Abstract:

Some of the earliest Ni-base superalloys were derived from alloys containing additions of Cr, Cu, Co and Fe, with properties superior to those of stainless steels, these single-phase Ni-base alloys exhibited good high-temperature strength and resistance to environmental degradation, like aerospace industry, chemical and the pulp and paper industry. Some of the important requirements for high-temperature structural materials are high-temperature tensile strength, creep rupture strength and oxidation resistance. Significant additions of molybdenum make Ni-Cr-Mo alloys highly resistant to pitting. They retain high strength and oxidation resistance at elevated temperatures, but they are used in the chemical industry primarily for their resistance to a wide variety of aqueous corrosives. As the levels of refractory alloying additions to these Ni-base superalloys increases to enhance high-temperature mechanical properties, grain defect formation, particularly the development of porosity and freckle chains, during directional solidification has become an increasingly important problem. The conditions of solidification affect the microstructure and the properties of the alloys. The macrosegregation due to the freckles defects result from the convective flow due to the density gradient in the mushy zone. The objective of this present study is analyzing the tendency to a freckle formation using PWA 1480 nickel base superalloy, which is prone to form freckles. The macrostructure and microstructure obtained after directional solidification was characterized using visual observations (VO), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).