microstructure of adiabatic shear bands in ti6al4vematweb.cmi.ua.ac.be/emat/pdf/1868.pdfmens have a...

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Microstructure of adiabatic shear bands in Ti6Al4V

J. Peirsa, W. Tirryb, c, B. Amin-Ahmadib, F. Coghec, P. Verleysena, L. Rabetc,D. Schryversb,⁎, J. Degriecka

aDMSE, Ghent University, Technologiepark 903, 9052 Zwijnaarde, BelgiumbEMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, BelgiumcCOBO, Royal Military Academy, Renaissancelaan 30, 1000 Brussels, Belgium

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +32 3 265 32 47; fE-mail address: [email protected] (D

1044-5803/$ – see front matter © 2012 Elseviehttp://dx.doi.org/10.1016/j.matchar.2012.10.0

A B S T R A C T

Article history:Received 12 June 2012Received in revised form28 October 2012Accepted 29 October 2012

Microstructural deformation mechanisms in adiabatic shear bands in Ti6Al4V are studiedusing traditional TEM and selected area diffraction, and more advanced microstructuralcharacterisation techniques such as energy dispersive X-ray spectroscopy, high angleannular dark field STEM and conical dark field TEM.The shear bands under investigation are induced in Ti6Al4V samples by high strain ratecompression of cylindrical and hat-shaped specimens in a split Hopkinson pressure barsetup. Samples from experiments interrupted at different levels of deformation are used tostudy the evolution of the microstructure in and nearby the shear bands.From the early stages of adiabatic shear band formation, TEM revealed strongly elongatedequiaxed grains in the shear band. These band-like grains become narrower towards thecentre of the band and start to fraction even further along their elongated direction tofinally result in a nano-crystalline region in the core. In fully developed shear bands, twinsand a needle-like martensite morphology are observed near the shear band.

© 2012 Elsevier Inc. All rights reserved.

Keywords:Ti6Al4VHigh strain rateShear bandMicroscopyTEMStrain localisation

1. Introduction

Adiabatic shear bands (ASBs) are a thermodynamic phenom-enon occurring at high strain rates and are characterised bylarge deformations localised in a narrow band of 5 μm to100 μm [1]. ASBs are observed in many applications such asmachine chips, forging, and ballistic impact loading. In mostcases, the occurrence of adiabatic shearing is undesirable asthe formation of ASBs causes the material to lose its loadcarrying and energy dissipation capacity. Moreover, adiabaticshearing is known to be a precursor to failure. On the other hand,recently developed adiabatic cutting and blanking techniquesintentionally use the ASB phenomenon. Therefore, it is impor-tant to understand thewhole process from localisation to failure.

The mechanism proposed by Zener and Hollomon [2],based on the destabilising effect of thermal softening due to

ax: +32 3 265 33 18.. Schryvers).

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plastic work converted into heat, is generally accepted as theexplanation for the formation of adiabatic shear bands [1].The hypothesis of a thermo-mechanical instability being atthe origin of ASBs is supported by the fact that all studiespresented so far agree about shear localisation being favouredby low strain and strain rate hardening, a pronounced thermalsoftening, and a low thermal conductivity [3]. Moreover, ananalysis based on thermo-mechanical instability is able tocapture the basic features of high speed machining of Ti6Al4V[4]. However, next to the thermal softening, other mecha-nisms leading to local softening of the material have beenproposed. For example, many researchers agree that in manymaterials dynamic recrystallisation (DRX) takes place duringdeformation inside shear bands [5–7]. Some researchersbelieve that, in some cases, DRX is not the result of theASB formation. Instead, they argue that ASB formation and

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propagation originates from DRX [8]. Also, the nucleation andgrowth of voids [9,10] could cause softening and subsequentlocalisation. Furthermore, the role of microstructural featuressuch as the size, shape and orientation of grains, inclusions, etc.is much less clear. Experimental studies, involving differentloading conditions together with in-depth microstructural in-vestigations are thus needed.

Ti6Al4V is known to be prone to the formation of adiabaticshear bandsand subsequent failure [1,11–18]. In Ref. [19], resultsof an extensive experimental programme show that ASBsdevelop at high strain rates, as well as at low strain rates andhigh temperatures. This is mainly due to its low density andheat conduction. Because of the great practical importance ofTi6Al4V, extensive research has already been conducted on thesubject of adiabatic shear banding in this material.

Yang et al. [20] studied the microstructure of ASBs inα-titanium in titanium/mild steel explosive cladding inter-faces. Inside the ASB, fine equiaxed grains with diameters ofapproximately 0.03 to 0.1 μm are found, near the boundary,highly elongated subgrains exist. Experiments andmodellingare used to describe the kinetics of the nanograin formationand recrystallisation within the ASBs.

