Microhardness evolution and mechanical characteristics of commercialpurity titanium processed by high-pressure torsion

Mahmood Shirooyeh a, Jie Xu a,b, Terence G. Langdon a,c,n

a Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USAb Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150080, Chinac Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK

a r t i c l e i n f o

Article history:Received 21 May 2014Received in revised form10 July 2014Accepted 11 July 2014Available online 19 July 2014

Keywords:High-pressure torsionMicrohardnessSevere plastic deformationTitaniumUltrafine-grained materials

a b s t r a c t

An investigation was conducted to evaluate the microstructural evolution and mechanical properties of acommercial purity (CP) titanium (grade 2) processed by high-pressure torsion (HPT) at roomtemperature. Microstructural analysis was performed to provide detailed information on the effect ofshear strain on the grain size. The results demonstrate that significant grain refinement is achievedthrough HPT processing with a reduction from �45 μm in the initial annealed condition to a grain sizeof �150 nm after 10 turns of HPT. Measurements of the Vickers microhardness show that the disks areessentially homogeneous after 5 or more turns of torsional straining. Analysis by X-ray diffraction (XRD)revealed an allotropic phase transformation from the α-phase (hexagonal closed-packed) to the ω-phase(hexagonal) during HPT processing. The results from mechanical testing at an elevated temperature of673 K are consistent with the microstructural observations thereby showing that ultra-fine grained CP Tiexhibits excellent mechanical properties.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

It is now well established that severe plastic deformation (SPD)is an important processing route for achieving exceptional grainrefinement in metals and alloys [1,2]. This grain refinement isa result of the imposition of a large shear strain into the materialwhere this is undertaken using special procedures which producelittle or no change in the overall dimensions of the sample. Animportant advantage of grain refinement is that it leads toenhanced mechanical properties including high strength and apotential for achieving superplastic elongations when testing athigh temperatures.

Several different SPD techniques are now available [3] but thetwo processing methods receiving the most attention are equal-channel angular pressing (ECAP) [4,5] and high-pressure torsion(HPT) [6,7]. In ECAP, the material is in the shape of a rod or bar andit is pressed through a die constrained within a channel which isbent through a sharp angle. In HPT, the sample is generally in theform of a thin disk and it is subjected simultaneously to a highpressure and torsional straining. Comparing the two procedures

of ECAP and HPT, experiments show that processing by HPT isadvantageous because it generally produces materials with smal-ler grain sizes [8,9] and, by comparison with processing by ECAP,the HPT materials have higher fractions of grain boundaries withhigh-angles of misorientation [10,11].

An important and promising application for SPD processing liesin producing ultrafine-grained (UFG) titanium metal and Ti alloysfor engineering applications including in the aerospace [12] andautomotive [13] industries and, in addition, for biomedical pur-poses as in dental and orthopedic implants [14,15]. Commercialpurity (CP) Ti has an excellent combination of properties suchas good specific strength, formability, high corrosion resistance,low electronic conductivity and good biocompatibility [16,17].However, the mechanical properties and strength of pure Ti arelower than many Ti alloys such as the Ti–6Al–4V alloy which iswidely used in aerospace and biomedical applications [12,14]. Thepresent investigation was therefore initiated in order to evaluatethe potential for achieving high strength and good mechanicalproperties in pure Ti through processing by HPT.

Table 1 displays a summary of earlier reports investigatingthe processing of CP Ti using HPT [18–37]. In Table 1, the HPTprocessing conditions are presented in terms of the numbers ofturns, N, the pressure applied to the disk, P, the absolute proces-sing temperature, T, and the diameter, D, and height, h, of the disk.In addition, Table 1 lists the Vickers microhardness values, Hv, and

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/msea

Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.07.0300921-5093/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: Department of Aerospace & Mechanical Engineeringand Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA.

E-mail address: langdon@usc.edu (T.G. Langdon).

Materials Science & Engineering A 614 (2014) 223–231

details of the annealing conditions including the temperature,total time of annealing and the atmosphere. By comparing theinitial grain sizes prior to HPT and the final grain sizes of CP Ti afterprocessing by HPT, given in the two penultimate columns inTable 1, it is apparent that significant grain refinement may beachieved from initial grain sizes in the range of �8–50 μm to finalgrain sizes within the range of �10–650 nm. It is also apparentthat the hardness values increase by HPT processing to valueswhich are generally comparable to those in commercial Ti alloys.

It is instructive to note that, despite the extensive researchon HPT processing of CP Ti recorded in Table 1, no systematicinvestigations are currently available describing the level of micro-hardness homogeneity attained in HPT processing or the effects ofprocessing on the mechanical properties and the ductilitiesachieved in tensile testing at elevated temperatures. Accordingly,the present research was initiated to provide detailed informationon the processing of pure Ti by HPT and the subsequent mechan-ical properties when testing in tension at a temperature of 673 K.

2. Experimental material and procedures

The experiments were conducted using CP grade 2 titaniumhaving a purity of 99.5%. The metal was received in the form ofrods having diameters of 10.0 mm and these rods were annealedin air at 873 K for 1 h before cooling to room temperature. Themicrostructure in the annealed condition is shown in the opticalmicrograph in Fig. 1 where the mean linear intercept grain sizewas measured as �45 μm.

