1. An affordable creep-resistant nickel-base alloy for power plant Franck Tancret Harry Bhadeshia

2. The problem Future power plant: 750°C But: Commercial superalloys are too expensive (Nb, Ta, Co, Mo…) New steels: 650°C => Use of Ni-base alloys => Design an affordable creep-resistant Ni-base alloy

3. Industrial requirements - Affordable - 100 000 h creep lifetime under 100 MPa at 750°C - Stable at service temperature - Forgeable - Weldable - Corrosion resistance - Toughness

4. Design procedure -Empirical nonlinear multiparametric modelling of mechanical properties as a function of composition and processing conditions (Gaussian processes) Based on a huge database on the properties of many existing alloys => captures trends and interactions -Phase diagram and segregation simulation (Thermo-Calc) -Processability -General metallurgy principles Materials Science & Technology, 19 (2003) Ni – 20Cr – 3.5 W – 2.3 Al – 2.1 Ti – 5 Fe – 0.4 Si – 0.07 C – 0.005 B

5. Experimental results

6. Next issue: PROCESSING  Melting and solidification (primary chemical segregation)  Forging (  ’-free temperature window)  Heat treatment (precipiation hardening…)  Welding  How modelling can be used to address these issues?  Is the designed alloy easy to process?

7. Phase diagram simulation (Thermo-Calc) forging window melting No undesirable phases at service temperature

8. Primary segregation simulation (Thermo-Calc) Scheil’s approximation:  homogeneous liquid  diffusion-free solids LIQUID T = T – 1 K SOLID(S) LIQUID  V and composition

9. Primary segregation simulation (Thermo-Calc) Simple dendrite geometrical models => composition profiles sphere cylinder plate

10. Primary segregation simulation (Thermo-Calc) EDS analysis profile

11. Primary segregation simulation (Thermo-Calc)

12. Precipitation hardening kinetics Solutionising 1175°C, WQ Isothermal heat treatments below  ’ solvus, WQ Vickers hardness

13. Precipitation hardening kinetics Diffusion-controlled growth model: 3 Ni + diffusing (Al, Ti) => Ni 3 (Al,Ti) Before ageing: solutionised  During ageing:  ’ C av

14. Precipitation hardening kinetics Diffusion-controlled growth model (Al + Ti) dN(i)dN(i-1)dN p distance from precipitate C(1) = C eq C(2)C(i-1)C(i)C(i+1)……  /  ’ interface surface S adjustable parameter

15. Precipitation hardening kinetics Diffusion-controlled growth model (Al + Ti) C d C av C eq t = 0 C d C av C eq t C d C av C eq t = 

16. Precipitation hardening kinetics Diffusion-controlled growth model (Al + Ti)

17. Precipitation hardening kinetics Diffusion-controlled growth model (Al + Ti)

18. CONCLUSIONS  Design of an affordable Ni-base alloy for power-plant  100 000 h creep lifetime under 100 MPa at 750°C  Corrosion-resistant, stable, forgeable, weldable…  Extensive use of modelling: Mechanical properties (Gaussian processes) Phase diagram simulation - Stability - Forgeability - Weldability - Age-hardening - Solidification segregation (Scheil’s model) Precipitation-hardening kinetics (diffusion model)

19. Present and future work  Multicomponent diffusion growth model  Welding  High temperature ductility / forgeability / recrystallisation…  Corrosion-resistance