In Ref. [21], the evolution of the microstructure associatedwithASB formation in ballistic impact plugging of Ti6Al4Vplatesis described. Bands with α′ martensite platelets accompany theASBs. However, these bands are not a necessary condition forASB formation. In the ASB, a DRX grain structure varyingfrom equiaxed, defect-free grains to equiaxed grains with highdislocation densities is observed.

Landau et al. [22] observed microstructural evolutions inTi6Al4V subjected to high-strain-rate deformation as a functionof the distance from the fracture plane or ASB. DRX grains arereported from the early stages of deformation, resulting frommicrostructural refinement from dislocation cells with increas-ing misorientation. In the β-phase, martensitic platelets areoccasionally observed; both stress-induced α′ hcp and α″orthorhombic martensite appear.

Although an abundant literature is available on ASBs inTi-alloys, a comprehensive study of adiabatic shear bands inTi6Al4V using different transmission electron microscopy(TEM) techniques is still lacking. Also, the respective contribu-tion of the α- and β-phases to the adiabatic shearing processremains unclear.

The presented study here, focuses on the microstructuralfeatures present in ASBs in Ti6Al4V. First, ASBs are generatedby dynamic compression of cylindrical and hat-shapedspecimens using a split Hopkinson pressure bar setup.Afterwards, a comprehensive TEM analysis in the shearregions of the deformed cylindrical and hat-shaped speci-mens is carried out. TEM reveals details of themicrostructureinside the shear band that are too small for optical andscanning electron microscopy (SEM). Based on a literaturereview and experimental observations, the following ques-tions are put forward:

1. What is the size and shape of the grains in the shear band?2. Are both α- and β-phases contributing to the shear band? Do

phase transformations occur in the shear band?3. Is there evidence for recrystallisation taking place before

and/or during the formation of the shear band?

4. How does the microstructure evolve during localiseddeformation?

2. Materials and Methods

2.1. Materials

Ti6Al4V is an α-rich α–β Ti-alloy at room temperature. Itsproperties provide a good balance between high strength,high toughness and positive response to heat treatment. Theα-phase of Ti6Al4V has a hexagonal (hcp) crystal structurewhereas the β-phase has a body-centred cubic (bcc) structure.Thermally-induced transformation of α into β phase typicallyoccurs between 600 °C and 995 °C [23]. In Ti6Al4V, smallamounts of β-phase exist at room temperature due to thestabilising effect of vanadium. The presence of other elementsthan V and Al such as O, N, and C (α-stabilising) and H, Mo, Fe,and Cr (β-stabilising) also plays an important role in themetallurgy of Ti6Al4V. Typically, all the Fe present in the alloyis confined to the β-phase [24]. Application of differentthermal and mechanical treatments results in a large numberof different possible microstructures and properties [25]. TheTi6Al4V used in this study is in the so-called mill-annealedcondition, with equiaxed α-grains (97 vol.%, average grain size11 μm) with small β-grains (3 vol.%, average grain size below1 μm) dispersed in between (Fig. 1a). The material wasdelivered as a rod (16 mm diameter), with a preferentialcrystallographic orientation of the c-axes in a directionperpendicular to the rod axis (Fig. 1b).

Two different specimen geometries are used in this study:cylindrical and hat-shaped specimens. The cylindrical speci-mens have a diameter of 10 mm and a length of 11.5 mm. Thespecimens are machined from the rod according to twoperpendicular directions: longitudinal specimens with axisof symmetry coinciding with the rod's axis and transversespecimens. Compressive loading of cylindrical specimensleads to the development of shear stresses on 45° inclinedplanes. El-Magd [27] and daSilva [12] found that shearingfailures develop in two different ways: (a) axis-symmetricshear cones originating from the friction at the bar–specimeninterfaces and (b) fracture along the 45° inclined shearingplanes.

The use of an axis-symmetric hat-shaped specimen is amore direct way to induce shear stresses. This technique wasoriginally proposed by Meyer and Manwaring in 1986 [28] andadopted by a large number of researchers [6,29–34]. Fig. 2bshows the specimen between the Hopkinson bars. Thespecimen can be divided into three regions: the upper hatpart, the lower brim part and the shear region in betweenwhere high shear strains develop when a compressive load isapplied. The axis of symmetry coincides with the rod's axis.The diameter of the hat, 8.20 mm, is slightly larger than thediameter of the brim, 8.00 mm, to obtain a hydrostaticcompressive stress next to the shear stress. The averageshear stress can be estimated by the ratio of the test force tothe shear area. The strain on the other hand cannot beobtained in a straightforward way. The high-speed deforma-tion of the specimen can be interrupted by placing a stopperring (SR) on top of the specimen. In this way, deformation of

Fig. 1 – a) Phase repartition; b) pole figure of the initial (0001) texture for the basematerial. The longitudinal direction correspondsto the direction perpendicular to the pole figure, while the transverse direction was chosen along the maximum c-axis texturecomponent.

