简介概要

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

来源期刊：Rare Metals2018年第9期

论文作者：Qing-Kun Meng Yu-Fei Huo Yan-Wei Sui Jin-Yong Zhang Shun Guo Xin-Qing Zhao

文章页码：789 - 794

摘要：In this paper, high density of dislocations, grain boundaries and nanometer-scale a precipitates were introduced to a metastable Ti-36Nb-5Zr alloy （wt%） through a thermo-mechanical approach including severe cold rolling and short-time annealing treatment. The martensitic transformation was retarded, and theβphase with low content ofβstabilizers was retained at room temperature after the thermo-mechanical treatment. As a result, both low modulus （57 GPa） and high strength （950 MPa） are obtained.The results indicate that it is a feasible strategy to control martensitic transformation start temperature through micros true ture optimization instead of composition design,with the aim of fabricating low modulusβ-type Ti alloy.

稀有金属(英文版) 2018,37(09),789-794

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

Qing-Kun Meng Yu-Fei Huo Wen Ma Yan-Wei Sui Jin-Yong Zhang Shun Guo Xin-Qing Zhao

School of Materials Science and Engineering,China University of Mining and Technology

GRINM Bohan (Beijing) Publisher Co.,Ltd.,GRINM Group Co.,Ltd.

School of Materials Science and Engineering,Jiangsu University

School of Materials Science and Engineering,Beihang University

作者简介：*Shun Guo,e-mail:shunguo@ujs.edu.cn;

收稿日期：14 March 2017

基金：financially supported by the National Natural Science Foundation of China (No.51601217);the Natural Science Foundation of Jiangsu Province (No.BK20160255);the Fundamental Research Funds for the Central Universities (No.2017QNA04);

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

Qing-Kun Meng Yu-Fei Huo Wen Ma Yan-Wei Sui Jin-Yong Zhang Shun Guo Xin-Qing Zhao

School of Materials Science and Engineering,China University of Mining and Technology

GRINM Bohan (Beijing) Publisher Co.,Ltd.,GRINM Group Co.,Ltd.

School of Materials Science and Engineering,Jiangsu University

School of Materials Science and Engineering,Beihang University

Abstract：

In this paper, high density of dislocations, grain boundaries and nanometer-scale a precipitates were introduced to a metastable Ti-36Nb-5Zr alloy (wt%) through a thermo-mechanical approach including severe cold rolling and short-time annealing treatment. The martensitic transformation was retarded, and theβphase with low content ofβstabilizers was retained at room temperature after the thermo-mechanical treatment. As a result, both low modulus (57 GPa) and high strength (950 MPa) are obtained.The results indicate that it is a feasible strategy to control martensitic transformation start temperature through micros true ture optimization instead of composition design,with the aim of fabricating low modulusβ-type Ti alloy.

Keyword：

Biomedical Ti alloys; Martensitic transformation; Low modulus; Short-time annealing;

Received： 14 March 2017

1 Introduction

Nowadays,titanium and its alloys have attracted considerable attention in the application of biomedical implants,which could be attributed to their superior properties such as light weight,high corrosion resistance,excellent biocompatibility and balanced mechanical properties [ 1] .Among the mechanical properties,elastic modulus is essential for biomedical application whose value should be as close as that of human bone [ 2] .The Young's modulus of the (α+β) Ti-6A1-4V is about four times that of human bone (～30 GPa),although it is substantially lower in comparison with that of 316L stainless steel or Co-Cr alloys (about 200 GPa) [ 3] .The large modulus mismatch between implant material and human bone would lead to the formation of stress shielding,which could result in bone resorption and thus failure of implant material [ 4] .Besides,the A1 and V irons released from Ti-6A1-4V are considered as toxic and could lead to long-term health problems [ 5] .In order to minimize the stress shielding phenomenon and enhance the biocompatibility,there has been a significant push to developβ-type Ti alloys with both low modulus and high strength,using non-toxic elements such as Nb,Zr,Ta and Sn [ 6, 7, 8, 9] .

