- Email: [email protected]
Structural effects of shot-peening in bulk metallic glasses
Journal of Alloys and Compounds 483 (2009) 256–259
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural effects of shot-peening in bulk metallic glasses F.O. Méar a,∗ , B. Doisneau b , A.R. Yavari b , A.L. Greer a a b
University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK SIMaP-CNRS UA29, Domaine Universitaire BP 75, Saint Martin d’Hères 38402, France
a r t i c l e
i n f o
Article history: Received 30 August 2007 Accepted 10 December 2007 Available online 13 February 2009 Keywords: Bulk metallic glass Shot-peening Stored energy of cold work Amorphization Crystallization X-ray diffraction Differential scanning calorimetry Transmission electron microscopy
a b s t r a c t Shot-peening induces surface compressive stresses in bulk metallic glass components, improving their plasticity. Structural changes in the peened surface of fully glassy and partially crystalline Zr55 Al10 Cu30 Ni5 are studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. An earlier study is extended by examining the effect of sample temperature. While fully glassy samples show no phase change under peening, partially crystalline samples show either amorphization or crystallization depending on temperature. Peening can induce very large stored energy in metallic glasses rendering them susceptible to crystallization below room temperature, a result which may be relevant for improving the plasticity of these materials. © 2009 Published by Elsevier B.V.
1. Introduction For conventional engineering alloys, mechanical surface treatments can have significant effects on near-surface properties: residual stress, hardness, roughness, microstructures, phase contents, porosity, texture, corrosion resistance, quasistatic and cyclic yield strength, thermal stability, etc. One of the most common treatments is shot-peening which induces compressive residual stresses in the surface layer, balanced by sub-surface tensile stresses. The maximum compressive stress is limited only by the yield strength of the surface-treated material [1]. Recent work shows that shotpeening can improve the plasticity of bulk metallic glass (BMG) components [2], an effect attributed to the compressive surface stress and to the generation of many shear bands helping to make subsequent deformation more uniform. Such effects are of interest because the main factor limiting the structural applicability of BMGs is their lack of tensile ductility associated with worksoftening and the localization of shear into very thin bands. In our earlier work [2], no phase change was detected in the peened material, Zr41.25 Ti13.75 Ni10 Cu12.5 Be22.5 BMG (Vitreloy 1). While there would be interest in optimizing the peening conditions for property enhancement, in the present work we choose a
∗ Corresponding author. Present address: Catalysis and Solid Chemistry Unit, Lille University, 59652 Villeneuve d’Ascq cedex, France. E-mail address: [email protected] (F.O. Méar). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2007.12.105
different, also well known, BMG, Zr55 Al10 Cu30 Ni5 (atomic%), and examine its structural changes under peening in greater detail. The latter composition was reported to form fully glassy samples up to 30 mm diameter [3], and has since been widely studied [4,5]. Its stability under heavy mechanical deformation, including shot-peening, has recently been examined [6]: in the absence of contamination, its glassy structure is stable under deformation and indeed deformation induces partial amorphization in annealed crystalline samples. The earlier work on Vitreloy 1 [2] suggested that the heating evident at longer peening times can have an annealing effect. In conventional crystalline alloys this heating can reduce the residual stress [7]. In the present work, therefore, sample temperature is regarded as an important variable. There are several reasons for interest in structural changes in metallic glasses under deformation, in particular crystallization, as recently reviewed [8,9]. It has been noted that mechanical deformation (usually by ball-milling) is an effective way to make nanocrystalline materials [8–10], the crystal diameter arising from a competition between crystal growth and break-up under deformation [11]. Based on results from low- and high-energy milling, and considering the effects of alloy composition and temperature, several authors [12–14] have noted that crystallization of metallic glasses cannot be a purely thermal effect (even though milling does induce significant heating). It has, for example, been suggested that milling can induce redistribution of solute through cyclic crystallization and amorphization [12], or generation of free volume enhancing diffusion [14]. The interpretation of deformation effects
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
257
