Materials Characterization 96 (2014) 151–157

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Materials Characterization

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Hardness characteristic and shear band formation in metastableβ-titanium alloys

Premkumar Manda a, Uday Chakkingal b, A.K. Singh a,⁎a Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad 500 058, Indiab Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

⁎ Corresponding author.E-mail address: singh_ashok3@rediffmail.com (A.K. Si

http://dx.doi.org/10.1016/j.matchar.2014.07.0271044-5803/© 2014 Elsevier Inc. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 June 2014Received in revised form 24 July 2014Accepted 30 July 2014Available online 31 July 2014

Keywords:Metastable β titanium alloysShear bandsHardnessPlastic deformation during indentationEBSD

Present work describes the formation of shear bands during Vickers indentation with indentation loads in therange of 0.1 to 20 kg in four metastable β titanium alloys in β water quench condition. Extent of solid solutionhas marked influence on the evolution of shear bands. All the four alloys exhibit both the primary and secondaryshear bands. The spacing between shear bands increaseswith increase in distance from the indenter edge. Crystalorientation mapping has been used to measure misorientations at different locations within the matrix, shearbands and at the interface between matrix and shear band.

© 2014 Elsevier Inc. All rights reserved.

1. Introduction

The use of metastable β-titanium alloys in the aerospace industryhas been increasing day by day due to their attractive properties [1].These are good strength, hardenability, fracture toughness and reasonableductility. These alloys do not formmartensite but retain the β phase uponquenching. As a result, the microstructure can be engineered during heattreatmentwith the help of the retainedβphasewhich in turn governs themechanical properties. The mechanical properties of these alloys arelargely affected by the volume fraction, morphology and distribution aswell as size of the hexagonal α phase precipitates within the matrix.These can be varied by varying heat treatments. For example, highhardenability in these alloys has been achieved in quenched and agedcondition with fine distribution of α-precipitates due to slow kineticsin thick sections of the material [2–4].

The first beta titanium alloy produced in the late 1950s was Ti–13V–11Cr–3Al (B 120 VCA). Thereafter, a large number of metastable β-titanium alloys have been developed for different applications with dif-ferent mechanical properties. These alloys have been produced in formof ingot, billet, bar, rod, plate, wire, sheet, strip and foil [5]. The latest ad-dition in this series is the alloy Ti–5Al–5Mo–5V–3Cr (Ti-5553) whichhas been developed to replace the alloy Ti–10V–2Fe–3Al (Ti-10-2-3)in high strength airframe components such as landing gear assembliesof the aircraft. It is to be noted that these alloys are extremely sensitiveto forming variables, even during isothermal processing. The alloy Ti-

ngh).

10-2-3 is sensitive to small fluctuations in both the temperature andthe strain rate. On the other hand, the alloy Ti-5553 has exhibited lesssensitivity to forming variables and offers a larger processing windowdue to the less steep β transus approach curve [2–4,6–10].

Shear bands around the edges of indenter have been observedmain-ly in bulk metallic glasses (BMGs). Formation of shear bands in BMGshas been reviewed recently [11]. However, observation of shear bandsduring indentation in crystalline titanium alloys is rather limited [12].Shear bands have been reported in different β-titanium alloys duringdifferent modes of deformation such as rolling, compression etc. [13,14]. It is known that shear band formation is one of the important defor-mation and failure mechanism and frequently occurs in short time [15].Therefore, it is quite useful to examine the formation of shear bandsduring different modes of deformation. The present work is thus con-cerned with the hardness characteristic and shear band formation dur-ing indentation in different metastable β-titanium alloys. The hardnesscharacteristics of the samewill also be useful to understand theflowbe-havior of these alloys during thermomechanical processing. The alloyTi-5553 and three other alloys which are derived from Ti-5553 by keep-ing Mo equivalent (8.15) constant have been used for this study.

