Superalloys – Life Cycle Design

Nickle base superalloy
Nickle base superalloy

Superalloys are used in high performance and demanding environments cherished because of their superior strength, temperature capability and environmental survivability as compared to ordinary (moderately alloyed).

You will find these used in cases when stainless and low alloy steels just won’t meet performance requirements

There are mainly three types of these in materials engineering

  • Iron Base Superalloys – Ni, Cr etc enhancements to stainless steel compositions. Chromia scale formation to protect against aqueous corrosion and oxidation. They exhibit moderate improvement in elevated temperature strengths.
  • Nickel-Base Superalloys – Ni is the highest single element content but significant amounts of Cr Co etc as well as Fe. They come in with enhanced corrosion and oxidation (& “Hot Corrosion”) resistance. Increased high temperature strength and especially creep and thermal fatigue resistance by additions of Al and Ti. A highly engineered class of materials. They are seen applied in aero and land based turbines owing to their Strength at High temperatures, and excellent Oxidation and/or Corrosion properties and resistance. What Gives these superalloys their excellent properties lies in their microstructure as we see below
    Microstructure-of-Nickle-base-superalloy
    Microstructure-of-Nickle-base-superalloy

     

  • Finally Cobalt-base Superalloys – Co plus Cr and Ni plus C and other elements. Originally used for superior oxidation resistance. Now mainly used for hard wearing applications … and medical applications!

It’s clear from the foregoing that their design and integration into systems requires a deep understanding of their chemistry and structures

They can be prone to failure however as a result of corrosion and stress corrosion cracking behavior (sometimes spurred on by chemical or heat conditions or both) seen for example in nickel base superalloys.

They can strengthened by engineered heat treatment practices or indeed forging. This can mean differing properties in service something that results from the properties of the eventual engineered microstructures. This important to understand in order to develop optimized high temperature capable structural alloys that are used to develop material properties in precipitation hardened Nickel base alloys.

When Problems develop, remedial approaches that can be used include:

  • Reapplying the solution treatment
  • Dissolving coarsened γ’ and GB carbide
  • Reprecipitating as heat treated γ’ and GB carbide structures

Among other remedial approaches is

  • Weld repair in allowable sections
  • Reheat treatment which even when it may not restore full initial life expectation in DS material but can ensure continued value in fractional life
  • Some cosmetic (and airflow) repair of SC components have been developed