简介概要

Relationship between microstructure and tensile properties on a near-β titanium alloy after multidirectional forging and heat treatment

来源期刊：Rare Metals2019年第4期

论文作者：Zhi-Dan Lü Chang-Jiang Zhang Zhao-Xin Du Jian-Chao Han Shu-Zhi Zhang Fei Yang Yu-Yong Chen

文章页码：336 - 342

摘要：In this study, a new near-beta titanium alloy, Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr, was prepared by induction skull melting（ISM） and multidirectional forging. The effect of aging heat treatment on microstructure and tensile properties of the alloy after solution treatment in the twophase（α + β） region was investigated. The micros tructure results show that the globular primary α phase（α_p） and the needle-like secondary α phase（α_s） are precipitated in the β matrix. The size of α_s increases with the increase in aging temperature,while the content of α_s goes up to a peak value and then decreases. The tensile testing results show that the strength increases first and then decreases with the increase in temperature. The variation of ductility presents the opposite way compared with the trend of strength level.When aged at 500 ℃, the alloy exhibits an excellent balance of tensile strength（1529 MPa） and elongation（9.22%）. And the relative mechanism of strengthening and toughening was analyzed and discussed.

稀有金属(英文版) 2019,38(04),336-342

Relationship between microstructure and tensile properties on a near-β titanium alloy after multidirectional forging and heat treatment

Zhi-Dan Lü Chang-Jiang Zhang Zhao-Xin Du Jian-Chao Han Shu-Zhi Zhang Fei Yang Yu-Yong Chen

Shanxi Key Laboratory of Advanced Magnesium-Based Materials, School of Materials Science and Engineering, Taiyuan University of Technology

School of Materials Science and Engineering, Inner Mongolia University of Technology

School of Mechanical Engineering, Taiyuan University of Technology

Waikato Centre for Advanced Materials, School of Engineering The University of Waikato

School of Materials Science and Engineering, Harbin Institute of Technology

作者简介：*Chang-Jiang Zhang,e-mail:zcj0408@163.com;

收稿日期：18 June 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 51504163, 51604191 and 51601099);

Relationship between microstructure and tensile properties on a near-β titanium alloy after multidirectional forging and heat treatment

Zhi-Dan Lü Chang-Jiang Zhang Zhao-Xin Du Jian-Chao Han Shu-Zhi Zhang Fei Yang Yu-Yong Chen

Shanxi Key Laboratory of Advanced Magnesium-Based Materials, School of Materials Science and Engineering, Taiyuan University of Technology

School of Materials Science and Engineering, Inner Mongolia University of Technology

School of Mechanical Engineering, Taiyuan University of Technology

Waikato Centre for Advanced Materials, School of Engineering The University of Waikato

School of Materials Science and Engineering, Harbin Institute of Technology

Abstract：

In this study, a new near-beta titanium alloy, Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr, was prepared by induction skull melting(ISM) and multidirectional forging. The effect of aging heat treatment on microstructure and tensile properties of the alloy after solution treatment in the twophase(α + β) region was investigated. The micros tructure results show that the globular primary α phase(α_p) and the needle-like secondary α phase(α_s) are precipitated in the β matrix. The size of α_s increases with the increase in aging temperature,while the content of α_s goes up to a peak value and then decreases. The tensile testing results show that the strength increases first and then decreases with the increase in temperature. The variation of ductility presents the opposite way compared with the trend of strength level.When aged at 500 ℃, the alloy exhibits an excellent balance of tensile strength(1529 MPa) and elongation(9.22%). And the relative mechanism of strengthening and toughening was analyzed and discussed.

Keyword：

Near-β titanium alloy; Multidirectional forging; Heat treatment; Microstructure; Tensile properties;

Received： 18 June 2017

1 Introduction

The beta titanium alloys are emerging as magnetically structural materials due to their potential advantages of the wide and unique range of strength-to-weight ratios,increased heat treatability,the best combination of strength and toughness and inherent ductility which is the result of their bcc structure[-].Hence,these alloys obtain a large interest for application in automotive parts,downhole services,sporting goods and military industries besides the conventional application in the aerospace industries in recent decades [ 5, 6, 7, 8] .However,beta titanium alloys cannot be employed in the higher strength level because of the poor ductility.For example,the first commercial beta titanium alloy,Ti-13V-11Cr-3Al used on the SR-71‘Blackbird'reconnaissance airplane,was employed at the1240 MPa strength level but with a minimum elongation of2% [ 8] .Hence,the acquisition of superior comprehensive mechanical properties (especially the high ductility) has become an increased research focus.

