MICROCHEMICAL INHOMOGENEITY IN A HIGHLY ALLOYED NICKEL-ALLOY

Highly alloyed, cast, and heat resistant nickel alloys are used in the domestic gas turbine equipment. These alloys possess high strength (s igma = 1050-1080 N/mm2) at t = 650-degrees-C for up to tau = 100 h. Th ese alloys are composed of Cr, Co, Mo, Al, Ti, Nb, and W along with mi croadditions of various elements which increase their mechanical prope rties. As the alloying becomes more complex, the plasticity of such al loys decreases. This is associated with an increase in the inhomogenei ty of the casting, with an increase in the amount of the gamma'-phase, and with its increased precipitation from the solid solution at high temperatures. In order to increase the plasticity, it is necessary tha t the alloys have low concentrations of gaseous impurities, nonmetalli c inclusions, structural defects, and that there is little segregation . However, in a number of cases, irregardless of the use of advanced m elting technology (vacuum induction followed by vacuum arc remelting) and homogenization of the casting, the amount of segregation remains h igh. In this article the chemical inhomogeneity of alloy EP975ID is ex amined after hardening.