To prepare specimens for HPT processing, the rods were slicedinto disks with thicknesses of �1.2 mm and these disks were thencarefully polished on both sides to final thicknesses of �0.83 mm.The HPT facility consisted of massive upper and lower anvils

having central depressions with diameters of 10.0 mm and depthsof 0.25 mm. For HPT processing, each disk was placed in thedepression on the lower anvil and then the two anvils werebrought together to impose an applied pressure. In this investiga-tion, all disks were processed at room temperature under anapplied pressure of P¼6.0 GPa. Torsional straining was achievedthrough rotation of the lower anvil at a rotation rate of 1 rpm.The disks were processed through selected numbers of turns, N,ranging from 1/4 to 10 revolutions. The HPT processing wasconducted under quasi-constrained conditions in which there isa small outflow of material between the two anvils during the

Table 1Summary of HPT processing of CP Ti.

Material HPT processing Hv (GPa) Annealing conditions Grain size Reference

N (turns) P (GPa) T (K) Dimensions D�h (mm) T (K) t (min) Atm. Initial (μm) Final (nm)

CP Ti VT1-0a –h 5 RT 10�0.3 �2.3–3.0 523–723 120 Vac 100–650 Popov et al. [18]CP Tib 3 1.5 RT, 723 20�2 �2.7–3.1 – – – 10 200–300 Stolyarov et al. [19]Ti powderc 3 0.3–1.5 573–773 20�2 �3.6 21 �75 Stolyarov et al. [20]CP Tid 5 RT 10 523–573 10 50 �120–150 Sergueeva et al. [21]CP Ti 1, 5 42 RT 623 10 20 �100 Valiev et al. [22]CP Ti VT1-0a,d 10i 5 RT 12�0.3 573 10 �120 Valiev et al. [23]CP Ti 5 6 12�0.3 1073 720 Ar �50 10–50 Faghihi et al. [24]Ti–0.05–0.12% O o10 1.5, 5 RT 10 or 20�0.85 �2.5–3.7 1073 60 Ar Todaka et al. [25]CP Ti 1/2–10 3–6 20�0.2 Ivanisenko et al. [26]CP Tie 5 RT 10 or 20�0.85 �3.5 1073 60 Ar 100–200 Todaka et al. [27]CP Ti Grade 4f 10 6 343–723 20 �3.8–4.5 40 105–120 Islamgaliev et al. [28]

293–723 60 150–310CP Ti 1/2–10 1.2–6 10�0.85 �2.6–3.4j 1073 60 Ar �150–200 Edalati et al. [29]CP Ti Grade 2d 10 3 RT 10�0.8 �2.5j 773 10 Vac 8.6 607 Wang et al. [30]CP Ti Grade 2 10 3 RT 10�0.8�0.85 �3.0j 8.6 130 Wang et al. [31,32]CP Tig 5 2 77–300 8�0.77 �2.4–2.7j 875 60 Vac 100–160 Podolskiy et al. [33]CP Ti Grade 2 10 6 100, 300 10�0.8 �2.6–3.4j 1073 60 45 54–118 Edalati et al. [34]CP Ti Grade 2 1–20 3, 6 RT 10�0.8 �3.0j 973 120 Vac 10.5 105–130 Wang et al. [35,36]CP Ti Grade 2 1–10 3, 6 RT 10�0.8 �2.9–3.1j 973 120 Vac 10.5 110 Nie et al. [37]

a VT1-0 is a Russian designation for CP Ti.b The CP Ti was processed by ECAP for 7 passes using processing route Bc with an initial temperature of 773 K prior to HPT.c The Ti powder was initially compacted in air in a temperature range of 573–823 K by HPT with an initial compaction pressure of 0.3–1.5 GPa.d The CP Ti was processed by HPT and followed by a subsequent annealing step.e The CP Ti was processed by HPT using monotonic (mHPT), cyclic (cHPT) and two-step (2sHPT) techniques.f The CP Ti Grade 4 was processed by ECAP at 723 K and forge-drawing at 573 K prior to HPT.g The CP Ti processed by HPT was subsequently annealed in vacuum in the temperature range of 425–545 K for 3 h.h The CP Ti was processed by HPT with a true logarithmic strain of 7 in a disk with a diameter of 10 mm and a thickness of �0.3 mm.i The total number of turns was calculated from the true logarithmic strain of 7 at a distance of 2 mm from the center of a disk with a diameter of 12 mm and a thickness

of �0.3 mm.j The Vickers hardness value was converted to MPa by multiplying by 9.807.

Fig. 1. Optical micrograph of the microstructure of pure Ti in the annealedcondition.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231224

processing operation [38–40]. An earlier report gave a detaileddescription of the processing operation and the same procedurewas followed in the present investigation except that a lubricantwas not placed around the central depressions prior to HPTprocessing [41].

In order to obtain information on the homogeneity of themicrostructure in each disk, microhardness measurements wererecorded using an FM-1e microhardness instrument equippedwith a Vickers indenter. Each disk was mounted and polished toa mirror-like surface prior to undertaking the microhardnessmeasurements. Then measurements were recorded using anindentation load of 500 gf and a dwell time of 10 s. Two differentprocedures were followed to obtain detailed information on themicrohardness variations.