can be complicated by associated contamination (often by Fe and O). In this respect, shot-peening is particularly attractive in inducing much less contamination [6] than the more widely studied ball-milling. Shot-peening, by repeated deformation of the same material, can also induce much greater total strains than techniques such as the rolling used in early studies of the effects of deformation [15]. Recent studies have given additional importance to crystallization during deformation, by showing that if it occurs, it may lead to improved plasticity of the metallic glass [16,17]. 2. Experimental methods In the present work samples of Zr55 Al10 Cu30 Ni5 were prepared in two laboratories, in each case starting by induction melting granules of the elemental constituents (purity of 99.9%). Casting was into copper moulds under high-purity argon, in one case to obtain plate approximately 10 mm × 40 mm × 2 mm, in the other case to obtain 3 mm diameter rod. Shot-peening was performed using a Guyson Euro 2SF system with a high gas velocity (at a pressure of 7 bar). The shot are 300–400 m diameter glass beads of composition 72.6%-SiO2 , 9.2%-CaO, 3.4%-MgO and 13.7%-Na2 O (Wheelabrator Ltd., Altrincham, UK). It is known from earlier work [2] that the peened layer has a depth of approximately 100 m and a surface roughness of Ra = 1.3 m [2]. Continuous peening times of up to 60 s were used and even in such times there is significant sample heating of up to about 50 K. Samples at the start of peening were either at room temperature (RT) or cooled by liquid nitrogen (liqN2 ) to approximately 77 K. The amorphicity of the samples was checked by X-ray diffraction (XRD) using a Philips PW 1830 generator and PW 1820 diffractometer with CuK␣ ( = 0.1542 nm) radiation and a Ni filter. Differential scanning calorimetry (DSC) used a TA instruments DSC Q1000 with a heating rate of 10 K min−1 ; the glass-transition temperature and the onset crystallization temperature were calculated using standard software. The DSC samples were prepared by mechanical thinning of the disks to a thickness of order 100 m with the intention that the entire sample should have been heavily deformed by peening. Transmission electron microscopy (TEM) was with a JEOL 3010 microscope operated at 300 kV. The TEM samples were prepared by grinding and polishing 3-mm diameter discs to a thickness of 100 m, followed by standard twin-jet electrochemical thinning using a solution of 1 vol. HClO4 –10 vol. C2 H5 OH at about −30 ◦ C [18]. Finally, the samples were thinned by a low-angle ion milling (5◦ ) for about 10 min to remove any chemical residues.
3. Results XRD of the as-cast plate and rod samples shows that while the plate samples are fully glassy (at least within the sensitivity of the XRD technique) (Fig. 1), the rod samples are partially crystalline (Fig. 2). This difference, attributed to differing levels of oxygen contamination (known to affect the critical cooling rate for glass formation, as reviewed in [19]), gives the opportunity to test the
Fig. 1. X-ray diffractograms of BMG Zr55 Cu30 Al10 Ni5 in plate form, as-cast (fully glassy) and after shot-peening at room temperature (RT) or cooled by liquid nitrogen (liqN2 ).
Fig. 2. X-ray diffractograms of BMG Zr55 Cu30 Al10 Ni5 in rod form, as-cast (partially crystalline) and after shot-peening at RT or liqN2 -cooled.
effect of starting structure on the subsequent behaviour. In the rod samples, some Bragg peaks correspond to CuZr2 and others are unidentified. The phase mixture appears similar to that indicated by XRD in other work on as-cast and annealed samples [4–6]. TEM of as-cast rod samples (Fig. 3a) shows that the crystals are small (10–30 nm), consistent with the broad peaks in XRD. Within the range of times and temperatures tested, shotpeening of the fully glassy plate samples does not produce any detectable change in phase (Fig. 1). The intense deformation induced by peening must influence the glassy structure (for example, deformation is widely considered to increase the free volume [14], and can be associated with significant stored energy [15]), but such effects are not observed directly in the present study. The absence of crystallization under peening is consistent with earlier work on Vitreloy 1 [2] and on Zr55 Al10 Cu30 Ni5 itself [6]. The response of the partially crystalline rod samples to peening is more complex (Fig. 2). Peening at RT clearly induces amorphization, while in liqN2 -cooled samples it induces crystallization. It is counter-intuitive that crystallization is favoured in the colder samples, so efforts were made to corroborate these XRD observations by TEM and by DSC. In rod samples peened at RT, TEM shows that the characteristic crystallite size decreases to 10 nm in the first 15 s and then stabilises. It is difficult to quantify the crystalline volume fraction from the TEM observations, but qualitatively the volume fraction does decrease on peening (Fig. 3b). In the samples liqN2 -cooled before peening, TEM (Fig. 3c) shows virtually complete crystallinity. Most remarkably, the crystal size has increased to a few micrometres. Furthermore, the crystals do not appear to be heavily deformed as they show clear interference-fringe contrast spanning the crystal diameter. DSC results from the as-cast rod (Fig. 4) shows a clear crystallization peak at 750 K. For the sample cooled to 77 K before peening, the crystallization peak has almost disappeared, corresponding to the near-complete crystallinity seen in TEM (Fig. 3c). It also confirms that the preparation is successful in thinning to leave a sample that consists essentially only of the peened layer. (The underlying undeformed bulk is expected to remain glassy and would give a crystallization peak if still present in the DSC samples.) Glasstransition temperatures Tg , onset crystallization temperatures Tx and heats of crystallization Hx are given in Table 1, and compared with values from [3]. Peening for 15 s at RT, moves the crystallization peak to higher temperature, sharpens it and increases its area, all effects consistent with greater initial amorphicity. The heat of crystallization shown by the as-cast sample is approximately 88% of that shown by the RT-peened sample. Thus, if the latter sample is
258
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
Fig. 4. DSC curves (heating at 10 K min−1 ) for BMG Zr55 Cu30 Ni5 Al10 in rod form, as-cast (partially crystalline) and after shot-peening at RT or liqN2 -cooled.
