Metallic Glasses

----SHRAVANA KATAKAM

Copper

Gold

Stone age

Iron

Bronze

Bronze age

Iron age

Steels Cast Iron

Super Alloy Alloys Steels Light Ti-, ZrAlloys Alloys

Steel technologies

Special Steels and New superalloys

Glassy Metals

Age of advanced materials

Glass (amorphous)

Crystalline

The viscosity increases with undercooling until the liquid freezes to a glass

‚bulk‘ metallic glass ‚conventional‘ metallic glass

Polymers

Critical cooling rate K/s

Max. thickness

Oxide glass (ceramics)

2500

Stress,  MPa)

Stress-stress curve 2000

Bulk metallic glass

1500 1000

Stainless steel

500 0 2%

0,5 %

Strain, 

Stored energy: area under stress/strain curve

Ur=y2/2E

Forming PtNiCuP T= 270C P=10 MPa 200 sec Tprocess

Tprocess

J. Schroers, JOM, 57, 34 (2005))

•Suppress

the nucleation and growth of crystalline

phases •Initial Rc was around 102-4 •New multi component alloys were developed for Rc aroun 0.1- 100k/s •New alloys developed have reduced glass transition Tm/Tg and wide supercooled region ΔTx (Tx-Tg) and these are the imp factors for GFA

Multicomponent eutectic alloys with strong negative heat of mixing

Large size mismatch between the components (destabilization of the crystalline phases) Zr41.2Ti13.8Cu12.5Ni10Be22.5

Tang et al., Nature 402, 160 (1999)

P. Haasen, Physical Metallurgy 3rd ed

•Three

emperical rules for good

GFA 1) Alloy system should contain more than three elements 2) Significant difference in atomic sizes 3) Components should exhibit negative heats of mixing •The models for GFA are mainly categoriesed into thermodynamics aspect, Kinetics aspect and Structural aspect

should be low for liquid to crystalline transformation •ΔG= ΔHf - ΔSf •Either ΔHf should be low or ΔSf should be high •When a multicomponent system is chosen, the entropy of the system increases since entropy is proportional to the microstates of the system. •At constant temp ΔG decreases due to low chemical potential caused by low ΔHf and high Tg/Tm and high liquid solid interfacial energy •Hence in a multicomponent system the ΔSf increases increasing the random dense packing and decreasing the ΔHf increasing the interfacial energy. •ΔG

I(T) = A ∙ T ∙ D ∙ exp (DG*/RT) with: D1/h : interfacial energy DG: driving force DG*: nucleation barrier r*: critical radius

r*2/DG

DG*=(16p3/3DGl-s2) f(θ

•Crystallization

kinetics are important to study •Parameters like viscocity and diffusivity of constituent atoms decied the GFA • In case of good GFA the viscocity increases due to the sluggish kinetics of the constituent atoms •As the mobility of atoms decreases the nucleation tendency of the crystalline phases reduces increasing the Glass forming ability

•Significant

difference in sizes result in high packing density which increases the solid-liquid interfacial energy and low diffusivity along with high viscosity. •XRD data is very similar to the XRD data of liquid phase • It has been confirmed for different alloy systems that the structure contains densely packed random structures •It is also confirmed by the density measurements of the multi-component systems compared with their crystalline counterparts •The atomic distances and coordination number change was observed after crystallization

•The

CCT curves determine the Rc (critical cooling rate) •Rc determines the GFA of the material The Rc decrease by using fluxing technique-[Chen 1978, Kui et al. 1984,1985)] •Rc

also depends on the type of moulds used •From the above two aspects it can be concluded that the Rc will depend on the amount of heterogeneous nucleation

“Conventional” metallic glasses (1959-1984) a) AuSi NbNi ZrCu critical cooling rates: 105106 K/s

FeB etc…..