As known,forβtitanium alloys,the traditional hot working inβphase field and subsequent processing in(α+β) phase region can be utilized to refine the grains [ 9, 10, 11] .Meanwhile,the solution treatment in either theβphase field or the (α+β) phase field can present better ductility level by controlling the amount and morphology of primaryαphase (α_p) [ 12, 13, 14] .And then aging at lower temperature in the (α+β) phase field can obtain higher strength level with sacrificing of the ductility due to the precipitation of finer secondaryαphase (α_s) [ 12] .In addition,the alloys after solution treatment in (α+β)phase field present higher level of ductility than in single phase field according to previous investigations [ 13, 14] .In general,among the thermomechanical processing ofβtitanium alloys,how to obtain fine microstructure is critical for the subsequent solution and aging treatment to improve the mechanical properties.As one kind of severe plastic deformation (SPD) technology,multidirectional forging(MDF) seems to be especially attractive for aluminum and magnesium alloys due to its significant role in refining the microstructure and dispersing the second phases [ 15, 16] .However,the relative study on MDF ofβtitanium alloys remains still lacking.

In this paper,a new near-beta titanium alloy,Ti-4Al-1 Sn-2Zr-5Mo-8V-2.5Cr,was designed independently in terms of Ti-5.7Al-5.1V-4.8Mo-1Cr-1Fe (VT-22) [ 17] .In order to enhance the thermostability and heat treatability and to decrease the segregation for the current alloy,the neutral elements Zr and Sn were added and theβstabilizer element Fe was eliminated,respectively [ 18] .Therefore,MDF and solution treatment in (α+β) phase field plus aging at lower temperature were employed to investigate the relationship between microstructure and tensile properties of the current alloy.

2 Experimental

The material used in the current experiment is Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr alloy which is a new near-βtitanium alloy.The current ingot was melted twice in induction skull melting furnace (ISM) to ensure chemical homogeneity.The reference and actual composition of current alloy are listed in Table 1,and its'moly'equivalent([Mo]_eq) calculated by Eq.(1) is 11.1 [ 19] .Theβtransition temperature measured by metallographic method is(798±5)℃.

where w is the content of elements in wt%.After casting,the rectangular samples with dimensions of 35 mm×35mm×70 mm for MDF were machined from the current ingot with 100 mm in diameter and 150 mm in height when the defective parts from top and bottom of the ingot were removed.The MDF processes were conducted in a hydraulic press with a 400-kN load limit.The pressing speed is 1 mm·s^-1,and each reduction of height was～50%.The schematic illustration of the MDF processes is shown in Fig.1 where F refers to force.After MDF processes,the samples were solution-treated at 775℃and then held for 1 h followed by air cooling.Following solution treatment,the samples were subjected to aging at temperature ranging from 420 to 540℃for 8 h and followed by air cooling.

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Table 1 Chemical composition of current ingot (wt%)

In order to research the mechanical properties of the current alloy,the uniaxial tensile testing at room temperature was performed on Instron 5500R testing machine with a constant crosshead speed of 0.5 mm·min^-1.The size of flat tensile specimens is exhibited in Fig.2.In order to verify the repeatability and accuracy of the results,the testing was repeated 3 times and the surface of the tensile specimens was polished with 360-1500 grid SiC paper in water,and then the mechanical properties were determined according to the average data.

The microstructure of current alloy after MDF and heat treatment was observed by scanning electron microscopy(SEM,Tescan MIRA 3 LMH).The samples for SEM were polished with 360-2500 grid SiC paper in water,followed by electrolytic polishing.Moreover,the modified Kroll's reagent (3 ml HF+5 ml HNO₃+92 ml H₂O) was also used to reveal the microstructure.

3 Results and discussion

3.1 Microstructure

Figure 3 exhibits the microstructure of Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr alloy after multidirectional forging.As shown in Fig.3a,the globular and clubbedα_p is evenly distributed inβmatrix,which is the typical microstructural characteristic ofβtitanium alloy processed in (α+β)region [ 14] .Meanwhile,the content ofα_p is measured to be～27 vol%and the average size of theα_p is～1.2μm,as shown in Fig.3b.And the average size and content ofα_pof the current alloy are comparable to those of Ti-5553alloy underwent traditional hot working andα/βsolution treatment [ 20] ,indicating that multidirectional forging is an effective way to refine microstructure of current alloy.