taken to be fully amorphous before the DSC run (i.e. assuming that peening induces complete amorphicity and that the DSC sample represents only the peened layer), then the heats of crystallization would indicate that the as-cast rod sample has a crystalline volume fraction of some 12%. In their initial work, Inoue and Zhang [3] reported Tg = 690 K and Tx = 780 K. In the present work (Table 1) the Tg of the rod sample peened at RT is essentially the same as for the fully glassy sample reported in [3]. On the other hand, Tx in the same sample is much lower, presumably because crystalline fragments facilitate nucleation of crystallization on heating. 4. Discussion
Fig. 3. Transmission electron micrographs of BMG Zr55 Cu30 Al10 Ni5 in rod form: (a) as-cast, dark-field (DF) image and selected-area diffraction pattern (SADP); and shotpeened; (b) 30 s at RT (DF); (c) 30 s liqN2 -cooled, bright-field (BF). Table 1 The glass-transition temperature Tg , onset crystallization temperature Tx , and heat of crystallization Hx for BMG Zr55 Cu30 Al10 Ni5 samples prepared and treated in the present work, compared with the literature values. Tg (K)
Tx (K)
T = Tx − Tg (K)
Hx (kJ mol−1 )
Present work, rod sample (3 mm diameter, ∼12% crystalline as-cast) As-cast 683 732 49 4.35 15 s @RT 690 733 43 4.96 15 s @liqN2 – 739 – 0.23 From Inoue and Zhang [3] (5 mm diameter rod, fully glassy as-cast) 690 780 90
XRD, TEM and DSC all show that the partially crystalline rod samples are amorphized by peening at RT. This effect is consistent with amorphization by heavy mechanical deformation by such processes as mechanical alloying, ball-milling and shot-peening itself [20,21]. In contrast to the work of Yamamoto et al. [6], the peening in the present work appears capable of inducing almost complete amorphization. This is also consistent with the structural stability in fully glassy plate samples (Fig. 1). The reduction in crystallite size and number are consistent with local melting of crystals in shear bands, followed by quenching into the glassy state. As noted in Section 1, there are many examples of deformationinduced crystallization in metallic glasses. In the present work, the crystallization induced by shot-peening rod samples cooled by liqN2 is remarkable in two respects: that the crystallization is favoured in colder samples, and that the crystals are large and undeformed. In contrast, shot-peening of crystalline alloys is known to induce refinement of grain size [22]. Also, shot-peening is expected to break up crystals formed during the peening [11,23]. Thus the observed crystals cannot be formed during peening. Fine, heavily deformed crystals, if formed during peening at low temperature, could recrystallize on subsequent heating to RT. However, the lack of crystallization, in contrast amorphization, induced by peening at RT argues against such a mechanism. Deformed metallic glasses can acquire significant stored energy while remaining glassy. It is likely that the excess energy of 209 J mol−1 , detected by Chen [15] in rolled samples, can be greatly exceeded in shot-peened material (a topic under investigation). On subsequent heating to RT, the relatively energetic and unstable deformed glass can undergo structural relaxation to a more stable glassy form or can crystallize. The present work suggests that the
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
preferred process is influenced by the starting structure. Crystal fragments, remaining from the initial partial crystallinity in the rod samples even after peening, may favour crystallization, presumably by eliminating the need for crystal nucleation. This is similar to the deformation-induced nanocrystallization made possible by pre-nucleation [17] and suggested to be important for improving the plasticity of metallic glasses. The high stored energy, presumed in liqN2 -peened samples, suggests that the glassy structure of Zr55 Al10 Cu30 Ni5 is not quite so stable under deformation as suggested by Yamamoto et al. [6], as it can crystallize on heating to RT. The possibility of crystallization after deformation has not been clear in earlier work, but this phenomenon may help to explain some earlier observations. 5. Conclusions Shot-peening can induce very high surface deformation, with little contamination compared to techniques such as ball-milling. In the present work the observations of Yamamoto et al. [6] on the stability of the bulk metallic glass, Zr55 Al10 Cu30 Ni5 under shot-peening have been extended to examine the effect of sample temperature. Fully glassy samples, whether initially at RT or liqN2 -cooled, remain fully glassy under extended peening. Partially crystalline samples are amorphized by peening at RT. In contrast to the earlier work [6], the amorphization can be essentially complete. Partially crystalline samples peened at liqN2 temperature show complete crystallization in XRD, TEM and DSC. The size and perfection of the surface crystals observed by TEM suggests that they must form after peening, presumably by crystallization of an unusually energetic glass on warming back to RT. This crystallization shows that glassy Zr55 Al10 Cu30 Ni5 is not always so stable under deformation as considered in earlier work [6]. The crystallization appears to require the presence of prior crystalline fragments, presumably to assist nucleation, and may therefore be related to the crystallization seen in shear bands in suitably pre-annealed samples and of interest in promoting plasticity of the BMG [17].
