Crystal orientation and morphology of α lamellae in wrought titanium alloys:On the role of microstructure evolution in β processing

Huojun ZHENG,Xiaoguang FAN,Xiang ZENG,Rui ZUO

State Key Laboratory of Solidification Processing,School of Materials Science and Engineering,Northwestern Polytechnical University,Xi'an 710072,China

KEYWORDS β forging;Burgers orientation relationship;Crystal orientation;Deformation band;Lamellar α;Titanium alloy

Abstract To obtain high-quality aviation forgings of titanium alloys,β forging is an essential processing step which must be considered throughout a production process.In this work,the effect of β forging on the crystal orientation and morphology of lamellar α was experimentally investigated in a two-phase titanium alloy.Strong dynamic recovery during β working resulted in the formation of low-angle grain boundary(LAGBβ)inside β grains.The lamellar α can penetrate through the LAGBβ,leading to similar intra α LAGBs on subgrain boundaries.Deformation banding occurs at high strain rates,and both diffusive and sharp boundaries of deformation bands can be observed.A continuous change of the β orientation in diffusive boundaries results in the formation of fine and disordered α lamellae without intra-lamellar boundary to hold the Burgers orientation relationship(OR).On sharp boundaries,it is prone to producing continuous grain boundary α(αGB)with a highly similar orientation along the boundaries.Meanwhile,there may exist several lower-angle boundaries within the grain boundary α for a smoother orientation change on the β grain boundary.

1.Introduction

Due to the high specific strength and good corrosion resistance,titanium alloys are widely applied in the aerospace industry.1,2The microstructure evolution throughout the entire production process needs to be considered for obtaining high-performance aerial forgings of titanium alloys.β forging of titanium alloys aims to homogenize and refine β grains.3,4After β forging,β phase transforms to a lamellar structure which consists of lamellar α phase and residual β phase during cooling.1,5To improve the comprehensive mechanical properties,subsequent(α+β)forging is often adopted to spheroidize α lamellae.Eventually,an equiaxed or bimodal structure can be formed.The spheroidization behavior is affected by the characteristics of α lamellae(crystallographic orientation,geometric orientation,lamellar morphology,etc.).6-8Meanwhile,the characteristics of the lamellar structure are greatly affected by β forging,4,9because β forging can determine the initial state of β grains prior to phase transformation.

Microstructural development in β forging is sensitive to deformation conditions.10,11Reported deformation mechanisms include dynamic recovery(DRV),12,13dynamic recrystallization(DRX),13,14and deformation banding(DB).12,15Initial β grains are upset in compression,and subgrains appear in DRVed grains after β forging.16DRX produces dislocationfree grains which are small and equiaxed.17DB divides grains into multi-parts with different morphologies and orientations.Meanwhile,it can produce two kinds of boundaries(diffusive and sharp boundaries).15Those initial structures may produce various lamellar structures.

Based on the calculation of the nucleation and growth rate of lamellar α,Teixeira et al.18built a model about the precipitation of lamellar α.They found that the grain boundary(GB)and misorientation of the prior β grains were related to the conditions of β forging,which affected the subsequent precipitating process of αGBand αWGB.Meanwhile,lamellae passed through LAGBβduring the growth process.It is noteworthy that the formation of a lamellar structure is also affected by other microstructural developments in β forging,i.e.,DB and DRX.He et al.19found that the Burgers OR ({0 0 0 1}α//{1 1 0}β, ＜11-20＞α//＜1 1 1＞β)was slightly affected by the applied strain,strain rate,and subsequent cooling rate in β forging.Meanwhile,the morphology and size of precipitating α phase were heavily affected by the strain rate and subsequent cooling rate.With the increase of the cooling rate and strain rate,the length of lamellae was significantly reduced,while the thickness of lamellae was reduced with an increasing cooling rate.However,the effects of the morphology and crystallographic orientation of prior β grains on lamellar α also need further investigation.

In this paper,β grains with various structures were produced using a series of hot working.The effect of the initial structure on the characteristics of α lamellae was investigated.It will provide theoretical guidance for microstructure regulation in primary hot working of titanium alloys.

2.Material and procedures

2.1.Material

The material employed is a TA15 titanium alloy with a measured chemical composition of Ti-6.68Al-2.25Zr-1.75Mo-2.26 V-0.14Fe(wt%),whose β transus is about 985°C.The as-received material was heated to 1020°C in an SXL 1200 resistance furnace,held for 1 h and furnace-cooled to obtain a colony structure,as shown in Fig.1,and the average size of initial β grains was about 750 μm.

