealed for 10 min to ensure that (α2+γ) lamellar colonies completely transform into the α phase.For all compositions,the heating rates are 300°C/min at 200–900°C and 50°C/min at 900°C to make the temperature rise smoothly and evenly.The stabilized temperatures are 1300°C in Ti–40Al and 1400°C in both Ti–45Al and Ti–49Al.Whether lamellar colonies are confirmed to transform into α phase completely is shown in the statement about helium quenching of alloys,and all the alloys are in the α phase at their stabilized temperature.The in-situ heating processes are presented in Fig.2.At a starting temperature of 200°C,every sample exhibits a polished surface.When samples are heated to 900°C,lamellar colonies with faint characteristics appear.This results from thermal expansion,i.e.,thermal etch.Then,the fine lamellae become too fuzzy to make out,and the coarse lamellar grooves remain on the surface.The (α2+γ) lamellae start to coarsen and decompose into α phase at 1146°C in Ti–40Al,1167°C in Ti–45Al and 1216°C in Ti–49Al.Strikingly,there are some differences between the in-situ obtained temperatures and the theoretical prediction in TiAl alloys.Lamellae in Ti–40Al start to transform into the α phase in the single α phase region above the α transus.The decomposition process in Ti–45Al begins in the α+γ region below the α transus.However,it occurs in the γ single phase region slightly below the γ solvus in Ti–49Al.This proves that the resistance to decomposition of (α2+γ) lamellae in TiAl alloys decreases with increasing Al content.

The microstructures in the single α phase region are displayed in Fig.3.The samples exhibit a bright surface of α grains accompanied by α phase grain boundaries and thermal etching grooves.The grain boundaries of the α phase migrate and absorb each other during annealing.The Al content has a significant effect on the grain size of the hightemperature α phase,and the Al-rich composition has coarser α grains than the Al-lean composition.The occurrence of lamellar thermal etching grooves does not mean the existence of(α2+γ) lamellar microstructures at the moment.Indeed,they are the residual shape of decomposed(α2+γ)lamellae,and lamellar colonies have transformed into the α phase completely.

The TiAl alloys were cooled at 0.3°C/s,1°C/s and 3°C respectively to research the effect of cooling rates on the CCT.As Fig.4 shows,the process in which γ lamellae start precipitating at different cooling rates is observed.The nucleation of γ lamellae begins with grain boundary γ allotriomorphs(γGBA),and γ lamellae grow into α grains in the form of plates [41].For all three compositions,γ lamellae arise from γGBAat all cooling rates.However,γGBAexhibit different morphologies in TiAl alloys.They clearly appear in α grain boundaries and wet partial grain boundaries in Ti–45Al and Ti–49Al,similar to the β/α transition in Ti alloys[42].But they are hardly detected in Ti–40Al before γ plates start to propagate.To derive the reason,the temperatures of the phase transformation are investigated.The starting temperature of γ lamellar precipitation is displayed in the conner of pictures.The moment at which obvious γ lamellae arise is regarded as the occurrence of γ lamellar precipitation reaction.Comparing the starting temperature of γ lamellar precipitation at 0.3°C/s with that obtained by DTA,it is found that the starting temperatures of Ti–45Al and Ti–49Al are lower than the α→γ transus provided by DTA.However,the one in Ti–40Al almost agrees with the DTA result.Accordingly,the morphology of γGBAinfluences the discrepancy between the actual precipitation temperature and the theoretical transus in TiAl alloys.γ lamellae nucleation at α grain boundaries needs that enough γGBAform and wet α grain boundaries in Ti–45Al and Ti–49Al.However,few γGBAappear in Ti–40Al"s nucleation process.Thus,γ lamellae are the main form of the γ phase when the temperature is slightly lower than the α→γ transus.A linear relation between cooling rates and starting/finishing temperature is confirmed in TiAl alloy CCTs[15].Thus,fittings between the cooling rateV(°C/s) and starting temperature of γ lamellar precipitationTs(°C) in three compositions are conducted,and the results areTs=1166.98–12.46Vfor Ti–40Al,Ts=1189.13–23.81Vfor Ti–45Al andTs=1230.21–18.29Vfor Ti–49Al.

Fig.5.In-situ observation of γ lamellae growing during CCT in TiAl alloys at different cooling rates.

Fig.6.Schematics of the formation of the deep-supercooling α phase caused by the anisotropy in γ lamella growth(a)and the alleviation of this phenomenon in TiAl alloys with finer α grains (b).

