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Amorphous Bulk Metals ―
Metallic Glasses
A. L. GreerDept. of Materials Science & Metallurgy
University of Cambridge
Materials on the HorizonCambridge, 9 December 2008
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Crystal
• regular atomicarrangement
• slip planes forplastic deformation
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Glass
• no periodicity, but
• density ~ same ascrystal
• local configurations~ same as crystal
• plastic flow is difficult
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How to make a glass?
• a glass forms if crystallization is avoided on cooling• the density of the glass depends on cooling rate
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The Glassy State
— is found for all classes of material:
• oxide (e.g. SiO2)• ionic (e.g. ZnF2)• polymeric• metallic• carbohydrates
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Metallic Glasses
• metals and alloys are naturally crystalline
• pure metals cannot form glasses — their simplestructure crystallizes too easily on cooling the liquid
• alloying can stabilize the liquid, and aids glassformation (“confusion principle”)
• for a binary alloy such as Fe80
B20
(atomic %), thecritical cooling rate for glass formation is
105 to 106 K s –1
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Bulk Metallic Glasses
• multicomponent compositions aid glass formation
• the critical cooling rate is much lower (~1 K s –1)
• glasses can be formed in bulk
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Metallic Glasses
• are now understood to be true glasses
Outline
• new insights on atomic-level structure
• metallic glasses as structural materials
• current progress on understanding plastic flow
• improving mechanical properties, and new horizons
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from The Times Higher Education Suppl . 3 Feb. 2006
Structure
John Desmond Bernal1901-1971
The dense random packing model forthe structure of liquid metals.
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Short-to-medium-range order in metallic glasses:― solute-centred clusters
H.W. Sheng, W.K. Luo, F.M. Alamgir, J.M. Bai & E. Ma, Nature 439 (2006) 419.
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Interpenetrating clusters in the efficient cluster packing model ofMiracle et al.
D B. Miracle, Acta Mater . 54 (2006) 4317.
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Dynamics in metallic glasses― atoms in icosahedral clusters are the least mobile
YQ Cheng et al ., Appl. Phys. Lett . (2008) 93, 111913
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YQ Cheng et al., Phys. Rev. B (2008) 78, 014207
Degree of icosahedral order increases on cooling, and is also differentfrom composition to composition, correlating with glass-forming ability.
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M.F. Ashby & A.L. Greer: Scripta Materialia 54 (2006) 321.(in Viewpoint Set on Mechanical Behavior of Metallic Glasses , edited by T.C. Hufnagel)
Elastic limit σ y plotted against density ρ for 1507 metals, alloys,metal-matrix composites and metallic glasses. The contoursshow the specific strength σ y / ρ .
Metallic glasses for structural applications
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Ce70Al10Cu20 — T g = 338 K, T x = 390 K
B. Zhang, D.Q. Zhao, M.X. Pan, W.H. Wang & A.L. Greer:“Amorphous metallic plastic”, Phys. Rev. Lett. 94 (2005) 205502.
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J Schroers et al ., Scripta Mater . (2007) 57, 341
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Microformability of BMGs
• of interest for micro-& nano-imprinting ofsurfaces
AFM and SEMimages of a patterned(100) Si die and a Pt-based BMG imprintedwith the die (10 MPa,550 K, 300 s)
Y. Saotome et al. “The micro-nanoformability of Pt-based metallic glass and thenanoforming of three-dimensional structures”, Intermetallics 10 (2005) 1241.
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J. Schroers: “The superplastic
forming of bulk metallic glasses”,JOM 57(5) (2005) 35.
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M.F. Ashby & A.L. Greer: Scripta Materialia 54 (2006) 321.(in Viewpoint Set on Mechanical Behavior of Metallic Glasses , edited by T.C. Hufnagel)
Fracture toughness and elastic limit for metals, alloys, ceramic,glasses, polymers and metallic glasses. The contours show theprocess-zone size d in mm.
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The world’s smallest motor
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from Materials Selection in Mechanical Design (2nd ed.)
M. F. Ashby, Butterworth-Heinemann, 1999
metallic glasses
— compared to metalsand alloys in general,
the glasses have highstrength σ and lowstiffness E , that is,unusually high elasticstrain —
σσσσ/E
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J.H. Tregilgas, “Amorphous titaniumaluminide hinge”Adv. Mater. Proc . 162 (Oct. 2004) 40.
MEMS Applications of Metallic Glasses
The Texas Instruments Digital
Light Processor (DLP) data
projector technology is based on
mirrors supported by amorphous
Ti-Al hinges. DLP devices with
>1.3 x 106
addressable mirrorsare in production, and the hinges
still show no fatigue failures after
1012 cycles.
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Pressure Sensors
Diaphragms
Annual production now nearly 50 million units
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from Materials Selection in Mechanical Design (2nd ed.)