First, microhardness measurements were recorded along arandomly selected diameter in each disk. The average values ofHv were determined from indentations taken at selected positionsfrom the center to the edges of each disk with the measurementpoints separated by a distance of 0.3 mm. For each reportedpoint, the average values of Hv were determined from fourseparate microhardness measurements uniformly located aroundthe selected point.

Second, the microhardness values were recorded over the totalsurface of each disk using points taken from a rectilinear gridpattern having intervals of 0.3 mm between each separate posi-tion. The hardness profiles were plotted in the form of color-codedcontour maps in order to provide a direct and simple visualpresentation of the hardness distributions over the total surfaceof each disk processed by HPT. A full description of these measur-ing procedures was given earlier [41].

A quantitative phase analysis was conducted on specimensboth in the annealed condition without HPT processing and afterprocessing by HPT. The analysis was conducted using X-raydiffraction (XRD) with Cu Kα radiation having a wavelength, λ,of 0.15418 nm.

For examinations of the microstructures, the annealed speci-mens were etched using a solution of 2% HF, 10% HNO3 and 88%H2O (vol%) for optical microscopy observations. For microstruc-tural observation by transmission electron microscopy (TEM), thinsections were taken from the mid-radius positions of the Ti disksafter processing by HPT through a lift-out technique in a Multi-Beam scanning electron microscope/focused ion beam (SEM/FIB)JEOL JIB-4500 microscope. A gallium liquid-metal ion source(LMIS) with an accelerating voltage of 30 kV was used to extractthe thin regions with dimensions of 4�5 μm2 and thicknesses of�80 nm. The extraction process occurred over a number of stepsto ensure nanoscale precision. In addition, beam alignment andpolishing techniques were employed to prepare the TEM speci-mens with minimal surface amorphization induced by the FIB. TheTEM micrographs were obtained using a JEOL JEM-2100F micro-scope operating under an accelerating voltage of 200 kV.

The mechanical properties were examined both in theannealed condition without HPT processing and after processingby HPT through 1, 5 and 10 turns. Two miniature tensile specimenswere cut from symmetric off-center positions in each disk usingelectro-discharge machining (EDM). This specimen configurationwas described earlier and it gives two specimens from each diskhaving gauge lengths and widths of 1 mm and with the centralpoints of each specimen located at distances of 2 mm from thecenter of the disk [42]. These tensile specimens were pulled intension to failure at a temperature of 673 K using an Instrontesting machine operating at a constant rate of cross-headdisplacement and with initial strain rates from 1.0�10�4 to1.0�10�2 s�1. The results were plotted as engineering stressversus engineering strain and the elongations to failure wereplotted against the initial strain rates.

3. Experimental results

3.1. Microhardness values after HPT processing

Fig. 2 shows the variation of the Vickers microhardness as afunction of distance from the centers of the disks after processingthrough 1/4, 1, 5 and 10 turns: the lower dashed line at HvE195shows the initial hardness in the annealed material without HPTprocessing. Inspection shows that the hardness increases signifi-cantly in the CP Ti even after 1/4 turn. Thus, after 1/4 turn thehardness is HvE300 at the edge and HvE220 in the center of thedisk. Furthermore, these hardness values increase with increasingnumbers of turns in HPT processing. After one turn, the valuesincrease slightly at both the center and the edge but after 5 turnsthere is a major increase in hardness in the center and onlya minor increase at the periphery of the disk. The hardness valuesafter 10 turns are also fairly similar although at the edge thehardness values increase from HvE370 after N¼5 turns toHvE390 after N¼10 turns. It is concluded that a reasonable levelof hardness homogeneity is attained in the disks of CP Ti after5 and 10 turns and this observation is consistent with severalearlier reports on the gradual development of hardness homo-geneity after 5 to 10 turns in different materials processed by HPT[43–46]. By contrast, it was shown recently that hardness homo-geneity was not attained in NiTi shape memory alloys even afterprocessing by HPT through 40 turns [47].

To evaluate the development of hardness homogeneity over thetotal surfaces of the disks processed by HPT, individual hardnessmeasurements were taken following a rectilinear grid pattern fordisks processed by HPT through totals of 1/4, 1, 5 and 10 turns. Theresults are displayed in the form of color-coded contour maps inFig. 3 where the coordinate system X and Y was selected arbitrarilyfor each disk such that it corresponds to two orthogonal axeswhere the point (0,0) is at the mid-point of each disk. These mapsprovide a valuable visual presentation of the hardness variationsover the disk surfaces where the hardness values are representedby a set of unique colors corresponding to values of Hv in therange from 200 to 450 in incremental steps of 50, as documentedby the key at the lower right in Fig. 3.

