简介概要

Phase precipitation behavior and tensile property of a Ti-Al-Sn-Zr-Mo-Nb-W-Si titanium alloy

来源期刊：Rare Metals2018年第12期

论文作者：Wen-Jing Zhang Xiao-Yun Song Song-Xiao Hui Wen-Jun Ye Wei-Qi Wang

文章页码：1064 - 1069

摘要：The characteristic of precipitation behavior of a2 phase and silicide, and the tensile properties at room temperature and 650℃after heat treatments in anovel TiAl-Sn-Zr-Mo-Nb-W-Si titanium alloy（BTi-6431 S） were investigated by microstructure analysis and mechanics performance testing. The results show that no second phase precipitates after solution treatment（980 ℃/2 h, air cooling（AC））. However, when the solution-treated specimens are aged at 600 ℃（600 ℃/2 h,AC）,α₂ phase precipitates in the primary α phase, and the size of α₂ phase increases with the aging temperature increasing to 750 ℃. Meanwhile, 50-100-nm S2-type silicide particles precipitate along lamellar phase boundaries of transformed β phase after aging at 750 ℃. BTi-6431 S alloy shows the best650 ℃ ultimate tensile strength（UTS） and yield strength（YS） when treated in solution treatment. However, aging treatment results in a decline in 650 ℃ ultimate tensile strength. This may be attributed to the loss of solution strengthening due to the depletion of Al, Si and Zr of the matrix caused by the precipitation of Ti₃ Al and（TiZr）₆ Si₃.Silicide is a brittle phase; therefore, its precipitation causes a sharp decrease in the room-temperature ductility of BTi-6431 S alloy.

稀有金属(英文版) 2018,37(12),1064-1069

Phase precipitation behavior and tensile property of a Ti-Al-Sn-Zr-Mo-Nb-W-Si titanium alloy

Wen-Jing Zhang Xiao-Yun Song Song-Xiao Hui Wen-Jun Ye Wei-Qi Wang

State Key Laboratory for Fabrication and Processing of Nonferrous Metals, General Research Institute for Nonferrous Metals

School of Materials and Metallurgy,Northeastern University

Baoti Group Co.,Ltd.

作者简介：*Xiao-Yun Song,e-mail:songxiaoyun82@126.com;

收稿日期：7 November 2014

基金：financially supported by the National Natural Science Foundation of China (No. 51201016);

Phase precipitation behavior and tensile property of a Ti-Al-Sn-Zr-Mo-Nb-W-Si titanium alloy

Wen-Jing Zhang Xiao-Yun Song Song-Xiao Hui Wen-Jun Ye Wei-Qi Wang

State Key Laboratory for Fabrication and Processing of Nonferrous Metals, General Research Institute for Nonferrous Metals

School of Materials and Metallurgy,Northeastern University

Baoti Group Co.,Ltd.

Abstract：

The characteristic of precipitation behavior of a2 phase and silicide, and the tensile properties at room temperature and 650℃after heat treatments in anovel TiAl-Sn-Zr-Mo-Nb-W-Si titanium alloy(BTi-6431 S) were investigated by microstructure analysis and mechanics performance testing. The results show that no second phase precipitates after solution treatment(980 ℃/2 h, air cooling(AC)). However, when the solution-treated specimens are aged at 600 ℃(600 ℃/2 h,AC),α₂ phase precipitates in the primary α phase, and the size of α₂ phase increases with the aging temperature increasing to 750 ℃. Meanwhile, 50-100-nm S2-type silicide particles precipitate along lamellar phase boundaries of transformed β phase after aging at 750 ℃. BTi-6431 S alloy shows the best650 ℃ ultimate tensile strength(UTS) and yield strength(YS) when treated in solution treatment. However, aging treatment results in a decline in 650 ℃ ultimate tensile strength. This may be attributed to the loss of solution strengthening due to the depletion of Al, Si and Zr of the matrix caused by the precipitation of Ti₃ Al and(TiZr)₆ Si₃.Silicide is a brittle phase; therefore, its precipitation causes a sharp decrease in the room-temperature ductility of BTi-6431 S alloy.