259
Acknowledgements This work was performed in the framework of the EU Network on Bulk Metallic Glass Composites (MRTN-CT-2003-504692) coordinated by A.R. Yavari. The authors thank Y. Zhang for useful discussions. References [1] I. Altenberger, in: Lothar Wagner (Ed.), Shot-peening, Wiley–VCH GmbH and Co., 2006, pp. 421–434. [2] Y. Zhang, W.H. Wang, A.L. Greer, Nat. Mater. 5 (2006) 857. [3] A. Inoue, T. Zhang, Mater. Trans. JIM 37 (1996) 185. [4] A.R. Yavari, A. Le Moulec, A. Inoue, W.J. Botta Filho, G. Vaughan, Å. Kvick, Mater. Sci. Eng. A 304–306 (2001) 34. [5] K. Soda, K. Shimba, S. Yagi, M. Kato, T. Takeuchi, U. Mizutani, T. Zhang, M. Hasegawa, A. Inoue, T. Ito, S. Kimura, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 585. [6] T. Yamamoto, T. Takahashi, H. Kimura, A. Inoue, J. Alloys Compd. 430 (2007)97. [7] S. Rouquette, E. Rouhaud, H. Pron, M. Franc¸ois, C. Bissieux, A. Roos, Mater. Sci. Forum. 524–525 (2006) 167. [8] J. Bednarˇcík, E. Burkel, K. Saksl, P. Kollár, S. Roth, J. Appl. Phys. 100 (2006) 014903. [9] P.P. Choi, I.V. Povstugar, Y.S. Kwon, E.P. Yelsukov, J.S. Kim, Mater. Sci. Eng. A 449–451 (2007) 1079. [10] B. Huang, R.J. Perez, P.J. Crawford, A.A. Sharif, S.R. Nutt, E.J. Lavernia, Nanostr. Mater. 5 (1995) 545. [11] M.L. Trudeau, J.Y. Huot, R. Schulz, D. Dussault, A. Van Neste, G. L’Espérance, Phys. Rev. B 45 (1992) 4626. [12] M.L. Trudeau, R. Schulz, D. Dussault, A. Van Neste, Phys. Rev. Lett. 64 (1990) 99. [13] Y. He, G.J. Shiflet, S.J. Poon, Acta Metall. Mater. 43 (1995) 83. [14] J. Xu, M. Atzmon, Appl. Phys. Lett. 73 (1998) 1805. [15] H.S. Chen, Appl. Phys. Lett. 29 (1976) 328. [16] S.-W. Lee, M.-Y. Huh, E. Fleury, J.-C. Lee, Acta Mater. 54 (2006) 349. [17] K. Hajlaoui, A.R. Yavari, A. LeMoulec, W.J. Botta Filho, G. Vaughan, J. Das, A.L. Greer, Å. Kvick, J. Non-Cryst. Sol. 353 (2007) 327. [18] Y.F. Deng, L.L. He, Q.S. Zhang, H.F. Zhang, H.Q. He, Ultramicroscopy 98 (2004) 201. [19] A. Castellero, S. Bossuyt, M. Stoica, S. Deledda, J. Eckert, G.Z. Chen, D.J. Fray, A.L. Greer, Scripta Mater. 55 (2006) 87. [20] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [21] D.M. Grant, S.M. Green, J.V. Wood, Acta Metall. Mater. 43 (1995) 1045. [22] G. Liu, S.C. Wang, X.F. Lou, J. Lu, K. Lu, Scripta Mater. 44 (2001) 1791. [23] Q.F. Cao, K. Tsuchiya, Y. Harada, D. Tomus, K. Morii, Y. Todaka, M. Umemoto, J. Jpn. Instrum. Met. 70 (2006) 473.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural effects of shot-peening in bulk metallic glasses F.O. Méar a,∗ , B. Doisneau b , A.R. Yavari b , A.L. Greer a a b
University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK SIMaP-CNRS UA29, Domaine Universitaire BP 75, Saint Martin d’Hères 38402, France
a r t i c l e
i n f o
Article history: Received 30 August 2007 Accepted 10 December 2007 Available online 13 February 2009 Keywords: Bulk metallic glass Shot-peening Stored energy of cold work Amorphization Crystallization X-ray diffraction Differential scanning calorimetry Transmission electron microscopy
a b s t r a c t Shot-peening induces surface compressive stresses in bulk metallic glass components, improving their plasticity. Structural changes in the peened surface of fully glassy and partially crystalline Zr55 Al10 Cu30 Ni5 are studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. An earlier study is extended by examining the effect of sample temperature. While fully glassy samples show no phase change under peening, partially crystalline samples show either amorphization or crystallization depending on temperature. Peening can induce very large stored energy in metallic glasses rendering them susceptible to crystallization below room temperature, a result which may be relevant for improving the plasticity of these materials. © 2009 Published by Elsevier B.V.