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the specimen is limited to the width of the gap indicated inFig. 2b. The main advantage of the use of a hat-shapedspecimen for the study of ASBs is that the specimen materialis forced to shearing failure at a known location. Evenmaterials that do not show shear localisation spontaneouslycan be forced up to a shearing failure. Additionally, since thedeformation is concentrated in a very narrow region, veryhigh shear strain rates are obtained (up to 5000/s). Moredetails about this experimental technique can be found in Ref.[35].

2.2. Mechanical Testing

Intermediate (0.1–50/s) and high (1500–2500/s) strain ratecompression tests at room temperature are performed onrespectively a classical servo-hydraulical type of compressiondevice (MTS) and a Split Hopkinson Pressure Bar (SHPB) setup,also known as Kolsky bars [26]. The cylindrical specimens aretested at intermediate strain rates, whereas the hat-shapedspecimens are tested dynamically. For the quasi-static tests,the sample–anvil interface is lubricated using Teflon-tape. Thetests are performed at constant strain rate by displacement

Fig. 2 – a) Principle of split Hopkinson pressure bar setup; b) princ) pictures of hat-shaped specimen.

control. Results were corrected for the elasticity of the anvilsand the compression rig. Previous research [11], based onthermocouple measurements in the centre of the specimen,has shown that for the considered material, adiabatic condi-tions are reached from strain rates as low as 0.1/s. Thehat-shaped specimens are tested on a SPPB setup. The setupbasically consists of two aligned bars, an input bar and outputbar, with the specimen placed in between (Fig. 2a). For thepresent experiments, aluminium bars are usedwith a diameterof 25 mm and lengths of 6 m and 3.25 m for the input andoutput bars, respectively. A compressive stresswave, generatedby an impact on the free end of the input bar propagates alongthis bar towards the specimen. In the Hopkinson tests, theamplitude of the incident loading wave is proportional to thevelocity of the striker, which is chosen between 8 and 25 m/s.When the incident wave reaches the specimen, the specimenmaterial is subjected to a high strain rate compressive load. Byinteraction with the specimen, part of the incident wave isreflected and part is transmitted to the output bar. The strainhistories corresponding with the loading, reflected and trans-mittedwaves aremeasured bymeans of strain gauges attachedto the bars. From those waves, the history of the force applied

ciple of hat-shaped specimenwith indication of shear zone;


on the specimen and the global deformation is calculated, basedon the principles of one-dimensional elastic-wave propagation inslender bars. The strain rate in the hat-shaped samples cannotstraightforwardly be calculated from the relative velocity of thespecimen/bar interfaces. Indeed, since the width of the shearzone is changing during the experiment, the strain rate canonly be estimated using shear zone widths measured after anexperiment [35]. If calculated using an estimation of the shearband width, values for the strain rate are obtained between 1000and 2500/s. Of course, once the adiabatic shear band is formed,locally much higher strain rates are obtained.

2.3. Microstructural Characterisation

For the conventional TEM analysis, a FEI Tecnai G2 microscopeoperating at 200 kV is used to perform conventional electrondiffraction, with bright and dark field (BF/DF) imaging. Analyticalmeasurements to obtain spatially resolved compositional infor-mation are performedwith the samemicroscope, but operated inscanning transmission electron microscopy (STEM) mode usingan energy dispersiveX-ray (EDX) detector. In order to increase theinformation resolution, the conical dark field (CDF) technique isused to determine size, shape and orientation of the grains insidethe ASB. When using CDF, virtual diffraction patterns can begenerated of smaller regions than with conventional TEMselected area diffraction. Hence, this technique shows a promis-ing potential to analyse nano-crystalline materials. For the CDFtechnique, a series of centred dark field images is recorded withdifferent tilt settings by a hollow-cone scan track with a parallelelectron beam. Crystals that light up in each DF image are in aBragg condition. By using the intensities of each pixel in thecomplete stack of DF images, the spot diffraction pattern from asmall region can be reconstructed [36,37]. This region canultimately go down to the pixel size of the DF image. For thepresentwork, a CDF script iswritten andused in a JEOL 3000F FEGinstrument operating at 300 kV.

Fig. 3 – a) TEM sample preparation from hat-shaped specimen; b) Tby FIB technique.

2.4. TEM Specimen Preparation

As illustrated in Fig. 3a, TEM samples from the hat-shapedspecimens are obtained by first cutting a slice containing thespecimen symmetry axis. From this slice a 3 mm disc is sparkeroded which is subsequently ground to a thickness of 120 μm.Next, dimpling is performed on the placewhere the shear band isexpected to occur, based on optical microscopy images (seebelow). Finally, a perforated sample with TEM transparent edgesismade by electro-polishing using a twin-jet system, followed byan extra ion-milling step to enlarge the electron transparent areawith the shear band within this area. However, the combinationof electro-polishing with ion milling might introduce someartefacts. Therefore, samples of undeformed Ti6Al4V are alsoused as a reference to validate that the findings of the TEM studyare characteristic of the shear band and not originating from thespecimen preparation. A typical TEM sample obtained from thehat-shaped geometry is also shown in Fig. 3a.

