Phase Identification in Multi-Phase Cu-Zn/Cu-Al Alloys with Macroscopic Concentration Gradients

In order to study concentration invariant phase transitions in Cu-Zn-Al alloys which occur in a very narrow concentration range samples with concentration gradients were produced. Cylinders made of brass and Cu-Al bronze were joined by diffusion bonding and subsequently annealed. The samples exhibit multi-phase regions (α, β, γ1, martensite) with changing compositions and very different chemical properties. Standard techniques for the metallographic preparation of Cu alloys usually target specific phases within limited concentration ranges. To evaluate the microstructure, a two-stage chemical etching method using sodium hydroxide solution and subsequent selective colour etching was successfully used to gain an overview of the phases present in different areas along the concentration gradient. For the characterization of martensite, its birefringent optical properties were utilized. In polarized light, details of the martensitic structure are revealed which would otherwise only be accessible by scanning electron methods.