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Stability of medium range order in Al-based metallic glass compacted by severe plastic deformation
Journal of Alloys and Compounds 561 (2013) 5–9
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Stability of medium range order in Al-based metallic glass compacted by severe plastic deformation Zs. Kovács a, P. Henits a, L.K. Varga b, E. Schafler c, Á. Révész a,⇑ a
Department of Materials Physics, Eötvös University, P.O.B. 32, H-1518 Budapest, Hungary Research Institute for Solid state Physics and Optics, Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest, Hungary c Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria b
a r t i c l e
i n f o
Article history: Received 26 October 2012 Received in revised form 31 January 2013 Accepted 1 February 2013 Available online 16 February 2013 Keywords: Metallic glass Medium range order Al-based Structure
a b s t r a c t High pressure torsion has successfully been applied to produce low-porosity, bulk specimens from Al-based metallic glass ribbons (Al85Y8Ni5Co2, Al85Ce8Ni5Co2 and Al85Gd8Ni5Co2). The compacted disks possess higher hardness than the original glass and have substantial glass fraction with nanocrystalline precipitations. Mechanical and thermal impacts have only minor effects on the glassy structure as demonstrated by the stability of the X-ray diffraction halo positions. Unchanged halos reveal that medium range order is a key characteristic of the amorphous state. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their low density, aluminium based alloys are widely used in engineering applications, particularly in the field of aerospace and automotive industry. Their low yield strength, however, cannot be increased over 500–600 MPa by classical strengthening mechanisms. In order to improve the mechanical properties of Al-based alloys, alternative mechanisms and structures have to be considered [1,2]. As a result of this research, aluminium alloys with non-periodic atomic structure have been discovered in the late 1980s, possessing a yield strength close to 1 GPa [3–5]. Later the 1 GPa limit has been exceeded by homogeneous precipitation of nanoscale Al-particles into the amorphous matrix. Recently, such nanoscale crystalline particles are induced either by heat treatment [6,7] or plastic deformation [8–10]. Since Al-based metallic glasses, even if designed for high stability [11], can only be produced in limited size [6,12,13], some kind of compaction of small metallic glass pieces is required for practical applications. For example, centimeter-scale Al85Ni5Y6Co2Fe2 bulk glassy alloys have been fabricated by warm extrusion of gas-atomized amorphous powder [14]. Unfortunately, the traditional compaction methods themselves can undermine the excellent mechanical properties of metallic glasses. However, record fracture strength of 1250 MPa can be achieved in Al86Ni6Y4.5Co2La1.5 glassy compacts produced by a specific method, i.e. spark ⇑ Corresponding author. Tel.: +36 1 372 2823; fax: +36 1 372 2811. E-mail address: [email protected] (Á. Révész). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.010
plasma sintering [15]. Alternatively, high pressure torsion (HPT), applying severe shear deformation can also be used as an alternative compaction method. Originally, HPT was invented for producing highly dense, bulk ultrafine-grained (submicron grain-sized or nanostructured) materials [16]. Later on, successful attempts were focused on producing massive samples by HPT from meltquenched amorphous ribbons by consolidation at room temperature [8,9,17]. Deformation induced crystallization of metallic glasses is a well known phenomenon [18–22], but despite of many experimental results, its basic mechanism is still unclear. In the Al–TM–RE system, if the first crystallization event is the precipitation of a-Al, the deformation usually promotes the nucleation of a-Al [23–25]. On contrary, we have reported primary crystallization of a-Al nanocrystals in a thermally eutectic-like crystallizing Al85Ce8Ni5Co2 amorphous alloy, when extremely large deformation was applied [26]. Stability of metallic glasses strongly depends on their composition. Fully amorphous alloys can be produced typically in narrow compositional range [4], in which topological constraints and chemical interaction stabilize the glassy structure [27,28]. Densely packed clusters with typical structural units determine the concentration ranges of stability [29]. These structural units are typical for a given family of glass and reflect the composition and chemical interactions between the constituents. In Al–TM–RE systems, Al–Al and energetically favored Al–RE pairs are generally the dominant pairs, but topological effects are also decisive on the short range ordering (SRO) of the clusters [30]. Powerful MD simulations
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Zs. Kovács et al. / Journal of Alloys and Compounds 561 (2013) 5–9
reveal that the chemically stable clusters (e.g. icosahedral cluster) with no possibility for periodic lattice configuration, form the glass structure with strong sharing between the neighboring cluster units [31]. In agreement with this constrained structure of the different clusters, metallic glasses possess structural stability and medium range order (MRO) as an inherent characteristic [32]. Extensive compaction processes could have both thermal and mechanical impacts on glassy alloys [10,33]. In this work, the effect of these impacts on the glassy structure of Al–TM–RE alloys is investigated, focusing on the atomic correlation beyond the average nearest neighbor distance.
