The effect of boron and zirconium on wrought structure and γ-γ′ lattice misfit characterization in nickel-based superalloy ATI 718Plus

The effects of boron and zirconium additives on the wrought structural characteristics of the nickel-based superalloy ATI 718Plus, including grain size, delta and γ′ phases, and γ-γ′ misfit were investigated. For this purpose, five alloys with different contents of boron (0.00-0.016 wt%) and zirconium (0.0-0.1 wt%) were cast via the VIM and then purified via the VAR. After the rolling process and the heat treatment cycles, the microstructural investigation by OM, FESEM-EDS and phase analysis by the XRD analysis was performed. Although the results indicated that boron and zirconium had a slight effect on the reduction of grain size, delta phase was increased by 41% and 50% in the presence of these elements, respectively. In fact, boron and zirconium decreased in time and increased the temperature of delta phase nucleation by shifting the nose of the TTP diagram obtained for that to the upper left. The XRD analysis results revealed an increase in the lattice constant of the γ and γ′ phases as well as constrained and unconstrained lattice misfit of them in the presence of boron and zirconium. The maximum γ-γ′ misfit was determined for the alloy with the maximum boron and zirconium content (16 B100Z) as −0.3424 (constrained) and −0.9821 (unconstrained). In addition, the coherent elastic strain between γ and γ′ phases, increased to a higher positive value in the presence of these elements.