1. Introduction For conventional engineering alloys, mechanical surface treatments can have significant effects on near-surface properties: residual stress, hardness, roughness, microstructures, phase contents, porosity, texture, corrosion resistance, quasistatic and cyclic yield strength, thermal stability, etc. One of the most common treatments is shot-peening which induces compressive residual stresses in the surface layer, balanced by sub-surface tensile stresses. The maximum compressive stress is limited only by the yield strength of the surface-treated material [1]. Recent work shows that shotpeening can improve the plasticity of bulk metallic glass (BMG) components [2], an effect attributed to the compressive surface stress and to the generation of many shear bands helping to make subsequent deformation more uniform. Such effects are of interest because the main factor limiting the structural applicability of BMGs is their lack of tensile ductility associated with worksoftening and the localization of shear into very thin bands. In our earlier work [2], no phase change was detected in the peened material, Zr41.25 Ti13.75 Ni10 Cu12.5 Be22.5 BMG (Vitreloy 1). While there would be interest in optimizing the peening conditions for property enhancement, in the present work we choose a
∗ Corresponding author. Present address: Catalysis and Solid Chemistry Unit, Lille University, 59652 Villeneuve d’Ascq cedex, France. E-mail address: [email protected] (F.O. Méar). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2007.12.105
different, also well known, BMG, Zr55 Al10 Cu30 Ni5 (atomic%), and examine its structural changes under peening in greater detail. The latter composition was reported to form fully glassy samples up to 30 mm diameter [3], and has since been widely studied [4,5]. Its stability under heavy mechanical deformation, including shot-peening, has recently been examined [6]: in the absence of contamination, its glassy structure is stable under deformation and indeed deformation induces partial amorphization in annealed crystalline samples. The earlier work on Vitreloy 1 [2] suggested that the heating evident at longer peening times can have an annealing effect. In conventional crystalline alloys this heating can reduce the residual stress [7]. In the present work, therefore, sample temperature is regarded as an important variable. There are several reasons for interest in structural changes in metallic glasses under deformation, in particular crystallization, as recently reviewed [8,9]. It has been noted that mechanical deformation (usually by ball-milling) is an effective way to make nanocrystalline materials [8–10], the crystal diameter arising from a competition between crystal growth and break-up under deformation [11]. Based on results from low- and high-energy milling, and considering the effects of alloy composition and temperature, several authors [12–14] have noted that crystallization of metallic glasses cannot be a purely thermal effect (even though milling does induce significant heating). It has, for example, been suggested that milling can induce redistribution of solute through cyclic crystallization and amorphization [12], or generation of free volume enhancing diffusion [14]. The interpretation of deformation effects
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
257
can be complicated by associated contamination (often by Fe and O). In this respect, shot-peening is particularly attractive in inducing much less contamination [6] than the more widely studied ball-milling. Shot-peening, by repeated deformation of the same material, can also induce much greater total strains than techniques such as the rolling used in early studies of the effects of deformation [15]. Recent studies have given additional importance to crystallization during deformation, by showing that if it occurs, it may lead to improved plasticity of the metallic glass [16,17]. 2. Experimental methods In the present work samples of Zr55 Al10 Cu30 Ni5 were prepared in two laboratories, in each case starting by induction melting granules of the elemental constituents (purity of 99.9%). Casting was into copper moulds under high-purity argon, in one case to obtain plate approximately 10 mm × 40 mm × 2 mm, in the other case to obtain 3 mm diameter rod. Shot-peening was performed using a Guyson Euro 2SF system with a high gas velocity (at a pressure of 7 bar). The shot are 300–400 m diameter glass beads of composition 72.6%-SiO2 , 9.2%-CaO, 3.4%-MgO and 13.7%-Na2 O (Wheelabrator Ltd., Altrincham, UK). It is known from earlier work [2] that the peened layer has a depth of approximately 100 m and a surface roughness of Ra = 1.3 m [2]. Continuous peening times of up to 60 s were used and even in such times there is significant sample heating of up to about 50 K. Samples at the start of peening were either at room temperature (RT) or cooled by liquid nitrogen (liqN2 ) to approximately 77 K. The amorphicity of the samples was checked by X-ray diffraction (XRD) using a Philips PW 1830 generator and PW 1820 diffractometer with CuK␣ ( = 0.1542 nm) radiation and a Ni filter. Differential scanning calorimetry (DSC) used a TA instruments DSC Q1000 with a heating rate of 10 K min−1 ; the glass-transition temperature and the onset crystallization temperature were calculated using standard software. The DSC samples were prepared by mechanical thinning of the disks to a thickness of order 100 m with the intention that the entire sample should have been heavily deformed by peening. Transmission electron microscopy (TEM) was with a JEOL 3010 microscope operated at 300 kV. The TEM samples were prepared by grinding and polishing 3-mm diameter discs to a thickness of 100 m, followed by standard twin-jet electrochemical thinning using a solution of 1 vol. HClO4 –10 vol. C2 H5 OH at about −30 ◦ C [18]. Finally, the samples were thinned by a low-angle ion milling (5◦ ) for about 10 min to remove any chemical residues.
3. Results XRD of the as-cast plate and rod samples shows that while the plate samples are fully glassy (at least within the sensitivity of the XRD technique) (Fig. 1), the rod samples are partially crystalline (Fig. 2). This difference, attributed to differing levels of oxygen contamination (known to affect the critical cooling rate for glass formation, as reviewed in [19]), gives the opportunity to test the
Fig. 1. X-ray diffractograms of BMG Zr55 Cu30 Al10 Ni5 in plate form, as-cast (fully glassy) and after shot-peening at room temperature (RT) or cooled by liquid nitrogen (liqN2 ).
Fig. 2. X-ray diffractograms of BMG Zr55 Cu30 Al10 Ni5 in rod form, as-cast (partially crystalline) and after shot-peening at RT or liqN2 -cooled.
effect of starting structure on the subsequent behaviour. In the rod samples, some Bragg peaks correspond to CuZr2 and others are unidentified. The phase mixture appears similar to that indicated by XRD in other work on as-cast and annealed samples [4–6]. TEM of as-cast rod samples (Fig. 3a) shows that the crystals are small (10–30 nm), consistent with the broad peaks in XRD. Within the range of times and temperatures tested, shotpeening of the fully glassy plate samples does not produce any detectable change in phase (Fig. 1). The intense deformation induced by peening must influence the glassy structure (for example, deformation is widely considered to increase the free volume [14], and can be associated with significant stored energy [15]), but such effects are not observed directly in the present study. The absence of crystallization under peening is consistent with earlier work on Vitreloy 1 [2] and on Zr55 Al10 Cu30 Ni5 itself [6]. The response of the partially crystalline rod samples to peening is more complex (Fig. 2). Peening at RT clearly induces amorphization, while in liqN2 -cooled samples it induces crystallization. It is counter-intuitive that crystallization is favoured in the colder samples, so efforts were made to corroborate these XRD observations by TEM and by DSC. In rod samples peened at RT, TEM shows that the characteristic crystallite size decreases to 10 nm in the first 15 s and then stabilises. It is difficult to quantify the crystalline volume fraction from the TEM observations, but qualitatively the volume fraction does decrease on peening (Fig. 3b). In the samples liqN2 -cooled before peening, TEM (Fig. 3c) shows virtually complete crystallinity. Most remarkably, the crystal size has increased to a few micrometres. Furthermore, the crystals do not appear to be heavily deformed as they show clear interference-fringe contrast spanning the crystal diameter. DSC results from the as-cast rod (Fig. 4) shows a clear crystallization peak at 750 K. For the sample cooled to 77 K before peening, the crystallization peak has almost disappeared, corresponding to the near-complete crystallinity seen in TEM (Fig. 3c). It also confirms that the preparation is successful in thinning to leave a sample that consists essentially only of the peened layer. (The underlying undeformed bulk is expected to remain glassy and would give a crystallization peak if still present in the DSC samples.) Glasstransition temperatures Tg , onset crystallization temperatures Tx and heats of crystallization Hx are given in Table 1, and compared with values from [3]. Peening for 15 s at RT, moves the crystallization peak to higher temperature, sharpens it and increases its area, all effects consistent with greater initial amorphicity. The heat of crystallization shown by the as-cast sample is approximately 88% of that shown by the RT-peened sample. Thus, if the latter sample is
258
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
Fig. 4. DSC curves (heating at 10 K min−1 ) for BMG Zr55 Cu30 Ni5 Al10 in rod form, as-cast (partially crystalline) and after shot-peening at RT or liqN2 -cooled.