2. Experimental Ingots of Al85Ce8Ni5Co2, Al85Y8Ni5Co2 and Al85Gd8Ni5Co2 were synthesized by induction melting of a mixture of high purity (99.9%) Al, Ce, Gd, Y, Ni and Co metals. Fully amorphous ribbon samples with width of 5 mm and thickness of 25 lm were obtained using a single roller melt spinning technique in inert atmosphere. Subsequently, the as-quenched ribbons were attrited into flakes with a characteristic diameter of 1 mm (Fig. 1a) and then pre-compacted into cylindrical disks with a diameter of 8 mm, thickness of 0.7 mm. The pre-compacts were placed between anvils of the HPT device and shear deformation was performed for N = 1 and N = 5 rotations with a 5 min revolution time under hydrostatic pressure of 6 GPa applying constrained condition. The maximum shear deformation achieved at the perimeter of the N = 5 disk is c 180. The detailed description of HPT processing can be found in Ref. [16]. The crystalline phase analysis was carried out on diffractograms performed by a Philips (PW1130) X-ray generator with a Guinier-chamber set-up. The chamber has a diameter of 100 mm and the patterns were recorded on FUJI Imaging Plate (BAS MS2025) with spatial resolution of 0.001° and high sensitivity span over 4–5 orders of magnitude. Morphology studies were performed on a FEI QUANTA 3D dual beam scanning electron microscope (SEM). The compositional changes of the surface related directly to contrast differences in the back-scattered electron (BSE) image were revealed and quantitatively determined by energy dispersive X-ray (EDX) analysis with a relative accuracy of 1%. For microhardness (HV) investigations and optical imaging the Al85Ce8Ni5Co2 disks were cut along the diameter, embedded in epoxy resin and polished to mirror-like surface. HV test was performed at room temperature on the polished surface along the diameter using a Shimadzu DUH-202 dynamic depth sensing ultra-microhardness tester with a diamond Vickers indenter at a load rate of 17.65 mN/s up to 200 mN. During the depth-sensing hardness measurements, the indentation depth and the load were recorded as a function of time by a computer. From these data the values of HV were determined using the Oliver–Pharr method [34] by averaging four indents. The porosity of the pre-compact and the HPT disks was estimated from the optical micrographs by counting the number of pixels corresponding to voids.
Fig. 1. Optical micrographs of the Al85Ce8Ni5Co2 metallic glass material at different stages of the compaction process. (a) Plane view of flake particles and the crosssectional view of (b) pre-compacted, (c) HPT N = 1 and (d) HPT N = 5 disks. The scale bar corresponds to all of the images.
3. Results and discussion Fig. 1b presents a typical optical micrograph of the cross-section of the pre-compacted Al-based metallic glass disk. As evidenced, the pre-compacts are porous and abundant in voids of different size and volume fraction of 18 ± 1%. Moreover, the as-attrited individual flake pieces can also be recognized in the whole image. The samples processed by HPT for N = 1 revolution are more compacted (pore fraction varies from: 2% to 9%), although numerous rives perpendicular to the torsion axis characterize the cross-section (Fig. 1c). The degree of compaction varies with the radius, i.e. the center of the disks possesses a sponge-like structure, while regions close to the perimeter are denser. The glassy material processed up to N = 5 rotations results in homogeneous porosity free disks (pore fraction: 1%), with a diameter of 8 mm and thickness of about 700 lm. Accordingly, HPT has successfully been applied to produce low-porosity bulk Al-based metallic glasses obtained from asquenched ribbons. Details of microstructural variation were monitored by SEM at different stages of the compaction process. As illustrated in Fig. 2a, the thickness of the as-cast ribbon is almost uniform. By introducing macroscopic plastic deformation, uniformity of the individual flake particles diminishes through Fig. 2b–d. Thickness variation of the particles and surface roughness of the rives observed in Fig. 2b are fingerprints of the highly localized strain in the sample. Additionally, the original flake particles having diameter of approximately 1 mm break into several smaller pieces with 100–200 lm in size. When external shear deformation is applied (N = 1), volumetric density of rives decreases and the surface roughness increases (Fig. 2c). Prolonged HPT deformation (N = 5) results in nearly full compaction of Al-based metallic glass material (Fig. 2d). Although, large fluctuations in shear deformation are present at micrometer level (see the light distorted tracer particle embedded in the Al85Ce8Ni5Co2 matrix in the high contrast SEM image), the total volume of the sample is subjected to severe plastic deformation of c 1. Microstructural features of the rapid-quenched Al85Ce8Ni5Co2, Al85Y8Ni5Co2 and Al85Gd8Ni5Co2 alloys can be simply characterized by the corresponding X-ray diffractograms (Fig. 3). In general, each pattern consists of two broad symmetric halos centered at around 2H = 18° and 2H = 37° in the range of 12–50°. The presence of the main halo and the lack of sharp crystalline peaks indicate that all three alloys contain a single homogeneous amorphous phase. The position of the main halo is directly related to the characteristic average near neighbor distances in the metallic glasses. In the AlTM-RE amorphous system, the basic structural units