M. F. Ashby, Butterworth-Heinemann, 1999
metallic glasses
materials for elasticenergy storage —
want to maximize
σσσσ2/E
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Strain→
S t r e s s →
Within the elastic (reversible) regime ―
σ σσ σ y
E
area = σ σσ σ 2/2E = elastic energystored per unit volume
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Strain→
S t r e s s →
to increase the elastic stored energy―
E
increase the yield stress, σ σσ σ y
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Strain→
S t r e s s →
E
decrease the Young modulus, E
to increase the elastic stored energy―
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T Fukushige & S Hata, J. Microelectro. Syst . (2005) 14, 243
MEMS Applications
A conical spring microactuator
with a long stroke of 200 mmnormal to the substrate. The
spring is a 7.6 µm thick film of
Pd76Cu7Si17 metallic glass.
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Golf clubs …. and tennis-racket frames, baseball bats, skis …
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from Materials Selection in Mechanical Design (2nd ed.)
M. F. Ashby, Butterworth-Heinemann, 1999
metallic glasses
materials for elasticenergy storage —
want to maximize
σσσσ2/E
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M.F. Ashby & A.L. Greer: Scripta Materialia 54 (2006) 321.(in Viewpoint Set on Mechanical Behavior of Metallic Glasses , edited by T.C. Hufnagel)
Fracture toughness and Young’s modulus for metals, alloys,ceramic, glasses, polymers and metallic glasses. The contoursshow the toughness G c in kJ m –2.
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M.F. Ashby & A.L. Greer: Scripta Materialia 54 (2006) 321.(in Viewpoint Set on Mechanical Behavior of Metallic Glasses , edited by T.C. Hufnagel)
Fracture toughness and Young’s modulus for metals, alloys,ceramic, glasses, polymers and metallic glasses. The contoursshow the toughness G c in kJ m –2.
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• the plastic flow stress in shear is proportional to the elastic shearmodulus — thus the shear modulus is a measure of the difficulty ofplastic flow
• similarly the bulk modulus is a measure of the difficulty of cracking
• thus high values of the shear-to-bulk modulus ratio µ / B should favourbrittleness and vice versa
• proposed by Pugh in 1954, and developed by others —
S.F. Pugh, Philos. Mag . 45 823 (1954).
A. Kelly, W.R. Tyson and A.H. Cottrell, Philos . Mag . 15 567 (1967).
J.R. Rice and R. Thomson, Philos. Mag . 29 73 (1974).
A.H. Cottrell, in Advances in Physical Metallurgy , edited by J.A. Charles and
G.C. Smith (Institute of Metals, London, 1990), pp. 181–187.
Metals: Plasticity or Brittleness?
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Compilation of all relevant and available data on as-cast(unannealed) metallic glasses (mostly, but not all BMGs)
J.J. Lewandowski, W.H. Wang & A.L. Greer, “Intrinsic plasticity or brittleness ofmetallic glasses”, Philos. Mag. Lett. 85 (2005) 77.
D f ti f M t lli Gl
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F. Spaepen: “A microscopic
mechanism for steady state
inhomogeneous flow in metallic
glasses”, Acta Metall . 25 (1977)
407.
Deformation of Metallic Glasses
Ambient temperature / high stress-- flow localization in shear bands
High temperature / low stress
-- homogeneous viscous flow
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Plastic deformation of a thin plate of a thin plate of Pd77.5Cu6Si16.5
glass in tension. Shear bands are consistent with work-softening.
H. Kimura, PhD Thesis (1978) Tohoku Univ.
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L.A. Davis & S. Kavesh, J. Mater. Sci . 10 (1975) 453.
Fracture surface of Pd77.5Cu6Si16.5 — characteristic vein pattern,
formed by Saffman-Taylor fingering in a liquid-like layer.
The thickness of the liquid-like layer must be at least severalµm.
2500
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0
500
1000
1500
2000
2500
0 0.01 0.02 0.03 0.04 0.05 0.06
True Strain
T r u e S t r e s s [ M P a ]
Vitreloy
0.1 MPa Hydrostatic Pressure
Yield/Fracture Strength = 1986 MPa
εεεεf = 0%
J.J. Lewandowski
At ambient temperature, metallic glasses in tension can appear
macroscopically brittle, despite extensive local deformation in theshear bands.
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Thickness of shear bands?
TEM studies consistently suggest a shear-band thickness of ~10 nm
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M. Chen, A. Inoue, W. Zhang & T. Sakurai: “Extraordinary plasticity of ductile bulk
metallic glasses”, Phys. Rev. Lett. 96 (2006) 245502.
Molecular dynamics simulations
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N.P. Bailey, J. Schiøtz &K.W. Jacobsen, Phys. Rev. B 73 (2006) 064108.
Molecular-dynamics simulations
— also show a shear-band thickness of ~10 nm
Q.-K. Li & M. Li, Appl. Phys. Lett . 88(2006) 241903.
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Operation of Shear Bands
TEM shows that the shear is sharply localized — — thickness of shear band = 10 to 20 nm
The origins of localization remain controversial — structural change, ortemperature rise?
Measurements of temperature rise 0.4 K to 1000 K
Predictions of temperature rise 40 K to 1000 K
Th f ibl ti th d
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The fusible-coating method
The operation of a shear band in a BMG generates a
hot plane and melts the coating (of tin). The total workdone by shear is proportional to the offset δ .
