technical trends in cemented carbides

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ITIA September 2012

Technical trends in cemented carbides

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Cemented carbides

• One of the most successful powder metallurgy products

• Balance between hardness and toughness: wide range of application

• Cutting tools, wear parts, rock tools, …

• Properties can be tailored within the material (e.g. gradients)

• Combination with coatings

• High performance required

• High added value material

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Cemented carbides: selected technical trendsDesign of microstructure at different scales: from nano to micro

Design of boundaries and interfaces by using adequate inhibitors, doping, …

Adjust morphology of binder phase structure, e.g. fcc/hcp ratios

Control of WC grains shape, size and distribution

Formation of gradients considering element distributions

Use of novel powders in morphology and composition

Use of recycled materials

Consideration of alternative binders to cobalt

Design of interfaces to coatings

Design assisted by modeling at different scales, from ab-initio to FEM

Use of high resolution characterization techniques

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Development of modern cemented carbides

=> Tailor properties at macro-, micro- and nano-scale for outstanding performance in defined applications, i.e. cutting tools, wear parts, rock tools, …

Multiscale modeling

high resolution characterization

Design at micro- & nano-scales

Properties &Applications

Raw materials & processing

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Microstructure design at different scales

• Design at micro- and nano-scales

grain boundaries and interfaces

binder phase structure

shape and distribution of WC grains

gradients

powders morphology and composition

interfaces to coatings

geometries

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Design assisted by modeling

• Use of multiscale modeling for design of microstructure

Models of mechanical properties / Neural networks

Finite element methods

Phase field simulations

Kinetic modeling of microstructure evolution

Thermodynamic predictions

Molecular dynamics

Ab-initio calculations 1 nm

1 μm

1 mm

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Modern characterization

• Use of high resolution characterization methods

HR-Transmission electron microscopy

Atom probe tomography

Synchrotron radiation

Neutron radiation

EBSD 3D tomography

Electron probe microanalysis

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Selected examples• Grain growth inhibitors at grain boundaries

HR-TEM, ab initio modeling, thermodynamic modeling

• Interfaces on WC-Co-Me systems

Atom probe tomography

• Formation of gradients

Thermodynamic and kinetic modeling

• Alternative binders

Thermodynamic and kinetic modeling

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Fine grained cemented carbides: effect of Cr addition to Co-WC

liquid+WC

fcc+WC WC+graphite

Mass-percent Carbon

Tem

pera

ture

Cel

sius

WC-Co

WC-Co-Cr

WC+M6C

Addition of 1.6at% Cr changes the equilibrium temperatures of the phase diagram and also affect the carbon content range

Phase diagram courtesy Susanne Norgren

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Fine grained cemented carbides: effect of Cr addition to Co-WC

WC-Co WC-Co-Cr

Strong grain growth inhibitor effectNo precipitation of Cr-carbides if content Cr below solubility limit J. Weidow, S. Norgren, H.O. Andren RMHM 27 (2009) 817

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Grain growth inhibition: Cr segregation?

0

50

100

150

200

250

300

350

400

450

0 2 4 6 8 10 12keV

CoInterface WC-Co

Co

W

Co

W

W

Cr

5,1 5,3 5,5 5,7keV

CoInterface WC-Co

Bulkbinder

Interface

Cr in Co 4.7% 6%

Probe size = 10 nm

WC-Co,Cr,C

Co15.4 W41.2 C41.8 Cr1.6 (at%)

Courtesy A. Delanöe

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Cr segregation to WC-Co interfaces

Courtesy Lay, DelanöeHigh Resolution TEM image

Co15.4 W41.2 C41.8 Cr1.6 (at%)

Cr segregates to grain boundaries of WC grains

CrC layer of few atoms form at the interface WC/Co

Grain growth inhibition, change of interfacial energies

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Effect of V addition to Co-WC Mechanism of the grain growth control ?

unstable

stable

Neq Nlimit

ΔγM

C

γfilm

∂γfilm

∂N = ΔgMC + eMC

Segregation of V to WC/Co interfaces

Thin cubic carbide layer at the WC/Co interface observed by HR-TEM

V profile

Yamamoto et al. Sci. Techn. Adv. Mater. 1 (2000) 97

S. Lay et al. Adv. Eng. Mater. 6 (2004) 811

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Effect of V addition to Co-WC Mechanism of the grain growth control ?•Can these thin films exist at high temperature liquid phase sintering conditions where a large part of the grain growth occurs?