As displayed in Fig.5,the growth of γ lamellae in TiAl alloys is observed in-situ.γ lamellae traverse the entire α grain from the γGBAand exhibit anisotropy in the growth direction.The main growth direction of γ plates is perpendicular to the lamella interface direction,[0001]α.The lateral interface of γ plates can hardly extend into the α phase.This kind of anisotropy can result in uneven distribution of γ lamellae and undercooling of the α phase.Additional discussion about the anisotropy of γ lamellar growth is presented in Fig.6(a).Because the lateral growth of the γ lamella is much lower than the propagation speeds,the remaining α phase between two parallel γ plates has difficulty in completing the transition through the lateral merger by adjacent γ lamellae.It transforms into γ phase until newly formed γ lamellae from the α grain boundaries traverse this area.This phenomenon occurs in all compositions at all cooling rates,and no influential factor on this is found.Besides of this,there is another kind of retained α phase,named nondominant area.In essence,the growth of γ lamellae is a kind of diffusion-controlled ledge migration,which is affected by the crystal orientation of the α parent phase.Another important factor,γGBA,which is the base of growing into the α phase for γ lamellae,allows α grain boundaries to influence the propagation of γ lamellae.These two factors can lead to nondominant area in α phase grains.As Fig.6(a)displays,a nondominant area appears near the α grain boundaries,and the α grain boundary here is normal to the crystal direction [0001]α.The poor lateral growth rate of the γ lamella makes that the retained α phase in the nondominant area cannot transform into γ lamellae massively.The appearance of γGBAin the perpendicular grain also cannot work.Only γGBApropagates to the position marked by the black arrow and produces γ lamellae,γ lamellae can appear in the nondominant area.Nevertheless,this phenomenon is of a low degree in Ti–40Al.This may be associated with the grain size of the α phase in the TiAl alloys.As Fig.6(b)shows,TiAl alloys with finer α grains have more grain boundaries.They can provide adequate nucleation bases to produce γ lamellae and avoid forming a deep supercooled α phase.Cooling rate seems to have little impact on the occurrence of this phenomenon,and the growth of γ lamellae instead of the transformation driving force may influence the consumption of the retained α phase.Because the driving force of transformation is governed by the cooling rate.The findings above provide new insight into the microstructure control of TiAl alloys.Currently,high cooling rates are considered a useful method to refine γ lamellae during their formation process [19,43–45].However,extremely high cooling rates will result in more formation of deep-supercooled α phases at the lamellar intervals or in nondominant areas.Then,the majority of the deep-supercooled α phase will freeze into the α2phase.This will decrease the fraction of γ phase in TiAl alloys and damage the mechanical properties.Based on the findings above,TiAl alloys with finer α grain sizes,i.e.,lamellar colony sizes,can suffer higher cooling rates to obtain smaller γ lamella spacings.

Fig.8.Nucleation number and nucleation rate of Ti–40Al,Ti–45Al and Ti–49Al suffered from different cooling rates.

Using the same method as the abovementioned method,the final stage of γ lamellar precipitation is studied,as shown in Fig.7,and the finishing temperatures are obtained for the TiAl alloys.The temperatures at which there is no obvious change on the surface are characterized as the finishing temperature.The relationship between the cooling rate and finishing temperature in each composition is researched,and linear regression is performed.The results show that the finishing temperature of γ lamellar precipitationTf(°C) and cooling ratesV(°C/s)conform to the equations ofTf=1058.19–28.07Vin Ti–40Al,Tf=1107.80–59.16Vin Ti–45Al andTf=1140.19–37.57Vin Ti–49Al.

To research the details of γ precipitation during CCT,the nucleation and growth of γ lamellae are investigated in-situ.Once γ lamellae start to appear,the numbers of γ laths are manually counted in the frame for each degree decline in temperature.The nucleation rate of γ lamellar precipitation is derived by the γ lamella number and temperature.The growth rate of γ lamellae is calculated by measuring the length in real-time pictures once per degree Celsius using Image-Pro Plus software,and then it is obtained by differential analysis.The nucleation number andnucleation rate of the TiAl alloys are presented in Fig.8.Actually,the curves of nucleation number and nucleation rate are not as smooth as the theoretical prediction.The γ lamella number fluctuates to some extent in the increasing process,especially in Ti–49Al,which makes numerous peaks appear on the curve of the nucleation rate.These fluctuations are consistent with the results mentioned above that the nucleation and growth of γ lamellae are discontinuous processes in TiAl alloys.The α phase in nondominant areas and lamellar interval contributes to the troughs of curves.The phenomenon that γ lamellae nucleate and grow discontinuously is slight in Ti–40Al because it has more α grain boundaries.Although Al content can influence the lamellar colony size and then make nucleation and growth processes complex,it has no significant effect on the maximum nucleation rate at the same cooling rate.Another important factor,the cooling rate,is able to increase the maximum nucleation rate.For the three compositions,the maximum nucleation rates are listed in Table 2.In addition,it seems to possess a positive coefficient linear relationship with the cooling rate,which has been confirmed in many studies[46,47].

