Influence of composition on microstructural parameters of single crystal nickel-base superalloys

Fourteen nickel-base superalloy single crystals containing a range of chromium (Cr), cobalt (Co), molybdenum (Mo), and rhenium (Re) levels, and fixed amounts of aluminum (Al) and tantalum (Ta), were examined to determine the effect of bulk composition on basic microstructural parameters, including γ′ solvus, γ′ volume fraction, topologically close-packed (TCP) phases, γ and γ′ phase chemistries, and γ–γ′ lattice mismatch. Regression models describing the influence of bulk alloy composition on each of the microstructural parameters were developed and compared to predictions by a commercially-available software tool that used computational thermodynamics. Co produced the largest change in γ′ solvus over the wide compositional range explored and Mo produced the biggest effect on the γ lattice parameter over its range, although Re had a very potent influence on all microstructural parameters investigated. Changing the Cr, Co, Mo, and Re contents in the bulk alloy had an impact on their concentrations in the γ matrix and to a smaller extent in the γ′ phase. The software tool under-predicted γ′ solvus temperatures and γ′ volume fractions, and over-predicted TCP phase volume fractions at 982 °C. However, the statistical regression models provided excellent estimations of the microstructural parameters and demonstrated the usefulness of such formulas.