目录结构

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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 Nucleation of SIMT process
3.2 Variant selection of SIMT process
3.3 Orientation dependence of SIMT process
4 Conclusion▲
图表▲

Rare Metals2020年第3期

Strain-induced martensitic transformation in biomedical Co-Cr-W-Ni alloys

Zi-Yi Zhu Li Meng Leng Chen

School of Materials Science and Engineering,University of Science and Technology Beijing

Metallurgical Technology Institute,Central Iron and Steel Research Institute

作者简介：*Leng Chen,e-mail:lchen@ustb.edu.cn;

收稿日期：4 September 2019

基金：financially supported by the National Key R&D Program of China (No.2017 YFA 0403804);

Strain-induced martensitic transformation in biomedical Co-Cr-W-Ni alloys

Zi-Yi Zhu Li Meng Leng Chen

School of Materials Science and Engineering,University of Science and Technology Beijing

Metallurgical Technology Institute,Central Iron and Steel Research Institute

Abstract：

The nucleation,variant selection,and orientation dependence of the strain-induced martensitic transformation(SIMT) process in biomedical Co-Cr-W-Ni alloys were investigated.The experimental results show that theε-hexagonal-close-packed phase was preferentially formed at the Σ3 twin boundaries and high-angle grain boundaries during the tensile process.The theoretical analysis shows that the variant selection of SIMT is governed by Schmid's law.However,the SIMTed ε-phase did not form equally on the two sides of the annealing twins,even though they had the same Schmid factor.This phenomenon is related to the mechanical work developed by the formation of the ε-phase.Only the side which has both high Schmid factor and high mechanical work can initiate the SIMT process.A strong <111> fiber texture was formed,and the ε-variants tended to appear in grains with orientations close to the<111> and <100> directions during the tensile process.These results can provide theoretical guidance for controlling the SIMT process of Co-Cr-W-Ni alloys to fabricate more reliable stents.

Keyword：

Biomedical Co-Cr-W-Ni alloys; Straininduced martensitic transformation; Nucleation; Variant selection; Orientation dependence;

Received： 4 September 2019

1 Introduction

Many types of metallic alloys,such as Ni-Ti [ 1] ,Ti-Nb [ 2, 3] ,Ti-6A1-4V [ 4, 5] ,and Mg-based alloys [ 6, 7] ,have been used in biomedical applications for bone implants and heart valves because of their high mechanical properties and high resistance to corrosion.Compared with these alloys,Co-Cr-W-Ni (CCWN) alloys are mainly used for stent elaboration since their fracture strain can exceed 40% [ 8, 9, 10, 11] .For example,certain stents need to undergo deformation of approximatelyε≈40%,whereas conventional Ti-6Al-4V alloys exhibit limited ductility(ε≈10%) [ 4] .The nominal chemical composition of CCWN alloys is 20 wt%Cr,15 wt%W,and 10 wt%Ni,with Co accounting for the remaining amount.Although CCWN alloys contain only 10 wt%Ni,their biocompatibility is sufficient for vascular implants,such as cardiovascular stents [ 12, 13, 14, 15] .The effects of heat treatment on microstructure and mechanical properties have been studied [ 16, 17, 18, 19, 20] .In equilibrium,theε-phase with hexagonal closed-packed (hcp) structure is stable at room temperature for pure Co [ 21] .However,a singleγ-phase with facecentered cubic (fcc) structure can be obtained as a metastable phase at room temperature after conventional casting or hot-working process because the 10 wt%Ni of CCWN alloys enlarged theγ-phase region.Such metastableγ-fcc-phase can transform into a stableε-hcpphase during plastic deformation,in a process called straininduced martensitic transformation (SIMT) [ 22, 23, 24, 25, 26] .The occurrence of SIMT inγ-fcc-phase is usually considered to proceed via the motion of a/6{111} Shockley partial dislocation [ 27, 28, 29] .In Co-Cr type alloys,the SIMTedε-phase is detrimental to coldworking because cracks initiate and propagate atε/γ-interfaces [ 30] .In the fabrication of biomedical stents,a cold drawing process is generally used to obtain the desired diameter and thickness.Therefore,studying the SIMT process during cold deformation has important industrial significance.To our knowledge,few studies have focused on the SIMT process of CCWN alloys.To develop more reliable biomedical CCWN alloys,we need to clarify the factors that affect the SIMT process in CCWN alloys.

