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FULL CRITICAL REVIEW

Size-dependent mechanical responses of metallic glasses

Pages 163-180 | Received 04 Oct 2017, Accepted 10 May 2018, Published online: 19 May 2018

References

  • Johnson WL. Bulk glass-forming metallic alloys: science and technology. MRS Bull. 1999;24(10):42–56.
  • Inoue A, Nishiyama N. New bulk metallic glasses for applications as magnetic-sensing, chemical, and structural materials. MRS Bull. 2007;32(8):651–658.
  • Greer A. Metallic glasses … on the threshold. Mater Today. 2009;12(1–2):14–22.
  • Wang W, Dong C, Shek C. Bulk metallic glasses. Mater Sci Eng R Rep. 2004;44(2–3):45–89.
  • Klement W, Willens RH, Duwez P. Non-crystalline structure in solidified gold-silicon alloys. Nature. 1960;187(4740):869–870.
  • Peker A, Johnson WL. A highly processable metallic-glass – Zr41.2ti13.8cu12.5ni10.0be22.5. Appl Phys Lett. 1993;63(17):2342–2344.
  • Inoue A, Gook JS. Fe-based ferromagnetic glassy alloys with wide supercooled liquid region. Mater Trans JIM. 1995;36(9):1180–1183.
  • Suryanarayana C, Inoue A. Iron-based bulk metallic glasses. Int Mater Rev. 2013;58(3):131–166.
  • Ma H, Xu J, Ma E. Mg-based bulk metallic glass composites with plasticity and high strength. Appl Phys Lett. 2003;83(14):2793–2795.
  • Inoue A. Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems. Prog Mater Sci. 1998;43(5):365–520.
  • Kumar G, Prades-Rodel S, Blatter A, et al. Unusual brittle behavior of Pd-based bulk metallic glass. Scr Mater. 2011;65(7):585–587.
  • Miracle DB. A structural model for metallic glasses. Nat Mater. 2004;3(10):697–702.
  • Inoue A, Zhang T, Masumoto T. ZR-AL-NI amorphous-alloys with high glass-transition temperature and significant supercooled liquid region. Mater Trans JIM. 1990;31(3):177–183.
  • Ding S, Liu Y, Li Y, et al. Combinatorial development of bulk metallic glasses. Nat Mater. 2014;13(5):494–500.
  • Tsai P, Flores KM. High-throughput discovery and characterization of multicomponent bulk metallic glass alloys. Acta Mater. 2016;120:426–434.
  • Zhong L, Wang J, Sheng H, et al. Formation of monatomic metallic glasses through ultrafast liquid quenching. Nature. 2014;512(7513):177–180.
  • Gupta PK. Non-crystalline solids: glasses and amorphous solids. J Non Cryst Solids. 1996;195(1):158–164.
  • Koch CC, Cavin OB, McKamey CG, et al. Preparation of ‘amorphous’ Ni60Nb40 by mechanical alloying. Appl Phys Lett. 1983;43(11):1017–1019.
  • El-Eskandarany MS, Saida J, Inoue A. Amorphization and crystallization behaviors of glassy Zr70Pd30 alloys prepared by different techniques. Acta Mater. 2002;50(10):2725–2736.
  • Nagase T, Hosokawa T, Umakoshi Y. Solid state amorphization and crystallization in Zr66.7Pd33.3 metallic glass. Intermetallics. 2006;14(8):1027–1032.
  • Yu H-B, Luo Y, Samwer K. Ultrastable metallic glass. Adv Mater. 2013;25(41):5904–5908.
  • Liu Y, Liu J, Sohn S, et al. Metallic glass nanostructures of tunable shape and composition. Nat Commun. 2015;6:7043.
  • Wang Q, Yang Y, Jiang H, et al. Superior tensile ductility in bulk metallic glass with gradient amorphous structure. Sci Rep. 2014;4:srep04757.
  • Nieh TG, Yang Y, Lu J, et al. Effect of surface modifications on shear banding and plasticity in metallic glasses: an overview. Prog Nat Sci Mater Int. 2012;22(5):355–363.