Bulk metallic glasses b) Ni-Pd-P [Turnbull, Drehman, Greer (1984)] La-Al-Ni [Inoue and co-workers (1991)] Mg-Cu-Y Zr-Al-Ni-Cu critical cooling rate: 100 K/s etc….. c)

Zr-Ti-Cu-Ni-Be [Johnson and co workers (1993)]

Zr-Ti-Al-Cu-Ni K/s

etc……

critical cooling rate: 1

from: W.L. Johnson, Materials Research Society Bulletin, October 1999

•For

multicomponent system a new supercooled liquid •Densely short range ordering and weak long range interactions due to different atomic sizes and high negative heat of mixing •High solid liquid interfacial energy preventing the growth of the solid

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Introduction Atomic structure Free volume Shear Transformation Zones Energy theory Strength vs Tg Yielding and Temp rise Ductility of Metallic Glasses Elastic Theory Energy criteria Plastic Deformation

Dense Randomly Packed Model Solute Centered Model Icosahedral(widely accepted)

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Total volume =Densely packed structure+ Free volume FV are the sites for structure destablization by Temp or Shear Responsible for Shear Localization

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Shear Deformation is Linear in liquids In solids a cluster of atoms move Results in internal stresses Analogous to nucleation of dislocation loop

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Strength of metallic glasses if related to physical and chemical properties of atoms The strength decreases on shear due to local reduction in viscocity near shear band as in case of Tg The relation between Tg and Strength is calculated for diff alloys

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Deformation Proceeds thr sliding of Shear band Temp is raised during sliding of shear band Microstructure has a typical vein pattern The temp raise in shear bands is equal to Tg hence low viscocity resulting in failure.

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Metallic Glasses high strength but no ductility Shear band initiate strain softening hence decrease in global plasticity Ductility depends on chemical composition and heat tratment Slight change in chemical composition may lead to uctile to brittle transition By annealing below recryatallizaton also shows britte nature hence its proved that ductility depends on chemical compo and physical proprties

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Two modes of deformation possible when stress is applied Normal stress resulting in brittle fracture Shear stress resulting in ductile fracture The bulk modulus B is the measure of Normal stress The shear modulus G is the measure of shear stress The G/B ratio determines which mode is operative

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At the onset of Shear band formation the applied force must be equal to the potential barrier for nucleation Hence a low potential is the charachteristic of good ductility The potential required for nucleation of shear can be calculated by calculating work done during the indentation o nucleate a shear band

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In crystalline materials many dislocations and their interactions cause ductility In MG the nucleation and propagation of Shear band are responsible are governing factors for plasticity Low potential can easily nucleate a shear band but this alone is not sufficient due to strain softening of shear band which cannot sustain a global plasticity The formation of multiple shear band results in the arrest and avoid catastrophic failure due to sliding of single shear band resulting in global plasticity. Factors causing arrest of shear bands are

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Inhomogneity may be caused due to phase separation, spinodal decomposition etc The inhomogneity nucleates a shear band and thus as the inhomogneity increases multiple shear bands are generated

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In some MG the deformation produces insitu nanocrystallization Nano crystallization along the shear band increases the global plasticity considerable The viscosity of the glass increases exponentially with the presence of nano crystals compensating viscosity decrease due to strain softening resulting global plasticity Insitu nano crystallization is not only due to rise in temp since the incubation period for crystallization is much more than shear band nucleation and propagation This phenomenon is not observed in all glasses but only in high GFA glasses which indicates that the configuration of atoms plays a role

Transmission electron microscopy (TEM)observations of shear bands in a deformed Zr50Cu50 BMG. (a) Bright-field TEM image of a narrow band with lighter contrast across the thin region of the TEM specimen. (b) Low-magnification high-resolution electron microscopy (HREM) image of a shear band with precipitate Nano particles.

a) Strain softening induces formation of nano crystals to form b)STZ also grow along with nano crystals c)STZ coalase to form SB and the nano crystal absorbs stress in matrix and undergoes twinnning.

a) Nano crystals after fracture surface b),c),d) TEM images showing the formation of twins in nano crystals

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Work hardening is believed to be due to exhaustion of shear bands After the arrest of shear bands by nano crystals inhomogneity etc new shear bands are to be nucleated The new shear bands are nucleated relatively at difficult nucleation sites To maintain a constant shear rate the rate of stress will be apparently more.

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Effect on alloying additions Laser processing of amorphous materials Influence of laser parameters Suggestion of a system with background theory

Questions??