are typically RE solute centered Al clusters which are responsible for the chemical and topological short range order [29,35]. Specifically, the dominant length is the Al-RE distance in these clusters. At the same time, an apparent trend can be observed in the intensity of the smaller halo varying with the rare earth component (in the order of Y, Gd to Ce). According to Miracle’s model, medium range order in pseudo ternary metallic glasses can be approximated by interpenetrating RE centered clusters [29,35]. The appearance of the less intense halo at lower 2H position in the diffractograms is related to the RE–RE correlation between neighboring clusters, henceforward, MRO is present in all the three investigated Albased systems. Mechanical or thermal impacts on metallic glasses usually lead to small changes of the glassy structure before devitrification. These changes are often characterized by minor variation of the position of the first halo [36,37]. The mechanical and thermal treatments, however, can have larger effect on the medium range order. So that, we selected the Al85Ce8Ni5Co2 alloy exhibiting the most intense second halo to investigate the effect
Zs. Kovács et al. / Journal of Alloys and Compounds 561 (2013) 5–9
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Fig. 2. SEM images of the Al85Ce8Ni5Co2 metallic glass material at different stages of the compaction process. Cross sectional view of (a) an as-cast ribbon piece with a Vickers indent, (b) a pre-compacted disk compressed at 800 MPa, (c) the outer part of a HPT disk compressed at 6 GPa with N = 1 full revolution and (d) a fully compacted HPT disk subjected to 6 GPa pressure and N = 5 revolutions. The direction of compression is indicated by arrows.
Fig. 3. X-ray diffractograms of the rapid quenched Al85Ce8Ni5Co2, Al85Gd8Ni5Co2 and Al85Y8Ni5Co2 alloys.
of heavy shear deformation and thermal treatment on the medium range order. Fig. 4a and b shows diffraction patterns acquired on the Al85Ce8Ni5Co2 alloy processed by HPT for N = 1 and N = 5 revolutions and by thermal treatments at temperatures near the glass transition (Tg), respectively. The diffractogram of the N = 1 disk is similar to that of the as-quenched ribbon, exhibiting the two amorphous halos (Fig. 4a). Applying N = 5 rotations, some crystalline peaks evolve which can be identified as Al11Ce3 and fcc-Al reflections, however, the main and second halos still dominate the pattern. In a recent study it was demonstrated by transmission electron microscopy that both the Al11Ce3 and fcc-Al phases nucleate from the amorphous matrix even after one HPT revolution [38]. Isothermal treatment obtained below the glass transition temperature (Tg = 560 K [7,26]) at Tiso = 542.5 K results in small distortion of
the halos and appearance of faint Bragg peaks (Fig. 4b). At slightly higher temperature (Tiso = 545 K) the glass, however, devitrifies into several crystalline phases accompanied with the total elimination of the amorphous halos. Accordingly, MRO is almost unchanged during severe plastic deformation and in the early stages of the thermal devitrification process; and it disappears with the devitrification of the amorphous state in the Al85Ce8Ni5Co2 alloy. Slight variation of the position (2H) and the width of the halo (D2H) related to the MRO can be envisaged in Fig. 4c. Quantitative analysis revealed that the position shifts to lower angle with torsion straining from 2H = 18.3° to 2H = 17.7°, accompanied with a significant halo broadening (from D2H = 6.5° to D2H = 8°). Isothermal treatment, on the other hand, results in an increase of the MRO halo position by about 0.4°. At the same time, a moderate halo narrowing occurs (D2H = 5.9°). The observed smaller width and higher position of the halo corresponding to the heat-treated state indicate that the RE–RE correlation between the adjacent RE-centered clusters is stronger and the apparent distance of the clusters becomes shorter. Conversely, the deformed sample (N = 5) exhibits higher degree of disorder and enhanced distance between the RE–RE clusters, indicating that interstitial vacancies are more abundant in this state [35]. In line with the cross-sectional morphology (see Figs. 1 and 2), mechanical properties of the Al85Ce8Ni5Co2 specimens also exhibit strong variation with the compactness of the HPT disk (Fig. 5). The average hardness value of the ribbon is HV = 244 with a scatter less than DHV = 11. Hardness measurements performed on the N = 1 specimen scatters in a large interval (from HV = 200 to HV = 300), indicating that softer and harder regions alter in the sample. As can be seen, slight shear softening occurs predominantly in the center of the disks, as was also observed in HPT-deformed [39] and compressed Vitreloy BMG [40]. When shear straining is processed up to N = 5 rotations, the hardness becomes higher (HV 295 ± 12) and more homogeneous across the cross section,
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Fig. 4. Effect of mechanical and thermal impacts on the microstructure of Al85Ce8Ni5Co2 metallic glass. (a) X-ray diffractograms of the as-cast ribbon and samples deformed by HPT up to N = 1 and N = 5 revolutions. (b) X-ray diffractograms of the samples heat treated at Tiso = 542.5 K, Tiso = 545 K and the as-cast ribbon as reference. (c) Positions and widths of the smaller halo for the as-cast ribbon, the HPT deformed samples (N = 1 and N = 5) and the sample heat treated at Tiso = 542.5 K.