  • Packard CE, Witmer LM, Schuh CA. Hardening of a metallic glass during cyclic loading in the elastic range. Appl Phys Lett. 2008;92(17):171911.
  • Tong P, Louca D, Yokoyama Y, et al. Investigating the effects of annealing on fatigue behavior in Zr-based bulk metallic glasses. MRS Proc. 2011;1300:u09-36.
  • Wada T, Louzguine-Luzgin DV, Inoue A. Preparation of Zr-based metallic glass nanowires and nanoparticles by selective etching. Scr Mater. 2007;57(10):901–904.
  • Lan S, Ren Y, Wei XY, et al. Hidden amorphous phase and reentrant supercooled liquid in Pd-Ni-P metallic glasses. Nat Commun. 2017;8:14679.
  • Bakke E, Busch R, Johnson WL. The viscosity of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy in the supercooled liquid. Appl Phys Lett. 1995;67(22):3260–3262.
  • Nieh TG, Wadsworth J, Liu CT, et al. Plasticity and structural instability in a bulk metallic glass deformed in the supercooled liquid region. Acta Mater. 2001;49(15):2887–2896.
  • Egami T, Poon SJ, Zhang Z, et al. Glass transition in metallic glasses: A microscopic model of topological fluctuations in the bonding network. Phys Rev B. 2007;76(2):024203.
  • Taub AI, Spaepen F. The kinetics of structural relaxation of a metallic glass. Acta Metall. 1980;28(12):1781–1788.
  • Busch R, Bakke E, Johnson WL. Viscosity of the supercooled liquid and relaxation at the glass transition of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy. Acta Mater. 1998;46(13):4725–4732.
  • Spaepen F. Microscopic mechanism for steady-state inhomogeneous flow in metallic glasses. Acta Metall. 1977;25(4):407–415.
  • Argon AS. Plastic-deformation in metallic glasses. Acta Metall. 1979;27(1):47–58.
  • Falk ML, Langer JS. Dynamics of viscoplastic deformation in amorphous solids. Phys Rev E. 1998;57(6):7192–7205.
  • Busch R, Schroers J, Wang WH. Thermodynamics and kinetics of bulk metallic glass. MRS Bull. 2007;32(8):620–623.
  • Lewandowski J, Wang W, Greer A. Intrinsic plasticity or brittleness of metallic glasses. Philos Mag Lett. 2005;85(2):77–87.
  • Schroers J. Bulk metallic glasses. Phys Today. 2013;66(2):32–37.
  • Egami T. Magnetic amorphous-alloys – physics and technological applications. Rep Prog Phys. 1984;47(12):1601–1725.
  • Tiberto P, Baricco M, Olivetti E, et al. Magnetic properties of bulk metallic glasses. Adv Eng Mater. 2007;9(6):468–474.
  • Carmo M, Sekol RC, Ding S, et al. Bulk metallic glass nanowire architecture for electrochemical applications. ACS Nano. 2011;5(4):2979–2983.
  • Oak J-J, Louzguine-Luzgin DV, Inoue A. Investigation of glass-forming ability, deformation and corrosion behavior of Ni-free Ti-based BMG alloys designed for application as dental implants. Mater Sci Eng C. 2009;29(1):322–327.
  • Chu JP, Liu T-Y, Li C-L, et al. Fabrication and characterizations of thin film metallic glasses: antibacterial property and durability study for medical application. Thin Solid Films. 2014;561:102–107.
  • Ashby MF, Greer AL. Metallic glasses as structural materials. Scr Mater. 2006;54(3):321–326.
  • Demetriou MD, Launey ME, Garrett G, et al. A damage-tolerant glass. Nat Mater. 2011;10(2):123–128.
  • Johnson WL, Samwer K. A universal criterion for plastic yielding of metallic glasses with a (T/Tg)^(2/3) temperature dependence. Phys Rev Lett. 2005;95(19):195501.
  • Greer AL, Cheng YQ, Ma E. Shear bands in metallic glasses. Mater Sci Eng R Rep. 2013;74(4):71–132.
  • Guo H, Yan PF, Wang YB, et al. Tensile ductility and necking of metallic glass. Nat Mater. 2007;6(10):735–739.