It is apparent from Fig. 3 that there is a gradual trend towardshigher values of hardness with increasing numbers of turns and,in addition, there is a higher level of hardness homogeneityafter larger numbers of turns. Thus, the hardness inhomogeneityobserved in the disk after processing through 1 turn is essentially

Fig. 2. Values of the Vickers microhardness across diameters of the disks processedby HPT for various numbers of turns; the lower dashed line shows the value of themicrohardness for the annealed condition.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231 225

lost after 5 turns. These color-coded maps confirm that a reasonablehardness homogeneity is achieved after processing through 5 and10 turns. The gradual evolution towards higher hardness and ahardness homogeneity is consistent with results reported for anumber of metals where hardening occurs without any significantrecovery within the microstructure [48].

3.2. Microstructural characteristics before and after HPT

The X-ray diffraction patterns presented in Fig. 4 show quanti-tative phase analyses of the Ti samples both in the annealedcondition and after processing by HPT through different numbersof turns. In the annealed condition all of the diffraction peaks

correspond to the alpha (α) phase which is the conventionalhexagonal close-packed (hcp) lattice structure at room tempera-ture. However, there is a phase transformation from hcp to anomega (ω) phase when processing by HPT even after 1/4 turn.

The microstructures of the CP Ti were studied by TEM afterprocessing by HPT. Fig. 5 shows an example in bright field for themid-radius position in a disk processed by HPT through 10 turns.These and other TEM observations showed the presence of highdensities of dislocations after HPT processing and after 10 turnsthe microstructure was well defined and the grains were essen-tially equiaxed. An average grain size of �150 nm was recordedafter 10 turns, thereby demonstrating significant grain refinementfrom the initial annealed grain size of �45 μm.

Fig. 3. The variation of microhardness over the surface areas of disks processed by HPT through (a) 1/4, (b) 1, (c) 5 and (d) 10 turns: the key for the colors is given at thelower right. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231226

3.3. Mechanical properties at elevated temperatures

Fig. 6 shows representative stress–strain curves for the CP Ti ata testing temperature of 673 K both in the annealed condition andafter processing by HPT through 1, 5 and 10 turns: the experimentswere conducted using initial strain rates of (a) 1.0�10�2, (b)1.0�10�3 and (c) 1.0�10�4 s�1, respectively. Close inspectionof the curves shows that CP Ti exhibits typical elevated tempera-ture mechanical behavior including some strain hardening anda reasonable level of ductility. It is also apparent that, for all threestrain rates, the maximum yield stresses increase with increasingnumbers of turns with the highest values recorded for the samplesprocessed after 5 and 10 turns. This is consistent with othermaterials processed by HPT including an Al alloy [49], pure Cu[50], a Cu alloy [51] and Armco iron [50]. Inspection also revealssome scatter in Fig. 6 in the ultimate tensile strengths afterprocessing samples by HPT for 5 or more turns. This scatter arisesfrom small variations in the final grain sizes of the CP Ti samples

after processing through a minimum of 5 turns together with theinherent difficulties in conducting tensile testing using the minia-ture specimens cut from the HPT disks.

To further investigate the role of HPT processing on ductility inthe CP Ti, Fig. 7 shows the measured elongations to failure plottedagainst strain rate for the annealed condition and after 1, 5 and10 turns. It is apparent that after 1 turn the ductility remainsessentially as in the annealed condition but the ductility thenincreases with further straining with the highest elongationsrecorded after 10 turns. The low ductility after processing by HPTfor one turn is due to the high level of microstructural inhomo-geneity as revealed in the hardness measurements in Figs. 2 and 3(b). The elongations to failure after processing through 5 and 10turns increase in a similar manner with decreasing strain rate. Thistrend is consistent with the microhardness measurements in Fig. 2for the samples processed after 5 and 10 turns and in Fig. 3(c) and(d). It is also consistent with earlier reports verifying a correlationbetween hardness measurements and microstructure [44,52–58].

The highest measured elongation in these experiments was�130% for the sample processed by HPT through 10 turns andtested at 673 K using an initial strain rate of 1.0�10�4 s�1. Thiselongation is not within the superplastic regime which requires aminimum elongation of 400% [59]. Thus, superplastic flow was notachieved in the CP Ti after HPT processing. This contrasts with theTi–6% Al–4% V alloy where elongations of 4500% were reportedafter HPT processing and subsequent tensile testing at the highertemperatures of 923 and 998 K [60,61].

4. Discussion

4.1. Microstructure evolution

The results in Fig. 2 indicate there is a gradual microhardnessevolution across the diameters of the disks after processing by HPTfor 5 and more turns. At first, the hardness is lower at the centerthan at the edges of the disks. However, with increasing numbersof turns in HPT processing, there is a trend towards a hardnesshomogeneity. It is also apparent that the disk after 10 turns hashigher average hardness values compared to the disk after 5 turns.Therefore, it appears that more straining may be required to reacha saturation condition and achieve a fully homogenous micro-structure. This observation is in agreement with other investiga-tions on various materials processed by HPT [45,46,54,62].