TEM samples from the cylindrical specimens are preparedby focused ion beam (FIB) thinning in a FEI Nanolab 200. Thedeformed cylindrical samples show fracturing along a conicalplane, although often the cracks do not fully propagate. Opticalobservation reveals that a crack propagates along a shear band(Fig. 3b). The shear band continues along the path of the crackand TEM samples aremade by FIB just beyond the tip of a crackas shown in Fig. 3b.

3. Results

3.1. High Strain Rate Experiments

3.1.1. Cylindrical SamplesIn all cylindrical samples, ASBs and fractures develop approx-imately along the 45° planes. However, clear differences arefound between the longitudinal and transverse specimens. The

EM sample at tip of crack from cylindrical specimen, prepared


fractured longitudinal specimens (main orientation of the unitcellswith reference to the loading direction is given in Fig. 4a (seealso the texture in Fig. 1b)) typically show several shear bandspresent in the material forming a conical V-shaped or X-shapedpattern (Fig. 4b and c). In contrast, there is generally only onewell-developed shear band and fracture in the transversespecimens (Fig. 4e). The main orientation of the unit cells withreference to the loading direction for the transverse samples isgiven in Fig. 4d. Another difference between the longitudinal andtransverse samples is that ASBs arise at lower strain rates in thetransverse sample (d�=dt ¼ 0:6 s−1) compared to the longitudi-nal sample ( d�=dt ¼ 9 s−1). A previous study using electronbackscattered diffraction (EBSD) measurements [11] has shownthat different deformation systems are activated in the trans-verse and longitudinal specimens leading to an anisotropicconstitutive material behaviour. This anisotropy seems to affectthe shear band nucleation and/or propagation characteristics.

3.1.2. Hat-shaped SamplesThe force–displacement curves of the tests with hat-shapedspecimens are characterised by a sudden drop in the loadcarrying capacity, which originates from the formation of anASB. The shear bands are formed along the line connectingthe inner and outer corner of the specimen. During deforma-tion, the shape and characteristics of the shear zone changecontinuously. In Ref. [35], five different deformation stages aredistinguished: (1) elastic deformation until plastic deforma-tion starts at the corners, (2) stable plastic deformation,(3) unstable deformation related to the initiation and propa-gation of the ASB, (4) deformation with fully developed ASB,and (5) plugging of the hat into the brim. These differentstages of deformation of a hat shaped specimen can be studiedby using interrupted experiments. Furthermore, unlike thespecimen geometry, the shear band is not axis-symmetric andhas a varying shape around the specimen. Thus, by studyingthe shear band in specimens at different levels of deformationand at different locations in the same specimen, the evolutionof the shear band can be observed.

In the following, two specimens are considered. The firstspecimen, denoted with Exp1, is interrupted in stage 3, before

Fig. 4 – a), b), and c) Longitudinal sample; d) and e) transversal samwith reference to the respective loading direction.

the drop of the force. The second specimen, denoted withExp2, is deformed to stage 4, beyond the peak stress in thestress–displacement curve as illustrated in Fig. 5a.

Fig. 5b and c shows optical images with polarised light ofthe shear region in specimen Exp1 but at a different locationalong the circumference of the specimen. In Fig. 5b, a ±30 μmwide band with elongated grains can be seen, while in Fig. 5c,a very narrow and faint band of ±2 μm (indicated with arrowson the figure) without visible grains is observed. The resolutionobtained with optical microscopy, however, is not sufficient toreveal details inside this band.

In the shear zone of the more deformed specimen Exp2, aband with a larger width of ±6 μm is observed, as can be seenin Fig. 5d. In this figure, the specimen has been etched withKroll's reagent, which causes the higher contrast betweenbright α- and dark β-grains. A qualitative comparison with thematerial outside the shear band seems to indicate that lessβ-grains are present inside the shear band.

3.2. TEM Observations

3.2.1. Hat-shaped Specimen: Initiation of Shear Band (Exp1)

3.2.1.1. Conventional TEM. Fig. 6 shows a bright field imageof the shear band in specimen Exp1. The shear band, markedwith two straight lines in the micrograph, has a finer structurecompared to the material next to the shear band. Informationabout the crystallographic orientation is obtained by means ofselected area diffraction (SAED). The diffraction patterns (DPs)shown in Fig. 6 are taken at three different locations indicatedwith circles in themicrograph. These regionshave a diameter ofapproximately 500 nm. A circle-like diffraction pattern, such asthe one in area 1, indicates the existence of multiple crystalorientations within the selected zone, in the ASB. In contrast,the pattern from area 3 shows clearly separated spots,indicating it originates from a single grain next to the ASB,in this case viewed along an [001] cubic orientation. Area 2 isan overlap of the central band and an adjacent single grainwith the interface between both regions clearly delineated inthe centre of area 2.

ple [8]. a) and d) indicate themain orientation of the unit cells

Fig. 5 – a) Average shear stress–displacement curve of two hat-shaped specimen experiments. The average specimendeformation speed is around 2 m/s in these tests b) and c) optical microscopy image from Exp1 and d) from Exp2.


