Toward the Understanding of CoAl2O4 Additions on the Formation of Microstructure in Alloy 718 Processed by Laser Powder Bed Fusion

Metallurgical and Materials Transactions A - This investigation rationalizes the impact of oxide particles on the melt pool and subsequent formation of microstructure in superalloy Inconel 718...     

The Microstructure Stability of Precipitation Strengthened Medium to High Entropy Superalloys

Metallurgical and Materials Transactions A - Medium and high entropy superalloys based on the Ni-Co-Fe system with strengthening L12 γ′ precipitates have been developed. The present...     

Understanding the Effects of CoAl2O4 Inoculant Additions on Microstructure in Additively Manufactured Inconel 718 Processed Via Selective Laser Melting

The effect of varying amounts of CoAl₂O₄ inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl₂O₄ resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl₂O₄ content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al₂O₃ particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl₂O₄ as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

     

On the Solidification and Phase Stability of a Co-Cr-Fe-Ni-Ti High-Entropy Alloy

In the present study, a Co1.5CrFeNi1.5Ti0.5 high-entropy alloy has been investigated for its high-temperature microstructural stability. This material is shown to possess mainly a face-centered cubic (FCC) structure; the η phase is present at the interdendritic region in the as-cast condition, and it is stable between 1073 K and 1273 K (800 °C and 1000 °C); γ′ particles are found throughout the microstructures below 1073 K (800 °C). Segregation analysis has been conducted on a single crystal sample fabricated by a directional solidification process with a single crystal seed. Results show that Co, Cr, and Fe partition toward the dendritic region, while Ni and Ti partition toward the interdendritic areas. Scheil analysis indicates that the solid–liquid partitioning ratio of each element is very similar to those in typical single crystal superalloys.     

On the creep and phase stability of advanced Ni-base single crystal superalloys

The present article examines microstructure stability and creep resistance of a 5th generation superalloy, which has Cr content at 4.6 wt%, 6.4 wt% Re and 5.0 wt% Ru, in comparison with that of a 4th generation superalloy (3.2 wt% Cr, 5.8 wt% Re and 3.6 wt% Ru). The aim is to elucidate the implication of increasing Cr, Re and Ru contents for future alloy developments. Experimental results have concluded that high Re + Ru content could promote formation of hexagonal δ phase at 900 °C; additional Cr and Re could enhance the precipitation of TCP phase at 1100 °C. Although an increase in lattice misfit between γ and γ′ in the 5th generation superalloy could strengthen the alloy against creep deformation under conditions at high temperatures (≥1000 °C) and low stresses (≤245 MPa) whilst the microstructural stability remained, the tendency to raft should be avoided during creep at lower temperatures and higher stresses.