2. Experimental details

Four experimental alloys with nominal compositions Ti–5Al–5Mo–5V–3Cr, Ti–5Al–3.5Mo–7.2V–3Cr, Ti–5Al–5Mo–8.6V–1.5Cr and Ti–5Al–3.5Mo–5V–3.94Cr were designated as A1, A2, A3 and A4, respec-tively. These alloys were prepared using non-consumable vacuum arcmelting. A total of 600 g pancake of each alloy was melted six times to

Table 1Chemical composition of the experimental alloys.

Alloy designation Nominal composition(wt.%)

Analyzed composition(wt.%)

Interstitial elements(wt.%)

Al Mo V Cr Ti Al Mo V Cr C O N H Ti

A1 5.0 5.0 5.0 3.0 Balance 5.17 4.96 4.85 2.98 0.021 0.10 0.007 0.016 BalanceA2 5.0 3.5 7.2 3.0 Balance 5.10 3.30 7.30 3.20 0.075 0.04 0.017 0.012 BalanceA3 5.0 5.0 8.6 1.5 Balance 5.25 4.52 8.75 1.50 0.025 0.12 0.005 0.005 BalanceA4 5.0 3.5 5.0 3.9 Balance 5.41 3.18 5.25 4.16 0.035 0.13 0.005 0.005 Balance

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ensure chemical homogeneity. The chemical composition of alloy A1 issame as that of Ti-5553, while the other three alloys (A2, A3 and A4)have been selected keeping Mo equivalent constant (8.15) with differ-ent combinations of Mo, V and Cr. The Mo equivalent is defined as thesum of the weighted averages of the elements (wt.%) present in thealloy [16]. This is given as

Moeq ¼ Moþ 0:67Vþ 0:44Wþ 0:28Nbþ 0:22Taþ 1:6Crþ 1:25Niþ 1:7Coþ 2:9Fe−1:0Al wt: %ð Þ: ð1Þ

The chemical compositions of all four alloys have been determinedby inductively coupled plasma-optical emission spectroscopy (ICP-OES)which is awet chemical technique. The analyzed chemical compo-sitions are given in Table 1. The interstitial elements such as C, O, N andHweremeasured using inert gas fusion technique and are also includedin Table 1.

The as-cast alloys were solution treated at 900 °C (β phase field) for30 min and then subjected to water quenching (WQ). In addition, thealloy A1 was also air and furnace cooled (AC and FC) after β solutiontreatment. The specimens for optical and scanning electron microscopy(SEM) were prepared following the standard metallographic techniqueused for titanium and its alloys and etched with Kroll's reagent (85 mlH2O, 10 ml HNO3 and 5 ml HF). The microstructural characterizationof the β WQ specimens was carried using a Leo 440i SEM in backscattered electron (BSE) and secondary electron (SE) imaging modes.

Fig. 1. Optical microstructures of βWQ samples: (a) A1, (b) A2, (c) A3 and (d) A4. The corresponly.

Electron backscatter diffraction (EBSD) experiments were carried outwith FEG-SEM, SUPRA™ 55, ZEISS make coupled with EBSD detectorsystemNORDLYS II andwith HKL channel 5 software. The X-ray diffrac-tion (XRD) patterns were recorded in a Philips PW 3020 diffractometerequipped with graphite monochromator operated at 40 kV and 25 mA.Bulk andmicro-hardnesses in terms of Vickers hardness number (VHN)of the β WQ alloys were measured using the AFFRI hardness tester(model: VRSD 270) and Matsuzawa Co. Ltd. hardness tester (model: AMMT-X7), respectively.

3. Results and discussion

The optical microstructures of all the four alloys along with BSE mi-crographs (inset) in β WQ condition are shown in Fig. 1 which clearlyexhibits the presence of single phase only. TheXRDpatterns also displaythe presence of single β phase in these samples (Fig. 2). The lattice pa-rameter a of all the alloys has been calculated from correspondingXRD patterns using Celn method [17] and is given in Table 2. It is clearthat the calculated values of a are smaller than that of the typical β-titanium (3.283 Å, from JCPDS: 89-4913) [18]. This can be attributedto alloying elements having smaller atomic radii compared to Ti.The atomic radii of alloying elements are Ti (1.47 Å), Al (1.43 Å),Mo (1.39 Å), V (1.34 Å) and Cr (1. 28 Å). The alloys A2 and A4 exhibitlarger values of a than those of A1 and A3. The hardness values of al-loys A2 and A4 (measured at 10 kg load) are higher than those of A1

onding BSE micrographs are also given in insets which reveal the presence of single phase