  • Hall EO. The deformation and ageing of mild steel .3. Discussion of results. Proc Phys Soc Lond B. 1951;64(381):747–753.
  • Chokshi AH, Rosen A, Karch J, et al. On the validity of the hall-petch relationship in nanocrystalline materials. Scr Metall. 1989;23(10):1679–1683.
  • Kumar K, Van Swygenhoven H, Suresh S. Mechanical behavior of nanocrystalline metals and alloys. Acta Mater. 2003;51(19):5743–5774.
  • Sheng HW, Liu HZ, Cheng YQ, et al. Polyamorphism in a metallic glass. Nat Mater. 2007;6(3):192–197.
  • Kazimirov VY. First-principles simulation of the elastic properties of multicomponent amorphous steels. Phys Rev B. 2009;80(21):214117.
  • Srolovitz D, Vitek V, Egami T. An atomistic study of deformation of amorphous metals. Acta Metall Mater. 1983;31(2):335–352.
  • Falk ML. Molecular-dynamics study of ductile and brittle fracture in model noncrystalline solids. Physical Review B. 1999;60(10):7062–7070.
  • Murali P, Guo TF, Zhang YW, et al. Atomic scale fluctuations govern brittle fracture and cavitation behavior in metallic glasses. Phys Rev Lett. 2011;107(21):215501.
  • Homer ER, Schuh CA. Mesoscale modeling of amorphous metals by shear transformation zone dynamics. Acta Mater. 2009;57(9):2823–2833.
  • Falk ML, Langer JS, Pechenik L. Thermal effects in the shear-transformation-zone theory of amorphous plasticity: comparisons to metallic glass data. Phys Rev E. 2004;70(1):011507.
  • Talamali M, Petäjä V, Vandembroucq D, et al. Strain localization and anisotropic correlations in a mesoscopic model of amorphous plasticity. C R Méc. 2012;340(4):275–288.
  • Gao YF. An implicit finite element method for simulating inhomogeneous deformation and shear bands of amorphous alloys based on the free-volume model. Model Simul Mater Sci Eng. 2006;14(8):1329.
  • Anand L, Su C. A theory for amorphous viscoplastic materials undergoing finite deformations, with application to metallic glasses. J Mech Phys Solids. 2005;53(6):1362–1396.
  • Rodney D, Tanguy A, Vandembroucq D. Modeling the mechanics of amorphous solids at different length scale and time scale. Model Simul Mater Sci Eng. 2011;19(8):083001.
  • Volkert CA, Donohue A, Spaepen F. Effect of sample size on deformation in amorphous metals. J Appl Phys 2008;103(8):083539–6.
  • Luo JH, Wu FF, Huang JY, et al. Superelongation and atomic chain formation in nanosized metallic glass. Phys Rev Lett. 2010;104(21):215503.
  • Tian L, Cheng Y-Q, Shan Z-W, et al. Approaching the ideal elastic limit of metallic glasses. Nat Commun. 2012;3:609.
  • Nakayama KS, Yokoyama Y, Ono T, et al. Controlled formation and mechanical characterization of metallic glassy nanowires. Adv Mater. 2010;22(8):872–875.
  • Chen DZ, Jang D, Guan KM, et al. Nanometallic glasses: size reduction brings ductility, surface state drives its extent. Nano Lett. 2013;13(9):4462–4468.
  • Magagnosc DJ, Ehrbar R, Kumar G, et al. Tunable tensile ductility in metallic glasses. Sci Rep. 2013;3:1096.
  • Shi YF. Size-independent shear band formation in amorphous nanowires made from simulated casting. Appl Phys Lett. 2010;96(9):121909.
  • Schroers J. Processing of bulk metallic glass. Adv Mater. 2010;22(14):1566–1597.
  • Wu XL, Guo YZ, Wei Q, et al. Prevalence of shear banding in compression of Zr41Ti14Cu12.5Ni10Be22.5 pillars as small as 150 nm in diameter. Acta Mater. 2009;57(12):3562–3571.
  • Jang D, Greer JR. Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses. Nat Mater. 2010;9(3):215–219.
  • Kumar G, Tang HX, Schroers J. Nanomoulding with amorphous metals. Nature. 2009;457(7231):868–872.