•At these temperatures and relevant doping conditions VCx is thermodynamically unstable.

Use of ab initio modeling

• Quantum mechanical calculations without experimental data as input parameters

• Limited by short length (nm) and time (ns) scale

• Can be used for:• Thermodynamics e.g. reaction enthalpies

• Interfaces e.g. stable configurations

• Interpreting experiments e.g. calculation of spectra

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Ab initio modeling : metal carbides in WC/Co interfaces• Ab initio calculations can be used to calculate which metal carbide thin films are stable in the

interface between WC and Co

• The metal carbides act as grain growth inhibitors, but also affect the mechanical properties of the cemented carbide

S. Lay et al. J Mater Sci 47 (2012) 1588

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Example: metal carbides in WC/Co interfaces

• Ab initio modeling shows that V containing layers are stable at the interface between Co and WC at liquid

• Films of atoms layers can be designed according to their stability at the interface WC-Co

• Understanding on control of grain size of WC

• Impact on toughness and plastic deformation

S. A. E. Johansson and G. Wahnström Phys Rev B 86, 035403 (2012)

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Interfaces in WC-Me-Co systems

Principle of atom probe tomography

Source: Oxford Materials

• Atomic resolution

• Reconstruction atom by atom

• Ideal method to study interfaces at nano scales

Interfaces between carbides and carbide-metal systems affect the properties of cemented carbides; need of high resolution technique to investigate interfaces

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WC/ and WC/WC interface in WC-TaC-Co2.3M atoms, z = 107 nm, d = 38 nm

WC

(Ta,W)C

SPECIMEN APT resultCourtesy J. Weidow

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WC/binder interface in WC-TaC-Co16.2M atoms, z = 178 nm, d = 122 nm

WC Co

Ta segregation

Co based binder

WC

Ta segregation at WC-binder interface

J. Weidow, H.-O. Andrén RMHM 29 (2011) 38

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/ interface in WC-TaC-Co

Layer consists ofCo 9446Cr 266Fe 34P 72Total 9818

2.7M atoms, z = 70 nm, d = 50 nm

Corresponds to 0.7 atom layer segregant atoms

(Ta,W)C

Co segregation at - interface

J. Weidow, H.-O. Andrén RMHM 29 (2011) 38

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WC/WC interface in WC-TaC-Co

Layer consists ofCo 2409Cr 63Fe 9Total 2481

Corresponds to 1.1 atom layer segregated atoms

5.9M atoms, z = 69 nm, d = 89 nm

Co

Ta

WC

No Ta segregation at WC-WC interfaceJ. Weidow, H.-O. Andrén RMHM 29 (2011) 38

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Summary of atom probe results at interfaces

Segregation of elements depends on element and type of interface

• Ti, V, Cr, Mn and Ta segregate to WC/binder phase boundaries.

• Segregation of V corresponds to approximately one monolayer of close packed VC.

• Segregation of Ti, Cr, Mn, Zr, Nb and Ta corresponds to a thin film with a thickness smaller than one monolayer assuming a MC structure.

• Co, Ti, Nb, Zr, Cr, Fe, segregate to WC/WC grain boundaries.

• Ta and Ni not observed to segregate to WC/WC grain boundaries

• Co, Fe segregate to /WC phase boundaries, where =fcc-MC and M = Ti, Zr, Nb, Ta.

• Co and Fe, segregate to (Ta,W)C/(Ta,W)C grain boundaries.