In the present study,the SIMT behaviors in biomedical CCWN alloys were investigated.Aside from nucleation at the annealing twin boundaries,high-angle grain boundaries(HAGBs) formed by the rotation of twin boundaries were found to be another nucleation site of the SIMTedε-phase.The variant selectivity of the SIMT process was governed by Schmid's law and mechanical work.A strong<111)fiber texture was formed during tensile deformation,and the SIMT process tended to occur in<111>and<100>-oriented grains.

2 Experimental

CCWN alloy ingot was obtained by vacuum smelting and secondary electroslag remelting,and then hot-rolled into a round bar ofΦ25 mm.The composition is as follows(wt%):0.070 C,19.570 Cr,10.660 Ni,14.550 W,1.200 Fe,1.280 Mn,0.100 Si,0.001 S,0.004 P,and the remainder as Co.The hot-rolled bar was aged in a nitrogen atmosphere(3 L-min^-1) at 1200℃for 2 h to obtain a singleγ-phase.The specimens used in tensile tests were first machined into the shapes with gage diameter and length of 5 and 60 mm,respectively,and then pulled at a strain rate of 1×10^-2s^-1.The tensile testing procedure was stopped at strain amplitudes of 20%,30%,and 40%.X-ray diffraction(XRD,Bruker D8) and electron backscatter diffraction(EBSD,Zeiss Ultra 55) analyses were performed on the cross sections in the middle of the tensile bars.The specimens used for EBSD observations were electrochemically etched with a 5 vol%perchloric acid alcohol solution.The FEI Tecnai G2 F20 at 200 kV was used to perform the transmission electron microscope (TEM) observation.

3 Results and discussion

3.1 Nucleation of SIMT process

XRD patterns of different tensile strains are shown inFig.1a.The intensities of diffractionpeaks became stronger with tensile strain increasing.On the other hand,the intensity of{022}γdecreased,which suggests that theγ-grains gradually rotated and texture was formed during tensile deformation.As shown in Fig.1b,the content of theε-phase increased monotonically with tensile strain.The content of theε-phase (x) was calculated by the following equation [ 31] :

(1)

whereI{200}γand are the integrated intensities of the{200}γand diffractions,respectively.

Figure 2 presents EBSD inverse pole figures (IPF),phase maps,and grain boundary distribution maps of the 0,20%,30%,and 40%strained specimens.It can be seen that the grain size of the original specimen is approximately120μm,and the grain boundaries are dominated byΣ3(60°<111>) annealing twin boundaries (ATBs),which account for approximately 60%.When the tensile strain reached 20%,the SIMTedε-phase was preferentially formed at theΣ3 twin boundaries,as shown by the arrow in Fig.2b'.However,only the twins with soft orientations can make the SIMT process occur.The twins with hard orientation gradually rotated and transformed the original straight twin boundaries into twisted HAGBs.This is the reason why the proportion ofΣ3 twin boundaries dropped from 60%to 11%.The preferential occurrence of the SIMT process at theΣ3 twin boundaries in CCWN alloys was also proposed in Ueki's study [ 22] ;however,the SIMTedε-phase started to nucleate at the HAGBs formed by the rotation of the ATBs when the tensile strain increased to30%,as indicated by the arrow in Fig.2c'.The proportion of HAGBs decreased when the tensile strain reached 40%because the nucleation of the SIMTedε-phase at the HAGBs transformed the original HAGBs of y-phase into theγ/ε-interfaces.Considering that the surface energy of HAGBs is approximately 0.65 J·m^-2 and the twin surface energy is 0.0127 J·m^-2 for pure Co [ 32] ,the higher surface energy of HAGBs can constitute the driving force for the nucleation of theε-phase.However,the SIMT process began to occur inside the grains when the tensile strain increased to 40%,as shown in Fig.2d'.

Figure 3 shows a nucleation model of the SIMT process that occurred at the∑3 twin boundaries.It can be seen that an ABA-type stacking structure of theε-hcp-phase existed at the∑3 twin boundary.This kind of stacking structure could be a nucleation site of theε-phase due to the low nucleation energy.Then,the motion of 1/6<112) Shockley partial dislocation on the{111}plane can change the stacking structure from the ABCABC (γ-phase) to ABA-BAB (ε-phase).Thus,the SIMT process preferentially occurred at theΣ3 twin boundaries in CCWN alloys.However,solute segregation is another possible reason for the preferential occurrence.In Co-Cr-Mo alloys,Koizumi et al. [ 33] examined the effects of solute segregation at∑3twin boundaries based on phase-field simulation and concluded that the segregation of Cr and Mo atoms atΣ3 twin boundaries promoted the phase transformation in Co-CrMo alloy because of the slightly decreasing energy of the hcp structure.

