Franck Tancret Harry Bhadeshia An affordable creepresistant nickelbase

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

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

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

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 Ni – 20 Cr – 3. 5 W – 2. 3 Al – 2. 1 Ti – 5 Fe – 0. 4 Si – 0. 07 C – 0. 005 B Materials Science & Technology, 19 (2003)

Experimental results

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

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

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

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

Primary segregation simulation (Thermo-Calc) EDS analysis profile

Primary segregation simulation (Thermo-Calc)

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

Precipitation hardening kinetics Diffusion-controlled growth model: 3 Ni + diffusing (Al, Ti) => Ni 3(Al, Ti) Cav Before ageing: solutionised g During ageing: g / g’

Precipitation hardening kinetics Diffusion-controlled growth model (Al + Ti) C(1) = Ceq C(2) … d. Np C(i) d. N(i-1) surface S g / g’ interface C(i-1) adjustable parameter distance from precipitate C(i+1) d. N(i) …

Precipitation hardening kinetics C Diffusion-controlled growth model (Al + Ti) C Cav t t=0 Ceq d d C Cav t= Ceq d

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

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)

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