Fig. 7 shows a higher magnification of the same regionwith corresponding DPs from inside the ASB. The latter areobtained from smaller regions with a diameter of approxi-mately 200 nm. Clearly, even in such small regions inside theASB, multiple crystal orientations are present. This impliesthat randomly oriented nanometre-sized grains are presentin the shear band.

In order to get an impression of the change in grain sizeand orientation, diffraction patterns are taken along a profileover the shear band. The locations are indicated in Fig. 7a.The corresponding DPs become less and less circular whenmoving away from the centre of the band. At positions 2 and3, the pattern is the most circular indicating the strongestfractioning of grains. A gradual decrease of grain size or

Fig. 6 – Bright field image of the centre of the shear band (Exp1). Loon the micrograph.

fractioning is observed up to a maximum close to the centreof the shear band.

In Fig. 8, a diffraction ring pattern obtained with a 700 nmdiameter aperture from within the nano-crystalline ASBregion is evaluated and compared with the lattice spacingsof the α- and β-phases. Only one β-spacing is indicated sincethe others are almost coinciding with those of the α-phaseand will thus not be visible separately. The pattern showsthat α is the major phase, but that in addition the β-phase isalso present. Since the β-phase also shows up more like aring, multiple β-orientations exist within the 700 nm region.

3.2.1.2. Conical Dark Field TEM. The region in which diffrac-tion patterns are generated with conventional SAED is too large

cations where the diffraction patterns are taken are indicated

Fig. 7 – a) A BF image of the region with nanometre-sized grains with indication of the positions where the diffraction patternsare obtained represented in (b).


to study individual nano-sized grains. Indeed, ring-like DPsare obtained that do not give much information about theorientation of individual grains. In the present case a stackof dark field images is recorded by precession and using themost prominent diffraction rings appearing in the presentmaterial. Virtual diffraction patterns from smaller regionscan then be reconstructed with the conical dark field TEMtechnique [36,37].

Fig. 9 shows reconstructed DPs of the shear region of sampleExp1 at three different locations, labelled in the correspondingmicrograph. Note that in this case, only a part of the shear bandis shown. Pattern (1) in Fig. 9 shows an orientation clearlybelonging to an [10, 11]hcp zone. Pattern (2) is identified asoriginating from a [111]bcc zone. Thus, the reconstructed DPsreveal the presence of α-phase as well as β-phase, which isconsistentwith the occurrence of rings of both phases in the ringpattern of Fig. 8. Pattern (3) in Fig. 9 is taken at a location closer tothe core of the shear band. This pattern cannot be assigned to asingle α- or β-grain, which means that the grains are smallerthan the size of the indicated box, approximately 50 nm.Furthermore, the band-like structure, still visible towards theedge of the band, is lost.

A reconstructed dark field imageof the same region as in Fig. 9using the DPs (1) and (2) is shown in Fig. 10. This virtual DF imageshowshigh intensities for crystalswith the sameorientationas at

positions (1) and (2) in Fig. 9. Most of the high intensity regionshave the shape of elongated bands. This implies that the crystalorientation is fairly constant within these bands, indicating aband originates from one grain. However, between those bandssmaller and less elongated structures can be found. Moreover,closer to the core of the shear band this elongated shape is lostand the grains are more fractioned, confirming the conclusionsbased on the analysis of Fig. 9.

From the CDF it can thus be concluded that on the outskirtsof the shear band the grains are clearly elongated and canextend over more than half amicron with amaximumwidth ofapproximately 100 nm while more fractioned structures areobserved whenmoving closer to the centre of the shear band.

3.2.1.3. EDX. Apart from conventional and advanced im-aging and diffraction methods, additional techniques areused to study the elemental composition of the materialwithin the shear band. In the present case, high angle darkfield (HAADF) scanning transmission electron microscopy isapplied. The contrast in such images is approximately propor-tional to the square of the atomic number of the element, Z2.Since the elemental compositions of the α- and β-phases areslightly different, HAADF images can thus be used to visualisetheα and β compositionsmore clearly than classical bright fieldimages.

Fig. 10 – Reconstructed DF images of the same region as inFig. 9 and from the orientations at the locations (1) and (2),shown in (a) and (b), respectively.

Fig. 8 – Analysis of a circular-like SAED pattern from withinthe nano-sized granular region obtained with a 700 nmaperture. Miller indices are listed at the respective rings.Some intensity of the (020) β-ring can be observed.