Keyword：

High-temperature titanium alloy; Precipitation behavior; α₂ phase; Silicide; Tensile properties;

Received： 7 November 2014

1 Introduction

Sky-rocketing development of high-speed vehicle has intensified the research on improving designs of hull materials.High-temperature titanium alloy with excellent properties,such as high specific strength and creep strength [ 1] ,is the right choice to be applied in high-speed vehicle.Concentrated efforts have resulted in the development of excellent titanium alloys to enhance the working temperature to about 600℃,and typical examples are IMI834 [ 2] ,Ti-1100 [ 3] and Ti-600 [ 4] .These alloys belong to TiAl-Sn-Zr-Mo-Si titanium alloy which has lessβ-stabilizing elements.In order to further increase the service temperature,a new-type Ti-Al-Sn-Zr-Mo-NbW-Si titanium alloy,BTi-6431S alloy was designed through adding more Mo,Nb and W elements together to enhanceβ-phase strength,improving temperature capability and tensile situation up to 650-700℃ [ 5] .It has been found that the ultimate tensile strength (UTS) of this alloy at650℃can reach more than 600 MPa,which is almost equal to the UTS of Ti-1100 and BT36 at 600℃(Ti-1100is 630 MPa;BT36 is 640 MPa) [ 6] .Therefore,BTi-6431S alloy is expected to be used for short-time structural components at 650-700℃in aerospace industry.

Since high-temperature titanium alloys typically contain4 wt%-8 wt%aluminum,aging may result in the formation of the ordered and coherent precipitation,α₂ phase (Ti₃Al) [ 7] .Madsen and Ghonem [ 8] investigated that the precipitation ofα₂ phase after aging in Ti-1100 alloy was largely responsible for the increase in the yield strength (YS) at593℃and decrease in ductility at room temperature (RT).In many titanium alloys,silicon is added as a solid solution strengthener to improve high temperature strength and creep resistance.Aging may also promotes the formation of silicide in the silicon-bearing titanium alloys.Two kinds of silicide were reported in earlier studies that is S₁((TiZr)5Si₃) and S₂ ((TiZr)6Si₃).Both of them are hexagonal but with different lattice parameters (for S₁,a≈0.78 nm,c≈0.54 nm;for S₂,a≈0.70 nm,c≈0.36 nm).The precipitation of different types of silicide depends on the composition and heat treatment.Usually,silicide is a brittle phase,and its precipitation causes a big loss in ductility [ 9, 10] .

In this work,BTi-6431S alloy contains relatively high A1 content and 0.2 wt%Si,so aging may produceα₂ phase and silicide in this alloy.Previous work mainly deals with the role ofα₂ phase and silicide on tensile properties of other titanium alloys under 600℃ [ 11] .Thus,few data exist on mechanical properties over 600℃.The present investigation aims to study the precipitation behavior ofα₂phase and silicide in BTi-6431S alloy after different heat treatments,and the effects of second phases on RT and650℃tensile properties.

2 Experimental

The composition of BTi-6431S alloy used in this research contains 6.21 wt%Al,3.15 wt%Sn,2.86 wt%Zr,1.19 wt%Mo,1.12 wt%Nb,0.45 wt%W and 0.20 wt%Si.The ingot was melted three times by the vacuum consumable electrode method,then forged and hot-rolled in(α+β) two-phase region to a 3-mm-thickness plate.Theβ-transus temperature of this alloy was 990-1000℃measured by metallographic techniques.The microstructure of as-received material is composed of lath-shaped primaryαphase (α_p) and lamellar transformedβphase (βt),as shown in Fig.1.

Three kinds of heat treatments,as designated in Table 1,were conducted on BTi-6431S plates using a resistance heating furnace.The tensile specimens with dimensions of3 mm×15 mm and a gage length of 50 mm were cut from the heat-treated plates along the rolling direction.RT and 650℃tensile tests were conducted on an Instron 5582testing machine at a strain rate of 0.04 and 0.005 min^-1,respectively.At least three specimens were tested for each treatment to make an average.Microstructures were observed by Axiovert 200 MAT Zeiss metallographic microscope (OM) and JEM2100 transmission electron microscope (TEM).Tensile fracture surface was observed by JSM-7001 scanning electron microscope (SEM).Compositions of each phase were measured on JXA-8100electron probe microanalyzer (EPMA).Specimens for OM were prepared by conventional mechanical polishing techniques and etched with a solution of 10 ml HF,30 ml HN0₃ and 70 ml H₂O.The foils for TEM observations were prepared by twin-jet polishing technique using a solution of 70 vol%methanol,21 vol%butanol and9 vol%perchloric acid,at a current of 55-60 mA and a temperature from-20 to-30℃.