Fig. 2. XRD patterns of β WQ samples: (a) A1, (b) A2, (c) A3 and (d) A4. Fig. 3.VHNvs load plot of theβWQsample (alloy A1). Opticalmicrostructures around theindenter are shown near the corresponding load. Variation of hardness as function ofsmaller loads and corresponding microstructures is displayed as inset.

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and A3. It appears that the presence of solute atoms has the same ef-fect on both the lattice parameter as well as hardness. The optical mi-crostructures around the indentation of Vickers hardness taken atdifferent loads exhibit the presence of shear bands (Figs. 3 to 6). Inorder to see the effect of load on the formation of shear bands, hard-ness of all four alloys in βWQ condition was taken as the function ofdifferent loads ranging from 0.1 to 20 kg. These values are given inTable 3.

The hardness values and corresponding microstructures around in-dentation are shown in Figs. 3–6which exhibit two trends: (1)Hardnessvalues reflect large and small variations between 0.1 to 2.0 kg and 5.0 to20.0 kg indentation loads, respectively. The later region consists of hard-ness valueswhich are to a certain extent independent of the indentationload. It is important tomention here that the hardness values of Ti-5553and similar alloys have been reported at 10 kg load. (2) Themicrostruc-tures around Vickers indentations display the presence of shear bandsin the range of 0.1 to 20 kg loads. The shear bands around the VHN in-denter have been reported frequently and investigated in detail inbulk metallic glasses [19–29]. On the other hand, formation of shearbands during indentation in crystalline materials is rather limited [12].In BMGs, shear bands are formed spontaneously within the bulk mate-rial when metals deform plastically because of shear stresses. These areinduced by highly localized inhomogeneous deformation during inden-tation. The presence of shear bands indicates excessive deformation dueto strain localization. As a result, semi-circular bands (primary shearbands) having wavy nature, originate from the edge of the indenterand end at the corner [22]. These bands extend beyond the indentedge but not away from the indent corner.

The stress distribution inside the hemi-spherical plastic zone and ra-dial stress components is responsible for the primary shear bands. Shearbands look like overlapping of upwardly displaced material. Deforma-tion in elastic-perfectly plastic solids occurs by the pile-up of the mate-rial against the faces of the indenter due to volume conserving nature ofplastic deformation. This pile-up is seen as discrete steps because of theinhomogeneous nature of plastic deformation. As a result, semi-circularshear bands are out-of-plane (plane stress) shear displacements. In

Table 2Calculated lattice parameters of experimental alloys inβ solution treat-ment (900 °C, 30 min) and WQ condition.

Alloy designation Calculated a (Å)

A1 3.2357 ± 0.0062A2 3.2493 ± 0.0051A3 3.2366 ± 0.0083A4 3.2445 ± 0.0031

addition, some bands originate inside the zone of primary shear bandsfrom edge of indenter and are called secondary shear bands [26]. Shearstress components are mainly responsible for secondary shear bands.

Present experimental alloys display shear bands around indentationin the range of 0.1 to 20 kg load (Figs. 3 to 6). The appearance of theseshear bands is similar to those observed in BMGs. The number of shearbands is less at smaller loads and increases with the increasing loads.The formationof shear bands at low loads indicates initiation of plastic de-formation along with the elastic components [24]. In contrast, the higherloads produce well defined shear bands. All the four alloys exhibit thepresence of both the primary aswell as secondary shear bands. The num-ber of bands in alloys A1 and A3 ismore compared to those of A2 and A4.These bands are present around all the four edges of the indenter for al-loys A1 and A3. In contrast, these bands are not uniformly present aroundall the four edges of the indenter of alloys A2 and A4. This difference canprobably be attributed to solid solution effect of alloying elements in thesingle β phase of these alloys. The solid solution effect appears to bemore predominant in A2 and A4 alloys, exhibiting both the higher latticeconstants and the higher VHN values. As a result, formation of shearbands in these alloys is not homogeneous. On the other hand, the alloysA1 and A3 with smaller lattice constant and VHN values (10 kg load) ex-hibit uniform development of shear bands around all four edges of the