Table 2 Maximum nucleation rates of γ lamellae in the TiAl alloys

Fig.9.The growth rates of γ lamellae of TiAl alloys at different cooling rates (a)and their regression with cooling rates in Ti–40Al(b),Ti–45Al(c)and Ti–49Al(d).

The growth rates of γ lamellae are exhibited in Fig.9 γ lamellae maintain a constant growth rate at a certain cooling rate for each composition.Moreover,a high cooling rate can enhance the growth rate of γ lamellae.However,the response of the growth rates to the cooling rate change is of different levels in the alloys.The added value of the growth rate in Ti–40Al is less than those in Ti–45Al and Ti–49Al at the same cooling rate.Acknowledgedly,the γ lamellae result from the α2phase in Ti–40Al,but they form from α phase in Ti–45Al and Ti–49Al.Thus,it can be inferred that γ lamellae grow slower from α2phase at higher cooling rates because it is associated with ledge movement.A power function relationship between the growth rate of acicular ferrite and the cooling rate is found by Yang et al.when they observe the process of acicular ferrite nucleation and growth in-situ by HTLSCM[48].Thus,power function fittings between the growth rate of γ lamellae and cooling rate are conducted in three alloys.The results are presented in Fig.9(b),(c) and (d),respectively.The growth rate of γ lamellae (G) and cooling rate(V)obey the law ofG=4.685V0.4365with R2=0.9449 in Ti–40Al,G=7.069V0.6189with R2=0.9676 in Ti–45Al andG=7.388V0.5659with R2=0.9110 in Ti–49Al.As a diffusional-displacive transformation governed growth progress,the growth rate of γ lamellae,G,follows Eq.(1)[6,49–51],

whereDis Al"s interdiffusion coefficient in the α phase,C0is the nominal Al concentration,CαandCγare the equilibrium Al concentrations of the α and γ phases,bis a geometrical factor andris the edge radius.It is easy to be found thatGmaintains a constant value during the growth process,which agrees with the results above.The cooling rate affects the growth rate of γ lamellae by changingr.When a γ lamella propagates into the αphase,its edge radiusris equal to the half of the lamellar thickness.As Table 3 displays,the lamellar spacing in each composition decreases with increasing cooling rate.The high cooling rate can inhibit lateral nucleation on γ lamellae and prevent γ lamellae from thickening.Thus,the γ lamellae become refined,andrdecreases.This leads γ lamellae to grow faster at a higher cooling rate.

Table 3 The relationship between lamellar spacing and cooling rates in TiAl alloys

Table 4 Vickers hardness of TiAl alloys suffered from different cooling rates

The precipitation behavior of γ lamellae under the rapid cooling condition is also investigated in-situ.The samples are heated to an α phase stabilized temperature respectively and held for 10 min.Then,they are quenched immediately using an atmosphere of helium gas.The results show that γ lamellar precipitation is completely avoided and that different phenomena occur in each composition.The evolution of Ti–40Al,Ti–45Al and Ti–49Al during the quenching process is exhibited in Fig.10,and the phase constitution of quenched samples in TiAl alloys is displayed in Fig.11.The samples in Ti–40Al and Ti–45Al experience a similar course in which the α phase is frozen into the α2phase.There is no change on the surfaces of the samples during quenching because the order-disorder phase transition is a kind of second-order phase transformation which does not concern volume change.A little different from the room-temperature microstructure of Ti–40Al,the one of Ti–45Al presents a few γ lamellae in the region adjacent to α grain boundaries.However,this process has not been observed in-situ by HTLSCM.The amount of γ lamella precipitation may be too low to result in enough surface change which can be detected by HTLSCM.Therefore,it has been neglected,and detailed starting and finishing temperatures cannot be obtained.It can only be concluded that a minority of γ lamellae are able to precipitate under the condition of helium gas quenching in Ti–45Al.For Ti–49Al,it has a different evolution compared with others.With XRD,the γ phase is confirmed as the dominant phase,and a few α2phases are preserved in the quenched microstructure.By investigating the quenching process in-situ,Widmannstatten laths appear on the surface at 1044°C,and the formation process stops at 983°C.However,only a minority of the α phase transform into Widmannstatten laths,and no surface relief change is observed in the spaced regions.By researching the microstructure of the quenched sample,Ti–49Al exhibits massively transformed microstructures as well as Widmannstatten γ laths.Therefore,massive transformation is proven to occur.In the massive transformation of TiAl alloys,the product phase has no strict orientation relationship with the parent phase,so the surface relief effect does not arise.Moreover,a massive microstructure is not the only morphology.Widmannstatten and feather structures which have a certain orientation relationship with the parent phase are reported in the massive transformed TiAl alloys and steel.Thus,it is concluded that a massive transformation in Ti–49Al occurs at 1044°C upon helium quenching by observing visible Widmannstatten γ laths.