  • Liu Z, Schroers J. General nanomoulding with bulk metallic glasses. Nanotechnology. 2015;26(14):145301.
  • Magagnosc DJ, Chen W, Kumar G, et al. Thermomechanical behavior of molded metallic glass nanowires. Sci Rep. 2016;6:19530.
  • Hasan M, Kumar G. High-throughput drawing and testing of metallic glass nanostructures. Nanoscale. 2017;9(9):3261–3268.
  • Xiao Q, Sheng HW, Shi Y. Dominant shear bands observed in amorphous ZrCuAl nanowires under simulated compression. MRS Commun. 2012;2(01):13–16.
  • Yuan F, Huang L. Size-dependent elasticity of amorphous silica nanowire: a molecular dynamics study. Appl Phys Lett. 2013;103(20):201905.
  • Shi Y, Luo J, Yuan F, et al. Intrinsic ductility of glassy solids. J Appl Phys. 2014;115(4):043528.
  • Zhang Q, Li Q-K, Li M. Key factors affecting mechanical behavior of metallic glass nanowires. Sci Rep. 2017;7:41365.
  • Singh S, Ediger MD, de Pablo JJ. Ultrastable glasses from in silico vapour deposition. Nat Mater. 2013;12(2):139–144.
  • Zhang Q, Li Q-K, Li M. Processing dependence of mechanical properties of metallic glass nanowires. Appl Phys Lett. 2015;106(7):071905.
  • Zhang Q, Li Q-K, Li M. Chemical segregation in metallic glass nanowires. J Chem Phys. 2014;141(19):194701.
  • Luan BQ, Robbins MO. The breakdown of continuum models for mechanical contacts. Nature. 2005;435(7044):929–932.
  • Tanguy A, Wittmer JP, Leonforte F, et al. Continuum limit of amorphous elastic bodies: a finite-size study of low-frequency harmonic vibrations. Phys Rev B. 2002;66(17):174205.
  • Maloney CE, Lemaître A. Amorphous systems in athermal, quasistatic shear. Phys Rev E. 2006;74(1):016118.
  • Leonforte F, Boissière R, Tanguy A, et al. Continuum limit of amorphous elastic bodies. III. Three-dimensional systems. Phys Rev B. 2005;72(22):224206.
  • Leonforte F, Tanguy A, Wittmer JP, et al. Continuum limit of amorphous elastic bodies II: linear response to a point source force. Phys Rev B. 2004;70(1):014203.
  • Luo WK, Sheng HW, Alamgir FM, et al. Icosahedral short-range order in amorphous alloys. Phys Rev Lett. 2004;92(14):145502.
  • Shi YF, Falk ML. Atomic-scale simulations of strain localization in three-dimensional model amorphous solids. Phys Rev B. 2006;73(21):214201.
  • Fang XW, Wang CZ, Yao YX, et al. Atomistic cluster alignment method for local order mining in liquids and glasses. Phys Rev B. 2010;82(18):184204.
  • Maloney CE. Correlations in the elastic response of dense random packings. Phys Rev Lett. 2006;97(3):035503.
  • Vempati UK, Valavala PK, Falk ML, et al. Length-scale dependence of elastic strain from scattering measurements in metallic glasses. Phys Rev B. 2012;85(21):214201.
  • Abell GC. Empirical chemical pseudopotential theory of molecular and metallic bonding. Phys Rev B. 1985;31(10):6184–6196.
  • Leishangthem P, Parmar ADS, Sastry S. The yielding transition in amorphous solids under oscillatory shear deformation. Nat Commun. 2017;8:14653.
  • Cheng YQ, Ma E. Intrinsic shear strength of metallic glass. Acta Mater. 2011;59(4):1800–1807.
  • Deng C, Schuh CA. Atomistic mechanisms of cyclic hardening in metallic glass. Appl Phys Lett. 2012;100(25):251909.
  • Shi YF, Falk ML. Simulations of nanoindentation in a thin amorphous metal film. Thin Solid Films. 2007;515(6):3179–3182.
  • Shi YF, Falk ML. Stress-induced structural transformation and shear banding during simulated nanoindentation of a metallic glass. Acta Mater. 2007;55(13):4317–4324.