2.2.Experimental procedures

Isothermal compression above the β transus temperature was employed to simulate β forging.Cylindrical samples with a size of ?10 mm×15 mm were adopted.Isothermal compression was carried out on a Gleeble-3500 simulator.The samples were heated to 1020°C with a heating rate of 10°C/s and held for 5 min before compression.To obtain β grains with various structures,hot compression was carried out at the temperature of 1020°C and strain rates of 0.01 s-1,1 s-1up to 50%height reduction.At low strain rates,DRX occurred easily,and equiaxed DRXed grains were obtained.14However,deformation band,elongated β grains,and β grains with LAGBβwere easily produced at high strain rates.12,15

Fig.1 Initial microstructure of TA15 alloy.

The compressed samples were cut axially for metallographic observation,and optical micrographs were taken on a Leica DMI3000 optical microscope. Scanning electron microscope(SEM)micrographs and electron backscattered diffraction(EBSD)data were obtained through a TESCAN MIRA3 XMU SEM equipped with a Nordlys Max EBSD detector.As the thickness of α lamellae was about 2 μm,the scanning step size was set to be 0.5 μm.The EBSD samples were electropolished for 40 s at 28°C with a voltage of 25 V in a solution of 65%methyl alcohol,30%butanol,and 5%perchloric acid.Channel 5 was used to analyze the EBSD data.

In this paper,a small fraction of residual β phase could be reserved after cooling down,and the residual β phase kept the original crystallographic orientation.Thus,the orientation of residual β phase was used to represent the orientation of the prior β grains.

3.Results

3.1.Microstructure of the undeformed material

Fig.2 shows the microstructure of the undeformed material,which is a typical basket-weave structure obtained by rapid cooling.α lamellae with diverse orientations intersect with each other.Lamellar αGBs are discontinuous and consist of pieces of short laths.From the pole figures of residual β phase and lamellar α phase in Fig.2(b)and(c),respectively,it can be seen that they obey the Burgers OR.19,20

The β phase in Grains 1 and 2 has a roughly close{1 1 0}crystallographic plane, as marked by the green circle in Fig.2(b),and the angle between the two normals of the planes is about 4°.Shi et al.21found that the{0 0 0 1}crystallographic plane of αGBwas almost parallel to the close{1 1 0}crystallographic plane in deformed grains to minimize interfacial energy and strain energy between neighboring β grains.However,the{0 0 0 1}plane of αGBisn't parallel to the specific{1 1 0}crystallographic plane in undeformed grains,as shown by the red circles and squares in Fig.2(b)and(c).

3.2.Microstructure of the β-deformed material

Fig.2 Microstructure of the undeformed material.

Previous work by the authors4suggested that the microstructure mechanism of the TA15 titanium alloy was greatly affected by the strain rate in β forging.The dynamic restoration mechanisms are diverse,resulting in prior β grains with different morphologies and substructures. As mentioned above,DRX occurs easily at low strain rates,and DRXed grains are equiaxed and dislocation-free.They are smaller than the initial β grains,as shown in Fig.3(a).DRX leads to a slight flow softening in the stress-strain curve at the strain rate of 0.01 s-1(Fig.4).At high strain rates,DRX will be restrained.It is also found in Fig.4 that the flow softening is replaced by a slightly strain hardening at high strain rates.DB becomes an important grain refinement mechanism,as shown in Fig.3(b).Boundaries of deformation bands(DBBs)are nearly perpendicular to the compression axis,splitting the original grain into strips with a very high aspect ratio.They often have a high misorientation angle above 30°.These DBBs can be sharp or diffusive.22The crystallographic orientation changes continuously in the diffusive boundary.The orientation change is often accommodated by the formation of cell structures.23On the other hand,the sharp boundaries are just like GBβs.DB is sensitive to grain orientation and deformation parameters.12,15Therefore,there are a lot of grains without a deformation band,which present an elongated morphology,as shown in Fig.3(c).In addition,as DRX is restrained at high strain rates,DRV is the main softening mechanism.24It often results in the formation of subgrains inside the original β grain.

