Evolution of Microstructure and Texture during Recrystallization of the Cold-Swaged Ti-Nb-Ta-Zr-O Alloy
W.Y.GUO,H.XING,J.SUN,X.L.LI,J.S.WU,and R.CHEN
The deformed microstructure and evolution of microstructure and texture during recrystalli-
zation of the cold-swaged multifunctional Ti-23Nb-0.7Ta-2Zr-1.2O(TNTZO,at.pct)alloy
were investigated by optical microscope,electron backscatter diffraction,and transmission
electron microscope.This alloy has been reported,by Saito et al.,to posss a specific dislo-
cation-free plastic deformation mechanism.In this study,the results show a curly grain or
swirled structure and a pronouncedfibrous110
h i texture along the swaging axis in the
cold-swaged TNTZO alloy.The normal to the swirled grain surface is near001
h i in the cross
ction of the rod.This characteristic microstructure can be considered to ari from the plane
strain deformation of the grains under applied stress,which is similar to that in ordinary bcc
metals after heavily drawing or swaging.It is also shown that recovery involves the redistri-
bution and partial annihilation of dislocations within the deformation bands,and recrystalli-
zation proceeds by a typical new grain nucleation-growth mechanism during annealing of the
TNTZO alloy.Thefibrous110
h i deformation texture is gradually replaced by random orien-
tations with increasing annealing time.Thus,it could be concluded that the TNTZO alloy
deforms by the traditional dislocation glide on111
h i110
f g,{112},or{123}slip systems,rather
than the dislocation-free mechanism.
DOI:10.1007/s11661-007-9433-x
ÓThe Minerals,Metals&Materials Society and ASM International2008
I.INTRODUCTION
T ITANIUM and its alloys have become one of the most attractive biomaterials due to their excellent strength,corrosion resistance,and high biocompatibil-ity.Recent concern with potential toxic alloying elements of V and Al in the widely ud Ti-6Al-4V alloy has led to the development of new b-type titanium alloys with a low modulus for biomaterials u,which only contain nontoxic alloying elements,such as Nb,Ta, and Zr.The b titanium alloys with composition of Ti-13Nb-13Zr,Ti-29Nb-13Ta-4.6Zr,Ti-35Nb-5Ta-7Zr, Ti-29Nb-13Ta-4.6Sn,and Ti-12Mo-6Zr-2Fe are typical examples.[1–4]
Recently,Saito et al.developed a group of multi-functional b titanium alloys with IVa,Va elements,and oxygen,which are basically expresd as Ti3(Ta+ Nb+V)+(Zr,Hf)+O.The alloys share three electronic numbers,(1)a valence electron number(e/a) of about4.24,(2)a bond order(Bo value)bad on the DV-X a cluster method of about 2.87,and(3)a d electron orbital energy level(Md value)of abo
ut 2.45eV,and simultaneously offer many super mechan-ical properties and drastic changes in physical proper-ties,such as super elasticity,super plasticity,super high strength,and Invar and Elinvar properties.In order to achieve the super properties,tho alloys must contain a certain amount of oxygen and be heavily cold worked. Moreover,tho alloys show much less work hardening than ordinary metals,which was attributed to a unique dislocation-free plastic deformation mechanism pro-pod by Saito et al.[5,6]Detailed studies have been performed by Kuramoto et al.[7]and Gutkin et al.[8]on the plastic deformation of the multifunctional b tita-nium alloys.The results showed that planar nanoscopic areas of local shear are typical elements of the defect structure of the deformed alloys,which can effectively be modeled as dipoles of nonconventional partial dislocations with arbitrary,nonquantized Burgers vec-tors,and a large amount of elastic strain energy is stored discretely and hierarchically during deformation in the alloys.[8]It is well known that the stored strain energy accumulated during the plastic deformation results in occurrence of recovery and recrystallization during the annealing process,which is strongly related to the redistribution and annihilation of dislocations and the formation of new strain-free grains in ordinary metals.However,little work about the recovery and recrystallization has been reported so far for the multifunctional b titanium alloys,which posss a unique plastic deformation mechanism.In the prent article,deformed microstructure and recrystallization of the multifunctional b titanium alloy wit
h a nominal composition of Ti-23Nb-0.7Ta-2Zr-1.2O(at.pct)alloy after cold swaging has been investigated.Emphasis will be placed on the evolution of microstructure and crystallographic texture during the recrystallization process.