Figure 4 presents low-magnification SEM micrographs of current alloy after solution treatment and subsequent aging treatment.In general,after solution heat treatment at775℃and succedent aging treatment,the microstructure mainly consists of globularα_p and fine needle-likeα_s,which are uniformly distributed inβmatrix.Meanwhile,what is worth mentioning is thatα_p content decreases significantly and theα_p tends to be refined in comparison with those of as-forged alloy.As shown in Fig.4,the content and average size of theα_p are～10%and 700 nm,respectively.And the obvious changes inα_p should be related to solution treatment at 775℃,because this type of microstructure can be generally obtained by (α+β)solution treatment for beta titanium alloys [ 21] .

Fig.1 Schematic design of MDF processes

Fig.2 Dimensioned schematic of tensile specimen

In order to better characterize the evolution of theα_swith aging temperature,high-magnification SEM micrographs are shown in Fig.5,where the fine acicularα_s in the form of triangles is distributed inβmatrix.Meanwhile,it is also noted that there are some precipitation-free zones in the vicinity ofα_p from Fig.5 a.The reason is possibly that the precipitation kinetics ofαabout current alloy is slower than that of otherβtitanium alloys due to the presence of sluggish species such as Mo and Cr [ 15] .In addition,the precipitation ofα_p increases the stability of theβmatrix by beta stabilizer elements and to somehow deplete the alpha stabilizers,which reduce the driving force of theα_s formation.With the increase in aging temperature,the area fraction of precipitation-free region decreases because higher aging temperature could provide more driving force of heat diffusion.As shown in Fig.5c,the precipitationfree region is eliminated completely when aged at 500℃.Moreover,the uniformity ofα_s size also decreases with the increase in aging temperature,especially aged at 540℃,where the smallest size is 100 nm and the largest size is773 nm in Fig.5d.

During aging treatment,the variations of content and size ofα_s which are dramatically affected by aging temperature are clearly shown in Fig.6.As shown in Fig.6a,theα_s content increases with the increase in aging temperature,which is associated with the disappearance of precipitation-free zone.When aged at 500℃,no-precipitation zone is eliminated completely andα_s content is up to68 vol%.Secondly,when aging temperature increases continuously,α_s content decreases because the fewer supercooling degree leads to the reduction in nucleation ofα_s.Especially at 540℃,α_s content is even down to 40vol%.As shown in Fig.6b,the average size ofα_s increases with the increase in aging temperature.The reason is possibly that higher aging temperature can provide more driving force of growth for a_s phase [ 22] .And the average size is even up to 270 nm when aging at 540℃.

3.2 Mechanical properties

The tensile properties of the current alloy with different processed states are listed in Table 2.It can be observed that the multidirectional forged alloy exhibits a moderate strength level (1072 MPa) with an excellent ductility(14.78%).And both the ultimate strength and the ductility are superior to those ofβtitanium alloy processed by traditional hot working [ 23, 24] .This result further confirms the significant role of multidirectional forging in the improvement of mechanical properties.After solution and aging treatment,it is obviously noted that higher strength can be obtained in comparison with that of as-forged alloy.The improvement in strength is partially due to the precipitation of needle-likeα_s,which can lead to the creation of a large number ofα/βphase boundaries which effectively inhibit the dislocation motion [ 25] .The yield strength is also associated with the content and average size ofα_s and can be expressed by the following rule of mixture [ 26] :

Fig.3 SEM images of alloys after multidirectional forging:a low-magnification view and b high-magnification view

Fig.4 Low-magnification SEM images of alloys treated at 775℃plus aged for 8 h at a 420℃,b 460℃,c 500℃and d 540℃

Fig.5 High-magnification SEM images of alloys aged treatment for 8 h at a 420℃,b 460℃,c 500℃and d 540℃

Fig.6 Evolution ofα_s with aging temperature:aα_s content and b width ofα_s

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Table 2 Tensile properties of current alloy processed at different conditions (AC:air cooling)

whereσ,fαp,fαs,d_αp) and d_αs are the yield strength,area fraction ofα_p,area fraction ofα_s,size ofα_p and size ofα_s,respectively.According to Eq.(2),the strength level of alloy after heat treatment is improved by the precipitation ofα_s and the finerα_p.