Fig.10.Helium quenching process of Ti–40Al,Ti–45Al and Ti–49Al

Fig.11.XRD patterns of quenched samples in TiAl alloys.

Fig.12.CCT diagraphs of Ti–40Al (a),Ti–45Al (b) and Ti–49Al (c) from the α phase and the effect of the cooling rate on the phase transformation in Ti–40Al (d),Ti–45Al (e) and Ti–49Al (f).

CCT diagrams of TiAl alloys are obtained based on the mentioned results,and the effect of the cooling rate on the phase transformation path from the α phase is proposed,as shown in Fig.12.Interestingly,combining the results of DTA and HTLSCM,it may be deduced that the γ phase is the primary phase in the formation process of the lamellar structure at moderate cooling rates in Ti–40Al.In addition,γ lamellae form at moderate cooling rates;α→α2occurs upon rapid cooling in Ti–45Al.For Ti–49Al,the lamellar structure can also come into being at moderate cooling rates,although Ti–49Al does not possess the so-called eutectoid,which further elucidates that a lamellar structure does not form by the eutectoid reaction.Additionally,the massive transformation will occur under the condition of helium gas quenching.

3.3.Hardness of γ-TiAl alloys cooled at different cooling rates

As acknowledged currently,the cooling rate has a great effect on the mechanical performance of TiAl alloys.Thus,it is of great significance to research the influence of the cooling rate on the properties of TiAl alloys.The Vickers hardness test is an easy method to evaluate the comprehensive mechanical properties of alloys.For some samples the tensile experiment is difficult to conduct,and therefore Vickers hardness test can be regarded as an indirect way to obtain the yield strength because the alloy"s yield strength has a linear relationship with the value of its Vickers hardness [52,53].The Vickers hardness results of Ti–40Al,Ti–45Al and Ti–49Al subjected to cooling rates of 0.3°C/s,1°C/s and 3°C/s are displayed in Table 4.It is evident that for all samples that their Vickers hardness is enhanced at high cooling rates.The reason may be that high cooling rates can result in a fine lamellar structure.For TiAl alloys with lamellar microstructures,their mechanical properties conform to Hall–Petch type relationships with lamellar spacing[54].The refinement of lamellar structures caused by high cooling rates improves the Vickers hardness of TiAl alloys,and this finding works in every composition.However,another variable,Al content,has no obvious impact on the Vickers hardness of the samples compared with the cooling rate.

The γ phase transformation behaviors of Ti–40Al,Ti–45Al and Ti–49Al at cooling rates of 0.3°C/s,1°C/s and 3°C/s were investigated by HTLSCM in this paper,and the conclusions are as follows:

(1) The starting temperature(Ts) of γ lamella precipitation in the alloys is obtained at cooling rates of 0.3°C/s,1°C/s and 3°C/s,and it has linear regressions with cooling rates(V) ofTs=1166.98–12.46Vfor Ti–40Al,Ts=1189.13–23.81Vfor Ti–45Al andTs=1230.21–18.29Vfor Ti–49Al.The finishing temperature(Tf)of γ lamella precipitation in Ti–40Al,Ti–45Al and Ti–49Al is also obtained at cooling rates of 0.3°C/s,1°C/s and 3°C/s,and it has linear regressions with cooling rates (V) ofTf=1058.19–28.07Vin Ti–40Al,Tf=1107.80–59.16Vin Ti–45Al andTf=1140.19–37.57Vin Ti–49Al.

(2) The quenching processes of the alloys in a helium atmosphere were observed in situ by HTLSCM.For both quenched Ti–40Al and Ti–45Al,there is no surface relief for the ordering transition of α phase.Massive transformation occurs in Ti–49Al,and massive microstructure together with a little Widmannstatten γ phase have appeared.The transus of massive transformation in Ti–49Al is approximately 1044°C.

(3) The nucleation and growth of TiAl alloys show a discontinuity because of the anisotropy of γ lamella growth,and grain boundary as well as crystal orientation has effects on it.The nucleation rates and growth rates of TiAl alloys are counted,and they increase with rising cooling rate.

(4) The CCT diagraphs of Ti–40Al,Ti–45Al and Ti–49Al were obtained.γ phase is the primary phase of(α2+γ)lamellae formation in Ti–40Al.Lamellar structure can form in Ti–49Al without a eutectoid line at moderate cooling rates.

(5) The Vickers hardness of the TiAl alloys increases with increasing cooling rate due to the decrease in lamellar spacing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [grant numbers 52001001 and 51425402] and the Major Special Science and Technology Project of Yunnan Province [grant number 202002AB080001-3].