  • Yang Y, Luo J, Huang L, et al. Crack initiation in metallic glasses under nanoindentation. Acta Mater. 2016;115:413–422.
  • Shi YF, Katz MB, Li H, et al. Evaluation of the disorder temperature and free-volume formalisms via simulations of shear banding in amorphous solids. Phys Rev Lett. 2007;98(18):185505.
  • Shimizu F, Ogata S, Li J. Yield point of metallic glass. Acta Mater. 2006;54(16):4293–4298.
  • Perepezko JH, Imhoff SD, Chen M-W, et al. Nucleation of shear bands in amorphous alloys. PNAS. 2014;111(11):3938–3942.
  • Ogata S, Shimizu F, Li J, et al. Atomistic simulation of shear localization in Cu–Zr bulk metallic glass. Intermetallics. 2006;14(8–9):1033–1037.
  • Cheng YQ, Han Z, Li Y, et al. Cold versus hot shear banding in bulk metallic glass. Phys Rev B. 2009;80(13):134115.
  • Greer JR, De Hosson JTM. Plasticity in small-sized metallic systems: intrinsic versus extrinsic size effect. Prog Mater Sci. 2011;56(6):654–724.
  • Wang C-C, Ding J, Cheng Y-Q, et al. Sample size matters for Al88Fe7Gd5 metallic glass: smaller is stronger. Acta Mater. 2012;60(13–14):5370–5379.
  • Shan ZW, Li J, Cheng YQ, et al. Plastic flow and failure resistance of metallic glass: insight from in situ compression of nanopillars. Phys Rev B. 2008;77(15):155419–6.
  • Schuster BE, Wei Q, Hufnagel TC, et al. Size-independent strength and deformation mode in compression of a Pd-based metallic glass. Acta Mater. 2008;56(18):5091–5100.
  • Dubach A, Raghavan R, Löffler JF, et al. Micropillar compression studies on a bulk metallic glass in different structural states. Scr Mater. 2009;60(7):567–570.
  • Cheng YQ, Cao AJ, Sheng HW, et al. Local order influences initiation of plastic flow in metallic glass: effects of alloy composition and sample cooling history. Acta Mater. 2008;56(18):5263–5275.
  • Shi YF, Falk ML. Shear localization and percolation of stable structure in amorphous solids. Phys Rev Lett. 2005;95:095502.
  • Shi YF, Falk ML. Structural transformation and localization during simulated nanoindentation of a noncrystalline metal film. Appl Phys Lett. 2005;86:011914.
  • Cheng YQ, Cao AJ, Ma E. Correlation between the elastic modulus and the intrinsic plastic behavior of metallic glasses: the roles of atomic configuration and alloy composition. Acta Mater. 2009;57(11):3253–3267.
  • Li QK, Li M. Assessing the critical sizes for shear band formation in metallic glasses from molecular dynamics simulation. Appl Phys Lett. 2007;91(23):231905–3.
  • Zhang Y, Wang WH, Greer AL. Making metallic glasses plastic by control of residual stress. Nat Mater. 2006;5(11):857–860.
  • Xiao Q, Huang L, Shi Y. Suppression of shear banding in amorphous ZrCuAl nanopillars by irradiation. J Appl Phys. 2013;113(8):083514.
  • Tönnies D, Maaß R, Volkert CA. Room temperature homogeneous ductility of micrometer-sized metallic glass. Adv Mater. 2014;26(32):5715–5721.
  • Magagnosc DJ, Kumar G, Schroers J, et al. Effect of ion irradiation on tensile ductility, strength and fictive temperature in metallic glass nanowires. Acta Mater. 2014;74:165–182.
  • Han Z, Wu W, Li Y, et al. An instability index of shear band for plasticity in metallic glasses. Acta Mater. 2009;57(5):1367–1372.
  • Wu Z, Zhang Y-W, Jhon MH, et al. Nanowire failure: long = brittle and short = ductile. Nano Lett. 2012;12(2):910–914.
  • Şopu D, Foroughi A, Stoica M, et al. Brittle-to-ductile transition in metallic glass nanowires. Nano Lett. 2016;16(7):4467–4471.