It is well known that the modulus of theβ-type Ti alloy is closely related to its phase stability which is controlled by the amount ofβ-stabilizing elements such as Nb,Mo,Ta [ 10] .The Young's modulus ofβphase decreases monotonically with the total content ofβstabilizers decreasing [ 11] .Thus,the lowest modulus of theβ-type Ti alloy could be achieved at such a composition that contains the least amount ofβ-stabilizing elements and keeps singleβphase simultaneously.However,a stress-inducedβ→α"martensitic transformation would occur during loading if the content ofβ-stabilizing elements was insufficient [ 12] .This would give rise to the"double yielding"phenomenon,and thus,the yielding stress could be quite low,e.g.,100-200 MPa [ 13] .It is not a desirable way to suppress martensitic transformation and improve yielding stress by conventional precipitation hardening,sinceαorωprecipitates have much higher modulus in comparison withβphase [ 11] .On the other hand,precipitation ofαphase could lead to the enrichment ofβ-stabilizers in the residualβmatrix,which is detrimental for obtaining low modulus [ 14] .

The martensitic transformation start temperature (M_s) is affected by both chemical composition and microstructure.It has been reported that martensitic transformation can be inhibited by grain refinement,high density of dislocations and nanometer-scale precipitates [ 15, 16] .In comparison,the elastic modulus of theβphase depends on its composition and is insensitive to the micros true ture.Therefore,it would be expected that both low elastic modulus and high strength could be achieved in aβ-type Ti alloy with low phase stability,if the martensitic transformation start temperature was decreased by modifying microstructure while keeping the chemical composition unchanged.In this paper,the microstructure of a Ti-Nb-Zr was optimized through a thermo-mechanical treatment,with the aim of obtaining combined low modulus and high strength.Also,the influence of the microstructural evolution on the martensitic transformation start temperature and mechanical properties was briefly discussed.

2 Experimental

An ingot with the nominal composition of Ti-36Nb-5Zr in wt%was produced via vacuum arc melting from highpurity Ti (99.99%),Nb (99.95%) and Zr (99.95%) materials.The ingot was re-melted four times to improve homogeneity and forged into a billet with a cross section of8 mm×60 mm at 1173 K.The hot-forged billet was encapsulated in quartz tube and then homogenized at1223 K for 5 h,followed by water quenching via breaking the quartz tube.The homogenized billet was cold-rolled to a thickness of 1 mm at a reduction of 87.5%without intermediate annealing.Specimens were cut from the coldrolled plate using electrical discharge machining.The coldrolled specimens were annealed at 673 K for 20 min and then quenched into water (referred to as CRA specimens hereafter).On the other hand,part of the cold-rolled specimens was solution-treated at 1073 K for 1 h in an evacuated quartz tube and finally quenched into water by breaking quartz tubes (referred to as ST specimens hereafter).

Uniaxial tensile tests were performed on an Instron 8801machine at room temperature with a strain rate of1×10^-4 s^-1.Tensile specimens have a rectangular cross section of 1.46 mm² and a gage length of 30 mm,with the rolling direction parallel to the loading axis.To ensure the accuracy of the Young's modulus,an extensometer with a gage length of 25 mm was used to measure the strain.Phase constitutions were determined using an X-ray diffractometer (XRD,Rigaku D/max2550) with Cu Kαradiation at an accelerating voltage of 40 kV and a current of 250 mA.Microstructural characterization was examined on a transmission electron microscope (TEM,FEI Tecnai G2 F20) operating at a voltage of 200 kV.TEM specimens were prepared by a twin-jet electro-polishing technique at about 243 K,using a solution of 9 vol%perchloric acid,21vol%n-butyl alcohol and 70 vol%methanol.Dynamic mechanical analysis (DMA) was conducted on a TA Q800in single cantilever mode with amplitude of 15μm,dynamic stress frequency of 1 Hz and cooling rate of5 K.min^-1.