L l lti f ti ti t h b d i th BMG
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Local melting of a tin coating at shear bands in other BMGs
(Cu50Zr50)92Al8 La55Al25Cu10Ni5Co5
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B. Yang, P.K. Liaw, G. Wang, M. Morrison, C.T. Liu, R.A. Buchanan & Y. Yokoyama:
“In-situ thermographic observation of mechanical damage in bulk-metallic glassesduring fatigue and tensile experiments”, Intermetallics 12 (2004) 1265.
Average measuredtemperature rise inshear bands = 0.4 K(for observed width of0.15 mm)
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Resolution of the fusible-coating method
• temporal resolution ≈ thermal diffusion time for coating thicknessincluding latent heat of melting, the resolution ≈ 30 ps
• spatial resolution ≈ scale of islands ≈ 100 nm
In contrast for direct infrared measurements the best reported resolutioncombinations are —
• for imaging 1.4 ms ~11 µµµµm
• for single detector ~10 µµµµs 100 µµµµm
1500
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0
500
1000
1500
-2 -1 0 1 2
T e m p e r a t u
r e R i s e ,
∆ ∆∆ ∆
T
( K )
Distance, x ( µµµµm)
H = 0.4 kJ m-2
H = 2.2 kJ m-2
7
50
1
0.2
10
50
167
1000∆∆∆∆T = 207 K
Minimum observedmelting half-width= 200 nm
Observed melting
half-width = 1 µµµµm
H = 0.4 kJ m
–2 H
= 2.2 kJ m
–2
Distance, x (µµµµm)
Half-profiles oftemperature at a typical
shear band evolving overtime (in nanoseconds)
― calculated fromindependently measuredthermal diffusivity
Profile when tin coating is
melted to maximum width
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How fast are shear bands?
Estimates of the time of shear vary enormously―
― as short as 0.2 ns(estimated from the offset and the speed of sound)
― as long as 100 ms
(estimated from serrations on stress-strain curves)
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DB Miracle, A Concustell, Y Zhang, AR Yavari & AL Greer:Thermal profiles around shear bands in metallic glasses, submitted
Mode III Shear
velocity of propagation V p
(velocity of the shear-band front)≤
90% of transverse sound speed
shear velocity V s(does work and sets local shear
time) V s
= 4γ V p
γ = 0.0267 (Johnson & Samwer)V s = 0.107 V p
Velocities less than ~50% of the maximumwould be insufficient to melt tin coatings
Length scales and times associated with shear-band heating
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[measured (from TEM) thickness of shear band = 10 to 20 nm]
1000thermal diffusion length at end of shear (nm)
1990
upper-bound estimate of temperature rise atshear-band centre (K)
20.8lower-bound estimate of shear duration, δt (ns)
[assumed shear velocity = 206 m s –1]
4.3shear offset calculated from H and τ y (µµµµm)
167time at melting limit (ns)
2.2calculated heat content of shear band, H (J m –2)
1 µµµµmobserved half-width of melted zone
Extreme conditions in shear bands
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Extreme conditions in shear bands
heating rate ≈ 1012 K s –1
shear rate ≈ 1010 s –1
cooling rate ≈ 109 K s –1
How to improve the mechanical properties of BMGs?
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How to improve the mechanical properties of BMGs?
One approach is to make a composite —
• very encouraging results by introducing a ductile crystalline phase into
the glassy matrix
• increases ductility• lowers strength (but better compromise of properties)
Another approach is to control the operation of shear bands within
the glassy phase —
• aim to deflect shear bands
• prevent failure by one dominant band
• proliferation of bands makes deformation more diffuse and increasesenergy absorption
• also reduces shear offset at each shear band → reduces cracking
• possibility of work-hardening?
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How to deflect shear bands or force their proliferation?
• any inhomogeneity in the amorphous phase
• embedded particles
• crystallites (produced by annealing)
• globules of second glassy phase (after phase separation)
• voids (bubbles / porosity)
Partially crystallized glasses
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y y g
— can have crystals smaller or larger than the shear-band thickness
When the crystallites are very small, shear-band operation continueslargely unchanged.
Effects of dispersed nanocrystals
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A. Inoue, W. Zhang, T. Tsurui, A.R. Yavari & A.L. Greer, “Unusual room-
temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass”, Philos. Mag. Lett . 85 (2005) 221.
Glassy Cu-Zr normally
shows a compressive
failure strain of ~1.5%.
As-cast Cu50Zr50, 1 mm
rod, with 10-15 %
volume fraction of 5-10
nm shows much better
plasticity.
True stress / true strain curveTest stopped at 52% strain
Record-breaking toughness in glassy-crystalline composites with
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DC Hofmann et al., Nature (2008) 451, 1085
optimised microstructural scale ―
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Metallic Glasses
• new insights on atomic-level structure
• metallic glasses as structural materials
• current progress on understanding plastic flow
• improving mechanical properties, and new horizons