W.Y.GUO and H.XING,Postdoctoral Students,J.SUN, Professor and Principal Rearcher,X.L.LI,Rearch Assistant, and J.S.WU and R.CHEN,Professors,are with the School of Materials Science and Engineering,Shanghai Jiao Tong University, Shanghai200240,PeopleÕs Republic of China.Contact e-mail: jsun@
Manuscript submitted February11,2007.
allerganArticle published online January26,2008
II.EXPERIMENTAL
The round rods of Ti-23Nb-0.7Ta-2Zr-1.2O (TNTZO,at.pct)alloy after cold swaging were ud as the starting material for this study.The reduction ratio of the rods is about90pct.Samples were carefully cut out from the rods by wire-cutting machine,and then subjected to a heat treatment in argon atmosphere at 800°C,820°C,and840°C for5,10,15,20,and 30minutes,respectively,followed by brine q
uenching rapidly.X-ray diffractions were performed to reveal that all of tho samples are single b pha without a¢¢martensite and x pha.
Samples were mechanically polished and then etched with a20pct HF+20pct HNO3+60pct lactic acid solution for optical microscope obrvations.The point counting method was ud to quantify the microstruc-ture,from which the recrystallized fraction of the alloy subjected to the different heat treatments can be determined.Samples for electron backscatter diffraction (EBSD)analys were electropolished in a3pct per-chloric acid+97pct ethanol solution at a clod circuit voltage of25V.Crystal orientation measurements were performed using EDAX*EBSD facilities attached to the JEOL**JSM-6460scanning electron microscope(SEM)
operating at20kV.The thin foils were prepared by twin jet electropolishing in a6pct perchloric acid+30pct butanol+64pct methanol solution chilled to about -30°C at30V,and then obrved using the JEOL JEM-2100F transmission electron microscope(TEM) operating at200kV.All samples prepared previously were cross-ctional ones,which are perpendicular to the swaging direction.Hardness was examined at a load of200gf.
III.RESULTS AND DISCUSSION
The optical micrograph of the deformed microstruc-ture of the TNTZO alloy after heavily cold swaging is shown in Figure1,from which a characteristic marble-like structure can be en clearly and the marblelike structure appears to consist of asmblies offine filamentary structures,as earlier indicated by Saito et al.[5,6]Figure2is an inver polefigure of the cold-swaged TNTZO alloy determined by crystal orientation measurement,which shows a pronouncedfibrous110badger
h i texture along the swaging axis.The TNTZO alloy before cold swaging was also checked by crystal orientation measurement,and it was shown to be texture free.The same result has been achieved by X-ray diffraction.[9] The TEM micrograph of the deformed microstructure of the cold-swaged TNTZO alloy taken along the longitudinal direction of the rod is shown in Figure3. Many deformation bands can be obrved,which are nearly parallel with each other.Although dislocations could not be clearly identified,theflecked contrast in thisfigure implies inhomogeneous elastic strainfields existing inside the bands.The lected area electron diffraction(SAED)pattern of the deformation band shown in the top left in Figure3indicates that the[110] zone axis of the deformation band is almost parallel to the swaging axis,and the normal to the swirled grain surface is near the001
h i direction.The SAEDs were also performed for neighboring deformation bands,and similar results were achieved for the bands.
It was propod that the formation of the marblelike structure in the cold-swaged TNTZO alloy is associated with the appearance of the deformation band.During cold swaging,the lenticular deformation bands ap-peared in the coar equiaxed grainsfirst,then
the Fig.1—Microstructure of the cold-swaged TNTZO
alloy.
Fig.2—Inver polefigure at swaging axial direction of the cold-swaged TNTZO alloy.
*EDAX is a trademark of EDAX Inc.,Mahwah,NJ.
**JEOL is a trademark of Japan Electron Optics Ltd.,Tokyo.
bands are gradually distorted with increasing cold
working ratio,and last the marblelike structure was formed after 90pct cold swaging.[9]The bands are
thought to form via a shear deformation without any aid
of dislocations when the local stress reaches the ideal stress of the alloy.Saito et al.[5,6]and Gutkin et al.[8]trunk是什么意思
propod a unique dislocation-free plastic deformation
mechanism for this kind of multifunctional b titanium
alloys.According to them,dislocation glides as a result
of shear deformation in the 111h i direction and on the
{110},{112},or {123}plane in ordinary bcc metals do
not occur in the multifunctional b titanium alloys.