Fig.1 a XRD patterns of specimens with different tensile strains and b content ofεphase

As mentioned above,the SIMT process can also occur inside the grains when the tensile strain increases to 40%.To clarify the nucleation mechanism that causes the SIMT process to occur inside the grains,we investigated the microstructures of 0%strained CCWN alloys by highresolution transmission electron microscopy (HRTEM).Figure 4a shows an HRTEM image of the stacking faults in the 0%strained CCWN alloys.Figure 4b shows the reverse Fourier transformed pattern of the white square shown in Fig.4a.The ABA-type and CBC-type stacking structures of theε-hcp-phase were contained in the stacking faults,as shown in Fig.4b.Thus,the stacking faults inside the grains could be another nucleation site of theε-phase due to the low nucleation energy.This is direct evidence that why the SIMT process can also occur inside the grains when the tensile strain reached 40%.Above all,we conclude that the SIMT process can be adjusted by controlling the density of stacking faults in CCWN alloys.

Figure 5 a shows a bright field TEM image of the 40%tensile deformation sample.The selected area diffraction(SAD) pattern of the whole area is shown in Fig.5c.The indices of diffraction spots are schematically presented in Fig.5d.Figure 5b shows the related dark field image (DFI)of the same area with the beam direction parallel to the zone axis.The changing contrast of theε-phase can be seen in the DFI,indicating the high density of planar defects or lattice distortion.TEM results also show that the orientation relationship between the y-matrix and SIMTedε-phase conforms to the S-N relationship-{0001}_ε||{111}γ,

3.2 Variant selection of SIMT process

The SIMT process in Co-based alloys is usually considered to proceed via the motion of Shockley partial dislocation [ 27] .Thus,we only consider the Schmid factors of Shockley partial dislocations.For an n-grain polycrystalline matrix,the orientation of the jth grain is defined as (h_jk_jl_j)[u_jv_jw_j].Since the EBSD test was conducted at the cross section of the tensile bar,[h_jk_jl_j]is parallel to the loading direction.Table 1 lists the calculated Schmid factors of{111}(112) partial dislocation slip systems in the three grains denoted in Fig.6b.The trace normal passes matching poles (TNPMP) relationship,as the name suggests,is a method to determine the crystallographic index of the crystal interface by measuring the EBSD data [ 34] .Considering that the orientation relationship between theγ-phase andε-phase has been identified to conform to the S-N relationship-{0001}_ε||{111}γ, in CCWN alloys,theplane index of theγ/ε-interface can be determined by the TNPMP relationship.Theγ/ε-interfaces in G1 grain are ,as shown in Fig.6c,d.By comparing theplane index with the calculated Schmid factor (SF),it is easy to identify whether the variant selectivity conforms to Schmid's law.

As shown in Table 1,the SF values of the B1 and C3slip system in G1 grain are maximum and sub-maximum.This is consistent with the experimental results that the variants appeared in G1 grain,as shown in Fig.6.The maximum SF value in G2 grain is 0.24,which is not high enough to promote the SIMT process.That is the reason why noε-phase appeared in G2 grain.The plane index of the y/ε-interfaces in Grain 3₁ was determined to be ,which includes the maximum SF value,as shown in Table 1.In conclusion,variant selection in biomedical CCWN alloys conforms to Schmid's law.In general,there is only one variant inside a grain.But when the submaximum value of SF is close to the maximum,there may be two variants,as shown in Fig.6b.It is worth mentioning that no SIMTedε-phase was formed in G3₂ (one side of the twin),although the maximum Schmid factor is similar to that of G3₁ (the other side of the twin).This phenomenon indicates that Schmid's law cannot explain why theε-phase did not form equally on the two sides of the annealing twins.

