tra-for-the

cans a

e to ionsedtheeen

theureloy.

ereere

, Zr,d infieds in

CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx

n of

the

1012

tion

gate

and

and

IRP.

G Model

CIRP-1156; No. of Pages 4

Investigation of the mechanical behavior of ion irradiated Ni-free Ti-basedmetallic glass by nanoindentation

D.A. Lucca (1)a,*, A. Zare a, M.J. Klopfstein a, L. Shao b, G.Q. Xie c

a School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK, USAb Department of Nuclear Engineering, Texas A&M University, College Station, TX, USAc Institute for Materials Research, Tohoku University, Sendai, Japan

1. Introduction

Metallic glasses (MGs) have been the subject of extensiveresearch since their first discovery over 50 years ago. The absenceof structural features such as dislocations and grain boundaries inMGs due to their lack of long range atomic order leads to desiredengineering properties such as high corrosion and wear resistance,high hardness, and high yield strength [1,2]. Newer classes of MGssuch as Ni-free, Ti-based MGs offer good biocompatibility and havepotential applications in biomedical components [3]. Despite theirmany advantages, the application of different types of MGs istypically limited by their intrinsic low ductility at room tempera-ture [1,2].

It has been widely accepted that local rearrangement of atomsis the basic process that accommodates applied shear strain duringthe deformation of MGs [1,2,4]. This rearrangement of atoms iscommonly described by localized shear distortion in disk-shapedvolume elements of the material, containing small clusters ofatoms, known as shear transformation zones (STZs). The formationof STZs creates further distortion in the surrounding material andgenerates larger deformed regions which result in shear bandformation [2,4]. Homogeneous deformation of MGs occurs whenSTZs are distributed throughout the entire volume of material.

local shear strain [2,4,5]. Therefore, MGs with higher concentions of free volume can exhibit increased homogeneous demation by accommodating local shear strain and deterring

propagation of shear bands.It has been reported that ion irradiation at room temperature

lead to an increase in the free volume content of MGs [5–7]. Aresult, ion irradiation has the potential to be used as a techniquimprove the ductility of MGs [6,7]. Reports on the effects ofirradiation on the mechanical properties of Zr-, Pt- and Cu-baMGs have appeared [5,6,8,9] however, investigations of

mechanical response of ion irradiated Ti-based MGs have blimited [7]. The objective of the present study was to investigateeffects of ion irradiation on the mechanical properties and the natof the plastic deformation response of a Ni-free, Ti-based MG al

2. Preparation of surfaces

Master ingots of the Ti40Cu32Pd14Zr10Sn2Si2 alloy (whcomposition is given in nominal atomic percentages) wprepared by arc melting, where mixtures of pure Ti, Cu, PdSn and Si elements with a purity of over 99.9 mass % were meltean Ar atmosphere purified using a Ti getter. Rapidly solidiribbon specimens were prepared by re-melting the alloy ingot

A R T I C L E I N F O

Keywords:

Surface

Nano indentation

Metallic glass

A B S T R A C T

The mechanical response of ion irradiated metallic glass specimens with nominal compositio

Ti40Cu32Pd14Zr10Sn2Si2 was investigated by nanoindentation. Surfaces of specimens fabricated by

melt spinning process were irradiated with 4 MeV Fe2+ ions at 25 8C over a range of fluences from 1 �

to 1 � 1015 ions/cm2. Irradiations were also performed at temperatures up to 300 8C. Nanoindenta

experiments were performed to obtain values of reduced elastic modulus and hardness and to investi

the material’s shear band behavior. Irradiation was found to result in a lower reduced elastic modulus

hardness, however was found to increase homogeneous deformation by suppression of shear b

activity.

� 2014 C

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http: / /ees.elsevier.com/cirp/default .asp

e offaceonsThe

by

Whereas inhomogeneous deformation occurs when atomicrearrangements are limited to the creation of shear bands [4].The operation of STZs is strongly influenced by local atomicarrangements where sites of relatively loosely packed atoms (i.e.,sites with higher free volume) would more readily accommodate

no- the* Corresponding author.