A HAADF image in the nano-crystalline region of Exp1 isshown in Fig. 11a. The faint horizontal streaks in the sheardirection indicate that the elemental composition is not uniformat the nanometre level.

Fig. 9 – Top: BF image of shear band in region 2 of the specimen Exp1, as labelled in Fig. 6; bottom: reconstructed DPs from theareas indicated by (1) and (2) that can be identified as originating from respectively the α- and β-phases, and (3) a strongerfractioned region with no clear orientation. The shear direction is indicated by white arrows on top of the micrograph.

Fig. 11 – a) HAADF STEM image of the nano-crystalline region in the sample Exp1; b) elemental distribution contour plotscorresponding with the squared area in (a) for Fe, V and Al; c) compositional profile along the black arrow in (a).


An EDX analysis is carried out to qualitatively identify theelements that are present in the shear band. Fig. 11b showscontour plots of individual elements Fe, V andAl taken from theregion in Fig. 11a marked with a squared box. These elementsare chosen because they can be assigned to either the α- or theβ-phase. As can be seen on the plots, the same band-likestructures than in the HAADF image are observed. Thisconfirms that the lines in Fig. 11a are indeed the result ofelemental variations.

An additional EDX analysis is performed to quantify theelemental composition. First, EDX measurements areperformed on the α- and β-phases of undeformed Ti6Al4V.In case of the α-phase, the composition is Ti: 89.4 wt.%, Al:8.0 wt.%, and V: 2.6 wt.% with no trace of Fe, while for theβ-phase, Ti: 79.9 wt.%, Al: 4.0 wt.%, V: 15.4 wt.% and Fe: 1.5 wt.%is found. These different compositions of the α- and β-phasesare explained by the fact that Al is an α stabiliser while V is a βstabiliser. Second, using EDX analysis in STEM mode, compo-sitional profiles are made in the nano-crystalline area ofdeformed Ti6Al4V. It is found that the local elemental compo-sition of the material along the path marked on Fig. 11acorresponds,withinmeasurement errors, with the compositionof either the undeformed α- or β-phase. This implies that thebands observed in Fig. 11a and b originate from the α- orβ-phases. The fast fluctuations of the compositional profilesshown in Fig. 11c indicate that thewidth of theseα- andβ-phasebands is smaller than 100 nm, confirming the aforementioned

CDF observations. Furthermore, the presence of material corre-sponding with the β-phase appears to be rather high.

3.2.2. Hat-shaped Specimen: Fully Developed Shear Band (Exp2)The more severely deformed specimen from Exp2 is subjectedto a similar study. Fig. 12 shows three micrographs obtainedat and near the shear band together with some correspondingSAEDs and an overview of the hole and electron transparentregion in the specimen.

In the first region, obtained from inside the shear band,narrow and fractioned grains are observed but not as severeas in the specimen Exp1. This is illustrated by the clearindividual spots in the corresponding diffraction pattern. Inthe second region, at ±50 μm from the approximate locationof the shear band (Fig. 12d), the symmetry of the accompanyingdiffraction pattern indicates the presence of a twinned grain.The twin is identified as a {10–11} α twin, which is notcommonly encountered in Ti6Al4V (conversely to the case ofCP titanium). However, Xu and Meyers [38] have reported thatdeformation twinning is the major mode of deformation inTi6Al4V during dynamic explosion. This can be due to thehigher strain rates reached by explosive loading (104/s) com-pared with our experiments (103/s). In the third region, at ±20 μm away from the shear band (Fig. 12d), a fine needle-likestructure is present with the typical morphology of α′martens-ite. This presence of martensite is consistent with the observa-tions of Murr et al. [21] and Landau et al. [22].

Fig. 13 – (a) Bright field image of the FIB sample obtained from thfrom the indicated regions. (b) Overall ring pattern with indexing

Fig. 12 – Summary of results for Exp2: (a) BF image and DP(500 nm) of the region with highest shear deformation; (b) BFimage and diffraction pattern of {10–11} twin and (c) BF imageshowing plate shaped grain morphology next to the shearband. (d) Shear region showing the hole in the centre of theelectron transparent area, the line indicating theapproximate location of the ASB (the labels correspond withthe respective images in this figure).


3.2.3. Cylindrical Specimen

3.2.3.1. Conventional TEM. The FIB sample obtained from thecylinder specimen shows a band of nano-crystalline materialsimilar to the one shown in Fig. 13. Diffraction patterns obtainedfrom region 1 again show circular patterns, indicating a stronglyfractioned granular structure. Region 2 shows clearer reflectionsindicating less fractioning. Comparison of the diffraction patternof region 1with the α- and β-phase spacings again reveals an hcpstructure with some bcc grains.