3 Results and discussion

Figure 1 shows XRD patterns of the ST and CRA specimens.A dual (β+α") phase constitution can be identified from the XRD pattern of the ST specimen,suggesting that the M_s is above the room temperature.The occurrence of martensitic transformation on the quenching of the ST specimen can be attributed to the insufficient content of 3-stabilizers in the Ti-36Nb-5Zr alloy.It is well known that previously developedβ-type Ti alloys for biomedical implant consist of singleβphase in ST state,e.g.,Ti-35Nb-5Ta-7Zr,Ti-29Nb-13Ta-4.6Zr and Ti-24Nb-4Zr-8Sn [ 2, 17, 18] .The Ti-36Nb-5Zr alloy possesses intrinsically low 3 phase stability and is thus beneficial to achieve low modulus.

Fig.1 XRD patterns of ST and CRA specimens

Theα"martensite vanishes after severe cold rolling plus annealing treatment,as shown from XRD pattern of the CRA specimen.This indicates thatα"martensite is transformed back to the parentβphase upon annealing treatment,andβα"phase transformation is suppressed during subsequent quenching,i.e.,the M_s of the CRA specimen decreases below room temperature.In addition toβphase,diffraction peaks corresponding toαphase are observed in CRA specimen,but the weak intensity of the peaks indicates that the amount ofαphase is quite low.Interestingly,the diffraction peak derived fromα{110}crystal plane appears to be broadening significantly.This might result from the successive crystal distortion fromβtoαstructure,as reported by a recent work from Wang et al. [ 19] .Moreover,it is worth noting that XRD angles of theβpeaks have not been shifted during annealing treatment.This indicates that the solute partitioning is subtle,which is important to improve the strength without sacrificing low modulus.Upon conventional annealing treatment,the precipitation ofαphase usually gives rise to the enrichment ofβ-stabilizing elements in the residualβphase,thus increasing the Young's modulus ofβphase [ 14] .However,the present thermo-mechanical treatment strategy does not result in the chemical stabilization ofβphase.Therefore,it is proposed that the decrease in M_soriginates from the microstructural change rather than the enrichment ofβ-stabilizing elements in parent phase.

Additional microstructural information of the CRA specimen is provided by TEM observation,as shown in Fig.2.From the bright-field image in Fig.2a,it can be seen that CRA specimen is not fully recrystallized from the as-rolled state,since apparent contrast presumably caused by dislocation tangles still exists.The corresponding selected area electron diffraction (SAED) pattern shown in Fig.2b exhibits nearly continuously diffraction rings,suggesting that the present thermo-mechanical treat leads to a considerable reduction in grain size.It was reported that the stress-induced martensitic transformation upon severe cold deformation combined with the subsequent reverse martensitic transformation could give rise to significant grain refinement for theβ-type Ti alloy with low phase stability [ 20, 21] .Considering that the present Ti-36Nb-5Zr alloy possesses lowβ-phase stability,it is believed that the grain refinement could be attributed to the stress-induced martensitic transformation upon cold rolling and the subsequent reverse transformation upon annealing treatment.Actually,our previous work indicated that the present thermo-mechanical strategy is feasible to introduce high density of dislocations and grain boundaries [ 12, 13] .

At higher magnification of the bright-field image,nanometer-scaleαprecipitates with about 50 nm in length and several nanometers in width can be identified,as shown by the white arrows in Fig.2c.This can be further demonstrated by the<100>βzone axis SAED pattern corresponding to Fig.2c,where the additional reflections at 1/2{110}βcould be indexed asαphase,as shown in Fig.2d.As is well known,αphase prefers to nucleate at defects ofβmatrix such as dislocations,grain boundaries and phase interfaces [ 22] .In the present study,numerous defects introduced by cold rolling could provide abundant heterogeneous nucleation sites forαprecipitates.On the other hand,the short annealing duration suppresses the growth ofαphase,leading to the formation of nanometerscale precipitates.Owing to the shear-like nature of martensitic transformation,microstructure can exert considerable influence on phase transformation process.In fact,high density of defects such as dislocations,grain boundaries and phase interfaces can suppress martensitic transformation in addition to chemical composition [ 23] .In the present work,high density of dislocations and grain boundaries as well as the fineαprecipitates are believed to be responsible for the decrease in M_s in the CRA Ti-36Nb-5Zr alloy.