Please cite this article in press as: Lucca DA, et al. Investigation of tglass by nanoindentation. CIRP Annals - Manufacturing Technolog

http://dx.doi.org/10.1016/j.cirp.2014.03.045

0007-8506/� 2014 CIRP.

quartz tubes, and ejecting the molten alloy with an over pressur35 kPa through a nozzle onto a Cu wheel rotating with a survelocity of about 40 m/s in an Ar atmosphere. The resulting ribbhad a thickness of about 30 mm and a width of about 2 mm.

amorphous structure of the specimens was confirmed

performing X-ray diffractometry (XRD) in reflection with mochromatic Cu Ka radiation. Thermal stability associated with

he mechanical behavior of ion irradiated Ni-free Ti-based metallicy (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.045

glasscantranfoun

Tover1.7 Mdiamso airrad300

to amectureautowasspecand

The

the

Matcalcwhi

3. E

Ncombothcomperfproc4. ThbaseexpetherThe

confiintewerthe

to inneousequ0.33maxdimat 1unlocons30 smaxunloatomafteinderesp

4. R

Nspecflue25 8on tperfcm2

peneelasto th

D.A. Lucca et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx2

G Model

CIRP-1156; No. of Pages 4

Plegla

s transition and crystallization was examined by differentialning calorimetry (DSC) at a heating rate of 0.67 K/s. The glasssition temperature and first crystallization temperature wered to be 703 K and 783 K, respectively [3].he specimens were irradiated with 4 MeV Fe2+ ions at 25 8C a range of fluences from 1 � 1012 to 1 � 1015 ions/cm2 using a

V tandem accelerator. The beam spot was about 4 mm ineter and was raster scanned over an area of 1.2 cm � 1.2 cm

s to provide a uniformly irradiated area for analysis. Additionaliations were performed at temperatures ranging from 100 to8C at a fluence of 1 � 1013 ions/cm2. The specimens were glued

filament-heated stage, to which a thermocouple washanically attached for temperature monitoring. The tempera-

readings were used to adjust the filament current formatic temperature control. In addition, an infrared camera

used to monitor the surface temperature of the irradiatedimen. The temperature difference between the thermocouplethe infrared camera was less than 2 8C during the irradiation.temperature fluctuated less than 10 8C over the total time ofirradiation. Using the SRIM (Stopping and Range of Ions inter) code [10] the projected range of the 4 MeV Fe2+ ions wasulated to be 1.5 mm. After irradiation, XRD was performedch showed no evidence of crystallization.

xperimental procedure

anoindentation was performed using a load-controlledmercial nanoindenter. Indentations were performed using

Berkovich and spherical diamond indenters. The instrumentpliance and indenter area function were obtained byorming indentations in fused silica and tungsten using theedure of Oliver and Pharr [11] and consistent with ISO 14577-e radius of the spherical indenter was estimated to be 1 mmd on the obtained area function. Prior to performing eachriment, the instrument and specimen were allowed to

mally equilibrate for 10–12 h inside a thermal enclosure.instrument’s drift rate was checked before indenting torm that it was less than 0.1 nm/s averaged over a five second

rval. The indentations performed with the Berkovich indentere used to investigate hardness and elastic modulus, whereasindentations performed with the spherical indenter were usedvestigate the onset of plastic deformation and the homoge-s versus inhomogeneous material response. The loadingence for the Berkovich indentations consisted of loading at

mN/s to a maximum force of 10 mN, a 60 s hold at theimum force to allow any time dependent plastic effects toinish, 10 s unloading to 10% of the maximum force, a 60 s hold0% maximum force to measure thermal drift and a 2 s finalading. The loading sequence for the spherical indentationsisted of loading at 0.05 mN/s to a maximum force of 10 mN, a

hold at the maximum force, 10 s unloading to 10% of theimum force, a 30 s hold at 10% maximum force and a 2 s finalading. Typical average surface roughnesses, as measured withic force microscopy (AFM), were 3 nm Ra before and 6 nm Ra

r irradiation. The reported data for the Berkovich and sphericalntations represent the average of 7 and 10 experiments,ectively.

