Functionally graded IN625 and IN738 microstructure and mechanical properties
February 12, 2025
In the Journal of Alloys and Compounds, researchers from MRL Materials Resources LLC, Xenia, Ohio, USA, and Savannah River National Laboratory, Jackson, South Carolina, have published a paper investigating the microstructure and mechanical properties of radially functionally graded (RFG) materials fabricated from nickel-based superalloys IN625 and IN738 using Directed Energy Deposition (DED) Additive Manufacturing.
The researchers successfully fabricated RFG cylindrical deposits with radial gradients of IN625-IN738 and IN738-IN625; these were confirmed to be crack-free through tailored composition grading, scan strategy and process parameters that minimised thermal gradients and solidification velocities. Microstructural analysis revealed distinct variations across the gradient, characterised by differences in dendrite arm spacing, microsegregation, and the formation of distinct γ’ precipitates and carbide/boride phases (MC, M6C, M2C, and MB2) in as-deposited (AD) conditions, which evolved into M23C6 and M2B upon subsequent heat treatment (HT).
EBSD showed predominant columnar grains with a [100] texture, while microhardness profiles demonstrated consistent variation across the gradients. Mechanical testing highlighted the synergy of the graded structure, combining IN738’s strength with IN625’s ductility, resulting in enhanced properties such as a yield strength of 1131 MPa, ultimate tensile strength of 1382 MPa, and elongation of 12 % for HT IN625-IN738 grading. Numerical simulations employing CALPHAD thermodynamic calculations and finite element thermal modelling are used by the team to establish phase evolution and microstructural feature correlation with thermal profiles, aiding in the optimisation of process-structure-property relationship.
‘Directed energy deposition of radially functionally graded nickel based superalloys IN625 and IN738: Microstructure and mechanical properties’ is available here.