3.2.3.2. Conical Dark Field TEM. The compiled reconstructedDF image shown in Fig. 14 is made using precession taking intoaccount the (10–10)hcp, (0002)hcp, (−1101)hcp and (110)bcc latticereflections. It can be seen that the structure becomes more andmore fractioned along the direction of the long arrow. At theleft edge, there is a larger non-fractioned area followed by aband-like morphology. Fractioning increases towards the coreof the ASB, as indicated by the presence of small grains. Someof these grains are indicated by small arrows.

The core structure can be further investigated in the BFimage in Fig. 15, in which the orientation of small grains canbe evaluated studied using CDF reconstructed DPs. The DPsshow that all of the grains with an approximate diameterbelow 50 nm have a clearly different orientation.

3.2.3.3. EDX. EDX profiles and elemental maps are alsoobtained for the cylinder sample. Fig. 16 shows a compositionprofile over the ASB, confirming again the presence of bandswith fluctuating V and Fe concentrations. An additionalmapping(Fig. 17) shows narrow bands with a β-phase composition.

4. Discussion

4.1. Grain Morphology

Obviously, a different grain morphology is observed in thehat-shaped specimens with low (Exp1) and high deformation(Exp2). In the early interrupted hat-shaped specimen, the shear

e cylindrical sample with two diffraction patterns obtainedof the respective α (full) and β (dashed) rings.

Fig. 14 – Compiled conical TEM image of the shear band inthe cylindrical sample, using (10–10)hcp, (0002)hcp, (−1101)hcpand (110)bcc lattice spacings.


band region exhibits grains of very small size. This region has awidth of only 1 to 3 μm (Fig. 6 and other images not shown). Thesmall width corresponds also with the observations from opticalmicroscopy (Fig. 5). The nano-sized grains are proven by(1) ring-like diffraction patterns using 200 nm aperture, (2)reconstructed DF images obtained with CDF and (3) fastfluctuating elemental composition measured with EDX. On theone hand, the EDX analysis shows that long bands with anelemental composition corresponding to either the α- orβ-phases are present. On the other hand, analysis by CDFshows that some of these bands consist of multiple α- andβ-grains with different orientations. These fractioned grains aremost frequently found near the core of the shear band. Evenwithin a 50 nm zone, multiple grains are observed with CDF.

Similar results are found for the cylindrical specimen.Away from the core of the specimen, band-like zones withuniform elemental composition and crystal orientation areobserved. These zones reveal strongly deformed and elongat-ed grains. On the other hand, in the core of the shear band, thegrains are more fractioned. In these regions, the elongatedmorphology has disappeared.

Fig. 15 – (a) BF image of the core in the ASB of the FIB cylinder sashown in (b).

In the more severely deformed hat-shaped specimen fromExp2, no nano-sized grains are found in contrast to thespecimen from Exp1. In the region of the shear band, narrowand fractioned grains are observed, but fractioning is not assevere as in the case of Exp1. This is quite surprising becausethe strain in Exp2 is higher than in Exp1. In regions near to theshear band, a needle-like martensite morphology is found.Martensitic transformations in Ti6Al4V require a temperatureabove 528 °C [39], which is indeed achieved in a developedshear band. Needle-like martensite can even be formed byshock loading at lower temperatures and cooling rates [24,40].Although the formation of a martensitic structure thus seemspossible in view of the very high temperatures and coolingrates occurring in shear bands, a more thorough analysis isnecessary to confirm this. Finally, further away (±50 μm) fromthe region of maximal shear, {10–11} twins are observed, whichare not commonly encountered in Ti6Al4V.

4.2. Contribution of α- and β-phases

The indexed diffraction patterns (Figs. 8 and 13) clearly showthat both phases are present in the nano-crystalline region ofthe shear band. Additionally, EDXmeasurements show that thedistance between the α- and β-zones in the shear band is of theorder of 100 nm, while in the undeformed material it usuallyreaches several microns. As a result of the small distancebetween the grains, the β-phase is clearly present in all imagestaken from the shear band, even though it only constitutes 3%of the undeformed material.

4.3. Microstructure Evolution

Based on the observed grain morphology and phase distributionin the shear band, a hypothesis is formulated to reconstruct theevolution of the microstructure, schematically presented inFig. 18.

Initially, the α- and β-grains aremore or less equiaxed in theundeformed material and are represented by squares in Fig. 18.After a large shear deformation, the grains become veryelongated and form bands. The distance between the grains ishereby strongly reduced, including the distance between the

mple indicating 4 regions of which the reconstructed DPs are

Fig. 16 – (a) HAADF STEM image of the FIB sample with the corresponding compositional profile given in (b).


α- and β-grains. It can be seen that within the same referencearea (circles in Fig. 18), there are more grains and orientationsnow than in the undeformed material. In the next step, theelongated grains break up by a recrystallisation process.Fractioning first occurs in the core of the shear band where thesmallest grains are observed as shown in Figs. 10, 14 and 15. Inthis nano-structured region, the orientations of single smallgrains differ completely, eliminating the possibility of grainfractioning by further deformation of subgrains.