esults and discussion

range was 1.5 mm, therefore the values obtained represent themechanical properties of the irradiated layer. Fig. 1 shows typicalforce vs. penetration depth curves obtained for the as-spunspecimen and for specimens irradiated with 4 MeV Fe2+ ions withfluences of 1 � 1013 ions/cm2 and 1 � 1015 ions/cm2 at 25 8C. Thecurves show loading, the hold at maximum force and unloading.The point at which the unloading curve intersects the depth axis isthe permanent depth of indentation. Discrete displacement bursts(an increase in penetration depth with no corresponding increasein force) can also be observed. This behavior becomes morepronounced at higher forces. Also shown on the curves are errorbars at the high end of loading representing the maximum andminimum deviation of the curves obtained for 7 repeat indenta-tions. The data enables the evaluation of hardness and reducedelastic modulus. The initial stage of unloading is fit to a power lawwhich then allows for the determination of the stiffness duringunloading, S = dP/dh, and the contact area, A at the maximumapplied force, Pmax. The hardness, H and reduced elastic modulus, Er

are then obtained by H = Pmax/A and Er = p1/2S/(2A1/2) where 1/Er

= (1 � n2)/E + (1 � ni2)/Ei and E and n are the elastic modulus and

Poisson’s ratio for the specimen and Ei and ni are the same values forthe indenter.

The values obtained for Er and H are shown in Table 1. Theresults shown represent the average values obtained for 7indentations along with their standard deviation. The as-spunspecimen was found to have Er and H values of 118 GPa and9.5 GPa, respectively. These values can be compared to thoseobtained in a recent study of a similar as-cast Ti-based MG wherereduced elastic modulus and hardness were reported to be 105 GPaand 7.5 GPa, respectively [12]. Ion irradiation at 25 8C is seen toreduce both Er and H. This is consistent with a study on the effectsof ion irradiation of a Zr-based MG, where the introduction ofexcess free volume resultant from irradiation was implicated asthe cause for the reduction in elastic modulus [5]. A similar effect ofexcess free volume on hardness could also be expected. Both Er andH are seen to increase with increasing irradiation temperature. It isaccepted that heating a MG below its glass transition temperature

Fig. 1. Force vs. penetration depth curves for Berkovich indentations performed in

as-spun and ion irradiated specimens.

Table 1Reduced elastic modulus and hardness.

Specimen irradiation Er (GPa) H (GPa)

as-spun 118 � 1% 9.5 � 3%

1 � 1012 Fe2+/cm2 at 25 8C 99 � 8% 7.4 � 15%

1 � 1013 Fe2+/cm2 at 25 8C 104 � 3% 7.3 � 6%

1 � 1014 Fe2+/cm2 at 25 8C 98 � 4% 6.8 � 7%

1 � 1015 Fe2+/cm2 at 25 8C 106 � 4% 7.9 � 5%

as-spun 118 � 1% 9.5 � 3%

1 � 1013 Fe2+/cm2 at 25 8C 104 � 3% 7.3 � 6%

1 � 1013 Fe2+/cm2 at 100 8C 112 � 3% 9.0 � 4%

1 � 1013 Fe2+/cm2 at 200 8C 117 � 1% 9.4 � 3%

1 � 1013 Fe2+/cm2 at 300 8C 127 � 4% 9.6 � 3%

anoindentation experiments were performed on an as-spunimen and on specimens irradiated with 4 MeV Fe2+ ions with

nces of 1 � 1012, 1 � 1013, 1 � 1014 and 1 � 1015 ions/cm2 atC. To investigate the effect of elevated irradiation temperaturehe resulting materials response, nanoindentation was alsoormed on specimens irradiated with a fluence of 1 � 1013 ions/at 100, 200 and 300 8C. The measured indentation force vs.tration depth curves were used to determine the hardness and

tic modulus at a depth below the surface which correspondede maximum force (220–250 nm). Recall that the projected ion

ase cite this article in press as: Lucca DA, et al. Investigation of the mechanical behavior of ion irradiated Ni-free Ti-based metallicss by nanoindentation. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.045

for to

thaturstanding

vs. the

theangDE),y ofrial

waspth

PDEen

eentedom

luesomith

cantionplesun,013

vs.ted

theear

otalhis

pun byces 8C,ous

spun

D.A. Lucca et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx 3

G Model

CIRP-1156; No. of Pages 4

leads to the annihilation of free volume [6] which is consistentwith reducing the effect of irradiation on Er and H. At the higherirradiation temperatures investigated, the irradiated specimensare found to exhibit mechanical properties comparable to the as-spun specimen.