Instead,planar nanoscopic areas of local shear (nano-
disturbances)are considered to be typical elements国际教师节
of the defect structure of the deformed alloys.The
directions of the local shear can be 111h i 112f g and
110h i 110f g .The nanodisturbances can effectively be
modeled as dipoles of nonconventional partial disloca-
tions with arbitrary,nonquantized Burgers vectors,and哥伦比亚大学校训
the generation of nanodisturbances reprents a specific
and effective mechanism for plastic deformation in this
kind of alloys.[8]
alex watsonAs a matter of fact,the marblelike structure appears
similar to the curly grain or swirled structures,which are
commonly obrved in the transver ctions of tung-
sten,iron,and niobium bcc metal rods heavily deformed
either by rotary swaging or by wire drawing.[10–12]It has
been suggested that the curly grain or swirled struc-
tures result from a favorable plane strain elongation of考研科目
the grains for bcc metals under tensile-dominated
applied stress.The multiple glide or double glide of
dislocations along the 111h i 110f g ,{112},or {123}
system leads to extension parallel to [011]and contrac-
tion parallel to [100]in a grain and thus develops
elliptical cross ctions (deformation bands)normal to
the fiber axis.Meanwhile,neighboring grains with different orientations would have to bend around one another to satisfy compatibility,effectively forming ribbon shapes in three dimensions.The prent results of the fibrous 110h i texture along the swaging axis and the normal to the swirled grain surface near the 001h i direction in the cross ction indicate that the traditional dislocation glide on the 111h i 110f g ,{112},or {123}slip system should occur in the cold-swaged TNTZO alloy.Before heat treatments,the b transus temperature of the TNTZO alloy was estimated by the following empirical equation:T (°C)=885-8.5[Nb]-1[Ta]-2[Zr]+200[O],[13,14]where the elements in square brackets denote their mass percent concentration in the alloy.The b transus temperature of the alloy is about 635°C.It has been reported that a primary recrystal-lization occurs in the 90pct cold-swaged TNTZO alloy during annealing at 700°C,and a complete recrystalli-zation was achieved after annealing for 30minutes.[6]However,in the prent study,the metallographic inspection by optical microscope showed that the 90pct cold-swaged TNTZO alloy does not recrystallize even afte
r annealing at 750°C for 30minutes.Figure 4shows the microstructure of the cold-swaged alloy annealed at 800°C for 5minutes.It can be clearly en that recrystallized new grains have nucleated along the filament-like structures.Figure 5further shows the evolution of microstructure in the cold-swaged TNTZO alloy annealed at 820°C for different times.Deforma-tion bands are favorable nucleation sites for the recrys-tallized new grains.With increasing annealing time,more recrystallized grains form and new grains grow further.A complete recrystallization is achieved for the alloy with fairly equiaxed grains after annealing at 820°C for 30minutes.Moreover,there is no abnormal grain growth during recrystallization of the TNTZO alloy.The recrystallization via nucleation of new grains and grain growth at the expen of the deformed structure in TNTZO alloy is similar to that in ordinary bcc
metals.Fig.3—TEM substructure of the cold-swaged TNTZO alloy taken
along the longitudinal direction of the rod.The SAED pattern
shows that the normal to the swirled grain surface is near 001h i
.
Fig.4—Microstructure of the cold-swaged TNTZO alloy annealed at 800°C for 5min.
The relationship between the recrystallized fraction and annealing time at different temperatures for t
he cold-swaged TNTZO alloy is plotted in Figure6.It can be en that the relationship of the recrystallized fraction vs annealing time can be described by a sigmoid curve. This implies that the recrystallization of the alloy is a typical new grain nucleation–growth process.The acti-vation energy of recrystallization can be estimated by the Arrhenius equation:1/t=A exp(-Q/R T),where t is the time for50pct recrystallization,A the constant,Q the activation energy of recrystallization,R the gas constant,and T the Kelvin temperature of annealing. Using the data from Figure6,the activation energy Q was calculated to be180kJ/mol for recrystallization of the TNTZO alloy,which is much higher than that of cold-rolled commercial purity titanium(64to88 kJ/mol).[15]The high activation energy of recrystalliza-tion of the alloy is considered to ari from the high addition of oxygen and transitional metallic elements with the high melting point in the alloy.
Figure7shows the crystallographic texture of the cold-swaged alloy annealed at820°C for different
times. Fig.5—Microstructures of the cold-swaged TNTZO alloy annealed at820°C for(a)5min,(b)10min,(c)15min,(d)20min,and(e)30min.