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Kinetic simulations of gradient formationDICTRA software coupled to ThermoCalc

nkj

nkjeff DfD )(

zc

DJ jn

j

nkjk

1

1

RTQ

RTMM kk

k exp0

Mko: frequency factor

Qk: activation energy factor

Labyrinth factor

law relating flux and concentration gradient given by the multi-component extension of Fick’s first law

aN atm = 0

JTi

JN

moving interface

FCC-freelayer

bulk

vacuumatmosphere

aN bulk > 0

aN atm = 0

JTi

JN

moving interface

FCC-freelayer

bulk

vacuumatmosphere

aN bulk > 0

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Kinetic simulations of gradient formation

Dictra simulation of fcc-free layer formation at 1450ºC and 2 h vacuum sintering: same mobility for all elements Modeling: 35 µm, Experimental: 20 µm

=> gradient kinetics too fast

Previous investigations [Ekroth et al. Acta Mat. 48 (2000) 2177] assumed the same mobility for all elements (W, Ti, Ta, Nb, N, C)

0.385.592.212.342.708.0balance

NCNbTaTiCoW

0.385.592.212.342.708.0balance

NCNbTaTiCoW

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Metallic element mobilities=> 2 times slower than that of C and N

Best fit with experimental results (thickness of -free layers and phase distributions)

okk MRTQM lnMobilities of the different elements in the cobalt binder phase at the sintering temperature must be optimized

J.Garcia, et al. RMHM 29 (2011) 256

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Modeling of kinetics of -free graded layer formation at different sintering conditions

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Raw materials: alternative binders

• Co => wettability to WC

=> low stacking fault energy (~20 mJ/m2) => formation SF (partial Shockley dislocations)

=> strengthening effect

• Ni => higher stacking fault energy (~125 mJ/m2) => formation of twins

=> reduced strength compared to cobalt

• Fe-Ni-Co => compositions with low SFE (”similar” properties to Co) => Invar alloys (Fe-36Ni)

=> / transformation, commercial alloys, e.g.70Fe-20Ni-10Co

=> C-control / C-solubility, => Austenite, Martensite, Bainite, …

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Raw materials: alternative binders• Binder is the “transport media” for the diffusion process in the formation of -free gradients

• How is the -free gradient formation influenced if we change the binder composition?

J.Garcia, RMHM 29 (2011) 306

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Raw materials: alternative binders• Thermodynamic predictions of N solubility on Fe-Ni-Co liquid binders (1450°C)

0

1

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3

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5

6

7

8

10-5

MA

SS_F

RA

CTI

ON

N

0 20 40 60 80 100MASS_PERCENT CO

0

1

2

3

4

5

6

10-4

MA

SS_F

RA

CTI

ON

N

0 20 40 60 80 100MASS_PERCENT FE

0

1

2

3

4

5

6

10-4M

ASS

_FR

AC

TIO

N N


A) B) C)

liquid liquid liquid

Liquid + N(g) Liquid + N(g) Liquid + N(g)

0

1

2

3

4

5

6

7

8

10-5

MA

SS_F

RA

CTI

ON

N


0

1

2

3

4

5

6

10-4

MA

SS_F

RA

CTI

ON

N


0

1

2

3

4

5

6

10-4M

ASS

_FR

AC

TIO

N N


A) B) C)

0

1

2

3

4

5

6

7

8

10-5

MA

SS_F

RA

CTI

ON

N


0

1

2

3

4

5

6

10-4

MA

SS_F

RA

CTI

ON

N


0

1

2

3

4

5

6

10-4M

ASS

_FR

AC

TIO

N N


A) B) C)

liquid liquid liquid

Liquid + N(g) Liquid + N(g) Liquid + N(g)

=> Fe-containing binders has a much higher solubility of N compared to pure Co and Ni

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Raw materials: alternative binders

=> For same sintering conditions, addition of Fe to Co-binders leads to the formation of thicker gradient layers

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Outlook• Design of microstructure with „tailored“ properties depending on application

• Use of high resolution characterization techniques

• Thermodynamic and kinetic modeling

• Complex interaction between different processes

• Deep understanding of metallurgy for prediction of microstructure formation

Acknowledgements• Chalmers, Göteborg, Sweden, First principles calculations

• Chalmers, Göteborg, Sweden, Microscopy, microanalysis

• SIMAP, Grenoble, France, Electron microscopy

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www.sandvik.com

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technical trends in cemented carbides

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