Fig.1 OM image of original material

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Table 1 Heat treatments of BTi-643 1S alloy

3 Results and discussion

3.1 Observation of microstructure

Figure 2 shows the microstructure of the alloys after different heat treatments.All the three specimens consist of bimodal microstructures,i.e.,α_p andβ_t.However,aging treatment changes the content ofα_p.The content ofα_p in the solution-treated (ST) specimen is about 40 vol%and increases to 42 vol%by the following aging at 600℃.With the aging temperature increasing,the content ofα_p in the STA2 condition further increases to 50 vol%.

The concentration of each element inα_p andβ_t after different heat treatments is listed in Table 2.α_p has more Al thanβ_t,the content of Al in t_p andβ_t decreases after follow-up aging treatment,and the decrement gets larger with the aging temperature increasing.Asβstabilizers,Mo,Nb and W mainly exist inβ_t [ 12] ,and their contents increase with aging temperature rising.

Fig.2 OM images of samples after different heat treatments:a ST,b STA1 and c STA2

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Table 2 Composition of each phase after different heat treatments by EPMA (wt%)

3.2 Precipitation characteristics of second phase

The precipitation features of second phases after different heat treatments were investigated by TEM.In ST condition,no second-phase particles are observed.After aging treatments,the selected area electron diffraction (SAED)pattern and corresponding dark field image (DFI) ofα_p are shown in Fig.3,indicating that large quantities of coherent and orderedα₂-phase particles precipitate inα_p.With the aging temperature increasing from 600 to 750℃,the size ofα₂-phase particle grows from 5 to 10 nm,as shown in Fig.3b,d.

Figure 4 exhibits the TEM images ofβ_t in STA1 and STA2 conditions.It can be found that no second phase precipitates inβ_t of STA1 sample,as shown in Fig.4a.However,elliptical second-phase particles with size of 50-100 nm precipitate along the lamellar phase boundaries in STA2 specimen,as indicated by the arrows in Fig.4b.From the corresponding SAED pattern in Fig.4b,this phase is indexed as hexagonal S₂-type silicide with[1216],a=0.698 nm and c=0.367 nm.This is well consistent with the results of IMI829,IMI834 and BT9 alloys,as reported by Singh et al. [ 13] .The composition of this phase was measured by TEM equipped with energy-dispersive spectroscopy (EDS) as 33Si-35Zr-32Ti (at%),which is also in accordance with (TiZr)6Si₃ (S₂-type silicide).

3.3 Tensile property

Figure 5 summarizes the tensile properties,including YS,UTS and elongation (EL),at RT and 650℃of BTi-643 1S alloy after different heat treatments.Compared with ST specimen,the RT tensile properties of the STA1 specimen almost remain constant,whereas the value of EL for STA2specimen decreases dramatically from 8.0%to 2.5%.On the contrary,the YS and UTS at 650℃drop gradually after aging at 600 and 750℃,while the EL of all specimens at 650℃is more than 20%.At 650℃,the values of YS and UTS are 400 and 605 MPa for ST specimen and350 and 545 MPa for STA2 specimen,respectively.The results indicate that aging treatment greatly affects the RT ductility and 650℃tensile strength.

3.4 Precipitation behavior of second phase

In the present investigation,aging at 600 and 750℃for2 h results in the precipitation of orderedα₂ phase,and the size ofα₂ phase gets larger when aging at higher temperature.It is reported that the precipitation ofα₂ phase is accompanied with element diffusion [ 14] .Therefore,increasing aging temperature could raise the diffusion rate of atoms,accelerate the growth and finally coarsenα₂-phase particles.

Fig.3 TEM images of BTi-6431S alloy:a SAED pattern ofα_p with

in STA1 condition,b DFI ofα_p in STA1 condition,c SAED pattern ofα_p with

in STA2 condition and d DFI ofα_p in STA2 condition

Fig.4 TEM images ofβ_t in different conditions:a STA1 and b STA2

On the other hand,aging at 750℃for 2 h results in the precipitation of silicide particles,while aging at lower temperature (600℃) for the same time could not.This could be understood in terms of the relative increase in the amounts of theβ-stabilizing elements in the reduced content ofβt with the increase in the aging temperature,which finally accelerates such precipitation [ 10, 11] .From the EPMA result in Table 2,it can be inferred thatβ_t has the highest content ofβstabilizers after aging at 750℃.The role of the higherβstabilizers in enhancing the kinetics of precipitation of silicide has been brought out recently [ 15] .In this alloy,only the S₂-type silicide precipitates predominantly along the lamellar phase boundaries inβ_t under the aging condition.This phenomenon is in agreement with the observation in IMI829 and BT9 alloys [ 13, 16] .