To accentuate the displacement bursts visible in Fig. 1,indentations were performed with a spherical indenter using alower loading rate than used for the Berkovich indentations. Usinga spherical indenter with large radius extends the elastic region tolarger depths of penetration. Slower loading rates have been foundto accentuate displacement bursts in a variety of MGs [4,13].

Fig. 2 shows the force vs. penetration depth curves obtained forindentations made with the spherical indenter to a maximum forceof 10 mN in the as-spun and 1 � 1015 Fe2+/cm2 ion irradiatedspecimens. The curves have been offset for clarity. Displacementbursts are observed for both of the specimens however they aremore clearly defined for the as-spun specimen. Displacementbursts have been observed for a variety of MGs and are attributedto deformation by shear bands [4,14]. The values obtained for theforce at the first displacement burst and the plastic deformationenergy (PDE) (to be discussed below) for selected specimens areshown in Table 2. The results shown represent the average valuesobtained for 10 indentations along with their standard deviation.For the as-spun specimen the first displacement burst occurs at anaverage force of 1014 mN. The ion irradiated specimens exhibitedmore variability in the force at which the first displacement burstoccurred however the average force values were significantlyhigher. Elevated irradiation temperature, as for Er and H, was seento reduce the effect of room temperature irradiation. For all of thespecimens the length of the displacement burst tended to increaseas the force increased which has been found for most MGs [4]. Theion irradiated specimens were found to have slightly fewerdisplacement bursts and the length of the bursts were of similarlength or shorter than displacement bursts at the same force in theas-spun specimen.

Fig. 3 shows the initial portion of a loading curve for anindentation in the as-spun specimen. For this indentation, the forceat which the first displacement burst occurs is 1035 mN. It has beenshown that the initial portion of the loading curve up to the firstdisplacement burst represents elastic deformation [15]. To further

specimen with a maximum force which was less than thatthe first displacement burst. The unloading curve was foundretrace the loading curve which further supports the report

the portion of the loading curve before the first displacement bis fully elastic. The displacement burst is caused by shear bformation and can be considered the onset of plasticity [14]. Usthe definition of mechanical work, the area under the forcepenetration depth curve from the initial force to the start offirst displacement burst is elastic work, We and the area underdisplacement burst is plastic work, Wp as shown in Fig. 3. Wet al. [15] demonstrated that the plastic deformation energy (Pwhich is based on the plastic work, is related to the ductilitMGs. The PDE is equal to Wp/Vp where Vp is the volume of matedisplaced during the displacement burst. The volume

calculated using the area function and the average contact deof the indenter during the displacement burst. Table 2 shows

values obtained for selected specimens. For the as-spun specimthe average value of PDE was 1.23 � 1010 J/m3. This value was sto decrease with irradiation at room temperature. Elevatemperature again was seen to reduce the effect of rotemperature irradiation. Wang et al. [15] report that lower vaof PDE are found in MGs with higher ductility. Irradiation at rotemperature was found to decrease PDE, which is consistent wan increase in ductility.

The effects of ion irradiation on the deformation of the MGfurther be investigated by considering the amount of deformathat occurs during the displacement bursts. Fig. 4 shows examof force vs. penetration depth curves for four specimens: as-sp1 � 1013 Fe2+/cm2 at 25 8C, 1 � 1015 Fe2+/cm2 at 25 8C and 1 � 1Fe2+/cm2 at 300 8C. Next to the original data is the forcepenetration depth curve with the displacement bursts subtracfrom the curve which shifts the curve to the left. The sum ofdisplacement bursts which is the deformation caused by shbands (inhomogeneous deformation) can be compared to the tplastic deformation by calculating the ratio of the two values. Tvalue expressed as a percent is shown in Fig. 4. For the as-sspecimen approximately 20% of the total deformation isinhomogeneous deformation and ion irradiation at 25 8C reduthis value to about 10%. For the specimen irradiated at 300approximately 30% of the total deformation is by inhomogene

Fig. 3. Force vs. penetration depth curve for initial loading portion of the as-

specimen.Fig. 2. Force vs. penetration depth curves for spherical indentations performed in

as-spun and ion irradiated specimens. Arrows denote the location of the first

observed displacement burst.

ted

confirm this, indentations were performed in the as-spun n oftheandilardy.

alsoion., anth as a

Table 2Force at first displacement burst and PDE.