3.2.1.Lamellar α in DRXed β grains

DRX produces new dislocation-free grains.The size of a DRXed β grain is about 100 μm at the strain rate of 0.01 s-1,as shown in Fig.5(a).The grain is mainly occupied by lamellar α with the same orientation,as indicated with blue in Grain 3 of Fig.5(b).These lamellae with a greater length are mainly nucleated at the GBβbetween Grains 3 and 4,while lamellae nucleated at the other GBβs with different orientations are much shorter.This may be caused by the preferred nucleation in the special GBβbetween Grains 3 and 4,which will be discussed below.The residual β and lamellar α obey the Burgers OR in the DRXed β grain,as shown in Fig.5(c)and(d),respectively.

Fig. 4 Stress-strain curves in β working of TA15 alloy at 1020°C.

Fig.3 Microstructures after deformation at 1020°C to a height reduction of 50%.

Fig.5 α phase in DRXed β grains.

The αGB(B1 in Fig.5(b))between Grains 3 and 4 has a good continuity.Although the misorientation angle of β phase between Grains 3 and 4 is about 45°,β phase has a close{1 1 0}crystallographic plane,as marked by the black square in Fig.5(c).This may promote the formation of continuous αGBand the preferred nucleation at grain boundaries.On the other hand,the αGBat the other GBβis discontinuous because a close{1 1 0}crystallographic plane doesn't exist between Grain 3 and other grains(e.g.,Grains 3 and 5 in Fig.5(b),between which the misorientation angle is about 40°).Moreover,the αGBobeys the Burgers OR with the prior β grain on one side of GBβ(Grain 5 in this work),as marked by arrows and red squares in Fig.5(c)and(d).In general,those α lamellae in DRXed grains are quite similar to those in undeformed grains.

3.2.2.Lamellar α in a deformation band

A typical deformation band is shown in Fig.6(a).A and B have a close crystal orientation,and the average misorientation angle is less than 10°.On the other hand,the misorientation angle between B and C is about 35°.This fulfills the definition of kink band in the deformation band,which involves a double orientation change in neighboring bands.22

The β phase shows a good orientation consistency within the deformation band,as shown by the pink pole points in Fig.6(b).The DB interior may be taken as an isolated grain.Most precipitating α lamellae inside the DB have similar geometric and crystal orientations.Those α lamellae are so long that they reach DBBs on both sides.

There is a narrow transition zone between the neighboring bands,in which the orientation of prior β grains changes dramatically.However,the orientation of the deformation band boundary α(αDBB)changes continuously along the normal direction of the DBB plane,as shown in Fig.6(d).Those αDBBs share the same{0 0 0 1}crystallographic plane,and the normal of their{11-20}crystallographic plane changes continuously,as shown in Fig.6(d).It can be found that the β grains on both sides of the DBB have a close{1 1 0}crystallographic plane,but a close{1 1 1}crystallographic plane does not exist.Those specific {0 0 0 1} and {11-20} crystallographic planes are formed due to variant selection.In addition,those αDBBs with continuously-changed orientations play a transitional effect between the A-C and C-B to reduce the strain energy.

The lamellar α on the DBB can also be fine and disordered,as indicated by DBB2 in Fig.7(a).Pole figures of residual β phase(Fig.7(b))suggest that this is a diffusive boundary where the β orientation changes continuously.Though the lamellar α phases are disordered,their orientations also change continuously(Fig.7(c)).It agrees reasonably with those orientations of residual β.Thus,the Burgers OR was also kept at the diffusive boundaries.

Fig.6 Morphology and crystal orientation of α phase in a deformation band.

3.2.3.Lamellar α phase in elongated β grains

Due to the high stacking fault energy of β phase,DRV is the most common restoration mechanism for β forging.24After β forging,elongated grains with inner substructures are produced.16Fig.8 is the IPF map of elongated grains containing two prior β grains.

In Grain 7,the prior β phase has two orientations,and the misorientation angle is about 4°between them,indicating the existence of an LAGBβ(Fig.8).Although there is an LAGBβin Grain 7,the Burgers OR is still strictly obeyed.

It is clear that the LAGBβis not the nucleation site for α lamellae,and it won't hinder the growth of α lamellae.α lamellae(e.g.,B4-B5 in Fig.8(a))grow directly across the LAGBβ.Besides,lamellar α phase keeps the Burgers OR with the corresponding β matrix.The orientation of lamellar α changes with the orientation of the β matrix,as shown in Fig.9.The pole points marked by blue squares in Fig.8(b)suggest that the β phase has a consistent orientation(the misorientation angle is less than 2°)in Grain 6.Meanwhile,the misorientation angle within α lamellae of Grain 6 is also less than 2°,as shown in Fig.9.