Fig. 5. Mechanical properties of the Al85Ce8Ni5Co2 specimens characterized by Vickers hardness measurements. (a) Variation of the HV values along the cross-section of the HPT samples deformed up to N = 1 and N = 5 revolutions. The average HV of the as-cast ribbon is also indicated. (b) Distribution of HV values for the individual measurements without averaging.
alike the homogenization of the microstructure during simultaneous compression and shear deformation. Noteworthy that hardness corresponding to the N = 1 specimen alters between that of the as-spun ribbon and the N = 5 disk, indicating that softer regions
are dominantly compiled of less deformed ribbon pieces, while harder areas are mechanically similar to the fully compacted alloy. Indeed, nanocrystal formation is induced by severe plastic deformation in the Al85Ce8Ni5Co2 metallic glass sample, as broadened
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diffraction peaks (see Fig. 4a) and electron microscope images indicate [38]. Typically, these forming Al nanocrystals lead to an increase of the average hardness of the glass [41]. Recently, a macroscopic quasi three-dimensional thermoplastic model, estimating the temperature rise generated by extensive shear deformation during HPT of metallic glasses has been developed [42,43]. Accordingly, for the present deformation parameters, i.e. revolution time, total number of revolutions and sample thickness, the temperature rises substantially and it approaches Tg for the N = 5 disk. In agreement with the estimated temperature rise, a more solid second phase in the matrix of the N = 5 disk appears and it accounts for the hardening of the partially devitrified glass [44,45]. Based on the numerical model, the temperature remains significantly lower in N = 1 disk, resulting in much less amount of crystalline precipitates, in line with the observed smaller average value of HV. According to the topological approach of Smedskjaer et al. that hardness of a glass is determined by constraints among the various structural units and if the number of constraints is low, the material softens [46], as it takes place at the center of the N = 1 disk. On the other hand, as the devitrification commences the number of structural constraints start to increase parallel to the nucleation of nanocrystals. This is related to the observed enhanced mechanical resistance in Fig 5. Theoretically, both severe shear deformation (c 1) and thermal treatment initializing devitrification cause strong changes in an atomically rigid glassy structure. However, the effect of these changes is hardly seen in the diffraction component of the glass phase, i.e. the MRO is not altered significantly after strong perturbation of the initial atomic structure (see Fig. 4a and b). This implies that (i) the glass has some stability against external perturbations, (ii) short and medium range atomic order are sustained by the glass itself, (iii) MRO may be preserved by the advent of thermal atomic motion, and (iv) MRO is not only a side feature of the glassy structure, but it is a key characteristic of the amorphous state. 4. Conclusions Al-based metallic glass ribbons with composition of Al85Y8Ni5Co2, Al85Ce8Ni5Co2 and Al85Gd8Ni5Co2 were subjected to high pressure torsion and have successfully been compacted to produce low-porosity, bulk disks. The compacted glassy disks have higher hardness (HV 295) than the original ribbon (HV 244) and composed of crystalline precipitations with substantial glass fraction. Mechanical and thermal treatments have only small effect on the glassy structure as demonstrated by the invariability of the diffraction halo positions. Unchanged halos reveal that glassy structure has some stability against external perturbations and medium range order is an inherent feature of the amorphous state. Acknowledgements Á.R. and Zs.K. are indebted for the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. We appreciate the support of the Austrian-Hungarian bilateral fund (TéT). The support of the Hungarian Scientific Research Fund under Grant No. 81360 is acknowledged. The European Union and the European Social Fund have provided financial support to the project under the Grant Agreement No. TÁMOP 4.2.1./B-09/1/KMR-2010-0003.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Stability of medium range order in Al-based metallic glass compacted by severe plastic deformation Zs. Kovács a, P. Henits a, L.K. Varga b, E. Schafler c, Á. Révész a,⇑ a
Department of Materials Physics, Eötvös University, P.O.B. 32, H-1518 Budapest, Hungary Research Institute for Solid state Physics and Optics, Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest, Hungary c Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria b
a r t i c l e
i n f o