  • Sha ZD, He LC, Xu S, et al. Effect of aspect ratio on the mechanical properties of metallic glasses. Scr Mater. 2014;93:36–39.
  • Lewandowski JJ, Greer AL. Temperature rise at shear bands in metallic glasses. Nat Mater. 2006;5(1):15–18.
  • Luo J, Shi Y. Tensile fracture of metallic glasses via shear band cavitation. Acta Mater. 2015;82:483–490.
  • Guan P, Lu S, Spector MJB, et al. Cavitation in amorphous solids. Phys Rev Lett. 2013;110(18):185502.
  • Luo J, Keblinski P, Shi YF. A model metallic glass exhibits size-independent tensile ductility. Acta Mater. 2016;13:587.
  • Suresh S. Fatigue of materials. 2nd ed. Cambridge: Cambridge University Press; 1998.
  • Luo J, Dahmen K, Liaw PK, et al. Low-cycle fatigue of metallic glass nanowires. Acta Mater. 2015;87:225–232.
  • Sha ZD, Qu SX, Liu ZS, et al. Cyclic deformation in metallic glasses. Nano Lett. 2015;15(10):7010–7015.
  • Wang C-C, Mao Y-W, Shan Z-W, et al. Real-time, high-resolution study of nanocrystallization and fatigue cracking in a cyclically strained metallic glass. PNAS. 2013;110(49):19725–19730.
  • Jang D, Maaß R, Wang G, et al. Fatigue deformation of microsized metallic glasses. Scr Mater. 2013;68(10):773–776.
  • Taub AI, Spaepen F. Ideal elastic, anelastic and viscoelastic deformation of a metallic glass. J Mater Sci. 1981;16(11):3087–3092.
  • Suzuki Y, Egami T. Shear deformation of glassy metals: breakdown of Cauchy relationship and anelasticity. J Non Cryst Solids. 1985;75(1):361–366.
  • Dmowski W, Iwashita T, Chuang C-P, et al. Elastic heterogeneity in metallic glasses. Phys Rev Lett. 2010;105(20):205502.
  • Harmon JS, Demetriou MD, Johnson WL, et al. Anelastic to plastic transition in metallic glass-forming liquids. Phys Rev Lett. 2007;99(13):135502.
  • Ju JD, Jang D, Nwankpa A, et al. An atomically quantized hierarchy of shear transformation zones in a metallic glass. J Appl Phys. 2011;109(5):053522.
  • Ju JD, Atzmon M. A comprehensive atomistic analysis of the experimental dynamic-mechanical response of a metallic glass. Acta Mater. 2014;74(Supplement C):183–188.
  • Chen N, Louzguine-Luzgin DV, Xie GQ, et al. Structural investigation and mechanical properties of a representative of a new class of materials: nanograined metallic glasses. Nanotechnology. 2013;24(4):045610.
  • Sarac B, Schroers J. Designing tensile ductility in metallic glasses. Nat Commun. 2013;4:2158.
  • Kim J-Y, Jang D, Greer JR. Nanolaminates utilizing size-dependent homogeneous plasticity of metallic glasses. Adv Funct Mater. 2011;21(23):4550–4554.
  • Jeong HW, Hata S, Shimokohbe A. Microforming of three-dimensional microstructures from thin-film metallic glass. J Microelectromech Syst. 2003;12(1):42–52.
  • Xie G, Zhang W, Louzguine-Luzgin D, et al. Fabrication of porous Zr-Cu-Al-Ni bulk metallic glass by spark plasma sintering process. Scr Mater. 2006;55(8):687–690.
  • Liu Y, Zhu Y, Li F, et al. Processing porous bulk metallic glass using prealloyed powders. Adv Eng Mater. 2010;12(11):1131–1136.
  • Hofmann D, Suh J, Wiest A, et al. Designing metallic glass matrix composites with high toughness and tensile ductility. Nature. 2008;451(7182):1085–10U3.
  • Liontas R, Greer JR. 3D nano-architected metallic glass: size effect suppresses catastrophic failure. Acta Mater. 2017;133(Supplement C):393–407.

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