It has been well known that plastic deformation of bulk metallic glasses
(BMGs) is localised in thin shear bands. So, initiation of shear bands and
related deformation should be studied for comprehensive understanding of
deformation mechanisms of BMGs. In this paper, indentation techniques are
extensively used to characterise elastic deformation of Zr-Cu-based metallic
glass, followed by a systematic analysis of initiation and evolution of shear
bands in the indented materials. Our results, obtained with a suggested
wedge-indentation technique, demonstrated initiation of shear bands in materials
volume

Anish Roy (1256436)

Vadim Silberschmidt (1248129)

Vahid Nekouie (7202819)

English

Loughborough University Institutional Repository

Proceedings of XLIII International Summer SchoolConference APM 2015Indentation study of mechanical behaviour ofZr-Cu-based metallic glassV. Nekouie, A. Roy, V.V. SilberschmidtV.Nekouie2@lboro.ac.ukAbstractIt has been well known that plastic deformation of bulk metallic glasses(BMGs) is localised in thin shear bands. So, initiation of shear bands andrelated deformation should be studied for comprehensive understanding ofdeformation mechanisms of BMGs. In this paper, indentation techniques areextensively used to characterise elastic deformation of Zr-Cu-based metallicglass, followed by a systematic analysis of initiation and evolution of shearbands in the indented materials. Our results, obtained with a suggestedwedge-indentation technique, demonstrated initiation of shear bands in ma-terials volume.Key words: Metallic glass; shear bands; indentation.1 IntroductionModern high-tech industries rely on the manufacture and synthesis of advancedmaterials that are stronger than conventional ones. Bulk metallic glasses (BMGs)have received much scientific and technological attention due to their advanced me-chanical properties such as a high ratio of elastic limit to the Youngs modulus andhigher fracture toughness, when compared to their crystalline counterparts of thesame composition. This is typically attributed to the absence of a long-range orderin their atomic structure and a lack of defects such as dislocations, which controlductility in traditional metallic materials. BMGs are brittle and exhibit negligibleplasticity in the macro-scale. Some recent experiments on sub-micron and nano-sized BMG specimens showed that the process of shear localisation become morestable and less catastrophic, when compared to a response exhibited by large-sizesamples [1]. These desirable and unique properties of metallic make them an idealcandidate for many applications such as MEMS (micro-electromechanical systems),miniaturised biomedical devices and implants as well as in micro-robotics. A numberof mechanistic theories have been proposed to describe the plastic flow and defor-mation behaviour of BMGs. Some popular theories are concepts of free volume andshear transformation zones (STZs) proposed in Argon and Spaepens model, basedon motion of atoms [2, 3]. The deformation mechanism of metallic glasses based onthese concepts is realised homogenously or inhomogenously, depending on the levelsof strain rate, temperature and applied stress [4]. A significant amount of experi-ments was carried out to understand deformation mechanisms. Prior studies showed291Indentation study of mechanical behaviour of Zr-Cu-based metallic glassthat shear bands with a characteristic thickness of the order of 10-20 nm were re-sponsible for deformation of BMGs at low temperatures and/or high stresses. Morerecently, deformation-induced crystallisation was observed in a number of BMGsthat led to substantial plastic deformation [5]. These changes in shear bands werenot only induced by bending or compression of BMGs [6], but were the results of theirnanoindentation or microhardness testing [7], ball milling or cold rolling process [8].However, various research groups suggested contradicting conclusions on shear bandcrystalisation, based on similar experiments performed with various BMG systems[4, 9]. Thus, a question on propensity for crystallisation is still open. Traditional in-dentation techniques have been used extensively over years to determine mechanicalproperties and deformation mechanisms of metallic glasses [10]. These techniqueshelped researchers to perform mechanical characterisation at micro-scale and anal-yse the mechanism of plastic flow in BMGs. Although shear bands typically initiatebeneath the indenter, in nano- and micro-indentation experiments, by their verynature, shear bands could be observed only after they evolved to the surface. So, awedge indentation experiment was designed to overcome the limitation of nano- andmicro-indentation to observe the initiation and propagation of shear bands underthe indenter surface [1]. As the length of the wedge indenter was considerably largerthan its width, the wedge indentation experiments also allow numerical modelling tobe simplified to a 2D formulation. There are different, sometimes contradicting hy-potheses about the deformation mechanisms of BMGs at microscale. Hence, furtherstudies are required to understand initiation and propagation of shear bands in thevolume and at the surface of metallic glasses. In this study, a Zr-Cu-based metallicglass is characterised