Article history: Received 26 October 2012 Received in revised form 31 January 2013 Accepted 1 February 2013 Available online 16 February 2013 Keywords: Metallic glass Medium range order Al-based Structure
a b s t r a c t High pressure torsion has successfully been applied to produce low-porosity, bulk specimens from Al-based metallic glass ribbons (Al85Y8Ni5Co2, Al85Ce8Ni5Co2 and Al85Gd8Ni5Co2). The compacted disks possess higher hardness than the original glass and have substantial glass fraction with nanocrystalline precipitations. Mechanical and thermal impacts have only minor effects on the glassy structure as demonstrated by the stability of the X-ray diffraction halo positions. Unchanged halos reveal that medium range order is a key characteristic of the amorphous state. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their low density, aluminium based alloys are widely used in engineering applications, particularly in the field of aerospace and automotive industry. Their low yield strength, however, cannot be increased over 500–600 MPa by classical strengthening mechanisms. In order to improve the mechanical properties of Al-based alloys, alternative mechanisms and structures have to be considered [1,2]. As a result of this research, aluminium alloys with non-periodic atomic structure have been discovered in the late 1980s, possessing a yield strength close to 1 GPa [3–5]. Later the 1 GPa limit has been exceeded by homogeneous precipitation of nanoscale Al-particles into the amorphous matrix. Recently, such nanoscale crystalline particles are induced either by heat treatment [6,7] or plastic deformation [8–10]. Since Al-based metallic glasses, even if designed for high stability [11], can only be produced in limited size [6,12,13], some kind of compaction of small metallic glass pieces is required for practical applications. For example, centimeter-scale Al85Ni5Y6Co2Fe2 bulk glassy alloys have been fabricated by warm extrusion of gas-atomized amorphous powder [14]. Unfortunately, the traditional compaction methods themselves can undermine the excellent mechanical properties of metallic glasses. However, record fracture strength of 1250 MPa can be achieved in Al86Ni6Y4.5Co2La1.5 glassy compacts produced by a specific method, i.e. spark ⇑ Corresponding author. Tel.: +36 1 372 2823; fax: +36 1 372 2811. E-mail address: [email protected] (Á. Révész). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.010
plasma sintering [15]. Alternatively, high pressure torsion (HPT), applying severe shear deformation can also be used as an alternative compaction method. Originally, HPT was invented for producing highly dense, bulk ultrafine-grained (submicron grain-sized or nanostructured) materials [16]. Later on, successful attempts were focused on producing massive samples by HPT from meltquenched amorphous ribbons by consolidation at room temperature [8,9,17]. Deformation induced crystallization of metallic glasses is a well known phenomenon [18–22], but despite of many experimental results, its basic mechanism is still unclear. In the Al–TM–RE system, if the first crystallization event is the precipitation of a-Al, the deformation usually promotes the nucleation of a-Al [23–25]. On contrary, we have reported primary crystallization of a-Al nanocrystals in a thermally eutectic-like crystallizing Al85Ce8Ni5Co2 amorphous alloy, when extremely large deformation was applied [26]. Stability of metallic glasses strongly depends on their composition. Fully amorphous alloys can be produced typically in narrow compositional range [4], in which topological constraints and chemical interaction stabilize the glassy structure [27,28]. Densely packed clusters with typical structural units determine the concentration ranges of stability [29]. These structural units are typical for a given family of glass and reflect the composition and chemical interactions between the constituents. In Al–TM–RE systems, Al–Al and energetically favored Al–RE pairs are generally the dominant pairs, but topological effects are also decisive on the short range ordering (SRO) of the clusters [30]. Powerful MD simulations
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reveal that the chemically stable clusters (e.g. icosahedral cluster) with no possibility for periodic lattice configuration, form the glass structure with strong sharing between the neighboring cluster units [31]. In agreement with this constrained structure of the different clusters, metallic glasses possess structural stability and medium range order (MRO) as an inherent characteristic [32]. Extensive compaction processes could have both thermal and mechanical impacts on glassy alloys [10,33]. In this work, the effect of these impacts on the glassy structure of Al–TM–RE alloys is investigated, focusing on the atomic correlation beyond the average nearest neighbor distance.