Grains 6 and 7 don't have the same{1 1 0}crystallographic plane.Most of the αGBs(B6 and B8)keep the Burgers OR with the prior β phase of Grain 6,and they have the same orientation.Meanwhile,the others(B7)keep the Burgers OR with the prior β phase of Grain 7,as marked by the green circle and square in Fig.8(b)and(c).There are very limited α variants on the grain boundaries,and orientations of those αGBs have a great similarity,as shown in Fig.8(a).

4.Discussion

4.1.Lamellar α in β-wrought grains with substructures

The substructures formed in β forging grains don't affect the obeying of the Burgers OR in β→α phase transformation,and the Burgers OR is well obeyed through changing the orientation and morphology of lamellar α.

Fig.7 Microstructure of DBB2 shown in Fig.6.

Fig.8 Microstructure of typical elongated prior β grains.

In a DRVed structure,the substructures are relatively large,and the LAGBβhas low misorientation. The LAGBβis directly penetrated by lamellar α (e.g., B4-B5 lamella in Fig.8).Meanwhile,the orientations of lamellar α change accordingly at the LAGBβ.In the diffusive DBB(Fig.6),the orientation changes continuously,and the accumulated orientation variation is large.Fine and disordered lamellae with diverse orientations are formed to keep the Burgers OR with the prior β grain which has plentiful substructures(bundles of small subgrains).A single lamella has a consistent orientation,and the orientation changes continuously from lamella to lamella.In general,lamellar α inherits orientations of prior β grains.

Fig.9 Misorientation variation along the scanning traces in Fig.8.

The crystal defect is little near subgrain boundaries formed by DRV,so the LAGBβis not the nucleation site for α lamellae.Lamellar α can grow across the LAGBβ.On the other hand,the orientation changes continuously,and a large number of LAGBβs exist in diffusive DBBs.Those LAGBβs accumulate in a small zone,and crystal defects increase significantly.Therefore,those LAGBβs are served as the nucleation sites.Fine and disordered lamellae with diverse orientations are formed.A large number of LAGBαs in α lamellae will lead to a remarkable increase of the strain energy,so a lamella including multi-LAGBαs could not be formed.Those α lamellae are restrained by the LAGBβs.

4.2.Effect of initial boundaries on the formation of lamellar α

4.2.1.α layer on grain and deformation band boundaries

Pre-deformation greatly affects the morphology and crystallographic orientation of αGB.The αGBis discontinuous and presents multi-orientations in undeformed grains,as shown in Fig.2.However,the αGBis more continuous and keeps consistent or continuously-changing orientations in the deformed microstructure,as shown in Figs.10 and 8,respectively.

Pre-deformation promotes the nucleation of lamellar α at the GBβdue to the concentration of crystal defects along the GBβ,so nucleation is easier at the deformed GBβthan that at undeformed ones.In addition,the orientations of adjacent β grains significantly affect the crystallographic orientation of αGB.21In Fig.10,adjacent β grains have a close{1 1 0}crystallographic plane.During β→α phase transformation,the{0 0 0 1}crystallographic plane of α phase tends to be parallel to the close{1 1 0}crystallographic plane in adjacent prior β grains to reduce the interfacial energy between α and β phases.25Thus,the αGBs nucleated at different locations of the GBβhave similar orientations.Those discrete αGBs with similar orientations grow along the GBβand ultimately produce a continuous αGBlayer.Moreover,the misorientation angle is low within αGBalong the normal direction to the boundary,as shown in Fig.11.On the other hand,the αDBBis not fully the same as αGB.The orientation of αDBBchanges continuously along the normal direction of the DBB,which serves as a transition zone.The DBB lamellae have multiinterfaces with a lower misorientation angle,as shown in Fig.11.

Fig.11 Misorientation variation along scanning traces indicated by the yellow and red lines in Fig.10.

The two kinds of boundary α are both produced on the basis of the existence of close{1 1 0}crystallographic planes.The close{1 1 0}crystallographic planes in adjacent β grains have been frequently observed by numerous scholars.21,25,26It seems that hot compression promotes the formation of a close {1 1 0} crystallographic plane in adjacent β-wrought grains.Zhao et al.20also observed a fiber texture in which the{1 1 0}direction parallels to the axial direction after forging in Ti60 alloy.The compression still makes the{1 1 0}crystallographic plane being perpendicular to the compression axis.In addition,adjacent β grains normally have the same stress state and deformation style.Therefore,close{1 1 0}crystallographic planes widely exist in adjacent β grains after severe deformation.