In titanium group metals, the omega (ω) phase is a metastablephase based on a regular hexagonal lattice structure where threeatoms per unit cell are positioned at (0,0,0), (1/3,2/3,1/2), and(2/3,1/3,1/2) where a and c correspond to the lattice parameters inthe basal plane and perpendicular to the basal plane, respectively,and the axial ratio is c/aE0.61. By contrast, the α-phase has twoatoms per unit cell at (1/3,2/3,1/4) and (2/3,1/3,3/4) with an axialratio of c/aE1.58. Thus, the ω-phase has an open structure and apacking ratio of �0.57 which is significantly lower than thepacking ratio of �0.74 for the α-phase [63]. In practice, thereare numerous reports of the occurrence of a phase transformationfrom α-Ti to ω-Ti when processing by HPT at different pressuresof 3 [26], 4 [29], 5 [25] and 6 GPa [34]. It was shown also that theomega phase is effective in increasing the hardness of pure Ti untilthe amount of omega phase reaches a saturation level withincreasing torsional straining.

Since the HPT processing in this investigation was performed ata constant pressure of 6.0 GPa, it is reasonable to expect theoccurrence of a pressure-induced phase transformation from alphato omega and this is visible in Fig. 4. Nevertheless, there are somereports showing an absence of an omega phase when processingwith a pressure of 6.0 GPa through 10 turns [35,36] and this was

Fig. 4. X-ray patterns of Ti samples in both the annealed condition and afterprocessing by HPT through various numbers of turns.

Fig. 5. Microstructure of pure Ti at the half-radius position after processing by HPTthrough 10 turns.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231 227

attributed to using a CP Ti containing a relatively high concentra-tion of oxygen. It is known that the presence of oxygen is effectivein suppressing the alpha to omega phase transformation byincreasing both the relative energy of the omega phase and theenergy barrier for the transformation [64].

The grain refinement observed in the TEM study for the sampleafter HPT processing through 10 turns is consistent with earlierreports listed in Table 1. For example, the grain size was reduced inCP Ti from �50 μm to a refined value of �120–150 nm afterprocessing by HPT using an applied pressure of 5 GPa [21].

4.2. Improvements in the mechanical properties after HPT

There are very few reports on the high temperature mechanicalproperties of CP Ti processed by HPT and this contrasts with theroom temperature mechanical properties of CP Ti which are listedin Table 2. Thus, Table 2 summarizes the reported data for theHPT processing conditions, the gauge dimensions of the tensilespecimens and also the tensile testing conditions including theimposed strain rate, the testing temperature, the yield stress, σy,the ultimate tensile stress, σUTS, and the measured elongations tofailure, δ. It should be noted that the yield stresses for the currentinvestigation correspond to the 0.2% offsets in the stress–straincurves. More details on the annealing conditions, grain sizes andthe hardness values for the various earlier reports are given inTable 1.

It is difficult to make a direct comparison between the experi-mental data in Table 2 because the results are based on samples ofCP Ti which were processed by HPT using different conditions andthen tested using different tensile conditions. Nevertheless, it is

Fig. 6. Plots of engineering stress against engineering strain for CP Ti pulled in tension to failure at 673 K in the annealed condition and after HPT through different numbersof turns at initial strain rates of (a) 1.0�10�2, (b) 1.0�10�3 and (c) 1.0�10�4 s�1.

Fig. 7. Plot of elongation to failure against strain rate at 673 K for CP Ti in theannealed condition and after processing by HPT through 1, 5 and 10 turns.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231228

apparent that higher tensile strengths are measured in the testsconducted at room temperature and higher elongations to failureare achieved in the tests conducted at elevated temperatures. Aclose inspection of Table 2 shows that HPT processing is effectivein improving the ultimate strength of these materials.

A recent report on the room temperature mechanical proper-ties of CP Ti showed that, at a strain rate of 1�10�2 s�1, thevalue for σUTS was increased from �660 MPa to �940 MPa afterprocessing by HPT through 10 turns under a pressure of 3 GPa butthe tensile ductility was decreased from 40% to 23%. [32]. Bycontrast, the results for the high temperature mechanical proper-ties in the current study show that both the tensile strength andthe ductility increase with increasing numbers of HPT turns.For example, the value for σUTS increases from �440 MPa to�470 MPa when the number of HPT turns is increased from 1 to5 when processing with a pressure of 6.0 GPa at room tempera-ture. In addition, an elongation to failure of 100% was achievedafter 5 HPT turns from an initial elongation of 70% after one turnwhen the tensile testing was conducted at 673 K at a strain rate of1.0�10�3 s�1. Similar improvements in the mechanical proper-ties at elevated temperature were also observed for samplesprocessed by HPT through 10 turns.

As shown in Fig. 6(a), a maximum tensile strength of �630 MPawas obtained for the sample processed by 5 turns and tested at673 K at an initial strain rate of 1.0�10�2 s�1. However, super-plastic elongations were not achieved in this investigation and thehighest elongation of �130% was recorded after processing through10 turns and testing at an initial strain rate of 1.0�10�4 s�1.It should be noted that this elongation is significantly higher thanthe reported elongations to failure for CP grade 2 Ti after processing

by ECAP at 573 K and then cold rolling at either room temperatureor 173 K [65].