2. Experimental Ingots of Al85Ce8Ni5Co2, Al85Y8Ni5Co2 and Al85Gd8Ni5Co2 were synthesized by induction melting of a mixture of high purity (99.9%) Al, Ce, Gd, Y, Ni and Co metals. Fully amorphous ribbon samples with width of 5 mm and thickness of 25 lm were obtained using a single roller melt spinning technique in inert atmosphere. Subsequently, the as-quenched ribbons were attrited into flakes with a characteristic diameter of 1 mm (Fig. 1a) and then pre-compacted into cylindrical disks with a diameter of 8 mm, thickness of 0.7 mm. The pre-compacts were placed between anvils of the HPT device and shear deformation was performed for N = 1 and N = 5 rotations with a 5 min revolution time under hydrostatic pressure of 6 GPa applying constrained condition. The maximum shear deformation achieved at the perimeter of the N = 5 disk is c 180. The detailed description of HPT processing can be found in Ref. [16]. The crystalline phase analysis was carried out on diffractograms performed by a Philips (PW1130) X-ray generator with a Guinier-chamber set-up. The chamber has a diameter of 100 mm and the patterns were recorded on FUJI Imaging Plate (BAS MS2025) with spatial resolution of 0.001° and high sensitivity span over 4–5 orders of magnitude. Morphology studies were performed on a FEI QUANTA 3D dual beam scanning electron microscope (SEM). The compositional changes of the surface related directly to contrast differences in the back-scattered electron (BSE) image were revealed and quantitatively determined by energy dispersive X-ray (EDX) analysis with a relative accuracy of 1%. For microhardness (HV) investigations and optical imaging the Al85Ce8Ni5Co2 disks were cut along the diameter, embedded in epoxy resin and polished to mirror-like surface. HV test was performed at room temperature on the polished surface along the diameter using a Shimadzu DUH-202 dynamic depth sensing ultra-microhardness tester with a diamond Vickers indenter at a load rate of 17.65 mN/s up to 200 mN. During the depth-sensing hardness measurements, the indentation depth and the load were recorded as a function of time by a computer. From these data the values of HV were determined using the Oliver–Pharr method [34] by averaging four indents. The porosity of the pre-compact and the HPT disks was estimated from the optical micrographs by counting the number of pixels corresponding to voids.
Fig. 1. Optical micrographs of the Al85Ce8Ni5Co2 metallic glass material at different stages of the compaction process. (a) Plane view of flake particles and the crosssectional view of (b) pre-compacted, (c) HPT N = 1 and (d) HPT N = 5 disks. The scale bar corresponds to all of the images.
3. Results and discussion Fig. 1b presents a typical optical micrograph of the cross-section of the pre-compacted Al-based metallic glass disk. As evidenced, the pre-compacts are porous and abundant in voids of different size and volume fraction of 18 ± 1%. Moreover, the as-attrited individual flake pieces can also be recognized in the whole image. The samples processed by HPT for N = 1 revolution are more compacted (pore fraction varies from: 2% to 9%), although numerous rives perpendicular to the torsion axis characterize the cross-section (Fig. 1c). The degree of compaction varies with the radius, i.e. the center of the disks possesses a sponge-like structure, while regions close to the perimeter are denser. The glassy material processed up to N = 5 rotations results in homogeneous porosity free disks (pore fraction: 1%), with a diameter of 8 mm and thickness of about 700 lm. Accordingly, HPT has successfully been applied to produce low-porosity bulk Al-based metallic glasses obtained from asquenched ribbons. Details of microstructural variation were monitored by SEM at different stages of the compaction process. As illustrated in Fig. 2a, the thickness of the as-cast ribbon is almost uniform. By introducing macroscopic plastic deformation, uniformity of the individual flake particles diminishes through Fig. 2b–d. Thickness variation of the particles and surface roughness of the rives observed in Fig. 2b are fingerprints of the highly localized strain in the sample. Additionally, the original flake particles having diameter of approximately 1 mm break into several smaller pieces with 100–200 lm in size. When external shear deformation is applied (N = 1), volumetric density of rives decreases and the surface roughness increases (Fig. 2c). Prolonged HPT deformation (N = 5) results in nearly full compaction of Al-based metallic glass material (Fig. 2d). Although, large fluctuations in shear deformation are present at micrometer level (see the light distorted tracer particle embedded in the Al85Ce8Ni5Co2 matrix in the high contrast SEM image), the total volume of the sample is subjected to severe plastic deformation of c 1. Microstructural features of the rapid-quenched Al85Ce8Ni5Co2, Al85Y8Ni5Co2 and Al85Gd8Ni5Co2 alloys can be simply characterized by the corresponding X-ray diffractograms (Fig. 3). In general, each pattern consists of two broad symmetric halos centered at around 2H = 18° and 2H = 37° in the range of 12–50°. The presence of the main halo and the lack of sharp crystalline peaks indicate that all three alloys contain a single homogeneous amorphous phase. The position of the main halo is directly related to the characteristic average near neighbor distances in the metallic glasses. In the AlTM-RE amorphous