4.2.2.Similarity of αWGBs in neighboring β grains

The misorientation angle of β phase is about 45°between Grains 8 and 9.The αWGBs on the boundary have different geometrical orientations.However,the αWGBs still have similar crystal orientations(Fig.10).The misorientation angle of αWGBs is about 9°.The αWGBs of adjacent grains selectively precipitate with the minimum misorientation angle,and the boundaries of the deformation band also show a similar behavior.

Fig.10 α phase at deformation band and grain boundaries.

Fig.12 Fraction of different variants in β grains produced with different mechanisms.

The amount of crystal defects increases in the vicinity of the GB of a deformed grain.Moreover,the nucleation and growth of α phase will be accelerated.Therefore,a large amount of αWGBoriginated from the nucleation at GBβand the subsequent growth.This is analogous to formation of a colony structure1at slow cooling.The orientation of αWGBnear GBβis similar to that of αGB.Those αWGBs with similar orientations near flat boundaries are frequently observed in this work,and the same behavior has been observed by Furuhara et al.27The lamellar αWGBs on both sides of GBβhave similar orientations,as the adjacent β matrix has the close{1 1 0}crystallographic plane and owns the common αGB.In addition,the lamellar αWGBs with similar orientations are beneficial to reduce the interfacial energy on both sides of the flat boundaries.21,28

4.3.Variant selection phenomenon

The previous study and experimental results have shown that the lamellar α phase obeys the Burgers OR with the prior β phase.According to the Burgers OR rule,there are 12 variants that may precipitate from prior-β grains,but the probability is different among these variants.29

Based on the experiments above,the variant selection phenomenon has been studied under different conditions.Fig.12 shows the percentage of variants in β grains produced with different deformation mechanisms.It can be seen that variants with misorientation angles of 10.5°, 60.83°, and 90° are reduced in all deformed microstructures compared to that of random,and the 63.26°and 60°variants increase highly in deformed grains.The different lamellar structures due to various variant selections will affect the subsequent(α+β)deformation mechanism and spheroidization behavior.30

The self-adaptive theory has been widely accepted to interpret variation selection.It is thought that changes in the size and shape of a variant occur in the process of β→α transformation.Strain is produced during the nucleation and growth of α phase.Through adaptive preference,the system could reduce the total strain energy in the precipitation process.For the self-adaptive theory,it is also manifested in the formation of a specific variant,which is mainly applied in the martensitic transformation by now.Wang et al.29studied the β→α transformation in pure titanium,and analyzed the selfadaptive theory in detail.Their results showed that misorientations of 60°and 63.26°were easier to meet the adaptability of a transformation process in transformation.Notably,studies by Lee et al.31indicated that the adaptive theory could also be applied in diffusive transformation.Their results showed that the misorientations of 60°and 63.26°increased greatly.The results of this paper show that internal variants in β-wrought grains precipitate more easily with misorientations of 60°and 63.2°,which are the same as the results of Wang et al.29.

5.Conclusions

This work studied the characteristics of lamellar α in βwrought titanium alloy.The following conclusions are drawn:

(1)Lamellar α appears in different morphologies and orientations in wrought β grains with different substructures in order to maintain the Burgers OR with the β matrix.α lamellae grow directly across the LAGBβin DRVed β grains with a low density of substructures,and similar intra-α LAGBs are formed on the prior β subgrain boundaries.Meanwhile,fine and disordered lamellae with a consistent intra-α orientation are formed in β grains with a high density of substructures.Those lamellar α inherit the orientation of prior-β.

(2)Under the unidirectional compression,adjacent prior β grains as well as deformation bands are prone to having a close{1 1 0}crystallographic plane which is parallel to the compression axis.It promotes the formation of continuous αGBwith a similar orientation and c-axis of αGBto parallel to the grain boundaries.The αDBBincludes some lower-angle boundaries and plays a transition role in β-wrought grains with a deformation band.

(3)The αGBin pre-deformed grains will grow towards the interior of β grains,resulting in an occurrence of a large fraction of αWGBwith the same geometric and crystallographic orientations on one side of a flat boundary.The αWGBon both sides of the prior β boundary has a similar crystallographic orientation because of the close{1 1 0}crystallographic plane and flat boundary between adjacent β-wrought grains.

Acknowledgement

The authors would like to gratefully acknowledge the support of the National Natural Science Foundation of China(No.51575449).