Optimization process for microstructural assessment of a Co-based superalloy processed via additive manufacturing

Co-based superalloys are a particular class of metallic materials widely used in aerospace industry for the production of jet engine components. These alloys provide great corrosion resistance at high temperatures, this combined with the retention of mechanical properties at elevated temperatures make these alloys the reference materials for applications in aggressive environments subject to high temperatures. The growth of additive manufacturing technologies has allowed these alloys to be used for the production of components of complex geometry, expanding their uses. Co-based superalloys are characterized by a γ phase, which is Face centered cubic (FCC) and an ε phase, which is Hexagonal closed packed (HCP). The HCP phase results in the embrittlement of the material when present in large quantities, so its presence needs to be monitored. This work starts precisely with the aim of studying and developing a chemical etching method able to assess the presence of the HCP phase, ideally with the LOM, in a Co-based superalloy obtained via Direct metal laser melting (DMLM). The method was then used for the microstructural assessment of the material following various steps of heat treatments typical of these alloys. From the development phase of the chemical etching method, carried out on samples specifically treated to induce a high presence of HCP phase, it was found that the best method to highlight the presence of HCP phase is the immersion for 15h in pure HCl. This method allows visualization by darkening the HCP phase. The degree of darkening is higher in some areas than in others, in these areas particularly subject to etching, it is possible to observe the acicular structure of the HCP phase, a condition verified both with LOM and SEM. After the development phase, the method was used for microstructural assessment of the material in the heat treatment steps followed by the alloy. The heat treatments used are intended to bring the alloy from the As-built state to an optimized state before it is put into use. The analysis revealed a microstructural evolution from a highly anisotropic microstructure in the As-built state to a homogeneous and isotropic microstructure, characterized by equiaxed grains, in the final state. In each step of heat treatment only a small amount of HCP phase was found, this makes identification by optical tools, such as LOM and SEM, complex. The results obtained demonstrate that the developed method allows the visualization of the HCP phase on samples where it is present in relevant quantities. It was verified that the HCP phase undergoes significant darkening in some areas where it is also possible to verify the presence of acicular structures typical of the HCP phase. On samples in which the phase is present in small quantities, however, the visualization is more complex. The only limitation found in the development of this method is that it only allows a qualitative assessment of the presence of the HCP phase, but does not allow a precise quantitative analysis using tools such as LOM and SEM, since the degree of etchant attack is not homogeneous and the darkening varies from area to area.