taken to be fully amorphous before the DSC run (i.e. assuming that peening induces complete amorphicity and that the DSC sample represents only the peened layer), then the heats of crystallization would indicate that the as-cast rod sample has a crystalline volume fraction of some 12%. In their initial work, Inoue and Zhang [3] reported Tg = 690 K and Tx = 780 K. In the present work (Table 1) the Tg of the rod sample peened at RT is essentially the same as for the fully glassy sample reported in [3]. On the other hand, Tx in the same sample is much lower, presumably because crystalline fragments facilitate nucleation of crystallization on heating. 4. Discussion
Fig. 3. Transmission electron micrographs of BMG Zr55 Cu30 Al10 Ni5 in rod form: (a) as-cast, dark-field (DF) image and selected-area diffraction pattern (SADP); and shotpeened; (b) 30 s at RT (DF); (c) 30 s liqN2 -cooled, bright-field (BF). Table 1 The glass-transition temperature Tg , onset crystallization temperature Tx , and heat of crystallization Hx for BMG Zr55 Cu30 Al10 Ni5 samples prepared and treated in the present work, compared with the literature values. Tg (K)
Tx (K)
T = Tx − Tg (K)
Hx (kJ mol−1 )
Present work, rod sample (3 mm diameter, ∼12% crystalline as-cast) As-cast 683 732 49 4.35 15 s @RT 690 733 43 4.96 15 s @liqN2 – 739 – 0.23 From Inoue and Zhang [3] (5 mm diameter rod, fully glassy as-cast) 690 780 90
XRD, TEM and DSC all show that the partially crystalline rod samples are amorphized by peening at RT. This effect is consistent with amorphization by heavy mechanical deformation by such processes as mechanical alloying, ball-milling and shot-peening itself [20,21]. In contrast to the work of Yamamoto et al. [6], the peening in the present work appears capable of inducing almost complete amorphization. This is also consistent with the structural stability in fully glassy plate samples (Fig. 1). The reduction in crystallite size and number are consistent with local melting of crystals in shear bands, followed by quenching into the glassy state. As noted in Section 1, there are many examples of deformationinduced crystallization in metallic glasses. In the present work, the crystallization induced by shot-peening rod samples cooled by liqN2 is remarkable in two respects: that the crystallization is favoured in colder samples, and that the crystals are large and undeformed. In contrast, shot-peening of crystalline alloys is known to induce refinement of grain size [22]. Also, shot-peening is expected to break up crystals formed during the peening [11,23]. Thus the observed crystals cannot be formed during peening. Fine, heavily deformed crystals, if formed during peening at low temperature, could recrystallize on subsequent heating to RT. However, the lack of crystallization, in contrast amorphization, induced by peening at RT argues against such a mechanism. Deformed metallic glasses can acquire significant stored energy while remaining glassy. It is likely that the excess energy of 209 J mol−1 , detected by Chen [15] in rolled samples, can be greatly exceeded in shot-peened material (a topic under investigation). On subsequent heating to RT, the relatively energetic and unstable deformed glass can undergo structural relaxation to a more stable glassy form or can crystallize. The present work suggests that the
F.O. Méar et al. / Journal of Alloys and Compounds 483 (2009) 256–259
preferred process is influenced by the starting structure. Crystal fragments, remaining from the initial partial crystallinity in the rod samples even after peening, may favour crystallization, presumably by eliminating the need for crystal nucleation. This is similar to the deformation-induced nanocrystallization made possible by pre-nucleation [17] and suggested to be important for improving the plasticity of metallic glasses. The high stored energy, presumed in liqN2 -peened samples, suggests that the glassy structure of Zr55 Al10 Cu30 Ni5 is not quite so stable under deformation as suggested by Yamamoto et al. [6], as it can crystallize on heating to RT. The possibility of crystallization after deformation has not been clear in earlier work, but this phenomenon may help to explain some earlier observations. 5. Conclusions Shot-peening can induce very high surface deformation, with little contamination compared to techniques such as ball-milling. In the present work the observations of Yamamoto et al. [6] on the stability of the bulk metallic glass, Zr55 Al10 Cu30 Ni5 under shot-peening have been extended to examine the effect of sample temperature. Fully glassy samples, whether initially at RT or liqN2 -cooled, remain fully glassy under extended peening. Partially crystalline samples are amorphized by peening at RT. In contrast to the earlier work [6], the amorphization can be essentially complete. Partially crystalline samples peened at liqN2 temperature show complete crystallization in XRD, TEM and DSC. The size and perfection of the surface crystals observed by TEM suggests that they must form after peening, presumably by crystallization of an unusually energetic glass on warming back to RT. This crystallization shows that glassy Zr55 Al10 Cu30 Ni5 is not always so stable under deformation as considered in earlier work [6]. The crystallization appears to require the presence of prior crystalline fragments, presumably to assist nucleation, and may therefore be related to the crystallization seen in shear bands in suitably pre-annealed samples and of interest in promoting plasticity of the BMG [17].