using nano- and micro-indentation techniques. A thoroughstructural characterisation of shear bands around the indented region was carriedout to understand the nature of shear banding in BMGs.2 Experimental procedureFor our studies, a beam-shaped alloy with nominal composition of Zr48Cu36Al8Ag8was prepared at IFW Dresden, Institute for Complex Materials, , Germany by arc-melting the pure elements (99.9% Zr, 99.99% Cu). BMG specimens were cut andpolished to mirror-like finish with the roughness of some 5 nm. Indentation testswere conducted to characterise the shear bands with a nano indentation test sys-tem (Micro Materials Ltd.) using a spherical and Vickers indenters. A series ofnano- and micro-indentations were conducted on the polished surface of the sam-ples with a loading rate of 2 mN/s. A wedge indenter made of high-speed steel,with a nominal angle of 60 and an edge radius of 19.5 m was designed and manu-factured in-house (Figure 1). The indentation tests reported here were conductedat ambient temperature. XRD analysis of as-cast samples was carried out to studyformation of crystalline phases. In order to reconfirm the crystallography, the sam-ples were thinned to electron transparency and observed using transmission electronmicroscopy (TEM). Scanning electron microscopy (SEM) was used to observe evo-lution of shear bands on the deformed surfaces.292Proceedings of XLIII International Summer SchoolConference APM 2015Figure 1: Experimental set-up for wedge indentation and its dimension3 Results and DiscussionThe amorphous nature of the supplied samples of BMG was initially investigatedusing X-ray diffraction (Figure 2); their microstructure was further characterisedwith TEM. The TEM results confirmed the amorphous nature of the alloy, as thefirst halo ring of a Selected Area Electron Diffraction (SAED) pattern did not showany presence of nanocrystals.Figure 2: (a) XRD pattern of as cast Zr48Cu36Al8Ag8, (b) corresponding SAEDpatterns.3.1. MicroindentationMultiple unloading-reloading experiments were conducted using a spherical in-denter with diameter of 50 m at loading rate of 2 mN/s to investigate the variationof elastic modulus with depth in BMG specimens. The maximum indentation depthranged from 6 m to 18 m, and three partial unloads down to 20% of the peak load ateach step were applied in these steps. Shear bands formed at loads in excess of 10 N.As shown in Fig. 3, shear bands moving from various initiation points crossed eachother; however, shear bands nucleated later were arrested by already nucleated ones.293Indentation study of mechanical behaviour of Zr-Cu-based metallic glassInstability of shear bands was observed in the form of nucleation of several secondaryshear bands from the primary ones in the course of deformation. A large plastic zonewas formed under the indenter tip during indentation [12, 13]. It contained a highdensity of shear bands; this is ideal for investigation of deformation-induced harden-ing and softening effects [14]. The obtained results showed a dependence of elasticmodulus on penetration depth, indicating a work-softening phenomenon in the stud-ied metallic glass, especially at microscale. The values of reduced (Er) and elastic(E) moduli obtained from the curve using the Oliver-Pharr method are given inTable 1. The reduced and elastic moduli decreased from 48 GPa to 38 GPa and41 GPa to 33 GPa, respectively. This phenomenon is often referred as indentationsize effect (ISE) [12, 15], manifested by a decrease in the elastic modulus with anincrease in the indentation depth. A large number of shear bands were activated byindentation; this reduced the reaction force on the indenter, leading to a reductionin material stiffness.Figure 3: SEM images of micro-indentation in Zr48Cu36Al8Ag8.Table 1: Reduced modulus (Er) and elastic modulus (E) of Zr48Cu36Al8Ag8 atvarious depth.Cycle Indentation depth (µm) Reduced modulus (GPa) Youngs modulus (GPa)1 6.2 48 41.72 12.1 40 34.83 18.2 38 33.063.2 Shear-band initiationThe aim of this experiment was to characterise initiation of the first plastic eventin order to calculate the required stress for shear bands initiation of the metallicglass investigated in this study. Based on the nano-indentation experiments [1],the first pop-in event occurred at applied force of 4 mN resulted in indentationdepth of 0.060 m. The corresponding total area of contact and stress required forthe first pop-in were 1.88 m2 and 2.12109 N/m2, respectively. It is necessary todetermine the contact area Ac in wedge-indentation experiments in order to predictthe approximate force necessary for initiation of shear bands. This force can beobtained using the following equation:F = σAc Eq.1294Proceedings of XLIII International Summer SchoolConference APM 2015It was found that the required load would be around 500 N; hence, testing wascarried out using the beam-shaped samples in a compression mode with a constantdisplacement rate of 0.05 mm/min using the wedge indentation technique and theload levels between 200 N and 500 N. To avoid contact problem at low loads, surfacesof the wedge indenter were prepared with the use of a surface grinder in an attemptto make the surface uniform while maintaining the 60 angle.Figure 4: Evolution of shear bands pattern with load under wedge