The 110h i deformation texture is gradually weakened
with increasing annealing time.Eventually,the defor-
mation texture is nearly replaced by randomly oriented
grains after annealing for 30minutes.The TEM obr-
vations were also applied to the sample of the cold-
rackspaceswaged TNTZO alloy after annealing.Figure 8(a)
shows an early stage of recovery of the cold-swaged
TNTZO alloy annealed at 800°C for 5minutes.
Dislocation clearing and sub-boundaries are obviously
obrved inside the deformation bands.The Burgers vectors of the dislocations were determined to be 1=2111h i by diffraction contrast technique.Addition-ally,the nuclei located at boundaries of the bands were often found at this annealing,as shown in Figure 8(b).All features of the substructure obrved in the cold-swaged TNTZO alloy annealed at 800°C for a short time are similar to tho in ordinary bcc metals in partially recrystallized state.The results showed that the recovery of the cold-swaged TNTZO alloy during annealing is related to the redistribution and partial annihilation of dislocations,which must result from the plastic deformation process.The recrystallization pro-ceeds through the formation of strain-free nuclei at the boundaries of the deformation bands,and the nuclei grow at the expen of neighboring bands during annealing.The further indicate that the traditional dislocation glide on the 111h i 110f g ,{112},or {123}slip system should occur in the cold-swaged TNTZO alloy.The isothermal softening curves for the cold-swaged TNTZO alloy annealed at 800°C,820°C,and 840°C are shown in Figure 9.The hardness of the cold-swaged alloy drops slightly at the early stage of annealing and then remains constant,which is almost independent of the annealing temperature and annealing time.The higher hardness in the cold-swaged TNTZO alloy aris from a large amount of dislocations and elastic strain energy accumulated in the alloy,and the quick relea of the elastic strain energy and partial annihilation
of Fig.6—Plots of recrystallized fraction vs annealing time of the cold-
swaged TNTZO
alloy.Fig.7—Inver pole figures at swaging axial direction of the TNTZO alloy annealed at 820°C for (a )5min,(b )10min,(c )15min,(d )20min,and (e )30min.
dislocations lead to the drop of the hardness of the alloy
at early stage of annealing.IV.CONCLUSIONS The deformed microstructure and evolution of micro-structure and texture during recrystallization of the cold-swaged multifunctional TNTZO alloy were inves-tigated,and the results can be summarized as follows.
1.The TNTZO alloy posss a curly grain or swirled
structure after heavily cold swaging.The deforma-
tion texture is a typical fibrous 110h i texture along
the swaging axis,and the normal to the swirled
grain surface is near 001h i .This characteristic
microstructure can be considered to ari from theonaccountof
newage
plane strain deformation of grains in the alloy under applied stress,which is similar to that in ordin-ary bcc metals after heavily drawing or swaging.2.Recovery and recrystallization occur in the cold-swaged TNTZO alloy after annealing at 800°C to 840°C.Recovery involves the redistribution and partial annihilation of dislocations within the defor-mation bands,and recrystallization proceeds by a typical new grain nucleation–growth mechanism.The fibrous 110h i deformation texture is gradually replaced by random orientations with increasing annealing time.The general features of substruc-tures in the annealed TNTZO alloy are similar to tho found in ordinary bcc metals.From the pre-ceding results,it could be concluded that the TNTZO alloy deforms by the traditional dislocation glide on the 111h i 110f g ,{112},or {123}slip system,rather than the dislocation-free mechanism.ACKNOWLEDGMENTS
The authors acknowledge the financial support from
the National Natural Science Foundation of China
(Grant No.50571063)and the Science and Technology
Committee of Shanghai Municipal (Grant No.
04JC14054),and u of the facilities of the Key
Laboratory of the Ministry of Education for High
Temperature Materials and Testing.REFERENCES 1.T.Ahmed,M.Long,J.Silverstri,C.Ruiz,and H.Rack:Titanium 95Õ:Science and Technology ,The Institute of Materials,London,1996,pp.
1760–67.Fig.8—Bright-field TEM images of the cold-swaged TNTZO alloy annealed at 800°C for 5m
in:(a )recovery inside the deformation bands and (b )nucleation near the boundaries inside the deformation
bands.
Fig.9—Relationship between the hardness and annealing time at
800°C,820°C,and 840°C for the TNTZO alloy.
