Tensile Properties and Deformation Behavior of Several Cast Ni-Based Superalloys Fabricated by Different Solidification Ways

The tensile properties and deformation behavior of several cast Ni-based superalloys, respectively, in the equiaxed, columnar-crystal and single-crystal styles are comparatively studied. The effects of solidification way, heat treatment and strain rate on the tensile properties are discussed in detail. It is found that the reduction of grain boundaries by the feasible solidification ways offers cast Ni-based superalloys the potential capability of improving the mechanical properties, the ultimate achievement of which is also confirmed to lie on the appropriate modifications of chemical composition and heat treatment. The prolongation of solid solution facilitates the precipitation of fine secondary gamma' phase, whereas the extension of high-temperature aging leads to the coarsening of secondary gamma' phase. The combination of these two aspects has a crucial influence on the tensile properties. Under tensile applied stress, the surface grains of DZ-A alloy deform slightly, while the inner grains deform heavily. This deformation inhomogeneity is ascribed to the occurrence of cracks or oblique grains near the surface of specimens and the sliding or decohesion of grain boundaries between the surface and inner grains. Regardless of strain rate, the ILTDM (intermediate-low-temperature ductility minimum) phenomenon always happens in the temperature range from 400 to 600 degrees C in all the investigated alloys, the occurrence of which is closely related to the strong strain-hardening behavior in the deformation process. Finally, the interaction of slip bands which are the main deformation mode below 600 degrees C is established to be the essential reason for the strain hardening.