indentation: (a)400 N, (b) 300 N, (c) 200 N.The wedge indenter has an edge radius of 8 m and height of 5 m. Evolution ofdeformation pattern on the front surface of the specimen is presented in Fig. 4. Theplastic depth increased from 5 m to 13 m by increasing the load from 200 N to 500N, and serrated semi-circular slip-steps formed by shear bands were observed. Theresults show that the nucleation and initial propagation of shear bands occurredat loads below 200 N; there were no shear bands at 100 N load. For 200 N, the295Indentation study of mechanical behaviour of Zr-Cu-based metallic glassindentation depth was 22.17 m before unloading obtained from F-D curve. Thecalculated stress was :σ =FAc= 0.886GPaAt 100 N, the shear band initiation stress was approximately 0.7 GPa, therefore,it can be estimated that, in wedge indentation the stress level required to initiateshear-band formation is between 0.7-0.9 GPa. Due to different shear bands morphol-ogy in wedge indentation, this value is not similar to the calculated stress requiredfor shear bands initiation in nanoindentation with spherical indenter.3.3 Comparison between glass and metallic glassThe purpose of this study was to compare fracture surfaces of soda-lime-silicaglass and the studied Zr-Cu-based metallic glass using the wedge indentation tech-nique at room temperature. The relationship established between mechanical be-haviour and fracture feature can assist in elucidating the fracture mechanism. Wedgeindentation was applied to both glass and metallic-glass specimens with dimensionsof 40 mm x 4 mm x 2mm using loads of 500 N, 1 kN and 1.3 kN. Fractography studiesshowed that fracture surfaces of materials that fail in a brittle manner from surfacecracks are characterized by a sequence of three distinct fracture features includingmirror, mist and hackle regions, depending on the loading mode. For instance, therewas no mist region observed on fracture surfaces formed in the mixed-mode failure[17]. A side view of wedge indentation for a glass specimen is presented in Fig. 5;here, classical concentric cone cracks were observed. The contact radius at maxi-mum pressure was just within the outermost surface ring in Fig. 5, confirming thatthe cone fractures formed in the region of weak tension outside the subsurface com-pression zone. In addition, there was no detectable deformation observed beneaththe contact circle; essentially, the material behaved as an ideally homogeneous solid.As shown in micrographs of subsurface damage at higher magnification in Fig. 5(c),hackle marking on the fracture surfaces of soda glass appeared as lances. Observa-tion of fracture surfaces indicated that the propagating crack did not experience anyenergy-dissipation process such as plasticity or crack bridging, which could result inretarding the crack growth in a substantial manner.As shown in Fig. 6, significant differences were found in the appearance of thefracture surfaces of specimens of the traditional and metallic glasses at microscale.In contrast to the former, shear bands in BMGs were not brittle and provided theability to deform plastically, with many semi-circular shear bands created beneaththe indenter. Shear bands bifurcate with increasing distance from the indentertip, indicating branching and healing mechanisms contributing to energy-dissipationprocesses, which led to plastic deformation at microscale.4 ConclusionsThe microhardness study perfomed on Zr48Cu36Al8Ag8 clearly indicated the depen-dence of its elastic modulus on penetration depth at microscale due to activation of296Proceedings of XLIII International Summer SchoolConference APM 2015Figure 5: Optical micrographs of indented soda-lime-silica glass after applying 1.3kN showing cone fracture form (a)and hackle fracture (b and c).Figure 6: Optical micrograph of 1 kN wedge indentation on as-cast Zr48Cu36Al8Ag8metallic glass (a) and soda-lime-silica glass (b).a large number of shear bands. A relatively new technique wedge indentation wasemployed to calculate the required stress for shear bands initiation of the metal-lic glass. It was estimated that the level of stress required to initiate shear bandswas between 0.7-0.9 GPa in wedge indentation. The wedge indentation techniquewas also applied to compare fracture surfaces of the soda-lime-silica glass and thestudied Zr-Cu-based metallic glass at microscale. Observation of fracture surfacesindicated that the propagating cracks did not experience any energy dissipation inthe traditional glass; on the contrary, the shear-band evolution in the metallic glassshowed branching and healing mechanisms contributing to the plastic deformation297Indentation study of mechanical behaviour of Zr-Cu-based metallic glassat microscale.References[1] Nekouie, V., Abeygunawardane-Arachchige, G., Roy, A., Silberschmidt V.V.:Indentation-induced deformation localisation in Zr-Cu-based metallic glasses. J.Alloys Compd. 615, 93-97 (2014)[2] Argon, A.: Plastic deformation in metallic glasses. 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Indentation study of mechanical behaviour of Zr-Cu-based metallic glass

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Indentation study of mechanical behaviour of Zr-Cu-based metallic glass

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