Fig. 4.VHNvs load plot of theβWQsample (alloy A2). Opticalmicrostructures around theindenter are shown near the corresponding load. Variation of hardness as function ofsmaller loads and corresponding microstructures is displayed as inset.

Fig. 5.VHN vs load plot of theβWQsample (alloy A3). Optical microstructures around theindenter are shown near the corresponding load. Variation of hardness as function ofsmaller loads and corresponding microstructures is displayed as inset.

Fig. 6.VHN vs load plot of theβWQsample (alloy A4). Optical microstructures around theindenter are shown near the corresponding load. Variation of hardness as function ofsmaller loads and corresponding microstructures is displayed as inset.

Fig. 7. SE SEMmicrostructures of alloys near one of the edges of alloy A1: (a) low magni-fication and (b) highmagnification showing discrete steps, A2: (c) lowmagnification and(d) high magnification showing discrete steps, A3: (e) low magnification and (f) highmagnification showing discrete steps and A4: (g) low magnification and (h) high magni-fication showing discrete steps.

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indenter. The high magnification SEMmicrographs of these shear bandsof four alloys are shown in Fig. 7.

These are discrete bands andwavy in nature. The appearance of thesebands is like rough and irregular semi-circular bandswhich are originatedfrom the edge of the indenter. Some of the bands are intersecting withother bands. Steps are also observed at higher magnification. The spacingbetween semi-circular shear bands increases with increasing distancefrom the edge of indenter. This is in agreement with the shear bands re-ported in BMGs. The angles of intersection of primary and secondarybands of A1, A2, A3 and A4 samples lie within the range of 66°–116°,84°–123°, 75°–110° and 75°–130°, respectively. It has been reported

Table 3Vickers hardness number (VHN) of the experimental alloys in β solution treatment (900 °C, 3

Alloydesignation

Heattreatment

Hardness (VHN) with standard deviation at inde

0.1 0.3 0.5 1.0

A1 β-WQ 292 ± 5.7 306 ± 1.0 308 ± 4.0 30A2 β-WQ 353 ± 6.5 299 ± 8.1 307 ± 3.5 30A3 β-WQ 309 ± 2.7 305 ± 3.2 297 ± 1.5 28A4 β-WQ 361 ± 8.0 336 ± 2.3 336 ± 3.5 33

that the average angle between the two families of shear bands is ~79°from the edge of a spherical indent in a Zr-based BMG. It is to be notedthatwhen the angle betweenmaximumshear directions is 90°, themate-rial obeys the von Mises yield criterion [22]. This clearly reflects that thedeformation under VHN indenter in the present sample does not followthe von Mises yield criterion.

Crystal orientationmapping of shear bands andmatrix near the edgeof the indenter is shown in Figs. 8 and 9 alongwith Euler angles and cor-responding nearest texture components. It clearly indicates that thecrystal orientations of shear bands and matrix are different. The values

0 min) and WQ condition.

ntation load (kg)

2.0 5.0 10.0 15.0 20.0

1 ± 4.4 331 ± 2.6 301 ± 4.5 298 ± 2.5 283 ± 2.6 286 ± 2.36 ± 1.5 355 ± 3.5 300 ± 2.0 317 ± 2.0 302 ± 2.0 312 ± 3.05 ± 1.0 312 ± 3.0 295 ± 2.6 293 ± 4.0 296 ± 3.0 292 ± 5.71 ± 2.1 381 ± 3.2 305 ± 6.0 304 ± 2.5 305 ± 5.0 297 ± 7

Fig. 8. Crystal orientationmaps: (a) band contrast and (b) local misorientations of the alloy A1. Euler angles at three different locations within thematrix, shear band and atmatrix–shearband interfaces are given with corresponding nearest texture components.