system, the basic structural units are typically RE solute centered Al clusters which are responsible for the chemical and topological short range order [29,35]. Specifically, the dominant length is the Al-RE distance in these clusters. At the same time, an apparent trend can be observed in the intensity of the smaller halo varying with the rare earth component (in the order of Y, Gd to Ce). According to Miracle’s model, medium range order in pseudo ternary metallic glasses can be approximated by interpenetrating RE centered clusters [29,35]. The appearance of the less intense halo at lower 2H position in the diffractograms is related to the RE–RE correlation between neighboring clusters, henceforward, MRO is present in all the three investigated Albased systems. Mechanical or thermal impacts on metallic glasses usually lead to small changes of the glassy structure before devitrification. These changes are often characterized by minor variation of the position of the first halo [36,37]. The mechanical and thermal treatments, however, can have larger effect on the medium range order. So that, we selected the Al85Ce8Ni5Co2 alloy exhibiting the most intense second halo to investigate the effect
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Fig. 2. SEM images of the Al85Ce8Ni5Co2 metallic glass material at different stages of the compaction process. Cross sectional view of (a) an as-cast ribbon piece with a Vickers indent, (b) a pre-compacted disk compressed at 800 MPa, (c) the outer part of a HPT disk compressed at 6 GPa with N = 1 full revolution and (d) a fully compacted HPT disk subjected to 6 GPa pressure and N = 5 revolutions. The direction of compression is indicated by arrows.
Fig. 3. X-ray diffractograms of the rapid quenched Al85Ce8Ni5Co2, Al85Gd8Ni5Co2 and Al85Y8Ni5Co2 alloys.
of heavy shear deformation and thermal treatment on the medium range order. Fig. 4a and b shows diffraction patterns acquired on the Al85Ce8Ni5Co2 alloy processed by HPT for N = 1 and N = 5 revolutions and by thermal treatments at temperatures near the glass transition (Tg), respectively. The diffractogram of the N = 1 disk is similar to that of the as-quenched ribbon, exhibiting the two amorphous halos (Fig. 4a). Applying N = 5 rotations, some crystalline peaks evolve which can be identified as Al11Ce3 and fcc-Al reflections, however, the main and second halos still dominate the pattern. In a recent study it was demonstrated by transmission electron microscopy that both the Al11Ce3 and fcc-Al phases nucleate from the amorphous matrix even after one HPT revolution [38]. Isothermal treatment obtained below the glass transition temperature (Tg = 560 K [7,26]) at Tiso = 542.5 K results in small distortion of
the halos and appearance of faint Bragg peaks (Fig. 4b). At slightly higher temperature (Tiso = 545 K) the glass, however, devitrifies into several crystalline phases accompanied with the total elimination of the amorphous halos. Accordingly, MRO is almost unchanged during severe plastic deformation and in the early stages of the thermal devitrification process; and it disappears with the devitrification of the amorphous state in the Al85Ce8Ni5Co2 alloy. Slight variation of the position (2H) and the width of the halo (D2H) related to the MRO can be envisaged in Fig. 4c. Quantitative analysis revealed that the position shifts to lower angle with torsion straining from 2H = 18.3° to 2H = 17.7°, accompanied with a significant halo broadening (from D2H = 6.5° to D2H = 8°). Isothermal treatment, on the other hand, results in an increase of the MRO halo position by about 0.4°. At the same time, a moderate halo narrowing occurs (D2H = 5.9°). The observed smaller width and higher position of the halo corresponding to the heat-treated state indicate that the RE–RE correlation between the adjacent RE-centered clusters is stronger and the apparent distance of the clusters becomes shorter. Conversely, the deformed sample (N = 5) exhibits higher degree of disorder and enhanced distance between the RE–RE clusters, indicating that interstitial vacancies are more abundant in this state [35]. In line with the cross-sectional morphology (see Figs. 1 and 2), mechanical properties of the Al85Ce8Ni5Co2 specimens also exhibit strong variation with the compactness of the HPT disk (Fig. 5). The average hardness value of the ribbon is HV = 244 with a scatter less than DHV = 11. Hardness measurements performed on the N = 1 specimen scatters in a large interval (from HV = 200 to HV = 300), indicating that softer and harder regions alter in the sample. As can be seen, slight shear softening occurs predominantly in the center of the disks, as was also observed in HPT-deformed [39] and compressed Vitreloy BMG [40]. When shear straining is processed up to N = 5 rotations, the hardness becomes higher (HV 295 ± 12) and more homogeneous across the cross section,
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Fig. 4. Effect of mechanical and thermal impacts on the microstructure of Al85Ce8Ni5Co2 metallic glass. (a) X-ray diffractograms of the as-cast ribbon and samples deformed by HPT up to N = 1 and N = 5 revolutions. (b) X-ray diffractograms of the samples heat treated at Tiso = 542.5 K, Tiso = 545 K and the as-cast ribbon as reference. (c) Positions and widths of the smaller halo for the as-cast ribbon, the HPT deformed samples (N = 1 and N = 5) and the sample heat treated at Tiso = 542.5 K.