It is difficult to determine the M_s of metastableβ-type Ti alloy by differential scanning calorimetry (DSC),since the entropy of theβ→α"phase transition is quite low [ 24, 25] .Therefore,DMA was used to characterize the M_sof Ti-36Nb-5Zr alloy in the present study.Figure 3 presents the evolution of the storage modulus as a function of temperature during cooling,accompanied with the oneorder derivative (the slope) of the modulus.It can be seen that the storage modulus of both ST and CRA specimen firstly increases,then deceases and finally increases on cooling.The first increase in the modulus originates from the aggrandizement of atomic bonding during cooling,while the decrease in the modulus is associated with the decreasingβphase stability and the final increase can be ascribed to the formation of stableα"martensite [ 26] .When the modulus decreases upon cooling,a change in the slop of the modulus can be easily observed from the derivative curves.The corresponding temperature was measured from the maximum value of the derivative and is termed M_s which is associated with the beginning of the martensitic transformation [ 27] .The M_s of the ST specimen (340 K) is clearly higher than room temperature(～300 K),implying that martensitic transformation will occur during quenching,which is consistent with XRD result,whereas the M_s of the CRA specimen is only 260 K and much lower than room temperature.These results provide unambiguous evidence that the martensitic transformation can be effectively retarded by appropriate thermo-mechanical treatment.

Figure 4 shows the tensile stress-strain curves of the ST and CRA specimens.The ST specimen exhibits typical double yielding behavior.The stress plateau at about110 MPa can be attributed to the stress-inducedβ→α” martensitic transformation and the reorientation of the martensite variants.It is apparent that the ST state is unsuitable for biomedical applications due to the low yielding stress.Interestingly,a considerable enhancement of the strength is achieved after cold rolling plus short-time annealing treatment since a yielding strength of 820 MPa and an ultimate tensile strength of 950 MPa are reached.As mentioned above,the M_s of CRA specimen decreases sharply through tuning microstructure,resulting in the vanishing of the low stress plateau.On the other hand,nanometer-scaledαprecipitates can serve as strengthening medium in theβmatrix by blocking dislocations in the sense of either the Orowan mechanism or the Friedel mechanism [ 28, 29] .Furthermore,the slip of dislocations becomes even more difficult with the presence of the high density of defects such as dislocations,grain boundaries and phase interfaces.Therefore,the significant improvement of the strength can be attributed to the suppression of martensitic transformation and dislocation movement.In addition to the high strength,the Young's modulus retains at a low level of 57 GPa due to the neglectable chemical stabilization of theβmatrix during annealing.Considering that mostβ-type Ti alloys have a modulus of about 80 GPa when the strength reaches 900 MPa [ 30] ,the present thermo-mechanical treatment is feasible to fabricateβ-type Ti alloy with balanced low modulus and high strength.

Fig.2 TEM images and corresponding SAED pattern of CRA specimen:a,b low-magnification and c,d high-magnification bright-field image

Fig.3 Storage modulus and its one-order derivative versus temperature during cooling for a ST and b CRA specimen

Fig.4 Tensile stress-strain curves of ST and CRA specimens

4 Conclusion

In this study,the micro structure design of the metastableβ-type Ti-36Nb-5Zr alloy was realized by the thermo-mechanical approach including severe cold rolling and shorttime annealing treatment,during which high density of dislocations,grain refinement and nanometer-scale precipitates were introduced while the chemical stabilization of theβmatrix was hindered.The M_s decreases significantly through the optimization of the micros true ture instead of the chemical composition.As a result,the stressinduced martensitic transformation is suppressed and the strength is greatly enhanced.Moreover,the modulus of the CRA alloy keeps at a relatively low value due to the retention ofβphase with low content ofβ-stabilizing elements.This study not only provides an optimal thermomechanical pathway to retard martensitic transformation by microstructure design,but also open a new avenue for design of novelβ-type Ti alloys with both low modulus and high strength.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51601217),the Natural Science Foundation of Jiangsu Province (No.BK20160255)and the Fundamental Research Funds for the Central Universities(No.2017QNA04).