The variation in strain across each disk processed by HPT isestimated from the equivalent von Mises strain, εeq, through thefollowing relationship [66–68]:

εeq ¼ 2πNrh

ffiffiffi

3p ð1Þ

where N is the number of turns, r is the radial distance measuredfrom the center of the disk and h is the initial thickness of the disk.Based on Eq. (1), the torsional strain is expected to be maximum atthe edge of the disk and equal to zero at the center of the diskwhere r¼0. However, in practice there is generally a gradualevolution towards microstructural homogeneity as the numbers ofturns increase and this has been demonstrated both theoretically[69] and experimentally [9,46,54].

It was shown earlier that there is a direct correlation betweenthe microhardness values and the equivalent strain as calculatedby Eq. (1) such that the various individual microhardness datumpoints shown in Fig. 2 may be effectively brought together ontoa unique curve by plotting the individual values of Hv against theequivalent strain [43]. In order to check this correlation, Fig. 8shows the microhardness values plotted against the equivalentstrain estimated from Eq. (1) where, for simplicity in presentation,the 95% error bars are shown only for the points obtained atthe highest strains. It is apparent from Fig. 8 that the values ofmicrohardness increase from Hv close to �200 for the annealedcondition and the individual values fall reasonably on a singlecurve with a saturation occurring at equivalent strains higher than�100. The saturation condition for the CP-Ti is given by a value of

Table 2Summary of mechanical properties of CP Ti processed by HPT.

Material HPT Gauge size (mm3) Strain rate (s�1) Testing temperature (K) σUTS (MPa) σy (MPa) δ (%) Reference

N P (GPa) T (K)

CP Tia 3 1.5 RT 5�2�0.8 730 625 25 Stolyarov et al. [19]723 RT 640 530 30

673 �265 �60873 �45 460

CP Ti – – – 5�2�0.8 RT 460 450 27 Stolyarov et al. [20]773 135 95 32

Ti powderb 3 1.5 723 RT – – 0673 260 250 2723 125 60 90773 80 45 115

CP Tic 5 RT 1�1�0.1 1�10�3 RT 950 790 14 Sergueeva et al. [21]CP Ti – – – 5�2�1 1�10�3 523 �250 �40 Valiev et al. [22]CP Ti 1 42 RT �820 �10

5 �820 �30CP Ti VT1-0c,d 10f 5 RT 1�1�0.3 1�10�3 RT 980 800 12 Valiev et al. [23]

5 RT 5�2�1 1�10�3 523 �800 �35CP Ti Grade 4e 10 6 RT 3�1�0.4 RT 1560 1400 �5 Islamgaliev et al. [28]

623 1600 1400 �5723 1400 1350 �7

CP Ti 10 1.2 1�1�0.5 3.3�10�3 870 17 Edalati et al. [29]2 875 196 1180 0

CP Ti Grade 2 – – – 2�1�0.6 1�10�2 RT 660 40 Wang et al. [32]CP Ti Grade 2 10 3 RT 940 23CP Ti Grade 2 1 6 RT 1�1�0.6 1�10�3 673 440 280 �70 This investigation

5 470 340 �10010 520 340 �120

a The CP Ti was processed by ECAP for 7 passes using processing route Bc with an initial temperature of 773 K prior to HPT.b The Ti powder was initially compacted in air in the temperature range of 573–823 K by HPT with an initial compaction pressure of 0.3–1.5 GPa.c The CP Ti was processed by HPT followed by a subsequent annealing step.d The VT1-0 is a Russian designation for CP Ti.e The CP Ti Grade 4 was processed by ECAP at 723 K and forge-drawing at 573 K prior to HPT.f The total number of turns was calculated from the true logarithmic strain of 7 at a distance of 2 mm from the center of a disk with a diameter of 12 mm and a thickness

of �0.3 mm.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231 229

HvE400. This plot is consistent with other recent reports showingsimilar correlations for several different materials processed byHPT [45,70–76].

5. Summary and conclusions

1. Disks of CP Ti (grade 2) were processed by HPT through up to10 turns at room temperature under an applied pressure of6.0 GPa. Processing by HPT reduced the grain size from aninitial annealed value of �45 μm to a grain size of �150 nmafter 10 turns of HPT.

2. It is shown from microhardness measurements that a reason-able level of hardness homogeneity is achieved in CP Ti after5 and 10 turns of HPT. The individual measurements of hard-ness correlate directly with the equivalent strain so that alldatum points fall on or about a single curve.

3. X-ray diffraction patterns show the occurrence of a phasetransformation from an hcp α-phase to an hexagonal ω-phasewhen processing under an HPT pressure of 6.0 GPa.

4. The results from mechanical testing at a temperature of 673 Kshow an increase in the elongations to failure at lower strainrates for specimens processed by HPT through 5 and 10 turns.The maximum recorded tensile elongation was �130% whichshows that superplastic flow is not achieved in CP Ti at a testingtemperature of 673 K.

Acknowledgments

This work was supported in part by the National ScienceFoundation of the United States under Grant no. DMR-1160966and in part by the European Research Council under ERC Grantagreement no. 267464-SPDMETALS.

References

[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103.[2] T.G. Langdon, Acta Mater. 61 (2013) 7035.[3] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, Y.T. Zhu, JOM 58

(4) (2006) 33.[4] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskii, V.I. Kopylov, Russ. Metall. 1

(1981) 99.[5] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881.[6] P.W. Bridgman, J. Appl. Phys. 14 (1943) 273.[7] A.P. Zhilyaev, T.G. Langdon, Prog. Mater. Sci. 53 (2008) 893.