If the material is even further deformed, the original shearband microstructure is destroyed by excessive temperaturesand friction of the fracture surfaces (not shown in Fig. 18).After rapid cooling, a new fine-grainedmicrostructure – includinga martensitic phase – is formed as seen in the highly deformedhat-shaped sample Exp2.

4.4. Recrystallisation

Recrystallised grains are characterised by an equiaxed shape andlow dislocation density [31]. Since recrystallised grains are softerthan deformed- and strain hardened-grains, they could cause

Fig. 17 – (a) HAADF STEM imagewith a compositionalmapping (b)and Fe maps clearly reveal the elongated β-grains.

instable deformation leading to strain localisation [8]. However,in the presented results here, no evidence for recrystallisationtaking place before the formation of the adiabatic shear bandcould be found.Mostly, elongated and strongly deformed grainsare observed within the shear band. The circular DP patternsare not sufficient to prove recrystallisation because a nano-crystalline structure can also arise by excessive sheardeformation without any recrystallisation taking place (seefirst step in Fig. 18). The presence of equiaxed grains at thecore of the shear band indicates that these recrystallisedgrains arise after strain localisation and not before. Indeed, ifthey would have been present before localisation and thesubsequent formation of the shear band, they should haveseen an important amount of shear deformation and wouldhave been unable to retain their equiaxed aspect.

5. Conclusions

Shear bands are generated in Ti6Al4V with two different testtechniques. Complementary information about the fracture

of the area indicated by the horizontal rectangle. Again, the V

Fig. 18 – Schematically represented α (light)–β (dark) band-like structures created by shearing. The elongated grains arefractioning in the last step.


behaviour, including the formation of adiabatic shear bands isobtained. The use of interrupted shear tests on hat-shapedspecimens enables studying the shear band formation atdifferent levels of strain. The experiments with cylindricalspecimenswith twomaterial orientations show the effect of theinitial texture on the sensitivity of the material to form shearbands.

An extensive TEM analysis has been conducted on the shearbands in two hat-shaped specimens and one cylindricalspecimen. In addition to conventional bright field imaging andSAED diffraction patterns, also more advanced TEM techniquesare used. The TEM study revealed many different microstruc-tures inside and nearby the band: very elongated grains,nano-crystalline equiaxed grains, martensitic structures andtwins. The following list summarises the most importantfindings of the TEM study:

• The microstructural characteristics of the shear band inthe hat-shaped specimen and cylindrical specimen aresimilar. Of course, for the hat-shaped specimens thelocation of the ASB is more or less fixed as there is analready very localised, yet initially homogeneous defor-mation. Starting from that zone of sheared material, justas for the compression samples, the forming of the ASB is afurther localisation phenomenon, with a thickness muchbelow that of the initially sheared zone in the hat-shapedspecimen.

• Nanometre-sized grains well below 100 nm are present inthe shear band. Most grains are elongated. Yet, equiaxedgrains are also found, most frequently in the core of theshear band. Thus, inside the ASB, a gradual decrease ingrain size seems to be present. The elongated grains at theexterior side of the band become narrower towards thecentre and start to fraction even further along theirelongated direction to finally result in a nano-crystallineregion in the core.

• In the region of strong elongated grains, one primaryorientation remains present within each grain.

• In the nano-crystalline region, the orientations of singlesmall grains differ completely, even within an area of 1 μm.

• EDX and CDF diffraction patterns reveal that inside thishighly fractioned region, α- as well as β-phases are present inthe form of elongated grains. In some cases the β-fraction ishigh, resulting in an α–β band-like structure. The fraction of

β-phase bands depends on the sample and location in thesample. Since, initially, only ±3% of β-phase is present, theoccurrence of β-phase might depend on whether or not aβ-grain was present close to the location before deformation.

• In a hat-shaped specimen deformed beyond the peak stress,no such nano-crystalline regions are found. However, twinsand a needle-like martensite morphology are observed nearthe shear band. Possibly, the high temperatures and frictionof the fracture surfaces have destroyed the previouslyformed nano-crystalline structure.

• For the studied specimens, no evidence of recrystallisationoccurring before strain localisation is found. It is morelikely that strain localisation originates from thermalsoftening while recrystallisation takes place afterwards inthe core of the shear band. Elongated grains are herebyfractioned into very small nano-sized grains with differentcrystal orientations.

Acknowledgements

The authors acknowledge funding of the Interuniversity Attrac-tion Poles Program (IUAP) of the Federal Science Policy of Belgiumand the partners of IUAP-VI (www.m3phys.be), and of the FWOresearch project G012012N “Understanding nanocrystallinemechanical behaviour from structural investigations”.

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microstructure of adiabatic shear bands in ti6al4vematweb.cmi.ua.ac.be/emat/pdf/1868.pdfmens have a...

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