The effect of thermal exposure on the mechanical properties of the directionally solidified superalloy TRW-NASA VIA

The nickel-base superalloy TRW-NASA VIA was studied in the directionally solidified (DS) condition utilizing metallographic and residue analysis techniques in conjunction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure conditions of 1000 h at temperatures from 1500 to 1900°F (816 to 1038°C) were investigated. Four minor phases (two varieties of MC, MgC and M₃B₂) plus gamma-prime were identified in the gamma matrix of the DS material. Significant variations were observed to occur in the mechanical properties with thermal exposure. Microstructural evaluation indicated that as in the equiaxed condition these variations were due principally to gamma-prime agglomeration or ripening. Comparison of the findings from the DS and the equiaxed process conditions indicated several factors which contributed to the property enhancement observed in the DS condition. These included the virtual elimination of the transverse grain boundaries by the DS process which improved the 1400 (760°C) and 1800°F (982°C) properties, the heterogeneous distribution of the blocky and the spherical-like gamma-prime which primarily improved the room temperature and the 1400°F (760°C) strength properties, the generally larger gamma-prime size as well as the intragranular columnar form which improved the ductility properties and the longitudinal grain boundaries containing the stabilized Hf enriched MC and the MgC carbides which in conjunction with the intergranular gamma-prime formation also improved the ductility properties.