Specimen irradiation Force at first displacement

burst (mN)

PDE (1010 J/m3)

as-spun 1014 � 3% 1.23 � 4%

1 � 1013 Fe2+/cm2 at 25 8C 2633 � 28% 0.96 � 7%

1 � 1015 Fe2+/cm2 at 25 8C 2618 � 17% 0.99 � 5%

1 � 1013 Fe2+/cm2 at 300 8C 1820 � 9% 1.14 � 5%

Please cite this article in press as: Lucca DA, et al. Investigation of tglass by nanoindentation. CIRP Annals - Manufacturing Technolog

deformation. Hu et al. [7] studied Ti40Zr25Be30Cr5 MG irradiawith C4+ and Cl4+ and found differences in the loading portiothe force vs. penetration depth curves. They attributed

differences to ion irradiation changing the formation

propagation of shear bands. It is plausible that a simmechanism is involved for the Ti-based MG used in this stuThe increase in free volume resultant from irradiation is

consistent with this shift toward more homogeneous deformatTo better understand the deformation mechanisms at work

examination of the area around the indentations performed wiBerkovich indenter was undertaken using AFM. Fig. 5 show

he mechanical behavior of ion irradiated Ni-free Ti-based metallicy (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.045

comforc3 mmrangspun1 �

Asurrfor

(Figindeindeirradof thpiledtop

arouAl90

asso

Fig. 5perfo

Fig. 4displ

D.A. Lucca et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx4

G Model

CIRP-1156; No. of Pages 4

Plegla

parison of indentations in four specimens all with a maximume during indentation of 10 mN. The scan area was

� 3 mm in the x- and y-directions and the height scaleed from 210 nm to 420 nm. The specimens shown are: (a) as-, ion irradiated with (b) 1 � 1013 Fe2+/cm2 at 25 8C, (c)

1015 Fe2+/cm2 at 25 8C and (d) 1 � 1013 Fe2+/cm2 at 300 8C.ll specimens exhibit pile-up i.e., material that is above theounding surface, however the nature of the pile-up is differentthe specimens. For the two specimens irradiated at 25 8C. 5b and c), the pile-up is mostly confined to the edges of thentation and does not extend much beyond the perimeter of thent. For the as-spun specimen (Fig. 5a) and the specimeniated at 300 8C (Fig. 5d), the pile-up extends beyond the edgese indentation in arcs around the indentation. Furthermore the-up material is mostly terrace-like with flat or slightly sloped

surfaces. Jiang and Atzmon [16] have seen similar featuresnd deeper indentations, 1 mm depth at maximum force, in

Fe5Gd5 MG which they attributed to shear bands and theciated displacement bursts. This is consistent with the results

of Figs. 1 and 2 where the specimens that were irradiated at 25 8Cshowed fewer displacement bursts than the as-spun specimen. It isalso consistent with the results of the spherical indentationsshown in Fig. 4 where the specimens irradiated at 25 8C showedfewer displacement bursts and more homogeneous deformation.

5. Conclusions

The mechanical response of ion irradiated Ni-free, Ti-based MGhas been investigated by nanoindentation. Ion irradiation per-formed at room temperature, which has been reported tointroduce excess free volume in MGs, was seen to result in areduction of elastic modulus and hardness, and to shift thedeformation mechanism toward less shear localization and morehomogeneous plastic flow. This observation is consistent with thefact that MGs with higher concentrations of free volume canexhibit increased homogeneous deformation by accommodatinglocal shear strain and deterring the propagation of shear bands.Elevated temperature below the glass transition temperature,which is known to lead to the annihilation of free volume, was seento reduce the effects of room temperature irradiation on the elasticmodulus and hardness as well as on the relative proportion ofinhomogeneous to homogeneous deformation.

