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Microstructure of Nickel, Cobalt and their Alloys

Lucas’ Reagent

  • 50 mL lactic acid
  • 150 mL HCl
  • 3 g oxalic acid

Lucas’s reagent for Fe-Ni, Ni- and Co-base superalloys. Use electrolytically at 1-2 V dc, for 10-20 s. Electrolyte can be made as a stock solution.

Molybdic Acid Reagent

  • 100 mL water
  • 100 mL HCl
  • 100 mL HNO3
  • 3 g molybdic acid

Molybdic acid reagent to reveal the dendritic structure of cast Ni-base superalloys. Mix and alloy to stand >1 h before use. Immerse specimen for several seconds. Can be stored.


Metallography of Superalloys


Superalloys are complex alloys of Fe-Ni, Ni, or Co-base compositions. Their microstructure can be quite complex due to the potential for a variety of phases that can be formed by heat treatment or service exposure conditions. The paper presents the use of new metallographic materials to prepare these alloys with emphasis on modern, four- and five-step practices. Different etchants are required to reveal the structure of these alloys properly. Examples will be presented showing the use of different etchants as a function of alloy composition, heat treatment, and microstructural phases.


Metallographic Techniques for Superalloys


Preparation of superalloys for microstructural examination is not exceptionally difficult. The procedures are similar to those used to prepare stainless steels. Because they are facecentered cubic “austenitic” alloys with exceptionally good toughness, machinability is poorer than for steels and the age hardened alloys, especially the cast alloys, can be more difficult to section than most steels when they have a very high ‘ content. FCC metals readily deform and work harden, consequently aggressive sectioning methods (e.g., power-hacksawing or band sawing) will introduce considerable damage which can be very difficult to remove in the subsequent preparation steps. If these procedures must be used, it is advisable to re-section the material with the correct abrasive cutoff wheel (consumable type) with abundant cooling.