The ductility of alloy after heat treatment is lower than that of forged alloy which may be contributed to the precipitation of fine needle-likeα_s,and the variation of ductility after aging treatment confirms such interpretation as well.Meanwhile,Chen et al. [ 21] have shown that the lower content of globularα_p is beneficial to the improvement of ductility ofβtitanium alloy.Hence,lower content of globularα_p compared to that of the forged alloy is also beneficial to the ductility level in some degree during aging treatment.

The influence of aging temperature on tensile properties of the current alloy after solution-treated at 775°C is shown in Fig.7.As exhibited in Fig.7a,the values of ultimate strength (UTS) and yield strength (YS) increase with the increase in aging temperature from 420 to 500°C,and then they decrease with the increase in aging temperature continuously.As shown in Fig.7b,the variation of ductility level (elongation (EI) and reduction in area (RA))presents the opposite way in comparison with strength.What is worth mentioning is that the better combination of strength and ductility alloy can be obtained than that of Ti-5553 which shows ultimate strength of>1200 MPa and elongation of 10%when aged at 600°C [ 20] .

As mentioned above,the content and size ofα_s have a significant influence on mechanical properties of beta titanium alloys [ 27] .The variation of strength level is consistent with the change ofα_s content,indicating thatα_scontent is a major factor affecting mechanical properties.As shown in Fig.5,some precipitation-free zones exist in theβmatrix and disappear when aging temperature is up to500℃,because the increase in fineα_s content can lead to larger number ofα/βphase boundaries which act as effective dislocation barriers.Especially aged at 500℃,the ultimate strength is up to 1529 MPa with a moderate elongation (9.22%),whereα_s grains with the highest content (68 vol%) and suitable average size (174 nm) are precipitated in theβmatrix.Subsequent,the lowestα_scontent leads to the drop in strength.Moreover,the increase inα_s size also leads to the drop in strength with the increase in aging temperature.Especially aged at the540℃,the coarsestα_s (the average size is 274 nm) leads to sharp reduction in strength as well.Meanwhile,the micros truc ture distributed non-uniformly probably has a negative influence on strength as well.

Fig.7 Tensile properties of alloys processed at different conditions:a strength and b ductility

Fig.8 SEM images of tensile fracture surfaces of alloys after a multidirectional forging and b aging treatment at 500℃

In this paper,the alloys after aging treatment present excellent ductility level.As analyzed above,the reasonable explanation should be attributed to the precipitation ofα_p.And the variation of ductility is opposite with the change ofα_s content,which also indicates that the better ductility can be obtained by lowerα_s content.In addition,largerα_s size has a significant influence on the improvement of ductility according to the research of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy [ 21] ,which confirms that current alloy obtains higher ductility (elongation of 12.78%) when aged at540°C.Meanwhile,according to Hall-Petch relationship [ 28] ,the small grain size and bi-model microstructure can greatly improve crack nucleation resistance,resulting in the improvement of the ductility.

The fracture surface of alloy after multidirectional forging and solution-treated at 775℃plus aged at 500°C is shown in Fig.8.As shown in Fig.8a,the fracture surface of the alloy after multidirectional forging,which exhibits excellent ductility level,presents numerous deep and small dimples.In contrast,Fig.8b presents relatively shallow and larger dimples.These results indicate that the number and morphology of dimples can affect the ductility strongly.And the finer and deeper dimples would lead to better ductility,which is associated with other investigations [ 29, 30] .

4 Conclusion

Firstly,the current alloy after multidirectional forging presents good comprehensive mechanical properties(strength of 1072 MPa and elongation of 14.78%) due to the precipitation of fineα_p with low content inβmatrix.Secondly,with the increase in aging temperature,the average size of secondaryαincreases due to the more driving force at higher temperature.The content ofα_sincreases first and then decreases because of the disappearance of the precipitation-free zone and the fewer supercooling degree.Lastly,the alloy after solution and aging treatment exhibits a good combination of strength and ductility.The strength values increase first and then decrease with the increase in temperature due to the variation of content and size ofα_s,and the ductility values present the opposite way.When aged at 500°C,the high strength of 1529 MPa is obtained with a good ductility of9.22%due to the highest content (68 vol%) and suitable size (174 nm) ofα_s.