[8] A.P. Zhilyaev, B.K. Kim, G.V. Nurislamova, M.D. Baró, J.A. Szpunar, T.G. Langdon,Scr. Mater. 46 (2002) 575.

[9] A.P. Zhilyaev, G.V. Nurislamova, B.K. Kim, M.D. Baró, J.A. Szpunar, T.G. Langdon,Acta Mater. 51 (2003) 753.

[10] J. Wongsa-Ngam, H. Wen, T.G. Langdon, Mater. Sci. Eng. A 579 (2013) 126.[11] J. Wongsa-Ngam, M. Kawasaki, T.G. Langdon, J. Mater. Sci. 48 (2013) 4653.[12] R.R. Boyer, Mater. Sci. Eng. A 213 (1996) 103.[13] O. Schauerte, Adv. Eng. Mater. 5 (2003) 411.[14] M. Niinomi, Mater. Sci. Eng. A 243 (1998) 231.[15] R.Z. Valiev, I.P. Semenova, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruzelka,

L. Dluhos, D. Hrusak, J. Sochova, Adv. Eng. Mater. 10 (2008) B15.[16] C.N. Elias, J.H.C. Lima, R. Valiev, M.A. Meyers, JOM 60 (3) (2008) 46.[17] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Prog. Mater. Sci. 54 (2009)

397.[18] A.A. Popov, I.Yu. Pyshmintsev, S.L. Demakov, A.G. Illarionov, T.C. Lowe, A.

V. Sergeyeva, R.Z. Valiev, Scr. Mater. 37 (1997) 1089.[19] V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.K. Islamgaliev, R.Z. Valiev, Nanostruct.

Mater. 11 (1999) 947.[20] V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.K. Islamgaliev, R.Z. Valiev, Mater. Sci. Eng.

A 282 (2000) 78.[21] A.V. Sergueeva, V.V. Stolyarov, R.Z. Valiev, A.K. Mukherjee, Scr. Mater. 45

(2001) 747.[22] R.Z. Valiev, I.V. Alexandrov, Y.T. Zhu, T.C. Lowe, J. Mater. Res. 17 (2002) 5.[23] R.Z Valiev, A.V. Sergueeva, A.K. Mukherjee, Scr. Mater. 49 (2003) 669.[24] S. Faghihi, F. Azari, A.P. Zhilyaev, J.A. Szpunar, H. Vali, M. Tabrizian, Biomater-

ials 28 (2007) 3887.[25] Y. Todaka, J. Sasaki, T. Moto, M. Umemoto, Scr. Mater. 59 (2008) 615.[26] Y. Ivanisenko, A. Kilmametov, H. Rösner, R.Z. Valiev, Int. J. Mater. Res. 99

(2008) 36.[27] Y. Todaka, M. Umemoto, A. Yamazaki, J. Sasaki, K. Tsuchiya, Mater. Trans. 49

(2008) 47.[28] R.K. Islamgaliev, V.U. Kazyhaniv, L.O. Shestakova, A.V. Sharafutdinov, R.

Z. Valiev, Mater. Sci. Eng. A 493 (2008) 190.[29] K. Edalati, E. Matsubara, Z. Horita, Metall. Mater. Trans. A 40 (2009) 2079.[30] C.T. Wang, N. Gao, M.G. Gee, R.J.K. Wood, T.G. Langdon, Wear 280 (2012) 28.[31] C.T. Wang, A. Escudeiro, T. Polcar, A. Cavaleiro, R.J.K. Wood, N. Gao, T.

G. Langdon, Wear 306 (2013) 304.[32] C.T. Wang, N. Gao, M.G. Gee, R.J.K. Wood, T.G. Langdon, J. Mater. Sci. 48 (2013)

4742.[33] A.V. Podolskiy, C. Mangler, E. Schafler, E.D. Tabachnikova, M.J. Zehetbauer, J.

Mater. Sci. 48 (2013) 4689.[34] K. Edalati, T. Daio, M. Arita, S. Lee, Z. Horita, A. Togo, I. Tanaka, Acta Mater. 68

(2014) 207.[35] C.T. Wang, A.G. Fox, T.G. Langdon, J. Mater. Sci. 49 (2014) 6558.[36] C.T. Wang, A.G. Fox, T.G. Langdon, Mater. Sci. Forum 783–786 (2014) 2701.[37] M. Nie, C.T. Wang, M. Qu, N. Gao, J.A. Wharton, T.G. Langdon, J. Mater. Sci. 49

(2014) 2824.[38] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Mater. Sci. Eng. A 528 (2011) 8198.[39] R.B. Figueiredo, P.H.R. Pereira, M.T.P. Aguilar, P.R. Cetlin, T.G. Langdon, Acta

Mater. 60 (2012) 3190.[40] R.B. Figueiredo, M.T.P. Aguilar, P.R. Cetlin, T.G. Langdon, J. Mater. Sci. 47 (2012)

7807.[41] M. Kawasaki, T.G. Langdon, Mater. Sci. Eng. A 498 (2008) 341.[42] A. Loucif, R.B. Figueiredo, M. Kawasaki, T. Baudin, F. Brisset, R. Chemam, T.

