Nickel-based superalloys

CALPHAD based computational tools allow for accurate analysis even on complex materials such as Ni-based superalloys, which typically contain 10 or more alloying elements.

Ni-based superalloys are an alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures, good surface stability and fatigue, resistance to oxidation and hot corrosion. The nickel–aluminium system is the binary basis for Ni-based superalloy compositions. As the amount of aluminium added is large enough, an ordered L12 phase (γ’) forms from the FCC matrix (γ) with the nominal composition of Ni3Al. Today's superalloys can also be based on cobalt or nickel-iron. All these kinds of alloys usually contain at least 10 alloying elements, with each one being added for a specific purpose. Due to this complexity in chemistry, it has traditionally taken a long time to optimise properties of existing alloys and to develop completely new alloys.

Thanks to computational thermodynamics or the so-called CALPHAD technique, it is now possible to perform accurate calculations even for these rather complex alloys, and thereby speed up the development time substantially. Some examples of what can be predicted are shown below.  All of them provide you with a better, more cost effective and faster way to tailor materials chemistry, optimise heat treatment schedules, enable quality assurance and so forth.

Some examples of what can be predicted:

  • Partitioning of alloying elements between γ and γ’ phases
  • Amount of phases at varying temperatures
  • Phase compositions and site occupancy within a crystal structure
  • γ’ solvus temperatures
  • Liquidus temperatures
  • Isothermal or vertical section phase diagrams
  • Avoidance of TCP phases
  • Minimise the γ/γ’ lattice mismatch
  • Lattice parameters for e.g. the disordered FCC phase
  • Densities of separate phases or the alloy itself
  • Scheil-Gulliver non-equilibrium solidification path
  • Thermal barrier coating

Ni-based superalloys have excellent strength and creep resistance, but in many applications a protective coating is required. During service, the coating degrades mainly due to the interdiffusion between the coating and substrate. It is therefore important to be able to study the interdiffusion fluxes occurring between the coating and substrate in order to optimise the lifetime of the coating in a specific application. The thermodynamically compatible kinetic data are fundamental in such a simulation using the Diffusion module (DICTRA). 

Further examples utilising the Diffusion module (DICTRA):

  • Interdiffusion in coating/substrate systems
  • Diffusion in ordered γ’ and B2 phases
  • Growth or dissolution of minor phases, such as TCP phases and carbides
  • Microsegregation during solidification
  • Homogenisation during heat treatments

Superalloys develop high temperature strength through solid solution strengthening. The most important strengthening mechanism is through the formation of secondary phase precipitates such as γ’ and carbides. Formation of the γ’ phase occurs in the solid state as the supersaturated solid solution of the matrix phase is cooled below its equilibrium solvus temperature. Hence, the precipitation and growth kinetics of the γ’ phase are highly sensitive to the rate at which the alloy is cooled through the solvus temperature. By using the Precipitation module (TC-PRISMA) and combining both thermodynamic and kinetic databases, typical concurrent precipitation in superalloys can be simulated under arbitrary heat treatment conditions. Some examples of outcoming results when studying γ’ and/or carbide precipitation are:

Examples related to precipitation modelling using the Precipitation module (TC-PRISMA):

  • Concurrent nucleation, growth/dissolution and coarsening of precipitates
  • Temporal evolution of particle size distribution
  • Average particle radius and number density
  • Volume fraction and composition of precipitates
  • Nucleation rate and coarsening rate
  • Time-Temperature-Precipitation (TTP) diagrams
  • Estimation of multicomponent interfacial energy

Thermodynamic and kinetic databases developed using the CALPHAD method have been successfully applied to the modeling and simulation of Ni-based superalloys for more than twenty years.

MOBNI4 is the specialised Nickel-based superalloys database and is intended for use with the TCNI8 database.