Acknowledgments

The support by NSF through Grant No. CMMI-1130606 (OSU)and Grant No. CMMI-1130589 (TAMU) is gratefully acknowledged.

References

[1] Miller M, Liaw P (2008) Bulk Metallic Glasses, Springer, New York, NY.[2] Trexler MM, Thadhani NN (2010) Mechanical Properties of Bulk Metallic

Glasses. Progress in Materials Science 55:759–839.[3] Zhu S, Xie G, Qin F, Wang X (2013) Effects of Minor Si Addition on Glass

Formation and Thermal Stability of Ni-Free Ti-Based Bulk Metallic Glass.Materials Science Forum 750:36–39.

[4] Schuh CA, Hufnagel TC, Ramamurty U (2007) Mechanical Behavior of Amor-phous Alloys. Acta Materialia 55:4067–4109.

[5] Raghavan R, Kombaiah B, Dobeli M, Erni R, Ramamurty U, Michler J (2012)Nanoindentation Response of an Ion Irradiated Zr-Based Bulk Metallic Glass.Materials Science and Engineering A 532:407–413.

[6] Raghavan R, Boopathy K, Ghisleni R, Pouchon MA, Ramamurty U, Michler J(2010) Ion Irradiation Enhances the Mechanical Performance of MetallicGlasses. Scripta Materialia 62:462–465.

[7] Hu Z, Zhao Z, Hu Y, Xing J, Lu T, Wei B (2012) Effect of Ion Irradiation onMechanical Behaviors of Ti40Zr25Be30Cr5 Bulk Metallic Glass. Journal of Materi-als Research 15(5):713–717.

[8] Luo WD, Yang B, Chen GL (2011) Effect of Ar+ Ion Irradiation on the Micro-structure and Properties of Zr-Cu-Fe-Al Bulk Metallic Glass. Scripta Materialia64:625–628.

[9] Carter J, Fu EG, Bassiri G, Dvorak BM, David Theodore N, Xie G, Lucca DA, MartinM, Hollander M, Zhang X, Shao L (2009) Effects of Ion Irradiation in MetallicGlasses. Nuclear Instruments and Methods in Physics Research Section B267:1518–1521.

[10] Ziegler JF, Biersack JP, Littmark U (1985) The Stopping and Range of Ions in Solids,Pergamon, New York. (Available from www.srim.org).

[11] Oliver WC, Pharr GM (1992) An Improved Technique for Determining Hard-ness and Elastic Modulus Using Load and Displacement Sensing IndentationExperiments. Journal of Materials Research 7:1564–1583.

[12] Fornell J, Van Steenberge N, Varea A, Rossinyol E, Pellicer E, Surinach S, BaroMD, Sort J (2011) Enhanced Mechanical Properties and in Vitro CorrosionBehavior of Amorphous and Devitrified Ti40Zr10Cu38Pd12 Metallic Glass. Jour-nal of the Mechanical Behavior of Biomedical Materials 4:1709–1717.

[13] Huang Y, Chiu YL, Shen J, Chen JJJ, Sun J (2009) Nanoindentation Study of Ti-Based Metallic Glasses. Journal of Alloys and Compounds 479:121–128.

[14] Schuh CA, Nieh TG (2004) A Survey of Instrumented Indentation Studies on

. Original force vs. penetration depth curves and the same curves with the

acement bursts removed.

. Atomic force microscopy images of residual impressions from indentations

rmed with the Berkovich indenter.

ase cite this article in press as: Lucca DA, et al. Investigation of tss by nanoindentation. CIRP Annals - Manufacturing Technolog

Metallic Glasses. Journal of Materials Research 19:46–57.[15] Wang K, Chen MW, Pan D, Fujita T, Zhang W, Wang XM, Inoue A (2008) Plastic

Deformation Energy of Bulk Metallic Glasses. Materials Science and EngineeringB 148:101–104.

[16] Jiang WH, Atzmon M (2003) Rate Dependence of Serrated Flow in a MetallicGlass. Journal of Materials Research 18:755–757.

he mechanical behavior of ion irradiated Ni-free Ti-based metallicy (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.045