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of Euler angles at different locations within thematrix reflect 3–5° mis-orientations. Similarly, Euler angles at different shear bands exhibit 3–5°misorientations. The different locations at the interface between shearbands andmatrix also display similar range ofmisorientations. Themis-orientations among different locations in matrix, shear bands and at

Fig. 9. Crystal orientationmaps: (a) band contrast and (b) local misorientations of the alloy A4.band interfaces are given with corresponding nearest texture components.

matrix–shear band interfaces support the inhomogeneous flow of ma-terials during indentation.

It has been reported that the plastic deformation in BMGs occurs bynucleation and propagation of shear bands [23]. There has been intensestudy in the area of shear band formation in the ultrafine grain

Euler angles at three different locations within thematrix, shear band and atmatrix–shear

Fig. 10. SEMmicrographs of β furnace cooled alloy A1: (a) backscattered electron image, (b) secondary electron image.

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microstructures andBMGswith thehelp of strain gradient plasticity [21,24–26]. Prasad et al. have reported the presence of shear bands duringindentation in Ti–40Al–2Mo alloy having fine basketweavemicrostruc-ture [12]. It has been mentioned in literature that below the surface ofthe material, a large strain gradient is expected due to inhomogeneousflow of the material. Verification of strain gradient plasticity throughgeometrically necessary dislocations or statistical introduction of dislo-cations during indentation is quite complex. The presence of shearbands in BMGs, ultrafine microstructures, fine basket weave micro-structure and in the present alloys (single β phase), therefore, clearlypoints towards the importance of structure, morphology and lengthscale of the microstructure. One of the common features of these mate-rials is the instability of microstructure. Even though, the experimentalalloys in the current study exhibit singleβ phase, a veryfinedistributionof athermal ω phase is present in the matrix. This is quite common inWQ metastable β-titanium alloys and the ω phase cannot be detectedby XRD and SEM. Instability of the β phase in metastable β-titanium alloys has been observed by transmission electron micros-copy in the form of streaks in selected area electron diffraction pat-terns (not shown) [2].

Shear band formation has been reported particularly in those mate-rials which do not exhibit strain hardening effect. The values of strainhardening exponent (n) of these materials are very close to zero. Then values of the present and other β-titanium alloys are also around0.07 which support the observation of shear bands around VHN inden-tation [30,31]. In order to validate this point, indentationswere taken inair and furnace cooled samples of the present alloys that contain dif-ferent volume fractions of α and β phases. The amount of α phase inthe sample increases with a decrease in cooling rate. The furnacecooled sample does not exhibit the formation of shear bands (Fig. 10)during indentation due to the presence of significant amount of αphase inβmatrix. It is to be noted that the n values ofα phase is around0.14 [32].

4. Conclusions

1. Formation of shear bands has been reported in metastable β-titaniumalloys in β WQ condition in the range of 0.1 to 20 kg loads. This hasbeen attributed to low strain hardening exponent value of the βphase.

2. Extent of solid solution in β matrix has marked influence on the de-velopment of shear bands.

3. All the four alloys exhibit both the primary and secondary shear bands.The alloys Ti–5Al–5Mo–5V–3Cr and Ti–5Al–5Mo–8.6V–1.5Cr showmore shear bands compared to Ti–5Al–3.5Mo–7.2V–3Cr and Ti–5Al–3.5Mo–5V–3.94Cr.

4. The spacing between shear bands increases with increasing distancefrom indenter edge.

5. The evidence of inhomogeneous plastic flow has been provided bymeasuring misorientations at different locations within the matrix,shear bands and at matrix–shear band interfaces.

Acknowledgments

The authors wish to acknowledge the Defence Research and Develop-ment Organization for financial support. We are grateful to Dr. Amol AGokhale, Director, Defence Metallurgical Research Laboratory for hiskind encouragement. The support provided by Dr. P. Ghosal and ShriVajinder Singh for EBSD measurements is acknowledged.

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