Technical trends in cemented carbides. ITIA September 2012

Transcription

1 Technical trends in cemented carbides ITIA September

2 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 2

3 Cemented carbides: selected technical trends Design 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 3

4 Development of modern cemented carbides Design at micro- & nano-scales Multiscale modeling Raw materials & processing high resolution characterization Properties & Applications => Tailor properties at macro-, micro- and nano-scale for outstanding performance in defined applications, i.e. cutting tools, wear parts, rock tools, 4

5 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 5

6 Design assisted by modeling Use of multiscale modeling for design of microstructure Models of mechanical properties / Neural networks 1 mm Finite element methods Phase field simulations Kinetic modeling of microstructure evolution 1 μm Thermodynamic predictions Molecular dynamics Ab-initio calculations 1 nm 6

7 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 7

8 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 8

9 Fine grained cemented carbides: effect of Cr addition to Co-WC Temperature Celsius liquid+wc WC-Co WC-Co-Cr WC+ M6C fcc+wc WC+graphite Mass-percent Carbon 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 9

10 Fine grained cemented carbides: effect of Cr addition to Co-WC WC-Co WC-Co-Cr Strong grain growth inhibitor effect No precipitation of Cr-carbides if content Cr below solubility limit J. Weidow, S. Norgren, H.O. Andren RMHM 27 (2009)

11 Grain growth inhibition: Cr segregation? W Co Co Interface WC-Co Co15.4 W41.2 C41.8 Cr1.6 (at%) W Probe size = 10 nm 200 Co W WC-Co,Cr,C 50 Cr kev Co Interface WC-Co 5,1 5,3 5,5 5,7 kev Courtesy A. Delanöe Bulk Interface binder Cr in Co 4.7% 6% 11

12 Cr segregation to WC-Co interfaces 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 High Resolution TEM image Courtesy Lay, Delanöe 12

13 Effect of V addition to Co-WC Mechanism of the grain growth control? Segregation of V to WC/Co interfaces V profile Thin cubic carbide layer at the WC/Co interface observed by HR-TEM unstable stable γ film N eq N limit ΔγMC γ film N = Δg MC + e MC S. Lay et al. Adv. Eng. Mater. 6 (2004) 811 Yamamoto et al. Sci. Techn. Adv. Mater. 1 (2000) 97 13

14 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. Quantum mechanical calculations without experimental data as input parameters Use of ab initio modeling 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 14

15 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)

16 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, (2012) 16

17 Interfaces in WC-Me-Co systems Interfaces between carbides and carbide-metal systems affect the properties of cemented carbides; need of high resolution technique to investigate interfaces Principle of atom probe tomography Atomic resolution Reconstruction atom by atom Ideal method to study interfaces at nano scales Source: Oxford Materials 17

18 WC/ and WC/WC interface in WC-TaC-Co 2.3M atoms, z = 107 nm, d = 38 nm WC (Ta,W)C SPECIMEN APT result Courtesy J. Weidow 18

19 WC/binder interface in WC-TaC-Co 16.2M atoms, z = 178 nm, d = 122 nm Co based binder Ta segregation at WC-binder interface Ta segregation WC W C Co J. Weidow, H.-O. Andrén RMHM 29 (2011) 38 19

20 / interface in WC-TaC-Co (Ta,W)C 2.7M atoms, z = 70 nm, d = 50 nm Layer consists of Co 9446 Cr 266 Fe 34 P 72 Total 9818 Corresponds to 0.7 atom layer segregant atoms Co segregation at - interface J. Weidow, H.-O. Andrén RMHM 29 (2011) 38 20

21 WC/WC interface in WC-TaC-Co 5.9M atoms, z = 69 nm, d = 89 nm WC Layer consists of Co 2409 Cr 63 Fe 9 Total 2481 Corresponds to 1.1 atom layer segregated atoms Co No Ta segregation at WC-WC interface Ta J. Weidow, H.-O. Andrén RMHM 29 (2011) 38 21

22 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. 22

23 Kinetic simulations of gradient formation DICTRA software coupled to ThermoCalc FCC-free layer a Nbulk > 0 J k n 1 j 1 D n kj c j z law relating flux and concentration gradient given by the multi-component extension of Fick s first law vacuum atmosphere J N J Ti D n kjeff ( f ) D n kj Labyrinth factor a Natm = 0 moving interface bulk M k 0 M k RT Q exp RT k M ko : frequency factor Q k : activation energy factor 23

24 Kinetic simulations of gradient formation Previous investigations [Ekroth et al. Acta Mat. 48 (2000) 2177] assumed the same mobility for all elements (W, Ti, Ta, Nb, N, C) W Co Ti Ta Nb C N balance 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 24

25 Kinetic simulations of gradient formation Mobilities of the different elements in the cobalt binder phase at the sintering temperature must be optimized M k Q k RT ln M o Metallic element mobilities => 2 times slower than that of C and N Best fit with experimental results (thickness of -free layers and phase distributions) J.Garcia, et al. RMHM 29 (2011)

26 Kinetic simulations of gradient formation Modeling of kinetics of -free graded layer formation at different sintering conditions 26

27 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, 27

28 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)

29 Raw materials: alternative binders Thermodynamic predictions of N solubility on Fe-Ni-Co liquid binders (1450 C) MASS_FRACTION N MASS_FRACTION N 4 5 Liquid + N(g) Liquid + N(g) Liquid + N(g) liquid 1 liquid 1 liquid MASS_PERCENT FE MASS_PERCENT FE MASS_PERCENT CO A) B) C) => Fe-containing binders has a much higher solubility of N compared to pure Co and Ni MASS_FRACTION N

30 Raw materials: alternative binders => For same sintering conditions, addition of Fe to Co-binders leads to the formation of thicker gradient layers 30

31 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 31

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