3.5 Influence of second phases on tensile properties

For titanium alloys,many investigations reported that the precipitations of orderedα₂ phase and silicide produce dispersion strengthening effect;however,their precipitation could weaken solid solution strengthening effect and RT ductility.So the strengthening effect is a balance between increasing and decreasing due to the second-phase precipitation when solution elements are taken out of solution.

Fig.5 Tensile properties at different temperatures after each heat treatment:a RT and b 650℃

Fig.6 SEM images of tensile fractographs at RT of BTi-6431S alloy with different heat treatments:a,b ST;c,d STA1;e,f STA2

In BTi-6431S alloy,α₂-phase precipitation in STA1specimen does not affect RT strength and ductility.However,the RT ductility of STA2 specimen with S₂-type silicide is lower than those of ST and STA1 specimens.This is obviously due to the detrimental role of the precipitation of brittle silicide.Ramachandra et al. [ 17] also found the same phenomenon in oil-quenched IMI685alloy aged at 800℃(only S₂-type silicide formed).The fractographs in Fig.6 bring out the distinct differences in the fracture behavior of the different specimens after tensile tests.The fracture surfaces of ST and STA1specimens contain dimples and fracture facets with tearing ridge surrounded,as shown in Fig.6a-d,indicating a mixture mode of ductile fracture and transgranular cleavage fracture.However,the fracture surface of STA2specimen is covered with fracture facets,as shown in Fig.6e,suggesting that quasi-cleavage fracture is the primary fracture mode.Amplifying the fractographs of specimen treated in STA2 condition,there are small voids distributed randomly along the lamellar phase boundaries inβ_t,as indicated by the arrows in Fig.6f.Ramachandra et al. [ 17] reported that the tendency for faceted fracture is significantly enhanced due to the silicide precipitation inβ_t.So the silicide precipitation in STA2 specimen promotes voids initiation,resulting in the cleavage facets inβ_t.

The 650℃tensile strength decreases after aging treatment,especially in STA2 condition.It proves that at650℃,the ineffective role of the second phases on strengthening is coined with the decrease in solid solution strengthening due to the depletion of Al,Si and Zr which are the main elements composing Ti₃Al and (TiZr)6Si_3.As shown in Table 2,STA2 specimen has the least contents of Al in a_p and Si and Zr inβt.

High-temperature tests do not show the trend toward reduced ductility after aging treatment.However,in contrast to RT results,the difference in ductility between unaged and aged specimens is much less pronounced.It could be concluded that the silicide precipitation does not damage 650℃ductility in BTi-6431S alloy.For BTi-6431S alloy,in order to guarantee the RT ductility and650℃strength,the single solution treatment is appropriate.

4 Conclusion

BTi-6431S alloy was heat-treated by solution and aging treatments.Aging causes the formation of two types of precipitates,α₂ phase and S₂-type silicide.The precipitation behavior and the effects of them on tensile properties at room temperature and 650℃were studied.In BTi-6431S alloy,large quantities of 5-nm-orderedα₂ particles precipitate withinα_p after aging at 600℃for 2 h,and their sizes increase to about 10 nm when aging temperature rises to 750℃.After aging at 750℃,due to the higher content ofβstabilizer inβ_t,some S₂-type silicide (TiZr)6Si₃ particles with size of 100 nm precipitate along the lamellar phase boundaries inβ_t.Compared with the solution-treated and 600℃aged alloys,the RT ductility of the alloy aged at 750℃decreases dramatically,because of the precipitation of incoherent brittle silicide.After aging at 600 and750℃,although Ti₃Al and (TiZr)6Si₃ could produce precipitation strengthening effect,their precipitation also results in the decrease in Al,Si and Zr contents dissolved in matrix,leading to the loss of solid solution strengthening effect.Consequently,the tensile strength at 650℃decreases gradually.