259
Acknowledgements This work was performed in the framework of the EU Network on Bulk Metallic Glass Composites (MRTN-CT-2003-504692) coordinated by A.R. Yavari. The authors thank Y. Zhang for useful discussions. References [1] I. Altenberger, in: Lothar Wagner (Ed.), Shot-peening, Wiley–VCH GmbH and Co., 2006, pp. 421–434. [2] Y. Zhang, W.H. Wang, A.L. Greer, Nat. Mater. 5 (2006) 857. [3] A. Inoue, T. Zhang, Mater. Trans. JIM 37 (1996) 185. [4] A.R. Yavari, A. Le Moulec, A. Inoue, W.J. Botta Filho, G. Vaughan, Å. Kvick, Mater. Sci. Eng. A 304–306 (2001) 34. [5] K. Soda, K. Shimba, S. Yagi, M. Kato, T. Takeuchi, U. Mizutani, T. Zhang, M. Hasegawa, A. Inoue, T. Ito, S. Kimura, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 585. [6] T. Yamamoto, T. Takahashi, H. Kimura, A. Inoue, J. Alloys Compd. 430 (2007)97. [7] S. Rouquette, E. Rouhaud, H. Pron, M. Franc¸ois, C. Bissieux, A. Roos, Mater. Sci. Forum. 524–525 (2006) 167. [8] J. Bednarˇcík, E. Burkel, K. Saksl, P. Kollár, S. Roth, J. Appl. Phys. 100 (2006) 014903. [9] P.P. Choi, I.V. Povstugar, Y.S. Kwon, E.P. Yelsukov, J.S. Kim, Mater. Sci. Eng. A 449–451 (2007) 1079. [10] B. Huang, R.J. Perez, P.J. Crawford, A.A. Sharif, S.R. Nutt, E.J. Lavernia, Nanostr. Mater. 5 (1995) 545. [11] M.L. Trudeau, J.Y. Huot, R. Schulz, D. Dussault, A. Van Neste, G. L’Espérance, Phys. Rev. B 45 (1992) 4626. [12] M.L. Trudeau, R. Schulz, D. Dussault, A. Van Neste, Phys. Rev. Lett. 64 (1990) 99. [13] Y. He, G.J. Shiflet, S.J. Poon, Acta Metall. Mater. 43 (1995) 83. [14] J. Xu, M. Atzmon, Appl. Phys. Lett. 73 (1998) 1805. [15] H.S. Chen, Appl. Phys. Lett. 29 (1976) 328. [16] S.-W. Lee, M.-Y. Huh, E. Fleury, J.-C. Lee, Acta Mater. 54 (2006) 349. [17] K. Hajlaoui, A.R. Yavari, A. LeMoulec, W.J. Botta Filho, G. Vaughan, J. Das, A.L. Greer, Å. Kvick, J. Non-Cryst. Sol. 353 (2007) 327. [18] Y.F. Deng, L.L. He, Q.S. Zhang, H.F. Zhang, H.Q. He, Ultramicroscopy 98 (2004) 201. [19] A. Castellero, S. Bossuyt, M. Stoica, S. Deledda, J. Eckert, G.Z. Chen, D.J. Fray, A.L. Greer, Scripta Mater. 55 (2006) 87. [20] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [21] D.M. Grant, S.M. Green, J.V. Wood, Acta Metall. Mater. 43 (1995) 1045. [22] G. Liu, S.C. Wang, X.F. Lou, J. Lu, K. Lu, Scripta Mater. 44 (2001) 1791. [23] Q.F. Cao, K. Tsuchiya, Y. Harada, D. Tomus, K. Morii, Y. Todaka, M. Umemoto, J. Jpn. Instrum. Met. 70 (2006) 473.