Fig.2 EBSD-IPF,phase maps,and grain boundary distribution maps of a,a',a"0%strained,b,b',b"20%strained,c,c',c"30%strained,and d,d',d"40%strained specimens

Fig.3 Schematic showing SIMT process at twin boundary,where blue rectangles represent stacking structure ofε-phase

Another possible factor influencing variant selectivity is the mechanical work developed by the formation of theε-phase.As Humbert [ 35] suggested,theγ-fcc-phase toε-hcp-phase transformation can occur when every adjacent(111)γplanes are displaced toward the direction by .By these successive displacements,the fcc lattice is sheared by .As shown in Fig.7,the reference frame is defined by the basis vectors .Based on Humbert [ 35] ,this plane strain can be expressed by matrix D:

In the standard reference frame X(x₁-x₂-x₃),matrix D_xcan be obtained by

D_x=M·D·M^-1 (3)

where M is the rotation matrix:

Fig.4 a HRTEM images and b reverse Fourier transformed pattern of area indicated by white square in a of 0%strained CCWN alloy

Fig.5 TEM images of CCWN alloy after 40%tensile deformation:a bright field image,b dark field image,c selected area diffraction of whole area in a

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Table 1 Schmid factors for possible slip systems of grain G1,G2,and G3 in Fig.2d'

and the corresponding deformation tensor is equal to

E_Dx=<D_x+^TD_x)-I (5)where T indicates the transposed matrix andIis the identity matrix.

Based on the above operations,the deformation tensor of the 12 variants has been calculated.The grain orientation can be acquired by EBSD data,which means that the deformation tensor in the macroscopical reference frame can be obtained using traditional tensor transformations.The mechanical work corresponding to a given strain is calculated by

E=(1/2)σ·ε_i (6)whereσis the local stress also expressed in the macroscopical reference frame.

In the calculation process,we assumed that the local stress is the same as the applied macroscopical stress.For calculation,a hypothetical uniaxial stress of 200 MPa was applied [ 35] .Table 2 lists the calculated strain work per volume unit of the potential 12 variants.The bold figures give the highest mechanical work of variants in Grain3₁ and Grain 3₂.It can be seen that theε3 variant in Grain3₁ has the highest mechanical work,which is the most energetically formed.Thus,we can conclude that besides the SF,mechanical work also plays an important role in the SIMT process.Only variants that have both high SF and high mechanical work can be formed preferentially.

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Table 2 Mechanical works calculated forσ=200 MPa

3.3 Orientation dependence of SIMT process

The texture development of CCWN alloys in response to uniaxial tensile deformation was evaluated by the acquired EBSD data.The ODFs in theφ2=45°sections of the specimens at different tensile deformations are shown in Fig.8.It can be seen that when the tensile deformation reached 40%,the texture was dominated by a strong<111>fiber texture.

In order to study the effect of crystal rotation on the SIMT,we traced the orientations of 55γ-grains in the specimens of different tensile strains,as shown in Fig.9.The solid and hollow circles represent the grain orientations with and without SIMTedε-phase,respectively.It can be seen that the grain orientation gradually turned to the<100>and<111>direction,while the grains with<110>orientation are substantially absent.This phenomenon may be related to the effective stacking fault energy (SFE),as suggested by Lee [ 31] .On the other hand,SIMT primarily occurred in the<111>and (100)-oriented grains when the tensile strain reached 40%.With increasing tensile strain,the dislocations gathered near HAGBs and high dislocation density was formed.The high dislocation density contributes to the occurrence of stacking faults at the HAGBs.These stacking faults gradually developed in the grain,and their overlap led to the evolution of the SIMTedε-phase.

Fig.6 EBSD results of 40%strained specimens:a IPF,b phase map,c{111}pole figure of Gl,and d{0001}pole figure of two variants in G1

Fig.7 Frame of reference

4 Conclusion

The nucleation site,variant selectivity,and orientation dependence of SIMT in biomedical CCWN alloys were investigated in this work.The SIMTedε-phase preferentially formed at theΣ3-type twin boundaries.When the tensile strain reached 30%,the SIMTedε-phase initiated at the HAGBs was formed by the rotation of twin boundaries.In the CCWN alloys,the SIMT process conforms to Schmid's law.Only one variant appeared in a grain unless the sub-maximum SF value is similar to the maximum.The phenomenon in which theε-phase did not form equally on the two sides of the∑3 twin is dominated by the mechanical work.Only the side with both high SF value and high mechanical work can initiate the SIMT process.In the tensile process,a strong (111) fiber texture was formed and the SIMT process tended to occur in<111>and<100>-oriented grains.