Fig. 5. Mechanical properties of the Al85Ce8Ni5Co2 specimens characterized by Vickers hardness measurements. (a) Variation of the HV values along the cross-section of the HPT samples deformed up to N = 1 and N = 5 revolutions. The average HV of the as-cast ribbon is also indicated. (b) Distribution of HV values for the individual measurements without averaging.
alike the homogenization of the microstructure during simultaneous compression and shear deformation. Noteworthy that hardness corresponding to the N = 1 specimen alters between that of the as-spun ribbon and the N = 5 disk, indicating that softer regions
are dominantly compiled of less deformed ribbon pieces, while harder areas are mechanically similar to the fully compacted alloy. Indeed, nanocrystal formation is induced by severe plastic deformation in the Al85Ce8Ni5Co2 metallic glass sample, as broadened
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diffraction peaks (see Fig. 4a) and electron microscope images indicate [38]. Typically, these forming Al nanocrystals lead to an increase of the average hardness of the glass [41]. Recently, a macroscopic quasi three-dimensional thermoplastic model, estimating the temperature rise generated by extensive shear deformation during HPT of metallic glasses has been developed [42,43]. Accordingly, for the present deformation parameters, i.e. revolution time, total number of revolutions and sample thickness, the temperature rises substantially and it approaches Tg for the N = 5 disk. In agreement with the estimated temperature rise, a more solid second phase in the matrix of the N = 5 disk appears and it accounts for the hardening of the partially devitrified glass [44,45]. Based on the numerical model, the temperature remains significantly lower in N = 1 disk, resulting in much less amount of crystalline precipitates, in line with the observed smaller average value of HV. According to the topological approach of Smedskjaer et al. that hardness of a glass is determined by constraints among the various structural units and if the number of constraints is low, the material softens [46], as it takes place at the center of the N = 1 disk. On the other hand, as the devitrification commences the number of structural constraints start to increase parallel to the nucleation of nanocrystals. This is related to the observed enhanced mechanical resistance in Fig 5. Theoretically, both severe shear deformation (c 1) and thermal treatment initializing devitrification cause strong changes in an atomically rigid glassy structure. However, the effect of these changes is hardly seen in the diffraction component of the glass phase, i.e. the MRO is not altered significantly after strong perturbation of the initial atomic structure (see Fig. 4a and b). This implies that (i) the glass has some stability against external perturbations, (ii) short and medium range atomic order are sustained by the glass itself, (iii) MRO may be preserved by the advent of thermal atomic motion, and (iv) MRO is not only a side feature of the glassy structure, but it is a key characteristic of the amorphous state. 4. Conclusions Al-based metallic glass ribbons with composition of Al85Y8Ni5Co2, Al85Ce8Ni5Co2 and Al85Gd8Ni5Co2 were subjected to high pressure torsion and have successfully been compacted to produce low-porosity, bulk disks. The compacted glassy disks have higher hardness (HV 295) than the original ribbon (HV 244) and composed of crystalline precipitations with substantial glass fraction. Mechanical and thermal treatments have only small effect on the glassy structure as demonstrated by the invariability of the diffraction halo positions. Unchanged halos reveal that glassy structure has some stability against external perturbations and medium range order is an inherent feature of the amorphous state. Acknowledgements Á.R. and Zs.K. are indebted for the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. We appreciate the support of the Austrian-Hungarian bilateral fund (TéT). The support of the Hungarian Scientific Research Fund under Grant No. 81360 is acknowledged. The European Union and the European Social Fund have provided financial support to the project under the Grant Agreement No. TÁMOP 4.2.1./B-09/1/KMR-2010-0003.
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