Microstructural development in PWA-1480 electron beam welds — an atom probe field ion microscopy study

The microstructural development in PWA-1480 superalloy electron beam welds (Ni11.0at%Al11.5%Cr1.9%Ti5.1%Co4.0%Ta1.3%W) was characterized. Optical microscopy revealed a branched dendritic structure in the weld metal. Transmission electron microscopy of these welds, in the as-welded condition, showed fine cuboidal (0.05-0.5 μm) L12-ordered γ′ precipitates within the γ grains. The average volume percentage of the γ′ precipitates was found to be ∼ 75%. Atom probe analyses revealed that the composition of the γ matrix was Ni24.6at%Al25.5%Cr0.4%Ti9.4%Co0.8%Ta2.9%W and that of the γ′ precipitates was Ni18.2at%Al2.6%Cr2.4%Ti2.8%Co7.1%Ta1.5%W. These compositions were compared with the previous APFIM analyses of commercial PWA-1480 single crystals that had received conventional heat treatments. Small differences were found in the chromium and aluminum levels and these may be due to the nonequilibrium nature of phase transformations that occur during weld cooling. No solute segregation was detected at the γ-γ′ interface. The APFIM results were also compared with the thermodynamic calculations of alloying element partitioning between the γ and γ′ phases using the ThermoCalc™ program.