19
Tuning Ductility for Refractory High-Entropy Alloys Sheng Guo (郭晟) Associate Professor Department of Materials and Manufacturing Technology Chalmers University of Technology, Gothenburg, Sweden

Tuning Ductility for Refractory High-Entropy Alloysshengguo.weebly.com/uploads/1/6/9/3/16931630/ichem2016... · 2018. 9. 7. · Conclusions we developed a new ductile RHEA, Hf 0.5

  • Upload
    others

  • View
    0

  • Download
    0

Embed Size (px)

Citation preview

  • Tuning Ductility for Refractory High-Entropy Alloys

    Sheng Guo (郭晟)Associate Professor

    Department of Materials and Manufacturing TechnologyChalmers University of Technology, Gothenburg, Sweden

  • City of Gothenburg

  • Outline

    Why refractory high‐entropy alloys (RHEAs)?What is the current issue with RHEAs? The concept of intrinsic ductility in refractory alloys Using the concept to ductilize RHEAs Conclusions

  • Why refractory high‐entropy alloys (RHEAs)?

    The Hotter the Engine, the Better

    (Perepezko, Science, 2009)

    “The opportunity is that an almost 50% increase in output power could result for operation without auxiliary cooling at about 1300°C.”

    Need for ultrahigh-temperature materials

  • (source: ATI Aerospace)(Perepezko, Science, 2009)

    Jet engine materials

  • Refractory High‐Entropy Alloys (RHEAs): New opportunities?

    Disordered bcc solid solution was reserved after annealing at 1400 oc for 19h

    (Senkov, et al., Intermetallics, 2011)

    460 MPa@1600 oCbetter than superalloys

  • Refractory High‐Entropy Alloys (RHEAs): New opportunities?

    (Miracle & Senkov, et al., Acta Mater, 2016)

  • Nb25Mo25Ta25W25 V20Nb20Mo20Ta20W20

    compression test at 25 oC

    Problems with RHEAs brittleness density oxidation

    (Senkov, et al., 

    Intermetallics, 2011)

    =13.75 g/cm3 =12.36 g/cm3

  • Refractory High Entropy Alloys

    (Wu et al., Mater Lett, 2014)

    Hf25Nb25Ti25Zr25 the first refractory HEAs reporting tensile ductility Hf20Nb20Ti20Zr20Ta20 it shows an impressive compressive ductility and higher

    strength compared to Hf25Nb25Ti25Zr25 tensile ductility has also been confirmed

    (Senkov et al., JAC,2011)

    (Dirras, et al., MSEA, 2016)

    Why?

  • The concept:intrinsically ductile refractory alloys (not HEAs, yet)

    V and Nb fail by shear deformationand they are intrinsically ductile

  • IV V VI

    Mo

    The concept:intrinsically ductile refractory alloys (not HEAs, yet)

    The intrinsic ductility of bcc refractory alloys depends on whether shear instability occurs before the ideal tensile stress is reached. 

    Tensile strain alters the band structures of these alloys and changes the position of the Fermi level. Alloying additions can shift the Fermi level and can therefore tune the critical strain for the shear instability and the corresponding intrinsic failure mode. 

    More specifically, decreasing the number of valence electrons shifts the Fermi level down relative to the band structure, so less strain is required and the shear instability occurs earlier. 

    4 5 6

    Mo: intrinsically brittle (6)MoNb: intrinsically ductile (5.5)MoZr: intrinsically ductile (5)

  • Refractory High Entropy Alloys (revisit) Hf25Nb25Ti25Zr25 the first refractory HEAs reporting tensile ductility Hf20Nb20Ti20Zr20Ta20 it shows an impressive compressive ductility and higher

    strength compared to Hf25Nb25Ti25Zr25 tensile ductility has also been confirmed

    (Senkov et al., JAC,2011)

    IV V VI4 5 6

    NbHfZrTi: ductile (4.25)TaNbHfZrTi: ductile (4.4)TaNbWMo: brittle (5.5)TaNbVWMo: brittle (5.4)

  • Proof of concept: can design ductile RHEAs or not?

    500 m

    seems to work!

    Hf0.5Nb0.5Ta0.5Ti1.5Zr (4.25)

  • Fractured surface of Hf0.5Nb0.5Ta0.5Ti1.5Zr 

  • (Sheikh, et al., JAP, 2016)

    Critical VEC for ductile RHEAs?

    this work

  • Precondition: achieve ductility in single‐phase bcc solid solution

    (Guo et al., Intermetallics, 2013)

    (Guo et al., JAP, 2011)

    get solid solution first,using  and Hmix

    then get bcc solid solution, using VEC

    Also, check the binary phase diagram

  • Proposed compositional space for ductile RHEAs

    (Sheikh, et al., JAP, 2016)

  • Conclusions we developed a new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, with a density of 

    8.13 g/cm3 and a yield stress of 903 MPa, a fracture stress of 990 MPa, and an elongation of 18.8%

    we proposed the mechanism and route for ductilizing RHEAs comprising groups IV (Ti, Zr, Hf), V (V, Nb, Ta) and VI (Cr, Mo, W) refractory elements, using the electron theory

    we ductilized RHEAs by alloying elements from group VI or group V, with elements from group V or group IV, or in other words, by decreasing the number of valence electrons (s+d electrons), in single‐phase bcc solid solutions

    oxidation resistance is still a big concern: long way to go. 

  • Thanks for your attention!Sheng Guo (郭晟)

    Materials and Manufacturing Technology DepartmentChalmers University of Technology

    Gothenburg, SwedenE‐mail: [email protected]