G. Langdon, J. Mater. Sci. 47 (2012) 7815.[43] A. Vorhauer, R. Pippan, Scr. Mater. 51 (2004) 921.[44] C. Xu, Z. Horita, T.G. Langdon, Acta Mater. 56 (2008) 5168.[45] A. Loucif, R.B. Figueiredo, T. Baudin, F. Brisset, T.G. Langdon, Mater. Sci. Eng. A

527 (2010) 4864.[46] M. Kawasaki, S.N. Alhajeri, C. Xu, T.G. Langdon, Mater. Sci. Eng. A 529 (2011)

345.[47] H. Shahmir, M. Nili-Ahmadabadi, Y. Huang, T.G. Langdon, J. Mater. Sci. 49

(2014) 2998.[48] M. Kawasaki, J. Mater. Sci. 49 (2014) 18.[49] C. Xu, S.V. Dobatkin, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 500 (2009) 170.[50] F. Wetscher, A. Vorhauer, R. Pippan, Mater. Sci. Eng. A 410–411 (2005) 213.[51] J. Wongsa-Ngam, M. Kawasaki, T.G. Langdon, J. Mater. Sci. 47 (2012) 7782.[52] A.P. Zhilyaev, S. Lee, G.V. Nurislamova, R.Z. Valiev, T.G. Langdon, Scr. Mater. 44

(2001) 2753.[53] A.P. Zhilyaev, K. Oh-ishi, T.G. Langdon, T.R. McNelley, Mater. Sci. Eng. A 410–

411 (2005) 277.[54] C. Xu, Z. Horita, T.G. Langdon, Acta Mater. 55 (2007) 203.[55] A.P. Zhilyaev, T.R. McNelley, T.G. Langdon, J. Mater. Sci. 42 (2007) 1517.[56] M. Kawasaki, B. Ahn, T.G. Langdon, J. Mater. Sci. 45 (2010) 4583.[57] C. Xu, Z. Horita, T.G. Langdon, Mater. Trans. 51 (2010) 2.[58] S. Sabbaghianrad, M. Kawasaki, T.G. Langdon, J. Mater. Sci. 47 (2012) 7789.[59] T.G. Langdon, J. Mater. Sci. 44 (2009) 5998.[60] A.V. Sergueeva, V.V. Stolyarov, R.Z. Valiev, A.K. Mukherjee, Scr. Mater. 43

(2000) 819.[61] A.V. Sergueeva, V.V. Stolyarov, R.Z. Valiev, A.K. Mukherjee, Mater. Sci. Eng. A

323 (2002) 318.[62] J. Wongsa-Ngam, M. Kawasaki, Y. Zhao, T.G. Langdon, Mater. Sci. Eng. A 528

(2011) 7715.[63] K.M. Ho, C.L. Fu, B.N. Harmon, W. Weber, D.R. Hamann, Phys. Rev. Lett. 49

(1982) 673.

Fig. 8. Values of the Vickers microhardness plotted against the equivalent strainafter processing by HPT through various numbers of turns.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231230

[64] R.G. Hennig, D.R. Trinkle, J. Bouchet, S.G. Srinivasan, R.C. Albers, J.W. Wilkins,Nat. Mater. 4 (2005) 129.

[65] V.L. Sordi, M. Ferrante, M. Kawasaki, T.G. Langdon, J. Mater. Sci. 47 (2012) 7870.[66] R.Z. Valiev, Y.V. Ivanisenko, E.F. Rauch, B. Baudelet, Acta Mater. 44 (1996) 4705.[67] F. Wetscher, A. Vorhauer, R. Stock, R. Pippan, Mater. Sci. Eng. A 387–389

(2004) 809.[68] F. Wetscher, R. Pippan, S. Sturm, F. Kauffmann, C. Scheu, G. Dehm, Metall.

Mater. Trans. A 37A (2006) 1963.[69] Y. Estrin, A. Molotnikov, C.H.J. Davies, R. Lapovok, J. Mech. Phys. Solids 56

(2008) 1186.

[70] K. Edalati, T. Fujioka, Z. Horita, Mater. Sci. Eng. A 497 (2008) 168.[71] K. Edalati, T. Fujioka, Z. Horita, Mater. Trans. 50 (2009) 44.[72] K. Edalati, Y. Ito, K. Suehiro, Z. Horita, Int. J. Mater. Res. 100 (2009) 1668.[73] K. Edalati, Z. Horita, Mater. Trans. 51 (2010) 1051.[74] K. Edalati, Z. Horita, Mater. Sci. Eng. A 528 (2011) 7514.[75] Y.Z. Tian, S.D. Wu, Z.F. Zhang, R.B. Figueiredo, N. Gao, T.G. Langdon, Acta Mater.

59 (2011) 2783.[76] S. Sabbaghianrad, T.G. Langdon, Mater. Sci. Eng. A 596 (2014) 52.

M. Shirooyeh et al. / Materials Science & Engineering A 614 (2014) 223–231 231