Effects of mushy zone tensile stresses on pore nucleation in superalloy 718

摘要

Inadequate feeding of shrinkage during solidification can impose large tensile stresses upon the mushy zone of a casting. These stresses can be relieved by (1) plastic deformation of the dendritic network and/or (2) nucleation of pores in the interdendritic liquid. Analogous to other nucleation processes, heterogreneous nucleation of micropores at non-wetting inclusions in the liquid is expected to be thermodynamically favored over either plastic deformation of the solid or homogeneous nucleation of micropores in the liquid. The stress-induced nucleation of pores in liquid metals is examined in the present paper using a thermodynamic model and compared with experimental mushy-zone tensile data of superalloy 718. The thermodynamic model predicts that a threshold stress should be exceeded before pores are nucleated at a substantial rate. Mushy zone tension tests were performed on superalloy 718 under constant strain rates of 0.0014/sec, 0.0026/sec, and 0.0046/sec. Increasing amounts of liquid in the mushy zone significantly reduced the total failure strain. Interrupted tests showed that the fracture of the experimental samples occurred by a pore nucleation, growth, and coalescence mechanism. The highest strain rates caused sample failure with very low levels of strain, presumably due to stress concentration effects from inhomogeneous deformation.